National Geophysical Research Institute (NGRI), a constituent Laboratory of CSIR, was established in 1961 with the mission to carry out research in multidisciplinary areas of Earth Sciences. The Institute plays a pivotal role in the exploration of Hydrocarbons, Mineral and Groundwater resources in addition to studies in Engineering Geophysics, Seismology, Geo dynamics and Geo environment. The Institute has a staff strength of 550 that includes, About 150 highly qualified scientists doing extensive research in Earth Sciences assisted by an equal number of highly skilled technical staff for data acquisition, data processing and field investigations. Engineering geophysics Groundwater management studies Natural resource exploration Marine geophysical surveys Airborne geophysical surveys Environmental studies

Groundwater is water stored under the surface of the ground in the tiny pore spaces between rock, sand, soil, and gravel. It occurs in two “zones” an upper unsaturated zone where most of the pore spaces are filled with air and a deeper saturated zone in which all the pore spaces are filled with water.

In most of the groundwater investigations which will be carried out by using the electrical resistivity method. In this method we have to identified the subsurface features and various geological formations and most of the geophysical methods, especially the electrical resistivity is used to identify the groundwater zones in different geological terrains. We are going to discuss below the electrical resistivity method the commonly used technique in groundwater prospecting and exploration studies. The ground water as a primary drinking supply but also as a supply of water for both agriculture and industrial use. Groundwater applications of near-surface geophysics include mapping the depth and thickness of aquifers, mapping 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.

The Movement of Ground Water most ground water moves relatively slowly through rock underground because it moves in response to differences in water pressure and elevation, water within the upper part of the saturated zone tends to move downward following the slope of the water table Movement of ground water beneath a sloping water table in uniformly permeablerock. Near the surface the ground water tends to flow parallel to the sloping water table.

How water occurs in beneath ground surface? Sedimentary Rocks Primary Porosity Secondary Porosity Igneous Rocks Metamorphic Rocks

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 of rocks can be correlated with electrical conductivity signatures. The electrical resistivity surveying is based on the principle that the distribution of electrical potential in the ground around a current-carry in electrode depends on the electrical resistivity and distribution of the surrounding soils and rocks. Resistivity methods can be useful in detecting bodies of anomalous materials or in estimating the depths of bedrock surfaces and ground features related to groundwater studies but should be verified and confirmed by drilling and geophysical logging.

What is Resistivity Surveying? Method of measuring electrical current in the ground to image beneath the ground surface ❚ Detects many kinds of features ❙ layering ❙ folds, faults ❙ bedrock ❙ voids and cavities etc

Data from resistivity of apparent surveys are customarily presented and interpreted in the form of apparent resistivity (ρ a ) values. Apparent resistivity is defined as the resistivity of an electrically homogeneous and isotropic half-space that would yield the measured relationship between the applied current and the potential difference for a particular arrangement and spacing of electrodes. An electrode array with constant spacing is used to investigate lateral changes in apparent resistivity reflecting lateral geologic variability or localized anomalous features. To investigate changes in resistivity with depth, the size of the electrode array is varied.

A Practical way of Measuring Resistivity By using an experimental configuration where the two current electrodes are placed relatively close to one another and using two potential electrodes place between the two current electrodes, we can now estimate the resistivity of our homogeneous earth. The configuration of the four electrodes for this experiment is shown below. Let the distances between the four electrodes be given by r1, r2, r3, and r4 as shown in the figure.

WENNER ARRAY SCHLUMBERGER ARRAY DIPOLE – DIPOLE ARRAY GRADIENT ARRAY The diagram showing the different types of arrays used in the resistivity method.

It is a type of electrical resistivity method to know the lateral variation of subsurface resistivity in a given geological medium. In this array parameters are kept constant and the depth of penetration therefore varies only with changes in the subsurface layering. Wenner array is the most commonly used array in the electrical profiling method. The resistivity sounding is another type of electrical resistivity method to know the vertical variation of electrical resistivity of the subsurface formation. In addition, it also measures the gradient or the sharp changes due to subsurface formation.

MATERIALS RESISTIVITY(Ω.m)CONDUCTIVITY(µS/cm) IGNEOUS AND METAMORPHIC ROCKS GRANITE BASALT SLATE MARBLE QUARTZITE SEDIMENTARY ROCKS SANDSTONE SHALE LIMESTONE SOIL AND WATER CLAY ALLUVIUM GROUNDWATER SEA WATER 5 X – x10 -2 – 4X X X X X X X X – 1.7 X X X X X X X – 0.1 5

The purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. The 2D Electrical Resistivity Tomography (ERT) is widely used for mapping the subsurface geological formations and different rock structures like namely fault, fractures, joints, etc. with much more precision compared to the conventional resistivity method. The true resistivity of the subsurface geological formations estimated based on the type of the geological terrain. The ground resistivity is related to various geological parameters such as the porosity and degree of water saturation of the rock matrix. In many geological situations, 2D electrical imaging surveys can give useful results that are complementary to the information obtained by other geophysical method.

In 2D models we have to interpret the subsurface geological bodies by using the 2D electrical resistivity data. This gives the lateral and horizontal distributions of the subsurface rock types. This 2D survey can provide the more information about the subsurface with dense data density. By comparing the models, In1D model resistivity survey is usually associated with readings. While in case of 2D resistivity method we collect more number of readings and they show the accurate subsurface geological models and also it will show the detailed picture of the subsurface both in lateral and vertical direction.

THE RELATIONSHIP BETWEEN GEOLOGY AND RESISTIVITY Geologically we know that the subsurface consists of both hard rocks and sedimentary rocks and the resistivity depends on the type of rock. This will give rise to different resistivity ranges in different geological medium where in most of the igneous and metamorphic rocks shows high resistivity values. Where ever sedimentary rocks which are having lower resistivity values because of these rocks having more porous and high water content.

2D ELECTRICAL IMAGING SURVEY In 2D models we have to interpret the subsurface geological bodies by using the 2D electrical resistivity data. This gives the lateral and horizontal distributions of the subsurface rock types. This 2D survey can provide the more information about the subsurface with dense data density. By comparing the models, 1D model resistivity survey is usually associated with readings. While in case of 2D resistivity method we collect more number of readings and they show the accurate subsurface geological models and also it will show the detailed picture of the subsurface both in lateral and vertical direction.

The presence of clay minerals tends to decrease the Resistivity because : 1. The clay minerals can combine with water. 2. The clay minerals can absorb cations in an exchangeable state on the surface. 3. The clay minerals tend to ionize and contribute to the supply of free ions. Factors control the Resistivity 1.Geologic Age 2.Salinity. 3.Free-ion content of the connate water. 4.Interconnection of the pore spaces (Permeability). 5.Temperature. 6.Porosity. 7.Pressure 8.Depth

Field survey method The Electrical Resistivity Tomography technique is usually carried out using a large number of electrodes and multi-core reversible cables and are planted on the ground prior to the data acquisition. In 2D tomography method with a number of electrodes along a straight line attached to a multi-core cable and by doing this type of arrangements alternate reading it gives the apparent resistivity values depending upon the type of the rock.

The Electrical Resistivity Tomography (ERT) investigation is done using Wenner array within the National Geophysical Research Institute, Hyderabad campus by using Electrical Resistivity Tomography (ERT) Technique. The study area is mainly covered by granitic terrain of Archaean age consisting of crystalline granitic rock. On the surface it is covered by soil cover followed by weathered granite, semi-granite, and the fractured and fresh granite as the basement rock.

FIELD PROCEDURE OF ERT This technique is advanced one and is used now days for groundwater prospecting and exploration. In this technique we used main instrument like Terrameter SAS- 4000, external battery, 4 cables, switcher unit, steel electrodes, connecting clips and cable to cable connector and laptop for complete set up of the ERT lay out. The electrode spacing between the 2 distance by increasing the electrode spacing, more of the injected current will flow to greater depths The maximum distance that current electrodes can be separated by is in part dictated by the size of the generator used to produce the current and it gives the apparent resistivity of the subsurface anomalies. By using RES2DINV software we can get this inverse model of 2D section Applications of ERT include mineral prospecting, monitoring of ground water flow and archeology.

INTERPRETATION OF 2D RESISTIVITY DATA It shows the true resistivity variation of the subsurface granitic rock formation. The 2D resistivity model shows the total subsurface information up to 26.2 m depth. Towards left and near the bottom of the profile the deep red colour indicates the fresh granite country rock. The yellow colour indicates the semi-weathered granite with a resistivity, sky blue colour at the bottom end and towards right of the profile indicates the weathered granite saturated with moisture with a resistivity of which is also connected to the deeper source of groundwater. The light green colour indicates the weathered granite, and the deep green colour shows the weathered granite with less moisture content. The middle also shows the basement rock is very shallow towards left side of the profile as compared to the right side of the profile with no tectonic fracturing.