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1D and 2D Resistivity surveys for obtaining subsurface information in parts of morena district, madhya pradesh, india

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Ⅰ、Introduction

In order to get the prevailing subsurface hydrogeological conditions beneath the mountainous region in parts of Morena district, Madhya Pradesh, India a geo hydrological studies have been conducted, using Geomative GD-10 (Supreme+) resistivity imaging equipment, around by M/s HiGeoTech Services, Bhopal during 5th June to 8th June, 2020. 

Location: 

Morena district is located in the northern part of the Madhya Pradesh, bordered by Rajasthan on the West and Uttar Pradesh on the north. The adjacent districts are Gwalior and Bhind in the east and Sheopuri Kalan in the South The study area is in Morena district in central India and is about 70 km from Morena in western direction. The prominent township nearby study area is Sabalgarh and Kalaras. The studied village is well connected with village road from MorenaSabalgarh road. It falls in Survey of India Toposheet No 54 F/12. The location map of study area and part of the toposheet of study area has been shown in figure 1.2.

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Ⅱ、Investigation area analysis

Physiographically, the area is represented by north east – south west trending ridges and valleys. The ridges are represented by sand stone and the valleys by shale. The maximum elevation around study area is about 336 m amsl where as the minimum elevation is around 200 m amsl with general slope towards all direction. Kunwari river, a tributary of Chambal River, is flowing toward north east from the south of study area. As the study area is completely hilly terrain several small drainages originates from the study area in all direction (figure-1.2). The overall drainage pattern in the district is dendritic.Laterite forms flat and slightly capping over the rocks of Vindhyan super group

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Photo: Hilly region physiography and drainage system

Geology and hydrogeology

Vindhian super group of rocks, sand stones, shales and laterite are the rock types exposed in the area. The sandstones are hard and compact with siliceous matrix and as such are devoid of primary porosity and permeability. But wherever they are weathered and jointed secondary porosity and permeability is developed and made them water bearing. It is observed that sandstones in general are poorly and moderately weathered ( 2 to 4 m) and at places they are jointed and do not posses sufficient ground water potential. The shales are fine grinded and compact and are porous but are not permeable. At most places in most of the area shales are devoid of ground water but near drainage beds they form water bearing due to the presence of bedding planes and joints. Surrounding the hilly area groundwater occurs under unconfined/confined condition in weathered portions and in jointed zones.

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Photos: Vindhyan shale and sandstone geology

Ⅲ、Geophysical studies

Surface Resistivity Sounding (1D-VES)

Resistivity sounding is a process by which the depth investigation is made. In this, the center of configuration is kept fixed and the measurements are made by successively increasing the electrode spacing. The apparent resistivity values obtained with increasing values of electrode separations are used to estimate the thickness and resistivity of the subsurface formations. In Schlumberger sounding arrangement all the four electrodes are kept in a line symmetrically over a point `0; with inner (Potential) electrodes kept closer. For increasing the depth of investigation the current electrodes A and B are moved apart symmetrically about the centre point `0’ keeping the potential electrodes fixed. The separation between the Potential Electrodes is changed only when the potential between them drops to allow value during the course of sounding. Half Schlumberger (Pole-Dipole) configuration has been used in this investigation.

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Resistivity Imaging/Tomography (2D-Profiling): 

Resistivity imaging also known as resistivity tomography is an advanced development of the resistivity method. Enhanced data quality and resolution provide continuous two-dimensional resistivity models. Fifty or more electrodes are set-out in a regularly spaced array, connected to a computer-controlled resistivity meter via multi core cables. Unit electrode spacing is determined by parameters that include profile length, desired resolution and targeted depth penetration. A switching unit takes a series of constant separation readings along the length of the electrode array. The separation between sampled electrodes is then widened to increase the effective depth penetration and the procedure is repeated automatically.Advanced data processing using specialist inversion software removes distortions caused by the effects of electrode geometry, to produce a high-resolution image of the variations in ground resistivity with depth. The model is contoured using a color scale to produce a two-dimensional cross-sectional model of ground resistivity. The geophysicists corroborate the local geology with the final resistivity imaging section to provide the subsurface ground conditions beneath the profile. The resistivity sections are correlated with ground interfaces such as soil and fill layers, aquifers or soil-bedrock interfaces, to provide detailed information on subsurface geological conditions. Figure below shows an example of the electrode’s arrangement and measurements sequence for 2-D electrical imaging survey.

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The ERT data have been processed with RES2DINV software. It is a computer program that will automatically determine a two-dimensional (2-D) resistivity model for the subsurface for the data obtained from electrical imaging surveys. This program is designed to invert large data sets collected with a system with a large number of electrodes. The 2-D model, used by the inversion program, consists of a number of rectangular blocks. The arrangement of the blocks is loosely tied to the distribution of the data points in the pseudo-section. The distribution and size of the blocks is automatically generated by the program using the distribution of the data points as a rough guide. The depth of the bottom row of blocks is set to be approximately equal to the equivalent depth of investigation of the data points with the largest electrode spacing. The survey is usually carried out with a system where the electrodes are arranged along a line with a constant spacing between adjacent electrodes.

A forward modeling subroutine is used to calculate the apparent resistivity values, and a non- linear least-squares optimization technique is used for the inversion routine. The program supports both the finite-difference and finite-element forward modeling techniques. This program can be used for surveys using the Wenner, pole-pole, dipole-dipole, pole-dipole, Wenner-Schlumberger and equatorial dipoledipole (rectangular) arrays. The largest electrode spacing can be up to 36 times the smallest spacing used in a single data set. It is noteworthy that the software has a facility for editing or removing any bad data point. The main purpose of this option is to remove data points that have resistivity values that are clearly wrong. Such bad data points could be due to the failure of the relays at one of the electrodes, poor electrode ground contact due to dry soil, or shorting across the cables due to very wet ground conditions. These bad data points usually have apparent resistivity values that are obviously too large or too small compared to the neighboring data points. The best way to handle such bad points is to eliminate them so that they do not influence the model obtained. Mean- while, the true resistivity of the subsurface is then determined from the apparent resistivity measurements.

Geophysical Equipment:

Geomative GD-10 (Supreme+) resistivity imaging/tomography resistivity meter have been used for conducting the investigation. It is 120 electrode system. GD-10 series is a new electrical geophysical exploration system with highly engineered & cutting edge technology with a revolutionary change. GD-10 series supports 1D VES, 2D ERI and 3D ERT Resistivity/IP measurement, which are the versatile geophysical method apply for a broad range of applications, as groundwater and mineral exploration, infrastructure site investigations and so on. The equipment has the power of 3200 W (800V*4A). The equipment has the provision for recording the resistivity measurement with 120 electrodes at a time. These digital resistivity meters have been designed for use in shallow as well as deep resistivity survey. The equipment has the following features:

Can measure the resistivity, time domain induced polarization method (IP) and 

spontaneous potential (SP);

1)High power, wide range, high precision; 

2)Transmitter and receiver designed in one unit, light and portable;

3)The unique sub-level centralized cable system realized the electrode switching by CL-10 cable leader, no electrode limit on measurement, which features light, low cost and easy operation.

4)Using a unique scrolling technology, which can detects deeper than comparable instruments with no repeated measuring point. And the scrolling function is realized during detection, which improve the working efficiency. 

5)The data management platform, Geomative Studio, breaks through the bottleneck of traditional embedded software, which enrich the software function greatly;

6)Designed with built-in GPS, can accurately determine the measurement coordinates;

7)Multi-electrode cable with bi-direction design and dual take-outs enable the parallel electrodes for high power applications;

8)Communication port for USB data transfer;

9) High reliability, security and robustness, can use in extremely harsh weather.

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Photo: GD-10 imaging system and its accessories.

Three Dimensional (3-D) model of investigated area: 

On the basis of available hydrogeological data of study area and its integration with interpreted results of (geoelectric layer parameter) of 1D-VES and depth wise  analysis of 2D-ERT profiles a three dimensional (3-D) model and a fence diagram of  investigated area have been prepared and shown in figure 5.1 and figure 5.2. It is 23 evident from the 3-D model and fence diagram there is a presence of alternate high  and low resistivity geoelectric layer which may be representing the occurrence of  alternate layer of Vindhyan sandstone and shale in the investigated area in Morena  district of Madhya Pradesh.

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Figure-5.1Three Dimensional model of investigated area, Morena district.

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Figure-5.2: Fence diagram of investigated area, Morena district.

Ⅳ、Conclusions

The results of geo hydrological study conducted by using Geomative GD-10  (Supreme+) resistivity imaging equipment in Morena district of Madhya Pradesh,  India indicate that the study area encompasses a hilly region occupied with Vindhyan  rock formation comprised of shale and sandstone. The altitude of in and around of  study area ranges from about 200 m to 336 m amsl. There are very less groundwaters structures exist in the study area. As the study area is quite elevated  hilly region from ground, no major aquifer system has been identified within 150 m  depth. The prevailing local hydrogeological conditions of surrounding area indicate  that there may be presence confined aquifer beyond 150 m depths in investigated  hilly region. However the low resistivity layer may be indicative of saturated  weathered shale/sandstone layers which may be providing some seepage from  these layers. From the topography of the area it can be inferred that the general  groundwater flow in the area will be southwest to northeast. It is observed that the  GD-10 (Supreme+) equipment is very useful for subsurface mapping.