groundwater project 8

1 . INTRODUCT ION1 . INTRODUCT ION

India is facing serious water constraints today. India is not on the whole a water

scarce country. The present per capita availability of water in India of

approximately 2200 cub. M. per annum (1998). India should be able to harness

and manage its water resources more effectively to support intensive agriculture,

to fulfill drinking water and sanitation needs of both rural and urban populations,

and also to satisfy the needs of industry.

Current water resource constraints in India, in terms of both quality and quantity,

can be expected to manifest themselves even more rapidly in the coming years.

In the past, with lower population and development levels, there was still

substantial room for each sector to satisfy its water needs and concerns

independently. Now, as the gap between the availability of water resources and

the demands on such resources narrows, the past approach to water

management pursued in India is no longer tenable. Competition for water

between agricultural and urban sectors will be a major challenge in the

forthcoming century. Further, expansion in irrigation, industry, and domestic

water demands will have serious implications for competing non-consumptive

uses, such as hydropower and navigation. Provision for environmental and

ecological concerns will have to be made.

During the last few decades, creation of surface reservoirs and the associated

irrigation canal networks had formed the backbone of our water management

strategy. Where the canal network could not reach, ground water has been

heavily exploited for irrigation. A piquant situation has, therefore, arisen; the

areas are facing declining groundwater levels. The need, therefore, is to increase

our ability to conjunctively manage ground water in a region.

The hard rock areas of the country face special problems due to the sub-surface

void space being localized to regions of fractures and fissures resulting in limited

ground water potential. Fractures may transmit large quantities of water; in other

areas, they may be nearly impervious. Because of the complex distribution of

fractures in almost every type of rock, no single method can unambiguously map

SRTRI GROUNDWATER - PROJECT PROPOSAL

fractures and their capacity for fluid movement. The need, therefore, is to develop

more efficient and reliable methods for locating zones of fractures and fissures

and estimating their water yielding potential both under natural as well as

stressed condition. In this connection, satellite remote sensing, electromagnetic

waves, electrical, optical and microwave sensors may prove to be advantageous

in identifying potential ground water areas.

In alluvial areas where ground water availability is not restricted to zones of

fractures and fissures, there has generally been large scale over exploitation of

this resource. All over the country the need is, therefore, to develop innovative

methods for conservation of rain water and renewal and reuse of waste water,

wherever possible. While conservation is necessary to evolve methods for its sub-

surface storage in large quantities in aquifers in an economical manner without

endangering their water quality. In this context it is worth noting that as a result

of over exploitation during the last four decades, both in hard rock and alluvial

areas, we have inadvertently created a void space the volume of which is several

times more than the total surface reservoir capacity in the country. Further, this

reservoir space is more where relatively more use of ground water takes place.

Successful exploitation of this space for artificial recharge of ground water under

Indian conditions with highly seasonal rainfall necessitates providing a temporary

storage for the large volume of runoff generated during short spells of high

intensity rainfall and accelerating its percolation to the water table.

2


2 . PROJECT RAT IONALE2 . PROJECT RAT IONALE

With increase in population and the demand more and more people are

dependent on the groundwater resources for Agriculture, Livestock, Domestic and

Drinking. Especially in the interior and higher parts of the Andhra Pradesh State,

the situation is acute because of low rainfall and variability. Presently many

regions are over exploited and have become critical areas where rate of

exploitation is higher as compared to recharge. Moreover there is impact of

Climate Change i.e., decrease in precipitation and increase in temperatures

effecting the semi-arid parts of Andhra Pradesh State. It is also evident that the

dependency on the groundwater resources for irrigation is ever increasing as

compared to other sources available (see Graph 1).

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

19

93

-94

19

94

-95

19

95

-96

19

96

-97

19

97

-98

19

98

-99

19

99

-00

20

00

-01

20

01

-02

20

02

-03

YEARS

LA

KH

HE

CT

AR

ES

Canals Tanks Tube Wells Other Wells Other Sources

Graph 1 Net Area Irrigated by Different Sources, Andhra Pradesh, 1993-94 to 2002-03

3


3 . AREAS OF CONCERN IN 3 . AREAS OF CONCERN IN

ANDHRA PRADESHANDHRA PRADESH

Limitations for drilling borewells

It is generally thought

that borewells can be

drilled every where and

any where. This is not

true. There is a wide

variation in the hydro-

geological conditions

within short distances.

Number of factors viz.

hydro-geological

conditions, spacing

between wells etc., have

to be considered for well

site selection. Similar is

the case for selection of

an artificial recharge

structures. At a particular

place an aquifer should

be available to recharge

it.

Based on the

groundwater estimation if

X wells are feasible in an

area, say a Mandal, it

Case Study 1 A farmer, Ananthapur District

Beneath the surface frivolity, though, is a frightening struggle for survival in a district which has seen four successive crop failures. Reddy's graveyard borewells, too, are yielding less than he had hoped for. In all, this Village Officer (VO) has spent over a million rupees in his search for water. His debts mount by the month. "Last week, I phoned on the government helpline," he says. "I cannot carry on like this. We must have some water." The helpline was set up by the State's new Government to deal with those in distress amid continuing farmers' suicides. In a State hit worse than any other by farmers' suicides, Anantapur district has seen the maximum number. Here, in the past seven years, there have been over 500 in the `official' count. And many times that number in other estimates. Reddy's call on the helpline should serve as a clear warning signal. He is in a vulnerable group, right in the danger zone. Dreaming of water, drowning in debt. The horticulture in which he has so heavily invested is in ruins. So are his many borewells. The very rich are well placed to exploit this kind of crisis. Private water markets have swiftly emerged. Desperate farmers can buy a "wetting" for their fields at a cost of Rs. 7,000 or more an acre. This might mean paying a neighbour who has managed to corral access to whatever water there is. You can also buy that resource by the tanker load for a wetting. In this setting, commerce long ago overrode community. "Can you imagine what all this does to our costs per acre?" asks Reddy.

4


may not be necessary that all the X number of wells can be sunk there. Detailed

surveys for site selection have to be taken up depending on the hydro-geological

conditions, well density, spacing between wells etc. Therefore, as mentioned

above in order to overcome these problems a basin approach for groundwater

development is necessary.

The present form of groundwater utilization has resulted in haphazard

development, and is reaching alarming proportions. Every sphere of activity, be it

domestic or municipal water supply; agriculture; or industry is dependent on

groundwater for its sustenance, either fully or partially. Coupled with this is

reduced natural recharge due to frequent erratic spells or even failure of

monsoon. This has resulted in over exploitation in certain areas. It is evident from

the fact that there is a deepening of water table, reduction in yields of wells,

drying up of many shallow wells especially dug wells, failures of new wells

constructed, salinity and quality problems.

For a developing society groundwater development is a must, but on proper and

scientific lines keeping in view the availability of the sources and requirement for

different sectoral needs; sustainability of the extraction structures and yield of

water there from, potentiality and demand and socio-economic conditions of an

area.

Future Strategies

Every one is aware of need for groundwater management, artificial recharge,

legislation etc. These tasks call for collection of a large volume of data

representing all the actual field conditions, we have in the State. Without correct

basic data and going ahead with proposed management practices, will not yield

desired results. Hence, the basic information should be cent percent, instead of

random nature of collection of data with sample checks. After getting the

information, we must proceed with further development, monitoring and

management including artificial recharge etc. As a part of this program

registration of wells is going on under WALTA.

5


Assessment of Groundwater resources in Andhra Pradesh

(See Annexure 1)

Type of Groundwater micro basins identified: Over Exploited - 118, Critical -

79, Semi-Critical - 192 and Safe - 710

Due to changing groundwater scenario and addition of more than 50,000

extraction structures every year, it is proposed to take-up revision of

estimates at periodic intervals as envisaged in the “National Water Policy” and

WALTA-02.

Andhra Pradesh Water Land and Trees Act (APWALTA) With this scenario in

place the Government of Andhra Pradesh has brought in WALTA-02. The people

intending to drill a borewell in their fields for irrigation needs should take

permission from the Groundwater Departments of the respective Districts and

having met certain criteria and procedures:

1. The particular village is not banned against drilling any new bore-wells as per

the GO issued this year.

2. A professional Geologist / Hydro-geologist / Geophysicist registered with the

Groundwater department need to explore the area using the Geophysical

prospecting instruments and should issue a certificate on the possibility of striking

the aquifer successfully and finding water.

3. Before drilling a borewell based on the report need to approach an Insurance

agency and pay the premium against failure of the borewell.

4. The borewell drilling companies are permitted to drill only after referring to the

above certificates and receipt of the Insurance premium paid.

5. Permission is also needed form the electricity department for power

connection.

6


The Gap

Most important lacuna in the whole process is the non-availability of the number

of professionals registered with the State Groundwater department. As of now

only around 250 members have registered. And most of them are concentrated

only in some areas and are not accessible to all areas. Even by rough estimate at

least 1000 such professionals are needed for serving all the 1126 Mandals in the

State for covering 21,908 Gram Panchayats.

Another lacuna is that professionals registered with the government are following

only Electrical Resistivity (ER) Method for prospecting and issuing the certificate

to the farmers. Although it is one of the most important dependable traditional

methods, it has its own advantages and disadvantages. The professionals need to

apply integrated tools and techniques for maximum probability of striking an

aquifer. There are other issues too like groundwater contamination, salinity, etc.,

and there is huge demand for such information too.

7


Article 1 State lacks muscle to regulate sinking of borewells

Wednesday, Sep 08, 2004, The Hindu. By K. Venkateshwarlu

HYDERABAD, SEPT. 7. Much as the Government and the well-meaning citizens may wish, implementation of the recently-amended Andhra Pradesh Water, Land and Trees Act (WALTA), aimed at regulating and spacing borewells to check overexploitation of groundwater, seems to have hit roadblocks, with grossly inadequate hydro geologists /geophysicists, ill-equipped machinery, lack of database and uncooperative rig owners.

"Going by the estimates of 50,000 borewell sites per annum, we require 140 teams of hydro geologists/ geophysicists, to complete the task of investigating these sites. This is against the available strength of 22 teams that will be enough for 8000 sites. The number of vacant posts of field officers is 59 with whom additional 10,000 sites could be investigated," said B. N. Prasad, Director of the Ground Water department.

Demand-supply gap

Apart from Government staff, at present, there were only 41registered private geologists/ geophysicists. Eleven of them were based in Hyderabad city and the remaining in districts leaving eight to nine districts unrepresented. In view of the huge task, the Department has also proposed creation of division-level offices. All this simply means a huge demand- supply gap.

The role of geologists has become important, as it has now been made mandatory under the Act, for the rig owner to drillbore wells only at sites identified by them. This was to guard against borewell failure, found to be one of the main reasons for farmers' committing suicide. The rig owners have to submit a certificate from department or registered geologists to the authorities.

Major obstacle

Another major obstacle in the implementation is lack of database of all groundwater structures in the State. However painstaking and laborious the task, registration of all wells and rigs needs to be taken up right away, an essential pre-requisite for regulating new bore wells, the space between them and over exploitation of ground water. The GW department has suggested that registration be taken up on a campaign mode, assigning the job to village secretary in rural areas and the Municipal Commissioner in urban areas, after a wide publicity.

8


4 . PROPOSED TRA IN ING CENTRE 4 . PROPOSED TRA IN ING CENTRE

AT SRTR IAT SRTR I

There is a huge demand and gap of professionals required; at least 750 people

are needed in the State to meet the demand for groundwater prospecting. It is

worth to note that even earlier there was demand for professionals, in the

absence of such professionals people were dependent on other means like water

divining, which is neither scientific nor dependable.

To meet the above demand SRTRI intends to establish a “Training Center for

Groundwater Prospecting” as a potential livelihood opportunity for the rural

youth. SRTRI was established in the year 1995 is committed to play a pro-active

role in the case of rural development. Institute has identified 40-45 (trades) most

viable rural friendly livelihood options for each District. The hands on training on

rural technologies are provided to rural youth; about 70 % of them have turned

into rural entrepreneurs.

This project proposal is prepared based on

1. The information collected from Literature survey & Secondary data

(information collected from various sources).

2. Critical analysis of the existing and appropriate tools and methodologies for

successful Groundwater prospecting suitable for the Geo-hydrological

conditions existing in Andhra Pradesh State.

3. Literature survey & secondary data collected on the tools and methodologies

existing like ER and VLF methods.

4. Study of scope and potential for reliable groundwater prospecting services.

5. Need for establishment of Groundwater Prospecting Training facility at SRTRI

campus for imparting trainings.

9


3.1 Training Strategy

The people identified for training would be youth who are qualified

graduates / postgraduates with Geology, Hydrogeology, Water Resources

Engineering, Geophysics and other appropriate sciences.

In this proposed center interested youth would be given training on

Groundwater Prospecting with specialization on hard rock areas.

Trainees would be oriented on all appropriate and latest technologies

available. Intensive training with practical exposure would be given on the

two important tools, Electrical Resistivity Method and Very Low-frequency

(VLF) Method. Presently most of the existing institutes are providing

training and exposure only on the traditional ER method.

The long term course would be of 3 weeks duration @ one training per

month, for the qualified youth. Trainings would be done in batches and

there would be about 20 members in each batch. Total number of trainings

/ batches covered in a year would be 12 nos and the trainees covered

would be about 240 nos in a year.

The short duration orientation trainings of 3 days duration is for orientation

to the professionals on the latest technologies. These are the

professionals who are already working in the field. The short duration

orientation trainings would be conducted once in every two months and at

least 120 professionals would be covered in a year @ 20 people per

batch.

3.2 Components of Training

Study of types of geological / geohydrological situations and the

groundwater conditions in the context of AP.

Study of various types of tools and technologies existing for ground water

prospecting, recharging, salinity and identification of contaminants, etc.

Theory and pratical exposure on the ER and VLF tools.

3.3 Livelihood Opportunities

This training would cover diverse components related to the prospecting of

groundwater in diverse geological conditions, with special reference ot hard rock

10


areas. The trainees would have multiple opportunities to choose after training,

like

As professionals can lead an independent consultancy service. All the

professionals would be immediately registered with the State Ground

water department.

Can join a consultancy group as a professional for groundwater

prospecting and recharging.

Environmental studies – contaminants, salinity, pollutants etc.

With stringent laws in place for eg. WALTA and dearth of professionals in the

State, there are ample livelihood opportunities.

Budget Requirement

A. Facilities Existing at SRTRI

Particulars

1. Lecture halls / training halls

2. Diverse topography for practicals and exposure.

3. Hostel - Boarding and Lodging facilities at SRTRI

4. Resource persons to train

B. Tools / Instruments

Aspect Quantit

y

Rate Amount

(Rs.)

Remarks

1 VLF instrument 1 @Rs

12,00,000

1200000 Based on the

make and

features

2 Electrical

Resistivity Meter

and accessories

5 nos @Rs. 40,000 200000 Best resistivity

meters

3 Computer 1 no @Rs. 25,000 25000 For data

analysis and

interpretation

11


4 Survey

instruments,

brunton compass

and other

accessories

5 nos @Rs. 5,000 25000 For laying grid

Total 250000

C. Training Material - Annual

Aspect Rate Amount (Rs.)

1 Books, Charts, Training

Material etc.

@ Rs. 1,000 per month 12000

Total 12000

D. Personnel Costs - Annual

Aspect Amount (Rs.)

1 Training expert/s for Long term

trainings

@Rs. 15,000 per batch

of training (total 12

trainings in a year)

180000

2 Training expert/s for short

term trainings

@Rs. 10,000 per batch

of training (total 6

trainings in a year)

60000

Total 240000

SUMMARY

12


PARTICULARS AMOUNT IN

RS.

REMARKS

A. Physical Infrastructure

Existing at SRTRI

- Contribution from SRTRI - the

basic infrastructure

B. Tools / Instruments 0 Repairs or maintenance as per

the warrentyC. Training Material - Annual 12000 Recurring annuallyD. Personnel Costs - Annual 12000 Recurring annually

Total Fixed cost (B) 24000

Total operation costs

(C+D)

252000 Excluding boarding lodging for

the 240 trainees, which is as

per the actualTOTAL COST (B + C + D) 300000

For establishing the proposed training center at SRTRI the total fixed costs are Rs.

1450000/- and the operational and other costs are Rs. 252000/- (excluding the

boarding and lodging for the trainees) to organize 12 batches of long term

trainings for one year and 6 batches of short term trainings.

Total budget required is Rs. 1702000

Or SAY Rs. 17,00,000/-

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5 . APPL ICAT ION OF GEOPHYSICS5 . APPL ICAT ION OF GEOPHYSICS

FOR GROUNDWATER STUDIESFOR GROUNDWATER STUDIES

Geophysical groundwater prospecting techniques, base themselves on the

detection of the abnormal physical fields associated with inhomogeneities that the

geological evolution has “printed” on the Earth’s crust structure and geological

composition (graph 2).

14


Graph 2 Inhomogeneous earth crust

The use of geophysics for both groundwater resource mapping and for water

quality evaluations has increased dramatically over the last 10 years in large

parts due to the rapid advances in microprocessors and associated numerical

modelling solutions.

Near-surface geophysics for groundwater investigations is usually restricted to

depths of 250m below the surface of the Earth. Specific groundwater applications

of the near-surface geophysics include mapping the depth and thickness of

aquifers, mapping the aquitards or confining units, locating preferential fluid

migration paths such as fractures and fault zones and mapping contamination to

the groundwater such as that from saltwater intrusion.

Many geophysical techniques have been applied to groundwater investigations

with some showing more success than others. In the past, geophysics has either

been used as a tool for groundwater resource mapping or as tool for groundwater

character discrimination. For groundwater resource mapping it is not the

groundwater it self that is the target of the geophysics rather it is the geological

situation in which the water exists. Potential field methods, gravity and

magnetics, have been used to map regional aquifers and large scale basin

features. Seismic methods have been used to delineate bedrock aquifers and

fractured rock systems. Electrical and electromagnetic methods have proved

particularly applicable to groundwater studies as many of the geological formation

properties that are critical to hydrogeology such as the porosity and permeability

15


of rocks can be correlated with electrical conductivity signatures. General

methods of practice have been produced for geophysical technqiues in

groundwater exploration; situations with complex geology and hydrogeology do

not lend themselves to this and require specific targeting of methods for

particular problems. Most geophysical techniques have been used for

groundwater characterization but once again it is with the electrical and

electromagnetic methods that the greatest success has been shown in

directly mapping and monitoring clean and contaminated groundwater.

The use of geophysics for groundwater studies has been stimulated in part by a

desire to reduce the risk of drilling dry holes and also a desire to offset the costs

associate with poor groundwater production. Today the geophysicist also provides

useful parameters for hydrogeological modelling of both new groundwater

supplies and for the evaluation of existing groundwater.

Designing a Successful Survey

Achieving a successful geophysical survey is reliant on three features:

implementing the geophysical survey early in the project planning stages,

designing the correct geophysical survey and choosing the appropriate

geophysical contractor.

Planning the Survey

Ideally the use of geophysics should be discussed early in the planning stages of a

survey in order to gain most benefit from the geophysics. Unfortunately this is not

always the case and geophysics is only used when all other investigation

techniques have failed. This has led to bad publicity for geophysics as if all else

has failed then it is unlikely that the geophysics will be successful. Geophysics is

only one tool that can be applied to a groundwater investigation and its success

must rely on the careful interpretation and integration of the results with the

other geologic and hydrogeologic data for the site. Only then will the geophysics

be a success.

Geophysics is typically used in one of two ways. Either it is used to project an

interpretation of the geology and hydrogeology from boreholes and surface

exposure into a formation or the geophysics is used in an area of unknown

geology and hydrogeology in order to better focus the direct sampling

16


programme. For both of these types of use, if the geophysics is discussed early in

the proceedings then the most appropriate techniques can be found and used in

the most cost effective manner.

Designing the Geophysical Survey

Paramount to designing a successful geophysical survey is the definition of a clear

set of objectives and the choice of appropriate methods. The objectives must be

based on reasonable, geophysically achievable criteria. For this it is important

that the geophysical target has physical properties that can be distinguished from

background signatures (geological and hydrogeological features) and background

noise (ambient cultural noise together with system induced noise). The next stage

in defining a project is to be able to provide an adequate site description along

with any previous data that has been collected, site maps or other data that

would pertain to the project. This includes logistical features such as access to the

site, noise sources and working restrictions. Finally if the results are not presented

in a manner that the client can fully understand and utilize then they are as good

as useless results.

Choosing the appropriate geophysical methods and applying the methods in an

appropriate manner is also critical to a successful survey. Only once the

objectives have been defined clearly and agreed on by both the client and the

professional can the appropriate geophysical methods be chosen. The incorrect

choice of technique and insufficiently experienced personnel conducting

the investigation has been cited as primary reasons for the failure of

many geophysical surveys.

Quality control throughout all stages of the work is paramount to a successful

outcome. Field quality control should include basic equipment calibration

procedures, accurate field reporting including field printouts of digital data,

checks for digital data recording and up-loading to computers and repeat

measurements at base or calibration sites. During processing this quality control

will include manual calculations of computer-processed data, documentation of

processing steps and separate data reviews by an independent person not

directly involved in the project.

Choosing a Professional

17


A successful survey requires the choice of an appropriate professional. This will be

one who has the general knowledge to be able to suggest the most appropriate

geophysical survey tools to meet the objectives. A good professional should also

possess the knowledge and professional integrity to admit the inadequacies of the

geophysics if it is not likely to meet the survey objectives. The professional should

then have sufficient specialist knowledge to be able to carry out the geophysics or

to suggest an expert who has the necessary specialized knowledge. It may be

that more than one professional is needed with experts for field acquisition, for

data processing and for data integration. It may often be beneficial to use more

than one professional on large investigations and have them conduct trial surveys

to test various methods before a commitment is made to a full survey

programme. This allows more precise models of the geology and geophysics to be

constructed in order to maximise the results of the geophysics. It is important

throughout the process of choosing a professional to be aware of who shall have

responsibility for the different parts of work.

6 . TOOLS AND TECHNIQUES6 . TOOLS AND TECHNIQUES

There are various tools and techniques available for groundwater prospecting

(see Annexure 2). The electrical resistivity and VLF are the two techniques found

to be most useful and effective in hard rock areas. In Andhra Pradesh State the

reliable groundwater prospecting is most needed in the drought prone, semi-arid

18


areas. Majority of such areas are located in the hard rock areas. Therefore these

two techniques are discussed here.

Electrical Resistivity

(see Annexure 3 for more details)

The electrical resistivity technique of geophysical exploration is one of the earliest

methods. The Schlumberger brothers in France in the early part of the 20th

Century did much of the early development. It is one of the most useful

techniques in groundwater hydrology exploration because the resistivity of a rock

is very sensitive to its water content. The resistivity of water is very sensitive to

its ionic content. Different stratigraphic units in a geologic section can also be

mapped as long as the units have a resistivity contrast. The utility of the method

is wholly dependent upon the size of the target and the differences between its

electrical resistivity and the resistivity of the rock surrounding the target. Often

this is connected to rock porosity and fraction of water saturation of the pore

spaces

The electrical resistivity method typically employs a direct current (DC) or a very

low frequency (<10 Hz) current which is applied to the ground using electrodes in

contact with the ground. The voltage potential is then measured between a

second pair of electrodes. A number of possible patterns of electrodes can be

used, depending upon the depth of penetration needed and the resolution

desired. A mathematical combination of the current, potential, and electrode

spacing yields the apparent resistivity of the subsurface.

Resistivity measurements are used to measure lateral or vertical changes in the

resistivity of the subsurface. To investigate the variation of resistivity with depth,

electrode spacings are gradually increased. A fixed electrode separation is

maintained along a profile line to determine lateral variations.

There are basically three types of arrays Schlumberger (pronounced “schlum-bur-

zhay”), Wenner arrays and Dipole-Dipole arrays.

The technique has been successfully employed for investigating:

19


1. Groundwater depth

2. Lithology favorable for groundwater

3. Well siting / Aquifer exploration

4. Detection of fractures and dikes

5. Location of sinkholes and cavities

6. Contamination of groundwater / Groundwater quality

7. Depth to freshwater-saltwater interface

8. Brine plumes

9. Seawater intrusion

10. Thickness of overburden

11. Geologic structure / General stratigraphic mapping

12. Archeological sites

13. Electrical grounding of large electrical installation

14. Electrical grounding of cell phone transmitting towers

Advantages of the technique are:

1. The equipment is light, portable and

inexpensive

2. Less costly than drilling

3. Qualitative interpretation of the data is

rapid and straightforward

4. Field expenses are minimal

5. It is flexible and can be used for various

purposes and depths of investigation

6. It can be used for both soundings and

profiling

7. Shallow investigations are rapid

Disadvantages of the technique are

1. Deep investigations require long cables

and consume much field time

2. Interpretation of complex geologic structures is difficult and ambiguous

3. Cultural problems cause interference, e.g., power lines, pipelines, buried

casings, fences, metal pipes, and electrical grounds can complicate

interpretation

4. Resolution.

5. Data acquisition can be slow compared to other geophysical methods,

although that difference is disappearing with the very latest techniques.

20

Figure VLF technique used for detecting the anomaly.


2. Very Low Frequency (VLF) Technique

(See Annexure 4 for More Details)

The VLF (Very Low Frequency) technique, can summarily be characterized as the

detection of electromagnetic anomalies (figure 1) caused by induction from a

primary magnetic field of worldwide distributed military use VLF transmitters

operating in the 10-30Khz range (Graph 3) and measures distortions created by

local changes in the underlying conductivity of the earth. VLF transmitters are

located throughout the world (Map1).

Graph 3 VLF as compared to other waves

21


Map 1 Very low frequency / low frequency site locations

Principles of Operation

VLF falls in the broad category of electromagnetic (EM) methods of geophysics.

The primary field (the transmitted radio signal) causes eddy currents to be

induced in conductive geologic units or structures. Faraday’s principle of

electromagnetic induction tell us that any oscillating magnetic field (e.g.,the radio

wave) will produce an electric field and hence an electric current in a conductive

media. Those eddy currents in turn create a secondary magnetic field which is

measured by the VLF receiver. The secondary or perturbed field may be phase

shifted and oriented in a different direction than the primary field depending on

the shape or geometry of the conductor, the orientation of the conductor, and the

conductivity contrast with the surrounding material (e.g., the host rock). The

instrument measures both the primary and secondary fields together (graph 4).

All VLF instruments measure two components of the magnetic field or

equivalently the “tilt angle” and ellipticity of the field. Some instruments also

measure the third magnetic component and/or the electric field. The electrical

field is measured by inserting two probes in the ground, spaced about 5 meters

apart, and measuring the potential difference at the transmitter frequency. The

22

Photo A VLF Instrument, (WADI, ABEM)


electric field provides additional information about the overburden thickness and

conductivity.

Interpretation

VLF interpretation is generally qualitative or subjective in nature. Anomalous

areas are identified and a gross characterization attached to the anomaly (e.g.,

steeply dipping conductor or thickening conductive overburden). Some simple

modeling may be carried out for simple geometric structures.

Field Procedures

VLF instruments are “back pack” portable and operated by one person generally

weigh 5-10 kg (10-20 pounds) (Photo 1). Productivity depends on the terrain and

vegetation, but generally several kilometers of line may be covered in a good day.

State of-the-art instruments include software to store the data with survey

coordinates, and may be dumped to a laptop computer at the end of the day.

Magnetic field measurements do not

require ground contact and can be

made in less than a minute at each

station. Station spacing may vary from

5 to 20 meters (15 to 60 feet)

depending on the geologic objective. If

electric field data are also acquired,

Graph 4 Primary and Secondary waves

23


probes must be pushed into the ground at each station and hence the

measurement time at each station is increased.

Advantages of VLF

Useful for detecting the water bearing zones, such as fracture or fault zones of

high electric conductivity acting as conductors, imbedded in high electric

resistivity geological formations. These features, along with the portability and

low cost effectiveness use of the VLF equipment, make it the ideal tool for

groundwater prospecting associated with deeper paths along sub-surface water

bearing bodies (fractures and fault zones) in hard rock geological environments.

They are also useful in mapping the extent of sedimentary basins (limestones,

sandstones) to define gross lithology and locating vertical faults containing water,

clay or other conductive materials.

Limitations / Disadvantages of VLF

VLF is used primarily as a reconnaissance tool to identify anomalous areas for

further investigation, either with other geophysical methods or drilling.

Weaknesses of the method include:

VLF measurements are sensitive to “cultural interference” from pipelines,

utilities, fences, and other linear, conductive objects.

Interpretation is generally qualitative in nature; quantitative modeling

requires a high data density and a well constrained model.

Topographic effects can bias the data, are difficult to remove, and are

model dependent.

VLF transmitters are subject to outages for scheduled or unscheduled

maintenance.

Unfavorable ionospheric conditions may compromise the quality of the

data.

24


7 . APPL ICAT ION AND7. APPL ICAT ION AND

L IM ITAT IONS OF VARIOUSL IMITAT IONS OF VARIOUS

GROUNDWATER EXPLORAT IONGROUNDWATER EXPLORAT ION

TOOLSTOOLS

Interpretation of the geophysics requires ground truth information in order to

calibrate the data. The noise can be electrical noise and metallic structures that

will influence electric, electromagnetic and magnetic techniques and vibration

noise that will influence the seismic techniques. In addition, geologic noise can be

defined as any signature other than the target that is present in the subsurface.

However, these small changes in physical property that are manifest in changes

in geophysical signatures, are often not evident when only considering one

geophysical signature. Thus a multi-technique approach is essential to any site

investigation. The multi-technique approach includes not only the combination of

different geophysical techniques but also the combination of both borehole

logging and surface techniques.

Surface geophysics should be used for initially locating the boreholes and for

providing information between boreholes. Integration of the surface geophysical

data and the point specific sedimentological data may be achieved by down-hole

logging of geophysical properties in cased boreholes.

The optimal ground water surveying method is no doubt drilling. This method

ensures that all necessary information is being brought up from the geological

formations. However, in order to obtain a desired degree of information from the

subsurface of a project area, drilling alone is normally not a feasible alternative.

There are a number of efficient and inexpensive geophysical surveying methods

available to the project hydrogeologist. It is worth noting at this point that these

25


are, without exception, indirect methods. This implies that neither method

measure directly what we are actually looking for. With geophysical surveys, the

target features are therefore invariably associated features. This implies that

unless we understand the water context of these features, our geophysical

surveys will be less than meaningful.

In addition, a number of complications and limitations apply. For example, during

interpretation of a resistivity survey, a thick resistive layer may have the same

signature as a thin low resistive layer; the principle of equivalence. Highly

conductive layers may limit the depths of investigation short of the target

features. A thorough knowledge of a method's limitations and assets is vital. As a

rule, considerable effects of synergy can be achieved if more than one method is

applied.

Rational for Choosing the Appropriate Method

The next logical step is then to find the most appropriate method. that fits the

project's Term of Reference, budget, as well as local conditions, the identified

target features, appropriate technology levels, logistics, etc.

A systematic approach is encouraged in the selection of adequate methods. There

are many considerations; some few examples of pitfalls are illustrated below:

A VLF survey is highly productive but would provide little useful

information if the target aquifer is a porous gravel aquifer.

A magnetic survey could be next to useless over homogenous and

unfractured sandstones.

A particular method may prove inappropriate with regards to technology

transfer within the project context.

Data acquisition and processing could be too expensive for the project

budget

Use of explosives for seismic surveys could prove to be impractical.

26


A N N E X U R E 1A N N E X U R E 1

G R O U N D W A T E R R E S O U R C E S &G R O U N D W A T E R R E S O U R C E S &

C L A S S I F I C A T I O N C L A S S I F I C A T I O N

A N D H R A P R A D E S HA N D H R A P R A D E S H

Total

Replenis

h able

Ground

Water

Resourc

es

(m.ha.m

/ Yr)

Provisio

n for

Domesti

c,

Industri

al &

Other

Uses

(m.ha.

m/ Yr)

Availabl

e

Ground

Water

Resourc

es for

Irrigatio

n In Net

Terms

(m.ha.

m/ Yr)

Utilizabl

e

Ground

Water

on

Resourc

e for

irrigatio

n in net

Terms

(m.ha.

m/ Yr)

Gross

Draft

Estimat

ed on

Prorata

basis

(m.ha.m

/ Yr)

Net

Draft

(m.ha.m

/ Yr)

Balance

ground

Water

Resourc

e for

Future

use in

net

terms

(m.ha.m

/ Yr)

Level o

ground

water

develo-

pment

(%)

Weighte

d

average

delta

(m)

Utilizabl

e

irrigatio

n

potentia

l for

develo-

pment

(m.ha)

3.52916 0.52938 2.99978 2.69981 1.01318 .070922 2.29056 23.64 0.047-

1.472

3.96008

Source: Central Ground Water Board

Years Type of Wells Yield Well

density/sq.km

1982 Dugwells 60 - 150 cu.m < 5

1983-84 Dugwells/ Dug cum borewells 60 - 150 cu.m 5 - 10

1984-94 Dugwells/ borewells 40 - 100 cu.m /

150 - 600 lpm

> 10

1994-98 Borewells/ Dug cum borewells 50 - 400 lpm / 30 -

60 cu.m

> 15

27


1998-04 Borewells/ Few dug cum borewells 50 - 150 lpm / 20 -

40 cu.m

> 20

Source: State Ground Water Board

Categorization of Over-Exploited and Dark Mandals, Andhra Pradesh

S.No Districts Over Exploited

Mandals

(>100%)

Dark Mandals (100 to 85%)

1 Anantpur - 1. Rolla, 2. Parigi 3. Yadiki

2 Chittoor 1. Tirupati 1. Chandragiri 2. Somala 3. Kammapalle

3 Cuddapah 1. Vempalli 1. Proddatur

4 Guntur - 1. Thullar

5 Karimnagar - 1.Bejjanki 2. Ramadugu 3. Veenavanka. 4

Elkathurthy 5. Metpalli 6. Kothalapur 7.

Boinappalli

6 Mahaboobnagar 1. Midjil -

7 Medak - 1.Daulatabad

8 Nalgonda - 1. Chityal 2. Marguda 3. Nutankal

9 Nizambad - 1. Armoor 2. Sirikonda 3. Kammarapally

10 Rangareddy - 1. Moinabad

11 Warangal 1 Duggondi 2

Ghanpur

1. Bachchannapet

12 West Godavari 1.

Undrajavaram

-

A N N E X U R E - 2A N N E X U R E - 2

G R O U N D W A T E R S T U D I E S - S U R V E Y T O O L S G R O U N D W A T E R S T U D I E S - S U R V E Y T O O L S

A N D T E C H N I Q U E SA N D T E C H N I Q U E S

The range of geophysical techniques used in groundwater investigations is only

briefly described herein in order to provide an introduction to the methods and

some useful literature references. All geophysical techniques measure variations

in a material's physical properties. For soils and rocks the properties can be

divided into a framework or matrix component and the pore content component.

28


Different materials exhibit different parameter signatures such as their resistivity

or its inverse conductivity, acoustic velocity, magnetic permeability and density.

These parameters are influenced by the mineral type, grain packing arrangement,

porosity, permeability, and pore content (i.e. gas or fluid type). In general no one

property is unique to any material, rather a material is described by ranges of

each property. In most geophysical surveys it is important that changes in the

geophysical parameters are measured and compared. The criteria such as fluid

type (100% fresh water saturated vs. saline water saturated),

porosity/permeability and mineral type for distinguishing different soil/rock or

aquifer/aquitard conditions is importance.

For groundwater investigations, the most significant parameters that have been

used for describing an aquifer system are ones that relate to the porosity and

permeability of the aquifer and surrounding aquitards. Electrical conductivity or

its inverse resistivity is the proportionality factor relating the electrical current

that flows in a medium to the applied electric filed. It is the ability of an electrical

charge to move through a material. It has been correlated with porosity. A

relationship often exists between electrical conductivity and the clay content or

fluid type.

Seismic velocity for both compressional waves and shear waves is related to the

elastic moduli and the density of a material. Compressional wave velocity has also

been correlated with porosity and used for determining fluid content.

The successful use of each geophysical technique is dependent not only on the

careful design of the survey but also on the consideration of a number of key

geological and cultural factors together with the geophysical data:

Nature of the target: The target geophysical signature must be different to that

of the background geology or hydrogeology.

Depth of burial of target: The depth of burial of the feature of interest is

important as different techniques have different investigation ranges. The depth

range is technique dependant however there is always a trade off between

penetration depth and resolution of the technique with respect to the feature of

interest. A technique that will look deep into the earth generally has lower

resolution than a technique that is only looking to shallow depths

29


Target size: An estimation of the target size is necessary prior to selecting

appropriate techniques.

The target size should be considered in conjunction with the depth range for

individual techniques.

Measurement station interval: This will depend on the burial depth, target size

and technique selected. Geophysical surveys have traditionally been conducted

along line profiles or on grids and therefore the station spacing along the lines

must be calculated together with the line separation in order to not miss a

particular target size or to result in spatial aliasing the target (REF). A rough rule

of thumb is that a geophysical anomaly will be approximately twice the size of the

object causing the anomaly so this will give the maximum line and station

spacing.

Calibration of the data: The key to success of any geophysical survey is the

calibration of the geophysical data with both hydrogeological and geological

ground truth information. Calibration data may be provided by both down-hole

geophysical logs in boreholes, samples derived from boreholes by continuous

sampling and by measuring what goes into and comes out of a system.

Magnetic (or geo-magnetic) Techniques

Magnetic techniques measure the remnant magnetic field associated with a

material or the change in the Earth's magnetic field associated with a geologic

structure or man-made object. They have been used for regional surveys since

the early 1900’s in the hydrocarbon industry and for longer in mineral prospecting

however little use has been made directly for groundwater studies. This is mainly

because magnetic properties have no direct relation to those properties that are

of concern to groundwater surveys. The main use for regional groundwater

investigations has been as part of combined surveys with gravity for defining

large-scale basin structures.

Magnetic surveys have also been used to identify basement faulting and other

locations of crustal weakness that may represent preferential fluid flow paths.

Large areas can be covered using airborne magnetic surveys with line and station

spacing tens of meters wide. Results of magnetic surveys are usually presented

as line profiles or magnetic anomaly maps. The airborne Magnetic and

Electromagnetic surveys were then conducted with a 100m line spacing in order

to identify the major structural controls and geologic boundaries. The airborne

30


geophysics was followed by a ground-based programme of magnetic and

electromagnetic surveying. The results showed that the major fault and shear

zones represented highly fractured good aquifers could be mapped in relatively

sparse groundwater regions. A recommendation to conduct future airborne

mapping at 50m line spacing was made. The application of magnetic surveying

for unconsolidated sequences has been somewhat limited as the magnetic

signatures for different sediment horizons are often weak.

Gravity

Common uses of gravity or micro-gravity surveys have been to record the

changes in density of material. While gravity methods have not been widely used

for groundwater applications, there are some notable examples of its use for

mapping the location of low density rocks (typically sedimentary sequences)

within more dense basement rocks. A combination of electromagnetics and

microgravity can be used to design a strategic approach to mapping karstic

features.

Other common applications are the detection of voids within the subsurface

where the small changes in the Earth's gravitational attraction caused by such

contrasts in density can be recorded with modern instrumentation. Interpretation

of gravity data however is difficult as the causes of the changes in gravitational

field can be many and varied. In addition, the collection of gravity data is typically

a slow process and thus expensive. Results are presented in a similar manner to

those of magnetic data as gravity maps and 3D models.

Electrical and Electromagnetic

Electrical and electromagnetic techniques have been extensively used in

groundwater geophysical investigations because of the correlation that often exist

between electrical properties, geologic formations and their fluid content. Most

electrical techniques induce an electrical current in the ground by directly

coupling with the ground. The resulting electrical potential is then used to

measure the variation in ground conductivity, or its inverse, resistivity.

Different materials, and the fluids within them, will show different abilities to

conduct an electric current. In general, sequences with high clay contents show

higher conductivity as do saturated sequences and especially sequences where

saline (or sometimes other contamination) fluids are present. Common field

31


practice for electrical surveying relies on directly placing an electrical current into

the ground (direct current electrical resistivity surveying) and measuring the

response (the electrical potential drop) to that current over a set distance.

The typical results of electrical surveys are electrical profiles or geo-electric

images and geo-electric depth soundings. The profile or transect method for

mapping lateral resistivity changes is now largely replaced by electromagnetic

techniques as the electrical technique is slow (when probes have to be placed

directly into the ground) and thus is not cost effective relative to the

electromagnetic techniques. Electrical methods are still widely used however for

conducting soundings and electrical cross-sections. Electrical techniques can be

divided into a number of types based on the configuration of the electrodes that

are used to input the electrical currents into the ground and the nature of the

electrical signature.

DC resistivity

The direct-current (DC) electrical resistivity method for conducting a vertical

electrical sounding (VES) has proved very popular with groundwater studies due

to the simplicity of the technique and the ruggedness of the instrumentation.

Before the vertical electrical sounding were used a failure rate of over 82% was

recorded for boreholes. With the geophysics and a combination of geological and

photogeological inspection this was dramatically reduced to less than 20%.

It has been demonstrated the potential of these techniques when combined with

those of TDEM soundings for regional schemes of hydrogeophysical

investigations. They illustrate this approach using large-scale surveys in Denmark

where widespread problems exist in supplying increasing quantities of high

quality drinking water. Of particular note in these studies is the ability to obtain

high data coverage over densely populated areas where cultural noise and man-

made conductors make geophysical surveys difficult. Also, they demonstrate the

efficient nature of the surveys where 10 to 15km of data can be obtained by a

two-person crew per day.

Induced polarisation (IP)

The measurement of induced polarisation (IP) is made using conventional

electrical resistivity electrode configuration where the voltage between electrodes

32


is measured as a decay function with time after the current has been switched off

or as the current is switched on. The technique has found most use in the search

for mineral deposits but has had some limited success in groundwater

applications.

Spontaneous Potential

The method of spontaneous potential or self potential geophysics uses naturally

occurring ground potentials from mineral bodies, geochemical reactions, and

groundwater movement. The techniques have most often been used in

exploration for mineral deposits and successful applications have been seen for

groundwater surveying in association with geothermal systems.

Telluric Methods

Telluric methods that utilize natural fluctuations in the Earth’s magnetoshpere

causing low frequency currents within the ground have been developed for

regional (deep) geologic studies over the last 30years. In the early 1970’s

controlled sources were introduced to the method (CSAMT) for increasing the

reliability of the source signatures.

Electromagnetic

Electromagnetic techniques have been extensively developed and adapted over

the last 15 years to map lateral and vertical changes in conductivity with some

spectacular examples of their use being shown for groundwater studies. While the

final output is similar to that from electrical techniques, several advantages with

the electromagnetic techniques result in an increased resolution and more cost-

effective application.

Two types of electromagnetic survey are currently practiced, i) time domain

electromagnetic (TDEM) surveys which are mainly used for depth soundings and

recently in some metal-detector type instruments, and ii) frequency domain

electromagnetic (FDEM) surveys that are used predominantly for mapping lateral

changes in conductivity. In both electromagnetic survey techniques no direct

contact is made with the ground and thus the rate of surveying can be far greater

than for electrical techniques where electrode probes must be placed in the

ground for every measurement. Both techniques measure the conductivity of the

ground by inducing an electric field through the use of time varying electrical

33


currents in transmitter coils located above the surface of the ground. These time-

varying currents create magnetic fields that propagate in the earth and cause

secondary electrical currents which can be measured either while the primary

field is transmitting (during frequency domain surveys) or after the primary field

has been switched off (for time domain surveys). Instrumentation exists to survey

to a range of depths in either transect mode or as discrete soundings. A detailed

description of the use of both FDEM and TDEM for conducting soundings is given

below.

Frequency-domain Electromagnetics (FDEM)

The technique is usually used to measure lateral conductivity variations along line

profiles either as single lines or grids of data. Further recent improvement in

FDEM has seen the integration of GPS technology with the FDEM instruments and

thus has led to a dramatic increase in the rate at which electromagnetic surveys

can be accomplished. Typical survey results for FDEM surveys are contour maps

of conductivity and 2D geo-electric sections showing differences in conductivity

along a line profile. Changes in conductivity are often associated with differences

between lithological sequences and over disturbed ground such as faulted or

mineralized zones.

Time-domain Electromagnetics (TDEM)

TDEM techniques produce one-dimensional and two-dimensional geo-electric

cross-sections in a similar manner to electric cross-sections. Survey depths for

TDEM are from 5m to in excess of 100s of meters with high vertical and lateral

resolution. The techniques do not however give high resolution from 5m to the

surface. It is also possible to conduct electromagnetic surveying using logging

tools in non-metal cased boreholes. This procedure has been shown to be

extremely sensitive to lithological changes and is important for the calibration of

the surface geophysics with sub-surface geology. Additional correlation between

electrical/electromagnetic measurements and physical samples can be obtained

by measuring resistivity in the laboratory on borehole samples.

Seismic

The use of seismic surveys in groundwater evaluation has been somewhat limited

for groundwater contamination studies however a number of examples have been

34


shown for groundwater exploration. These studies have traditionally relied on

seismic refraction techniques using compressional waves rather than seismic

reflection as used in the hydrocarbon industry because of the high costs

associated with the very close spacing necessary for the recording intervals when

looking at shallow depths. A few recent studies have used both shear waves and

surface waves but until relatively recently the technologies were not available for

correctly processing data generated from these wave types.

The majority of seismic studies have used the refraction technique to better

define the geometry of an aquifer system, that is to map the geometric relation of

the soil and rock. Some further use has been made with compressional wave

seismic for mapping the water table as there is a significant velocity increase

across the water table.

Since the late 1980’s the increase in power of personal computers and the

decrease in cost of digital acquisition systems has led to an increase in the

number of seismic reflection surveys that have been conducted. However, for

most shallow aquifers (less than 100m deep) a very close refection shot and

receiver spacing, typically less than 3m, is necessary to avoid spatial aliasing of

the data. This close spacing means that costs for conducting seismic reflection

surveys are still high. Furthermore, the processing of the seismic reflection data is

still a complex task that requires

Ground Penetrating Radar

Ground penetrating radar has seen a significant increase in use through the

1990's in near surface investigations with a number of case histories now

recorded for groundwater surveys. The increase in use has in part be stimulated

by an increase in computing power and the decrease in cost of computing.

Ground penetrating radar is an electromagnetic technique for measuring the

displacement currents in the ground. Displacement currents are defined by the

movement of charge within the ground by polarization and can be related to the

applied electrical field by the electric permitivity of the ground or the dielectric

constant.

35



E L E C T R I C A L R E S I S T I V I T Y M E T H O DE L E C T R I C A L R E S I S T I V I T Y M E T H O D

The electrical resistivity method is used to map the subsurface electrical

resistivity structure, which is interpreted by the geophysicist to determine

geologic structure and/or physical properties of the geologic materials. The

electrical resistivity of a geologic unit or target is measured in ohm-meters, and is

a function of porosity, permeability, water saturation and the concentration of

dissolved solids in pore fluids within the subsurface. Electrical resistivity methods

measure the bulk resistivity of the subsurface as do the electromagnetic methods.

The difference between the two methods is in the way that electrical currents are

forced to flow in the earth. In the electrical resistivity method, current is injected

into ground through surface electrodes, whereas in electromagnetic methods,

currents are induced by the application of time-varying magnetic fields.

Advantages A principal advantage of the electrical resistivity method is that

quantitative modeling is possible using either computer software or published

master curves. The resulting models can provide accurate estimates of depths,

thicknesses and electrical resistivities of subsurface layers. The layer electrical

36


resistivities can then be used to estimate the electrical resistivity of the saturating

fluid, which is related to the total concentration of dissolved solids in the fluid.

Limitations of using the electrical resistivity method in ground water pollution

investigations are largely due to site characteristics, rather than in any inherent

limitations of the method. Typically, sites are located in industrial areas that

contain an abundance of broad-spectrum electrical noise. In conducting an

electrical resistivity survey, the voltages are relayed to the receiver over long

wires that are grounded at each end. These wires act as antennae receiving the

radiated electrical noise that in turn degrades the quality of the measured

voltages. Electrical resistivity surveys require a fairly large area, far removed from

powerlines and grounded metallic structures such as metal fences, pipelines and

railroad tracks. This requirement precludes using this technique at many ground

water pollution sites.

However, the electrical resistivity method can often be used successfully off-site

to map the stratigraphy of the area surrounding the site. A general “rule of

thumb” for electrical resistivity surveying is that grounded structures be at least

half of the maximum electrode spacing away from the axis of the electrode array.

Electrode spacings and geometries or arrays (Schlumberger, Wenner, Dipole-

dipole) are discussed in detail in the section below entitled, “Survey Design,

Procedure, and Quality Assurance”. Another consideration in the electrical

resistivity method is that the fieldwork tends to be more labor intensive than

some other geophysical techniques. A minimum of three crew members are

required for the fieldwork. Instrumentation Electrical resistivity instrumentation

systems basically consist of a transmitter and receiver. The transmitter supplies a

low frequency (typically 0.125 to 1 cycles/second or “Hertz”) current waveform

that is applied across the current electrodes. Power for the transmitter can be

supplied by either batteries or an external generator depending on power

requirements. In most cases, the power requirements for most commonly used

electrode arrays, such as Schlumberger (pronounced “schlum-bur-zhay”) and

Wenner arrays, are minimal and power supplied by a battery pack is sufficient.

Other electrode configurations, such as Dipole-dipole arrays, generally require

more power, often necessitating the use of a power generator. The sophistication

of receivers range from simple analog voltmeters to microcomputer-controlled

systems that provide signal enhancement, stacking, and digital data storage

capabilities. Survey Design, Procedure, And Quality Assurance Survey design

depends on the specific characteristics of the site and the objective of the survey.

The three most common modes of electrical resistivity surveying are profiling,

37


sounding, and profiling-sounding, each having its own specific purpose. If the

purpose of the survey is to map the depths and thicknesses of stratigraphic units,

then the electrical resistivity data should be collected in the sounding mode.

Lateral electrical resistivity contrasts, such as lithologic contacts, can best be

mapped in the profiling mode. In cases where the electrical resistivity is expected

to vary both vertically and horizontally, such as in contaminant plume mapping,

the preferred mode is profiling-sounding. Sounding Mode: The two most common

arrays for electrical resistivity surveying in the sounding mode are the

Schlumberger and Wenner arrays. Electrode geometries for both arrays are shown

below. The depth of exploration is increased by increasing the separation of the

outer current electrodes, thereby driving the currents deeper into the subsurface.

Profiling Mode

The two most common arrays for electrical resistivity surveying in the profiling

mode are the Wenner and dipole-dipole arrays. The electrode geometry for the

Wenner array is the same as the sounding mode — the difference is that in

profiling mode the entire array is moved laterally along the profile while

maintaining the potential and current electrode separation distances.

The electrode geometry for the dipole-dipole array is shown in Figure 9-1. In the

profiling mode, the distance between the potential and current dipoles (a dipole

38


consists of a pair of like electrodes) is maintained while the array is moved along

the profile. Profiling-Sounding Mode: As in the profiling mode, the Wenner and

dipole-dipole arrays are the most common arrays used in the profiling-sounding

mode. As the name implies, this mode is a combination of the profiling and

sounding modes. In the Wenner array the typical field procedure is to collect the

data in a succession of profiles, each having a different electrode separation. The

resulting data therefore contains information about the lateral and vertical

electrical resistivity variations.

39


In the dipole-dipole array, the typical field procedure is to transmit on a current

dipole while measuring the voltages on up to six of the adjacent potential dipoles.

When the data collection is completed for the particular transmitter dipole, the

entire array is moved by a distance equal to one dipole separation and the

process is repeated.

The most frequent source of inaccuracy in electrical resistivity surveying is the

result of errors in the placement of electrodes when moving electrodes and/or

expanding the electrode array. These distance measurement errors are easily

detected on apparent electrical resistivity versus electrode separation curves and

for this reason the apparent electrical resistivities should be plotted as the data is

40


acquired in the field. A qualified field geophysicist will recognize these errors and

direct the field crew to check the location of the electrodes. The second most

common source of error in electrical resistivity surveying is caused by the

electrical noise generated by powerlines. The most effective means of reducing

powerline noise is to minimize the contact electrical resistance at the potential

electrodes. This can be easily accomplished by using non-polarizing potential

electrodes along with wetting the soil under the electrode with water. Non-

polarizing electrodes are recommended instead of metal potential electrodes,

because the metal electrodes generate electrical noise due to oxidation reactions

occurring at the metal-soil (pore water) interface. Resistivity Data Reduction And

Interpretation Reducing electrical resistivity data is a simple process in which the

apparent electrical resistivity is calculated by dividing the measured voltages by

the applied current and then multiplying this quotient by the geometric factor

specific to the array used to collect the data. Once the apparent electrical

resistivities have been calculated, the next step in the interpretation process is to

model the data in order map the geologic structure. The method used to model

the apparent electrical resistivity data is specific to each data acquisition mode.

Electrical resistivity data acquired in the sounding mode, using either the Wenner

or Schlumberger array, can be modeled using master curves or computer

modeling algorithms. When using master curves, the interpreter attempts to

match overlapping segments of the apparent electrical resistivity versus electrode

separation plots with a succession of two-layer master curves. This modeling

method provides coarse estimates of the model parameters, is time consuming,

and requires skill on the part of the interpreter. An alternative method of

modeling sounding resistivity data is to use readily available computer modeling

software packages (Sandberg, 1990).

There are a variety of different types of algorithms; some assume discrete

electrical resistivity layers while others assume that electrical resistivity is a

smooth function of depth. The discrete layer algorithms require interaction on the

part of the interpreter, but allow for constraining model parameters to adequately

reflect known geologic conditions. The continuous electrical resistivity algorithms

are automatic, that is, they require no interaction on the part of the operator, and

therefore geologic constraints cannot be incorporated into the models. The

modeling of profiling and profiling-sounding mode data is much more involved

than in the case of sounding data. The profiling-sounding data reflects electrical

resistivity variations in the lateral and vertical directions, resulting in a much

more complicated computer simulation of the potential fields. The computer

41


techniques capable of simulating these fields are finite difference, finite elements

and integral equation algorithms. All of these techniques are extremely time

consuming, and therefore expensive, and require a detailed understanding of the

underlying physical principles on the part of the interpreter. For these reasons

most profiling-sounding mode data is interpreted in a qualitative manner, with the

accuracy of the interpretation being based solely on the experience of the

geophysicist. Presentation Of Results Listings of the electrode separations, current

amplitudes, measured voltages and reduced apparent resistivities should be

included in the report. Any specific information regarding the manner in which the

data were reduced or modeled should outline in the report. As with data

interpretation, presentation of the final results are specific to the mode of data

collection. Sounding Mode: The electrical resistivity data collected in the sounding

mode are presented as a bilogarithmic plot of electrical resistivity versus the

distance from the current electrodes to the center of the array. If the d ere

modeled, the apparent electrical resistivities, as calculated from the model,

should be presented on the bilogarithmic plot with the observed apparent

electrical resistivities. In addition, the model should be presented in a section plot.

Profiling Mode: Data collected in the profiling mode are presented in a plot of

apparent electrical resistivity versus distance. Any modeling results, either using

computer algorithms or by “rule-of-thumb” methods should be presented and

include a legend indicating any parameter values. Profiling-Sounding Mode: Data

collected in the profiling-sounding mode are presented in psuedosection format in

which the apparent electrical resistivity is plotted as a function of position and

electrode separation. Any modeling results either using computer algorithms or

qualitative methods should be presented and include a legend indicating

parameter values.

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V E R Y - L O W F R E Q U E N C Y ( V L F ) M E T H O DV E R Y - L O W F R E Q U E N C Y ( V L F ) M E T H O D

The very-low frequency (VLF) electromagnetic method detects electrical

conductors by utilizing radio signals in the 15 to 30 kiloHertz (kHz) range that are

used for military communications. The VLF method is useful for detecting long,

straight electrical conductors, such as moderate to steeply dipping waterfilled

fractures or faults.

The VLF instrument compares the magnetic field of the primary (transmitted)

signal to that of the secondary signal (induced current flow within the subsurface

electrical conductor). In the absence of subsurface conductors the transmitted

signal is horizontal and linearly polarized. When a conductor is crossed, the

magnetic field becomes elliptically polarized and the major axis of the ellipse tilts

with respect to the horizontal axis (McNeill, 1988). The anomaly associated with a

conductor exhibits a crossover. As with other frequency domain electromagnetic

systems,both the in-phase (“real” or “tilt-angle”) and the outof-phase

(“imaginary”, “ellipticity”, or “quadrature”) components are measured.

A number of VLF transmitting stations operated by the military are located

worldwide; the most commonly used in North America are Annapolis, Maryland

(21.4 kHz), Cutler, Maine (24.0 kHz), and Seattle, Washington (24.8 kHz) stations.

Commercially available VLF systems utilize one or more of these transmitting

stations for survey applications.

Advantages

The VLF method is very effective for locating zones of high electrical conductivity,

such as water-filled fractures or faults within the bedrock. Structures often act as

conduits along which ground water and contaminants flow. The information from

a VLF investigation can be used to optimally locate monitor and/or treatment

wells in order to intercept these hydrologic conduits. Another advantage of VLF is

that data collection is fast, inexpensive and requires a field crew of only one or

two people.

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Limitations

The VLF method is affected by all electrical conductors, including those that are

man-made (powerlines, wire fences, pipes, and so on). The bearing or direction

from the VLF transmitting station to the intended target must be located nearly

parallel to strike (or long axis) of the conductor, or intended target for it to be

detected. Unfortunately there are only a limited number of transmitting stations

available with enough primary field strength to be usable, thus limiting the

direction that traverses can be collected. Therefore, the geometry of the target,

the survey traverses and the bearing to the VLF transmitting station(s) must be

resolved in the survey plan.

VLF transmitting stations often shut down for scheduled and unscheduled

maintenance. If this happens, another transmitting station may have to be used

or data collection may have to be halted until the transmitting station resumes

operation. Care must be taken to make sure that the antenna of the VLF receiver

is correctly and consistently oriented (always oriented in the same direction for all

stations of a traverse).

Instrumentation

VLF instruments have historically fallen into two types. Early instruments were

hand-held, and measured the tilt-angle of the major axis of the magnetic field

polarization ellipse. This angle is obtained by rotating the instrument until a null is

obtained (indicated audibly through a speaker); then, the angle is read from an

inclinometer mounted on the instrument case. Some instruments of this type also

could provide reading indicating the magnitude of the maximum inphase

component.

More recent instruments are either belt or backpack mounted due to the

increased weight of batteries needed for microprocessors which control these

devices. These instruments measure both in-phase and quadrature components

of the ratio of horizontal-to-vertical magnetic field. Some instruments have real-

time interpretive capability for use while still collecting data.

In either case, the measured quantity is such that variations in the source field

over time (from atmospheric fluctuations or actual signal-strength changes) are

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normalized out and the resulting information is repeatable hour-to-hour or day-to-

day.

Survey Design, Procedure, and Quality Assurance VLF data are normally collected

along traverses, and anomalies are correlated from traverse to traverse.

When planning a VLF survey several considerations must be taken into account.

First, is the direction of strike of the target. Traverses must be located

perpendicular to strike so that anomalous zones can be compared to background

levels. Every effort should be made to avoid putting traverses in areas that

contain a number of cultural features that may mask anomalies associated with

the intended target. Second, consideration must be given to which transmitting

stations are available for use during the survey. The direction toward the

transmitting station must be nearly perpendicular to the traverse (or in line with

the strike of the target).

When designing a survey, several traverses should be placed parallel to one

another and close enough (25 to 50 feet apart) so that anomalies can be

correlated from traverse to traverse. It is crucial that traverses are long enough

that the entire anomaly caused by the target is covered and the readings return

to a background level. Data can be collected on a grid; however, the data must be

collected along grid lines that are perpendicular to the target. Station spacing

should be close enough together that the entire form of the anomaly can be

observed (15 to 30 feet).

Each traverse must be accurately located on a map and related to a point or

landmark that can be recovered later.

During data collection, care must be taken to properly orient the VLF receiver

antenna and to consistently collect data facing the same direction. Failure to do

so will result in anomalies that do not “cross-over” in the proper sense and could

result in improper interpretation of these data. Careful field notes should be kept

while collecting data, noting the location of any cultural features (including buried

pipes, wire fences, powerlines, fieldstone or concrete walls, and building

foundations). Keeping careful and observant field notes will save time when

interpreting the data.

If the transmitter stops transmitting during data collection, another transmitter

may have to be used. If this happens, the entire traverse should be read again

using the new transmitter station. In some cases, another transmitter that is

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located in the correct orientation may not be available. In this instance, data

collection will have to cease until the transmitter station resumes operation. It is

best if the same transmitter station can be used during the entire survey, because

strength and orientation of different transmitters can lead to slightly different

shaped anomalies, making the data more difficult to interpret.

To ensure data quality and to help in data interpretation, it is suggested that

readings be taken along the traverse using more than one transmitting station.

This does not add significantly to the amount of time it takes to collect data, and

often improves the accuracy of the interpretation.

Data Reduction and Interpretation

Most commonly used VLF interpretation methods are qualitative. Data collected in

the field can be interpreted without further data reduction. By plotting the “real”

and “imaginary” components versus distance along a traverse, an experienced

geophysicist can often interpret where fractures or zones of high electrical

conductivity are located.

Filtering techniques are often used to enhance data and make tilt-angle

crossovers easier to identify. Two commonly used filtering methods include the

Fraser filter (Fraser, 1969) and the Karous-Hjelt filter (Karous and Hjelt, 1985). The

Fraser filter simply converts tilt-angle crossovers into peaks. The Karous-Hjelt

filter calculates the equivalent source current at a given depth, commonly known

as current density.

This current density position can aid in the interpretation of the width and dip of a

fracture with depth. Commercial programs are available to calculate and plot data

using the Karous-Hjelt filter. Using such a program, current density can be plotted

with respect to depth and gray-tone plots can be created to further aid in

interpretation.

In order to determine the strike direction of a fracture it is necessary to have two

traverses (preferably more) close enough to one another so that the same

anomaly can be correlated from one traverse to the other. By stacking sets of

profiles it is then possible to correlate fractures or conductive zones across the

entire survey area. Once the strike direction of a fracture has been determined,

the fracture can be projected along strike to determine if it intersects any areas of

interest. Projecting fracture zones along strike can also aid in determining where

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to place monitor and/or treatment wells, or where contaminants can migrate in a

fracture-flow system.

More quantitative methods of interpretation include curve matching. Vozoff and

Madden (1971) developed a number of interpretive curves which can help in the

interpretation of VLF data. Simple, numerical forward modeling can be

accomplished done using formulas found in Telford and others (1976). It must be

emphasized that when modeling, a number of assumptions are made, some of

which may be incorrect in a given situation.

If enough parallel traverses are collected it is possible to contour the data to

further aid in identifying zones of increased conductivity. If the data is to be

contoured, filtered data should be used so that the zones of increased

conductivity correspond to “highs” on the contour map.

Presentation of Results

The report should explain the methods and the reasoning behind the methods

used for data collection.

Explanations for what transmitting station was used, the traverse station spacing

and field procedures should be discussed in the report. Any problems encountered

during data collection (such as a transmitting station shutting down, or excessive

atmospheric interference) should be noted.

The most common way to present VLF data is to plot the “real” and “imaginary”

component values on the y-axis and distance along a traverse on the x-axis of a

plot.Plots for each traverse should appear in the appendix of the report. All of the

plots should be drafted at the same vertical and horizontal scales for consistency

and ease of comparison. The location of cultural features, as well as areas

interpreted as fracture zones should also be indicated on annotated plots.

The locations of the traverses should be shown on a base map. It is also useful to

identify anomalies interpreted as fracture zones on the map. The correlation of

anomalies from traverse to traverse should also be indicated on the map, in order

to delineate the continuation of interpreted fractures.

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Case Study Groundwater exploration using VLF

Study in Granitic terrains of Northwestern Portugal

A groundwater exploration program using the VLF technique was carried out in

the Vieira do Minho area (Northwestern Portugal). Geological setting is dominated

by biotite-rich coarse-grained porphyritic granite crossed by quartz veins and

some basic rock dikes. Several sets of fractures break up the granite massif

yielding a chaotic relief constituted by the individual rock blocks generated by

rock fracturing.

The VLF data was collected with WADI, a two component magnetic receiver

developed by ABEM Corporation that operates in the frequency range 15-30 kHz.

The measurements of the VLF campaign over the area were carried out with

various profiles of varying length. Readings along those profiles stepwise 5 m

intervals. Another important issue involved in the planning of the surveys was the

necessity to maintain the orientation of the profile when taking the readings along

its length. So, a system was constructed to help in this matter, consisting of a wire

marked every 5 meters tightly stretched along the profiles.

The profile schematic applied on the area consisted of grid and isolated profiles.

The first aimed at determining the spatial distribution, accordingly to the terrain

possibilities, of the promising structures detected by the exploration goal of the

seconds. Thus far, the VLF campaign over the study area has generated 21

profiles between grid and isolated forms. The data of these profiles that,

summarily, is analyzed on the base of the relation between the horizontal primary

magnetic field and a vertical secondary magnetic field originated by induction on

a sub-surface conductor, was performed automatically with ABEM’s SECTOR

program. The Karous Hjelt filter offers the possibility to generate current density

pseudo-sections which, by showing the distribution of the apparent current

density along the depth, provides a pictured image that can give an idea of the

conductors geometry that originated the anomaly.

The data thus far gathered suggests that most of the anomalies detected are

small and shallow. They also seem more to translate a series of high density

fracture zones, poorly penetrating, rather than one isolated big fracture or fault

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zone, being difficult to extract a general dip orientation. Supporting this

conclusion is a grid with 10 profiles oriented N240º that, when analyzed with filter,

depths below 10 meters depict two more conspicuous lineaments with a general

orientation of 190º-200º, thus requiring a reorientation of the profiles in order to

explore an adequate perpendicular direction to those lineaments. The current

density pseudo-sections for these profiles reveal a rather small depth reach of

most conductors, although in some profiles some conductors reach as much as 30

to 40 meters deep. Also, a grid consisting of 3 parallel profiles orientated N-S

shows two lineaments approximately perpendicular to that orientation. In one

case, there is a profile with an orientation N52º that clearly depicts a shallower

(20 meters deep aprox.) conductor at 120-130 meters in length indicating a dip

towards NE and a deeper (40 meters aprox.) vertical conductor at 150-170 meters

in length.

The interpretation of the data so far gathered in the VLF prospecting campaign

enables to define some promising areas of hydrogeological potential, on the basis

of preferential lineaments identified and also taking in to consideration the

general depth of the conductors they contain.

As an end remark, a reference should be made to the difficulty of selecting an

adequate transmitter whenever the profiles require their orientation to be around

NW. This leads to the conclusion that the prospecting potential of this equipment

could greatly be enhanced with a portable VLF primary magnetic field generator

with the inherent disadvantages that such equipment would carry.

By Dr. N. Sai Bhaskar Reddy

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