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Groundwater Investigation Techniques-Geophysical Methods.

Unit-II

SEMINAR ON :

PRESENTED BY GOWRI V PRABHU

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•Water is one of the mankind’s most vital resources.

• In areas where surface water is not available, Ground water constitutes significant part of active fresh water resources of the world and is dependable source for all the needs.

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Geophysical Investigations•Geophysical investigations involve simple

methods of study made on the surface with the aim of ascertaining subsurface detail. This is achieved by measuring certain physical properties and interpreting them mainly in terms of subsurface geology.

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Geophysical Investigations•Groundwater Exploration project pass

through various surveys.•The main objective of these surveys is to

study and understand the hydrological cycle of the region, to understand overall concept of type, nature, no: aquifers and quality of groundwater.

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Importance of Geophysical Investigations• Geophysical methods are gaining importance

very rapidly because of their success in solving a vast variety of problems.

• These investigations are carried out quickly. This means large area can be investigated in a reasonable short period and hence time is saved.

• The geophysical instruments used in the field are simple, portable and can be operated easily. This means fieldwork is not laborious.

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Importance of Geophysical Investigations•Different inferences to suit different

purposes can be drawn from the same field data, for example electric resistivity data can be interpreted for knowing subsurface of rock types, geological structures, groundwater conditions, ore deposits depth to the bed rock, etc. Hence the investigations are multipurpose.

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Applications of Geophysical Investigations• Geophysical explorations are numerous,

important and widely varied. • Investigations aimed at locating and

estimating economically important mineral deposits.

• Investigations aimed at locating and assessing groundwater potential and its quality

• Investigations aimed at solving problems connected with geology.

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Geophysical Methods• Geophysical exploration is the scientific measurement

of physical properties of earths crust for investigation of mineral deposits or geologic structures.

• Geophysical methods detect differences in physical properties within the earth’s crust.

• Density, Magnetism ,Elasticity and Electrical Resistivity are

properties most commonly measured

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Electrical Methods•Electrical method are numerous and more

versatile, they are more popular because they are successful in dealing with a variety of problems like groundwater studies, and subsurface structure.

TYPES

•Electric methods are based on the fact that the subsurface formation, structures, ore deposits, etc. possess different electrical properties. These differences are investigated suitably and exploited to draw the necessary conclusion.

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Electrical Resistivity Method•All geological formations have a property

called electrical resistivity which determines the ease with which electric current flows through them. This resistivity is expressed in the units of Ώm ohms meter and is indicated by the symbol ᵖ

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Electrical Methods ELECTRICAL RESISTIVITY METHOD• The electrical resistivity of a rock

formation limits the amount of current passing through the formation when an electrical potential is applied.

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♦Electrical resistivity is the resistance of a volume of material to the flow of electrical current.

♦ current is introduced into the ground through a pair of current electrodes

♦ Resulting potential difference is measured between another pair of potential electrodes

♦ Apparent resistivity is then calculated as:

IVa2a

V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes).

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Electrical Resistivity method

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Electrical Resistivity method

•Profiling and Sounding are two types of resistivity investigations. Profiling is done to detect lateral changes in resistivity. This study reveals the changes in the subsurface lithology or structure from place to place.

•Sounding is done to determine the vertical changes in resistivity, this study reveals changes in lithology, at a particular place with increasing depth.

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Symmetrical Electrode Configurations • In this set-up, four electrode are placed in line

(i.e.. collinear) on the surface, two for energizing the earth and the other two for measuring the resultant voltage (potential difference). These are arranged symmetrically on either side of the point of investigation. The Wenner and Schlumberger configuration belong to this category.

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The Wenner Method• This was developed by Wenner in 1915. In this

configuration, the outer electrode C1 and C2, are used to send current into the ground and the inner electrode P1 and P2 are used to measure the potential. The important feature of this set-up is that the distance between any two successive electrodes is equal. The apparent resistivity measured with the wenner array is given by pa= 2 ∏a[∆v] /I ,

IVa2a

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The Schlumberger Method• This was developed by

Schlumberger in 1916. This method measures the potential gradient rather than the potential difference. For this purpose, potential electrodes are kept at smaller separation compared to the current .Here A and B are the current electrode and M and N are the potential electrodes. Electrodes in general MN≤1/5 AB relation is maintained in this investigation.

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The Schlumberger configuration

IV

b2/b2/L 22

a

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Sounding• This method is popularly known as vertical electrical

sounding. It is also described as ‘depth probing’. ‘electrical coring’ etc.

• In this method, Resistivity value ‘pa’ is measured at the same place by increasing the distance between the current electrodes each time after taking reading.

• This kind of successive increasing in distance makes the current penetrate more and more deeply. Hence the change in pa value measured indicate the vertical variations in the subsurface at the investigated point.

• Thus the sounding technique is useful in investigating only horizontal or gently inclined structures. This is so because, under such conditions only pa value shows variation when the successive readings are taken.

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Inverse Slope Method

Electrode spacing, a

Inverse slope method

• This semi-empirical method is mostly used in drought prone areas

where the water table is very deep. It is simple to operate and gives

good results.

• In this method Plotting are made by electrode space ‘a’ in X axis

and electrode separation divided by apparent resistivity values in

Y axis. The point of intercepts give depth of various interfaces.

a/p

a

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• In seismic method of prospecting, artificial exploration are made and elastic deformation are induced in rock present in the ground. The propagation of such seismic(elastic) waves through the geological formation is studied.

Seismic Refraction Method

Geophone

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• Depending upon whether reflected waves or refracted waves are used in the investigation, there are two types of methods, namely, seismic reflection method and seismic refraction method.

Seismic Refraction Method

• The basic components of seismic instruments are meant to take into account the chief functions involved in prospecting. These chief functions are (i) detection of signal, (ii) differentiation of signal followed by its amplification, (iii) recording of that signal. These functions are performed by the geophones, amplifier and galvanometer.

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Instruments Used in Seismic Studies• The geophone (detector), which is planted in the

ground, picks up the signal and, depending on the consequent ground displacement velocity (i.e. the intensity of disturbances suffered by the geophones), gives out a proportional voltage.

• This Voltage, after undergoing unwanted frequency filtration, is amplified in electrical circuit (i.e. signal). This is done by amplifier.

• The voltage is fed to the seismic galvanometer which has the mechanism to give out a trace of the signal on photographic paper. This photograph is the seismic record which is used for interpretation.

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Seismic Refraction Method

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Conclusion•Although groundwater cannot be seen on

earth’s surface, a variety of technique can provide information concerning occurrence of groundwater and even its quality from surface or above-surface locations.

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THANK YOU