Dr. Mahadev Unde, et. al.

International Journal of Engineering Research and Applications

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ISSN: 2248-9622, Vol. 10, Issue 5, (Series-I) May 2020, pp. 25-30

RESEARCH ARTICLE OPEN ACCESS

Soil Resistivity Measurement and Interpretation Technique

Dr. Mahadev Unde*, Vishal Tathe**
Department of Electrical Engineering, Zeal College of Engineering & Research, Pune
Corresponding Author: Dr. Mahadev Unde

ABSTRACT
Soil resistivity is one of the important key parameters which affects the performance of substation grounding
system. It is a function of depth. The substation grounding performance parameters like ground resistance,
ground potential rise (GPR), touch and step voltages, all are dependent on soil resistivity. Therefore, soil
resistivity measurement is very essential at the site of all generating stations and large substations to develop
suitable soil model which is like actual one. In this paper, various soil resistivity measurement techniques are
discussed. Experimentation has been carried out for measurement of soil resistivity. Further, it modelled for
various soil layer using various techniques. Modelling of soil structure makes the substation grounding cost
effective by means of optimizing length of ground rods. It also enhances the safety of the substation operators
and equipments.
Keywords -Soil model, Soil resistivity, Grounding, Ground rod, Wenner method.
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Date of Submission: 28-04-2020 Date of Acceptance: 11-05-2020
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against the inter electrode spacings. By visual
I. INTRODUCTION inspection, the rough estimate of number of layers,
Electrical conduction in soils is essentially their resistivities along with layer depth is possible.
electrolytic. The surface soil layer consists of clay However, computer with software simulation should
mixed sand and often mixed with decayed vegetable be used to obtain the desired accurate soil model
matter also. When dry this soil may not conduct which is close to actual true soil resistivity data
much electricity. However, in the presence of [8 ,9]. Soil resistivity varies horizontally as well
moisture, ionic conduction takes place according to vertically. It varies region to region and within the
the type of slots present in the water contained soil same substation. Soil resistivity also changes season
[1]. Soil resistivity is dependent on physical and to season. During dry season, it increases whereas it
chemical composition of soil, grain size, soil decreases during rainy season. In mountain and hilly
compactness, porosity, moisture content and areas, during winter due to ice fall, moisture in the
temperature. Resistivity of soil can vary within soil freezes and soil resistivity increases dramatically
extremely wide limits, between 1 Ω m and 100,000 high. Therefore, substation grounding grid designed
Ω m. It depends on type and nature of the soil [2-4]. in one season which is safe, may become unsafe in
The grounding performance parameters like another season [10].
ground resistance, GPR, touch and step voltages, all Based on soil resistivity measured data, soil
are dependent on soil resistivity. Therefore, soil models have been obtained. The result revels that for
resistivity measurement is very essential at the site a given data four-layer soil model is the most
of all generating stations and large substations to suitable. The safe and cost-effective substation
develop suitable soil model which is like actual one grounding is possible if the ground rods reaches the
[5]. The most popularly used method is equally low soil resistivity third layer. The paper is
spaced four probe Wenner‟s method. It is necessary organized as follows. The introduction followed by
that the measurements are made by increasing probe methodology is given in section II where the soil
spacing geometrically from small values up to about resistivity measurement methods are elaborated. The
extent of substation grounding grid. The soil resistivity measurement has been depicted in III
measurement should carry out at various profiles so whereas soil modelling has derived in section IV.
that it covers entire substation area [6,7]. The concluding remarks are drawn in section V.
The shallow depth soil resistivity is a II. METHODOLOGY
concern of surface potential whereas the deep level
resistivity is a function of grounding grid resistance. Soil resistivity is one of the important key
It is necessary to plot the graph of soil resistivity parameter which affects the performance of
grounding system. Many tables on soil resistivity are

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Dr. Mahadev Unde, et. al. International Journal of Engineering Research and Applications
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ISSN: 2248-9622, Vol. 10, Issue 5, (Series-I) May 2020, pp. 25-30

available in the literature which shows the range of the information of soil resistivity at a depth equal to
resistivity for various types of soils and rocks. But it probe spacings.
yields rough information on resistivity. Further,
resistivity of soil changes geographically from one 2.1.1 Interpretation
region to other. Therefore, actual measurement of
The current tends to flow near the surface
soil resistivity is imperative. It is very essential to
of earth for the small probe spacing, whereas more
know the soil resistivity at substation site for good
of the current penetrates deeper soils for large
estimation of soil structure and hence the design of
spacing. Since, grid is generally buried near to the
substation grounding [11].
surface of the soil, few readings are important at a
2.1 WENNER METHOD small probe spacings. Further, for effective use of
ground rods, the knowledge of deeper soil resistivity
Dr. Wenner‟s equally spaced four
is important [13]. The soil resistivity should be
electrode/probe method is most popularly used for
measured along number of profiles at different
soil resistivity measurement. The four probes must
locations, so that whole substation area is covered.
be inserted at equal distances along the straight line
The measurement should be carried out at
known as profile. The arrangement is as shown in
exponentially increasing probe spacings. The
figure 1. The battery-operated meter circulates the
question may come in once mind that how many
current I between the outer probes and . The readings are sufficient and to what extent must be
current (I) is injected into ground through probe C 1 the probe spacings? Answer to this question is very
and it is collected at probe C2. The potential simple ie there must be at least two profiles at one
generated is measured between inner probes and location. Typically, if the extent of substation is say
. The earth tester directly reads the ratio of V and 240 -300 m, then the probe spacing may be 0.5 m, 1
I that is resistance R and not the resistivity. The m, 2 m, 3 m, 5 m, 7 m, 10 m, 20 m, 30 m, 50 m, 70
outer electrodes and are known as current m, 100 m, 200 m, 300 m and may be up to 500 m.
electrodes whereas the inner two electrodes and While hammering the probes into hilly area, it‟s
contact with soil may become loose. To have a firm
are designated as potential electrodes [12].
contact it is advised to pour 100- 200 ml of water
around the probes. If the probe spacings becomes
more than 100 m, the current probes can be replaced
by multiple probes to reduce probe earth contact
resistance. If two probes are used, they may be
inserted one meter apart & short circuited by thick
copper wire. They can also be arranged in triangular
position one meter apart.
According to principle of reciprocity
theorem, the user can exchange current and potential
Figure 1. Equally spaced four probe Wenner method
electrode positions as shown in figure 2. When the
If the probe spacing is „a‟ and depth of probe in the
potential electrodes are moved outside and current
earth / ground is „b‟ as shown in figure 1, then the
electrodes inside; the measured result will not be
apparent soil resistivity is given by
altered [14].
The test wires should be insulated and should not
have joints. There should be firm contact to test
However, the depth of installation of electrodes / probes and terminals of the earth tester. As far as
probes is very small as compare to their separation. possible, wires from potential terminal may not run
It should not exceed 0.1 a. Then, the user can parallel to the current terminal wires. Otherwise,
assume b=0 and equation (1) will become meter will read additional resistance known as
mutual resistance.

It is necessary to take the number of readings at A set of readings taken at different profiles
exponentially increasing probe spacings from a can be gathered together to find the arithmetic
small value to up to the extent of the grid electrode. average of resistivity at the same probe spacings.
To speed up the measurement process, put the When the apparent resistivity plotted against probe
potential probes at half the desired probe spacing spacings yields valuable rough information like
and current probes at 1.5 times the probe spacing different soil layers, thickness of layers and
away from reference center axis. The method gives resistivities.

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ISSN: 2248-9622, Vol. 10, Issue 5, (Series-I) May 2020, pp. 25-30

unequal spacing four probes Schlumberger - Palmer

method is recommended.
In this method, the potential probes are
placed close together, and current probes are placed
farther apart as shown in figure 3. The Wenner
method requires all four probes to be reinstalled for
each soil resistivity measurement whereas;
Schlumberger- Palmer method requires only the
Figure 2. Interchanging current and potential outer probes to be reinstalled for measurements [15].
probes, Wenner method If the depth of burial of probes is small, as compare
2.1.3. Derivation to their spacing a and c, and , then apparent
soil resistivity is given by
If current probes are hemispherical then, the
potential at any arbitrary point (x), from current
source is given by Equation (10) indicates the soil resistivity at the
approximate depth of (2a+c) / 2 which is the
distance of current probe from centre axis.
Since, the size of current electrode is much smaller
as compared to inter electrode spacing, the current
distribution in the ground may be considered as a
radial. During soil resistivity measurement, the
current discharged into earth from electrode is I
and that collected at electrode is –I. As a result,
the voltage is induced at electrodes and .
Referring the figure 2, the potential generated at Figure 3. Unequally spaced Schlumberger-Palmer
electrode is given by method
2.2.1 Derivation
If the current probes are hemispherical,
Similarly, the potential at electrode is The potential at probe

Then, potential difference between and is Similarly, the potential at electrode is

given by

Then, potential difference between and is

given by
Where, Potential difference between electrodes
and .
Where, Potential difference between electrodes
and .

Where, is the apparent resistivity. It is an

average weighted resistivity at a depth equal to
electrode spacing „a‟.
2.2 SCHLUMBERGER- PALMER METHOD III. EXPERIMENTATION
One of the drawbacks of Wenner method The soil resistivity has been measured by
is; while measuring the soil resistivity at large using Wenner‟s four probe method at Ahmednagar
spacing, the voltage between potential probes near MIDC and results are listed in table 1. The
decreases very rapidly and instruments are unable to apparent soil resistivity Vs probe spacing curve has
measure such low voltage. Therefore, to measure the depicted in figure 4. For the uniform soil model
apparent soil resistivity at large spacing / depth, the analysis, the average soil resistivity =156.12 Ω m.
Based on this resistivity, grid resistance, touch and

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ISSN: 2248-9622, Vol. 10, Issue 5, (Series-I) May 2020, pp. 25-30

step voltages are calculated using IEEE STD 80 - 4.1 Multilayer Soil Model
2013 empirical formulae A typical multilayer soil model is as shown
Table 1: Soil resistivity measured data using in figure 5. The numbers of numerical methods for
Wenner method the analysis of multilayer soil model suggested by
Sr. Probe Probe Resistance Resistivity different authors are available in the literature as per
No spacing depth „ρ‟ (Ω-m) the methodology used. The method includes least
„a‟(m) „b‟(m) „R‟ (Ω)
square method, using Greens function based on
1 0.30 0.10 140 263.76 Bessel‟s function of first kind, Gauss Newton‟s
method, Simpsons rule, Fredholm‟s equation. Zhang
2 0.60 0.10 37 139.47
et al. presented the multilayer soil structure analysis
3 1.0 0.10 22 138.16 by using complex image method.
The curve that obtained from the actual soil
4 2.0 0.10 12 150.72
resistivity measurement data reflects the distinct
5 3.0 0.15 7.46 140.54 number of layers. The predefined standard curves
6 4.0 0.15 6.0 150.72 based on calculated apparent soil resistivity of
different soil structures having number of layers are
7 5.0 0.15 5.0 157.0 stored in the computer. Then actual number of layers
8 7.5 0.15 4.0 188.5 can be determined by comparing he measured data
with standard curves. For example, figure 6 indicate
9 10 0.15 3.0 188.5 the different kinds of curves for three-layer soil
10 12.5 0.20 2.0 157 model.
11 15 0.20 1.29 121.52
12 17.5 0.20 1.0 109.95[av1
56.12]

Figure 5. Typical multilayer soil model

Figure 4. Variation of apparent soil resistivity with

probe spacing
IV. SOIL MODEL
After successfully conducting the test for
soil resistivity measurement at the various profiles in
the substations, it is necessary to determine the
arithmetic average resistivity corresponds to each
probe spacings. Thus, a table is to be prepared
showing probe spacing „a‟ and average measured Fig 6. Typical three-layer soil resistivity curves (16)
apparent soil resistivity. The graph of average The characteristics of three-layer curves can be
apparent soil resistivity against probe spacing (ρ-a distinguished as follows.
curve) gives us a visual aid to find the number of Curve 1: Initially increases, reaches to maximum
soil layers. The number of soil layers equal to and then decreases
number of points at which the slope of apparent soil Curve 2: Increases continuously with probe spacing
resistivity curve changes greatly. The numbers of
graphical and numerical methods are available in the Curve 3: Decreases continuously with probe spacing
literature to determine the soil models.

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Curve 4: Initially decreases, reaches to minimum V. CONCLUSION

and then increases
Various soil resistivity measurement
Where,
techniques are demonstrated. The curve of soil
resistivity against probe spacing clearly reflects the
Where, four distinct layers without knowing the soil
resistivity of top layer up to depth resistivity of each layer and its depth. However, use
of multilayer soil model, provides all the information
resistivity of second layer up to depth
of soil resistivity and its depth for each layer. The
result table shows that the four-layer soil model is
best suited for given soil resistivity data.

is the resistivity of third layer up to depth The knowledge of multilayer soil model has
The soil model having n layers, need to determine paramount importance while design of substation
(2n-1) number of soil model parameters. The depth grounding grid. The most stringent safety criterion
of nth layer is infinity. The two sets of apparent soil such as touch & step voltages are the functions of
resistivity data using least square method can be top layer soil resistivity whereas ground resistance
presented as [16] and GPR depends on low soil layer resistivity. The
optimum length of ground rods can be obtained so
that they can reach to the low soil resistivity layer
and discharge large fault current resulting in
reduction in touch & step voltages, reduction in
ground resistance, GPR which enhances of safety of
measured apparent soil resistivity for k number
substation operators & costly control equipments.
of probe spacing and calculated apparent soil This research will be useful to researchers and power
resistivity for k number of probe spacing. engineers working in electrical utility and industries.
The parameters can be obtained by minimizing
function Ψ. This is a unconstrained nonlinear least REFERENCES
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Kavita Joshi. “A Survey on Real Time

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