This document is a lab report detailing an electrical resistivity test conducted by the Department of Civil Engineering at IIT Delhi. The objective was to explore subsurface conditions for groundwater potential evaluation using geophysical methods, specifically the Wenner and Schlumberger arrangements to determine subsurface resistivity distribution. Results indicated the presence of multiple layers of soil with varying resistivity and thickness, useful for assessing aquifer characteristics and groundwater quality.

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Department of Civil Engineering, IIT Delhi
Submitted By:
Abhinav Kumar
Soil Engineering Lab
REPORT TITLE (07)
Electrical Resistivity Test
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Objective: To perform geological exploration by the geophysical method of electrical resistivity near
block IV area of IIT Delhi
Equipments :
Resistivity Meter: An instrument used to carry out
resistivity surveys that usually has a current
transmitter and voltage-measuring circuitry.
Electrodes

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Measuring tape, Hammer, Pegs, Wires for connections.
Theory:
The purpose of the electrical resistivity survey is to determine the subsurface resistivity distribution by
making measurements on the ground surface. It measures the resistance to the flow of electrons. This
method does not say anything about the engineering properties/strength of the soil. From these
measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related
to various geological parameters, such as the mineral and fluid content, porosity, and degree of water
saturation in rock.
It has the following purposes:
a. To rapidly explore the subsurface conditions in order to locate groundwater, the
thickness of overburden, depth to different rock types, and stratigraphic
features.
b. To delineate the zones of seepage & identify the source around various
structures of river valley projects.
c. Assessment of groundwater potential, quality, and determination of aquifer
characteristics.
d. To correlate data from resistivity survey with those obtained from borehole and
trial pit logs.
Based on the relative location of the current and potential rods, there are various
arrangements that can be adopted for conducting this test. Two of the most popular
arrangements are:
a. Wenner method
b. Schlumberger method
This method primarily measures the resistance to the flow of electrons. This method does not say
anything about the engineering properties / strength of soil. As the flow of current is effected by the
presence of ions, thus electrical resistivity is sensitive to moisture and salinity. The more the moisture
content, the lesser the resistivity. Thus, dry soil has the highest resistivity while the saturated soil has
the least. Also, the more the concentration of dissolved salts, the lesser the resistivity. The value of
electrical resistivity also gets effected by the presence of underground conducting material like drains,
pipes etc. The graph below shows value of Resistivity for different type of material.

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Fig- 1: Resistivity values for different material.
Based on the relative location of the current and potential rods, there are various arrangements that
can be adopted for conducting this test. Two of the most popular arrangements are:
(i) Wenner Arrangement
(ii) Schlumberger Arrangement
Fig No.2: Wenner Arrangement
The distance between current rods = 3 times the distance between the potential rods

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Fig No.3: Schlumberger arrangement
Testing Procedure, Results and Discussion:
Wenner Arrangement:
Testing is carried out by Group1 Group2 combinedly for a particular line AB parallel to block IV
building.
In the study area, an electrical current was run through a pair of electrodes (A and B) placed at varying
spacings expanding symmetrically from a central point, while an additional pair of electrodes (M and N)
measured the surface expression of the resulting potential field. The distance between M & N i.e., a is
varied as a= 1m, 2m,4m,6m and 8.0m and Resistance as well as Apparent Resistivity are measured by
Resistivity meter.
The results of Wenner arrangement are shown in the table below:
Table 1: Results of Wenner arrangement
SL No a (m) AB Resistance R (Ohms) Apparent resistivity
(ohms-metre) pa
a/ƿa
Remarks
1 1 3 5.19 32.5 0.030769
2 2 6 2.25 28.32 0.070621
3 4 12 1.55 39.05 0.102433
4 6 18 0.907 34.19 0.17549
5 8 24 0.59 29.52 0.271003

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Inverse Slope Method: Using inverse slope method, Now the graph between ‘a’ in x axis and a/ƿa
in Y axis is plotted.
Fig No. 6, Inverse slope method graph, Wenner Arrangement
From the graph, it is observed that there are 3-line segments, which indicates presence of 3 layers
Now using Inverse slope method, from graph (Fig. No. 6):
Resistivity of layer 1 = 26.67 Ohm-m, depth (d1) = 0.075 m (Thickness of first layer)
Resistivity of layer 2 = 80 Ohm-m, depth (d2) = 0.1 m, Thickness of second layer= (0.1-0.075) = 0.025 m
Resistivity of Layer 3= 22.85 Ohm-m, depth (d3) = 0.275 m , (0.275-0.1)m, Thickness of third
layer=0.175 m
Schlumberger Arrangement:
Testing is carried out by Group3 & Group 4 combinedly for a same line AB parallel to block IV building.
In the study area, an electrical current was run through a pair of electrodes (A and B) placed at varying
spacings expanding symmetrically from a central point, while an additional pair of electrodes (M and N)
measured the surface expression of the resulting potential field. The distance between M & N i.e. a is
varied as a= 1m, 2m,4m,6m and 8.0m and Resistance as well as Apparent Resistivity are measured by
Resistivity meter.
The results of Schlumberger arrangement are shown in the table below:
0
0.05
0.1
0.15
0.2
0.25
0.3
0 1 2 3 4 5 6 7 8 9
a/pa
a
Layer : 1
Layer : 2
Layer : 3
d3
d2
d 1

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Table No. 2 : Results of Schlumberger arrangement
SL
No
a
(m)
MN(m) AB
(m)
AB/2 Resistance R
(Ohms)
Apparent
resistivity
(ohms-metre)ƿa
(AB/2)/ƿa
Remarks
1 1 1 5 2.5 2.8 57.78 0.043
2 1.6 1.6 8 4.0 1.49 45.03 0.089
3 2.2 2.2 11 5.5 1.05 43.54 0.126
4 2.8 2.8 14 7.0 0.69 36.56 0.191
5 3.4 3.4 17 8.5 0.52 33.72 0.252
6 4.0 4.0 20.0 10.0 0.42 31.49 0.318
Inverse Slope Method: Using inverse slope method, Now the graph between ‘AB/2’ in x axis and
(AB/2)/ƿa in Y axis is plotted. From the graph it is clear that, in Schlumberger arrangement also we can
find presence of 3 layers of soil
Figure No. 7, Inverse slope method graph, Schlumberger Arrangement
From the graph, it is observed that there are 3-line segments, which indicates presence of 3 layers
Now using Inverse slope method, from graph (Fig. No. 7):
Resistivity of layer 1 = 22.22 Ohm-m, depth (d1) = 0.060 m (Thickness of first layer)
Resistivity of layer 2 = 53.33 Ohm-m, depth (d2) = 0.085 m, Thickness of second layer
= (0.085-0.060) = 0.0275 m
Resistivity of Layer 3= 31.37 Ohm-m, depth (d3) = 0.23 m , (0.23-0.085)m, Thickness of third
layer=0.1425 m
0
0.025
0.05
0.075
0.1
0.125
0.15
0.175
0.2
0.225
0.25
0.275
0.3
0.325
0.35
0 2 4 6 8 10 12
(AB/2)/pa
AB/2
Layer : 1
Layer : 2
Layer : 3
d1
d 2
d 3

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Resistivity VS AB/2 graph, Schlumberger Arrangement
Comparison of Wenner and Schlumberger Arrangement
SL No Wenner Schlumberger
1 0.075 m 0.060 m Thickness of 1st Layer
2 0.025 m 0.0275 m Thickness of 2nd Layer
3 0.175 m 0.1425 m Thickness of 3rd Layer
4 26.67 Ohm-m 22.22 Ohm-m Apparent Resistivity
(1st Layer)
5 80 Ohm-m 53.33 Ohm-m Apparent Resistivity
(2nd layer)
6 22.85 m 31.37 Ohm-m Apparent Resistivity
(3rd Layer)

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Electrical Resistivity Tomography:
All the groups combinedly have performed Electrical Resistivity Tomography on Demo basis. In this
process, 18 electrodes have been placed in the ground @ 1.0 m interval. The model generated by
using software is shown below as
Fig No. 8: Model developed by Electrical Resistivity Method
As per above model, the apparent resistivity varies from 29.2 Ohm-m to 52.9 Ohm-m.
Discussion:
1. The objective of this research is to evaluate the water potential of the study area by
investigating the shallow subsurface aquifer material properties and moisture distribution using
2D ERT techniques. During the shallow tube well design and development in the study area,
some patches of land does have good potential and some does not, which motivated to
understand the aquifer properties, aquifer material, and response. Hence the 2D ERT was
carried out, oriented around the pond in the center and five ERT profiles.

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Reference:
IS 15736: 2007