introduction - earth science india archival/download.php?file=pdf-… · ground magnetic method...

e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011

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Geophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site Characterization forforforfor Mining Dump WasteMining Dump WasteMining Dump WasteMining Dump Waste Rolland Andrade and Karunakar Goud.B

Any mining activity results into generation of disintegrated waste materials “Tailings”.

Tailings are also known as slimes or leach residue which are the left over materials after

the process of separating the valuable fraction of radioactive as U3O8 in Uranium mining.

Mine tailings are usually produced from the mill in slurry form, which demands proper

disposable sites, as it generally consists of trace quantities of metals/radioactive elements.

Hence, surface waste disposal site has to be characterized for its sub-surface nature to

understand its role in environmental impact due to the loading of waste materials. The

objective of this article is to provide a description of the site investigation techniques and

approaches that can be used to characterize the flow of subsurface water, map the lithology

at near surface disposal facilities. Surface geophysical techniques like ERT (2D resistivity

tomography), ground magnetic and borehole seismic studies in integration with each other

can be an ideal tool in mapping the geophysical characteristics of sub-surface formations

in and around the disposal site.

INTRODUCTION

Mining generates a large quantum of tailings otherwise termed as slimes or leach residue,

basically a mixture of fine disintegrated mineral particles and fluid, which needs to be disposed

safely without causing any environmental hazard like leaching and erosion by wind or water.

The global legacy of mining and disposal of tailings had been for more than few centuries.

Tailings facilities consist of tailings ponds or lagoons, tailings dams and tailings transport

systems (generally pipelines). Usually a very large area is required to contain the tailings

which is man-made, and is the most critical elements of these facilities. The surface disposal

site is to be characterized for its sub-surface nature in order to understand its role in

environmental impact due to the loading of tailings. Geophysical exploration plays a pivotal

role in mapping the subsurface geological formation in terms of its geophysical parameters in

and around the waste disposal site.

Several near surface geophysical explorations methods like magnetic survey, resistivity

investigations, shallow seismic survey etc., can be adopted in integration with geological

mapping or litholog analysis to better understand the subsurface geological settings below the

waste disposal site. The thickness of weathered zone (overburden), presence of intrusive

bodies, dipping beds etc., are some of the geological inferences expected from geophysical

investigations. Geophysical investigation results are generally represented in terms of

resistivity contrast, density contrast, susceptibility contrast etc., for the subsurface lithology

(geological settings). These inferences are to be corroborated with hydrological and

geochemical results from the study area to concretize the geophysical findings.


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PONDING AREA

Tailings ponds are the areas of refused mine tailings where the water borne refuse material

is pumped into a pond to allow the sedimentation (meaning separation) of solid particles from

the water. The pond is generally impounded with a dam, and is known as tailings

impoundments or tailings dams. It was estimated in 2000 that there were about 3,500 active

tailings impoundments all over the world. There are different methods for dumping mining

waste which includes valley impoundments, ring dikes, in-pit impoundments, and specially-

dug pits. The most commonly adopted method is the valley pond, in which the natural

topographical depression of the ground is taken into utility. Large earthen dams are

constructed, which may be either upstream, downstream or centerline construction, which are

later filled with tailings. In all possible instances for dump site selection, due consideration

must be given to the probable possibility of groundwater contamination and other

environmental issues. The tailings (mining waste) are generally in a semi-liquid state with

considerable amount of water present in it. Dewatering treatment is an important part of pond

storage, as the tailings are added to the storage facility, the water is removed generally by

draining into decant tower structures. Dump site (Tailings Pond) for mining waste disposal are

generally selected over a considerable areal space with supportive geomorphological settings.

Once the tailings are added, over time the intrinsic properties of the pre-existing soil and

shallow subsurface geological parameters are prone to change due to several mechanism like

adsorption, diffusion, mechanical dispersion etc. Also loading of tailings over a period, builds

the pressure on the ground surface, which is transferred to the deeper levels causing widening

of preexisting fractures or weak zones. These may behave as conduits for the tailings to

migrate to deeper levels causing groundwater contamination over time. Hence before

commissioning any waste dump site, mapping the site area through near surface geophysical

techniques would give an understanding of the sub-surface geological setting, over which a

considerable thickness of tailings is going to sit permanently.

GEOPHYSICAL INVESTIGATIONS – An overview

Exploration geophysics is the practical application of principles of physical methods (such

as seismic, gravity, magnetic, electrical and electromagnetic) to remotely sense the subsurface

anomalies. Analysis of these measurements reveals the variation in the physical properties of

the earth’s interior. Integrating the knowledge of geology, these measurements can be useful in

understanding and interpreting the anomalies with meaningful geological inference. With the

advancement in instrumentation the application of geophysical prospecting is extended in

monitoring environmental studies, investigating buried archaeological and civil engineering

sites and interplanetary imaging etc. There are different geophysical techniques as enlisted

below:

� Seismic methods, such as reflection seismology, seismic refraction and seismic

tomography.

� Magnetotellurics.

� Transient electromagnetic (EM).

� Radio frequency electromagnetic propagation (e.g., ground penetrating radar).

� Electrical techniques, including Electrical resistivity tomography and induced polarization.

� Magnetic techniques, including aeromagnetic surveys and ground magnetic.

� Gravity and Gravity Gradiometry.


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� Geodesy.

� Remote Sensing.

The use of various near surface geophysical methods which can be adopted for tracking

the movement of hazardous waste and groundwater pollution sites is often a rapid, cost-

effective means of preliminary evaluation. The information gained from a surface geophysical

survey can be used to choose optimal locations for the placement of boreholes, monitoring

wells etc., and also to correlate geology between existing boreholes. Combination of two or

more exploration techniques used in integration for interpreting the results can reduce the

degree of ambiguity. A comprehensive knowledge of the local geology and site conditions is

necessary in order to select an effective geophysical exploration technique in order to plan a

survey, and to interpret the acquired data.

NEAR SURFACE GEOPHYSICAL EXPLORATION

� Ground Magnetic Method (Total Intensity Mapping):

Ground magnetic investigations are carried out using magnetometer, an instrument which

measures magnetic field strength in units of gammas or nanotesla (1 gammas = 1 nanotesla =

0.00001 gauss). The earth’s magnetic field strength gets distorted from its regional equilibrium

state due to localized subsurface features, like buried bodies, lithological contacts, presence of

dyke or sills etc., causing anomalous signatures. The basic objective of conducting a magnetic

survey at a mining dump site or groundwater pollution site is to map these localized anomalies

and delineate the area of existence or burial of the source for these anomalies. Magnetic survey

is generally practiced perpendicular to the strike direction of host country rock or exposed /

buried features like dykes, and is carried out along transverse profile lines, aligned in a grid

format to map a2D or 3D perception of the subsurface geologic features as mentioned earlier.

There are several types of magnetometers with different working principles used for

specific problem oriented investigations, of which the total-field proton-precession

magnetometer (PPM) is most commonly used. It generally measures the total magnetic field

strength at a point of measurement, hence a scalar quantity consisting of the sum of the earth’s

field, the anomaly caused by the magnetic source if any, and the variations of the field caused

by diurnal drift, magnetic storms and micro pulsations. Of the two configuration (Transverse

profiling and Grid), traverse magnetic profiles are usually aligned in a northerly orientation in

order to define the asymmetric anomaly usually associated with buried ferromagnetic material.

Grid or traverse coordinates must be surveyed from a known land mark with GPS

(preferentially ‘DGPS’ Digital Global Positioning System) locations, such as Bench mark

points, Road milestones etc., which can be recovered at a future date. Looping magnetic survey

method is generally adopted, since the magnetic readings are repeated at the same location, it is

expected to be consistent under normal circumstances over a short period of time.

After incorporating appropriate corrections followed by processing, the analytical data is

represented either as an X-Y plot or is contoured to give an image for the gridded study area.

In practice if the terrain over which the magnetic survey measurements are carried out is highly

undulating with elevation difference of more than 3-5mts within short interval, then the

elevation correction is also incorporated. Contour image represents the total magnetic field

strength over an area of investigation, clearly delineating the anomalous zones with contrasting

field intensity change as shown in Figure.1. Further in order to quantitatively analyze the


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depth, dimension and orientation of any anomalous body, individual grid profiles are analyzed.

Usually conductive bodies like dykes etc., are mapped, which are considered to be natural

hydrological barriers. Hence an appropriate site recommendation based on magnetic

investigation results can be useful in retarding the surface water seepage into the groundwater

and further migration of pollution plume.

Fig.1: A contour image showing various anomalous zones (dyke intrusions) mapped through

magnetic survey in and around a proposed waste dump site– case study from

sedimentary terrain of Cuddapah basin.

� Electrical Resistivity Method (Vertical Electrical Sounding):

Electrical resistivity exploration method maps the subsurface electrical resistivity structure,

which is interpreted to determine appropriate geologic structure and/or physical properties of

the subsurface. The electrical resistivity of a geologic unit or target is measured in ohmmeters,

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

solids in pore fluids within the subsurface. It measures the bulk resistivity of the subsurface

which is regarded as apparent resistivity. In this survey method the current and potential

electrodes can be placed in various configurations. However, there are three commonly used

configurations called Wenner, Schlumberger and Dipole for carrying out resistivity soundings.

Selection of suitable configuration is made based on the type of investigation and area of study.

The depth of investigation and depth of resolution are two independent properties

associated with each individual configuration. Selection of appropriate configuration for

investigation requires supplementary information like well inventory report, local geology,

hydrological conditions etc., in order to have appropriate site selection. The site for carrying

out a resistivity sounding should be in an area with more or less flat terrain and also without

any visible lateral surface inhomogeneities like dykes, veins etc. Presence of appreciable

thickness of soil cover along the sounding profile line will facilitate good galvanic contact

between the ground and various electrodes. Using VES (Vertical Electrical Sounding) it is

possible to infer the depth, thickness and the nature of various layers in the sub surface at a

particular place of investigation.


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Depending upon the total area of investigation, number of VES is done either randomly or

along a profile, with particular orientation (normally perpendicular to the strike of the host

formation) and spread length. Further, the interpreted results are reduced to the mean sea level,

by incorporating elevation for each electrical sounding location. Bore well site

recommendations are made based on the VES data, which is used in conjunction with

resistivity interpreted results to prepare a 2D or 3D lithological section for the entire

investigated site as shown in Figure.2. This kind of attempt can be a useful guide for an

engineer or planner to assess a site for construction of dam axis or storage tanks for decant

pond etc.

Fig.2. Geological section prepared based on VES (inset) and bore / dug well litholog analysis-

case study from granite terrain of Nalgonda district.

� Electrical Resistivity Tomography (Two Dimension):

The new advancement and development in recent years is the use of 2-D electrical

imaging/tomography surveys to map the study line as a whole with variable depth. Hence a

more realistic model accommodating the resistivity changes in the vertical direction, as well as

in the horizontal direction along the survey line is obtained. However, it is assumed that the

resistivity change in the direction that is perpendicular to the survey line on either side is less

influential. 2-D resistivity surveys are the most practical economic compromise for obtaining

accurate results and keeping the survey costs down.

Typical 1-D resistivity sounding surveys usually involve about 10 to 20 readings, while 2-

D imaging surveys involve about 100 to 1000s of measurements, hence denser the data

acquisition, better is the resolution. In many geological situations, 2-D electrical imaging

survey can give more useful information that are complementary to the information obtained

by other geophysical methods. Two-dimensional electrical surveys should be used in

conjunction with seismic or GPR surveys as they provide complementary information support

about the subsurface. The result of a 2D ERT (Electrical Resistivity Tomography) is a pseudo-

depth representation of the area investigated in terms of its resistivity variation as shown in the

Figure.3 (Inset).


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Fig.3: 2D resistivity image showing different subsurface geology (inset; image-1 and image-2)

and a 2D Geological section prepared based resistivity results and borehole lithologs-

case study from sedimentary terrain of Cuddapah basin.

In areas of investigations, similar to the VES sounding, the orientation of profile is kept

perpendicular to any anomalous buried / exposed body to map its dimensions. Also the depth

of required information depends on suitable protocol (schlumberger, wenner, dipole etc.,) and

length of profile is selected. As discussed earlier, the data acquisition is basically a

combination of VES and profiling, hence it is possible to deduce true resistivity variation with

depth pertaining to a given electrode position. If a number of 2D resistivity investigations are

executed in a grid format, resistivity data pertaining to desired electrode position and depth can

be integrated to form a 2D or 3D section of the area of study as shown in Figure.3. This kind of

2D perception helps in deciding appropriate sites for structure making like reservoir dams,

check dams etc.

� Shear wave mapping (MASW):

Surface seismic techniques used in groundwater studies are largely restricted to either

seismic refraction or seismic reflection method. These methods usually measures the travel

time of acoustic waves propagating through the subsurface and its attenuation pertaining to the

subsurface geology is reflected. In the refraction method, the travel time of waves refracted

along an acoustic interface is measured, were as in the reflection method the travel-time of a

wave which reflects off an interface is measured. These waves are detected at the surface by

small receivers or sensors (geophones), which transform mechanical energy (ground vibration)

into electrical voltages. The voltages are relayed along cables to the seismograph, which

records the voltage output versus time, similar to an oscilloscope.

There are many types of seismic sources used to impart sound into the earth. The most

common type of source in shallow seismic investigations is a sledgehammer and strike plate.

The types of sources used are dependent on the signal versus noise ratio in the survey area.

Noise can come from vehicular traffic, people or animals walking near the geophones,

electrical current in the ground (electromagnetic interference which affects the geophone


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cables), low-flying aircraft, or any sound source. Generally, the noise can be overcome by

using a larger source, which effectively increases the signal or by proper filtering technique.

Survey design is site dependent and must be planned so that the geometry of the geophone

spread will allow the target to be resolved. The spacing of the geophone stations within the

spread can vary from several feet to tens of feet, depending on the depth of the geologic layer

and required resolution. A closer spacing of geophones within the spread is chosen when a

higher resolution of a shallow target is the objective. Shot points should extend along the entire

traverse length and show a redundant sampling of the resolved interfaces. Small differences in

horizontal displacements can cause a considerable change in the interpretation. The geophone

stations should lie along as straight a line as possible (for profile data). Deviations from a

straight path will result in ray path projection inaccuracies. Shot point and geophone elevations

are necessary in case were there is very high undulation or rugged terrain. The interpreted

results are presented either as a 2D cross section or as 3D block model as shown in Figure.4,

when a grid of data is collected within a given study area.

Fig.4: Representation of Shear wave velocity map in sedimentary and hard rock terrain and its

presentation in 2D cross section and 3D block image.

CONCLUSION

In this paper the potentiality of geophysics as an exploratory tool for monitoring the

conditions, and site evaluation for mine tailings dumping is briefly elaborated. Evaluation of a

selected site area for dumping mining waste can be done prior to its commissioning through

near surface geophysical investigations in integration with drilling program, hydrological

investigations and geological mapping studies. Presence of natural barriers like dykes, conduits

like faults etc., can be mapped and also its dimensions can be quantified. This helps in better

understanding the site and to take respective precautionary measures to avoid groundwater /

environmental pollution. It is always necessary to have periodic monitoring of tailings pond

and dam, once it is in active state and also assess the transient variation in the geophysical

signature/ parameters with natural inputs like rainwater, tailings etc. Deeper investigations

through near surface geophysical tools over the tailings can be carried out to monitor in case


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there is any leakage to the groundwater system. Such an integrated approach will be a

beneficial tool for planners and engineers to make suitable designs as per recommendations.

Acknowledgement: The authors are highly thankful to Dr.Y.J.Bhaskar Rao, Director of National Geophysical

Research Institute for his encouragement and support towards the submission of this article for publication. The

authors also like to extend their gratitude towards Dr.R.Rangarajan, for his kind support and encouragement.

BIBILIOGRAPHY

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About the authors

Dr. Rolland Andrade joined National Geophysical Research Institute, Hyderabad, in 2002 as a research

fellow under the RGNDWM Project. Since then he was associated in successful completion of several

projects both In-house and sponsored. His main area of specialization is water management and exploration.

Currently he is working on Geohydrological characterization of nuclear waste dumping sites.

E-mail: [email protected]

Mr. Karunakar Goud joined National Geophysical Research Institute, Hyderabad as a research fellow in

groundwater division under the sponsored project entitled “Integrated approach for hydro geological

management at Tummalapalle uranium project area in Cuddapah district, Andhra Pradesh”. He was also

associated in network project “Sustainable development and management of water resources in different

problematic terrain” in Ananthapur District, Andhra Pradesh

E-mail: [email protected]

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