6. DETAILED SURVEY-METHODOLOGICAL APPROACH
ELECTRICAL RESISTIVITY
Purpose:
To identify groundwater yielding, their geometry, variation in quality (salinity) of groundwater and direction
of groundwater movement.
Resistivity method is consider as the superior in groundwater exploration because:
· It is very cost effective method.
· It has low maintenance during the transportation
of the equipments viz. cables, Resistivity meters, electrodes etc.
· Whether it is sounding or profiling it takes
very less time with respective to other methods.
· It has greater depth of penetration (not considering
seismic method)
· As far as groundwater is concerned, it gives
maximum probability of accurate interpretation.
· It informs correctly about the subsurface
geologic formations with respect to density with greater depth of investigation.
Principle:
Electrical Resistivity of any material is defined as numerically equal to the resistance(unit: ohm) offered
between two opposite faces of a unit cube of the material. It is independent of shape and size. The conventially used unit
of resistivity is Ohm-m.
Bulk Resistivity of water bearing geological formation depends on its ability to conduct electric current through interstitial
water present in the pore spaces and through matrix. That is the bulk Resistivity varies with the amount, distribution and
salinity of interstitial water and lithology. Thus, the variation in Resistivity of any water bearing formation manifest combindly
the variation in litho logy and characteristics of groundwater present.
Using Ohm’s law electrical resistivity of sub-surface geological formation is determined through artificially energizing
the subsurface and carrying measurements on the ground surface. Contrast in Resistivity of a layer with the surrounding or
effective presence(dependant on its relative resistivity and thickness)makes it detectable.
Significant contrast in
resistivity occurs between dry and water saturated formations, and formations with fresh and brackish/saline water. In general,
there are defined ranges of resistivity of different formations, e.g., sands of various grain size, clays, weathered and fractured
granites and gneisses, sandstones, cavernous lime stones, vesicular basalts etc.As a result of the combined effect of quality
of formation water and the formation matrix, there are overlaps.
In this regard the Resistivity method is superior, at least theoretically, to all the other electrical methods, in groundwater
prospecting because quantitative results are obtained by using a controlled source of specific dimensions. The chief drawback
is its high sensitivity to minor variations in conductivity near surface; in electronic parlance the noise level is high.
This limitation, added to the practical difficulty in dragging several electrodes and long wires through rough wooded terrain,
has made the electromagnetic method more popular than Resistivity in mineral exploration. Nor is Resistivity particularly
suitable for oil prospecting. However it is by no means obsolete .The search for geothermal reservoirs normally involves Resistivity
surveying and it is also employed routinely in groundwater exploration, which is of increase worldwide importance.
Apparent Resistivity:
In general, the subsurface resistivity is given by
Where the parameter p has to do with the electrical geometry. By measuring Δv and I and knowing the
electrode configuration, we obtain a resistivity ρ.
Over homogeneous isotropic ground this Resistivity will be constant for any current and electrode arrangement.
If the ground is inhomogeneous, however, and the electrode spacing is varied, or the spacing remains fixed while the whole
array is moved, then the ratios will in general change. this results in a different value of ρ for each measurement.
this measured quantity is known as the apparent resistivity ρa.
ASSUMPTIONS: Interpretation of the measurement is based on some assumptions.
They are as follows,
1) The subsurface consists of a finite number of layers separated from each
other by horizontal boundary plane and are laterally extensive. In field it would suffice if the dip is within 15 degree,
and with respect to the station of measurement the layer are radially extensive at least within a circle of radius not less
than the maximum separation attained between the end electrodes that in turn obviates lateral inhomogeneties.
2) Each of the geoelectrical layers are electrically homogeneous and isotropic.
3) The last bottom layer possesses infinite thickness.
Electrode Configuration in Resistivity Sounding:
A) Schlumberger Array
B) Wenner Array
C) Pole –Dipole array
D) Double-Dipole array
E) Lee- partitioning etc.
Field methods adopted(Considering schlumberger methods only)
All Resistivity methods employ an artificial source of current which is introduced into the ground through
point electrodes or long line contacts; the later is rarely used nowadays. The procedure is to measure potentials at other
electrodes in the vicinity of the current flow. Because the current is measured as well, it is possible to determine an effective
or apparent Resistivity of the surface.
A variety of configurations exists to place the current and the potential electrodes, viz., Schlumberger, Wenner, dipole-dipole,
two electrode, half-Schlumberger, Lee-partitioning etc. They are used for sounding as well as profiling.
Since Jodhpur lies in the arid region , with only some places having hard rock , the probability of finding fresh groundwater
suitable for drinking purposes lies in the fracture zones in the hard rock layers below the alluvium deposit. This leaves
us with the task to cover at least 40m depth to get any trace of water bearing formations. For this task vertical sounding
technique was applied to measure variation of Resistivity with depth.
In
vertical sounding, the potential electrical electrodes remain fixed while the current electrode spacing is expanded symmetrically
about the axis of the spread. This procedure is more convenient than Wenner configuration because only two electrodes need
to move. Of the several electrode arrays used in Resistivity methods the one used in my groundwater prospecting was Schlumberger
configuration because of its several advantages over the other arrays.
Schlumberger Configuration:-
In practice the potential electrode spacing is kept not more than 1/5th of the current
electrode spacing . Successive length of current electrode spacing is usually increased in geometric progression. As such
there should be an equal distribution of 4 to 8 points in each cycle of double log paper used for plotting apparent Resistivity
curve. The current electrodes can be moved outwards symmetrically at an increment of 2 to5 m also., without changing
the potential electrode positions to study the breaks in the curve. The current electrodes are conventially indicated as A
and B electrodes and the spacing as AB and potential electrodes as M and N electrodes and the spacing as MN .
Fig-1
The
geometric factor for Schlumberger configuration is :-
For symmetrical array the apparent Resistivity is given by :-
Equipment for Resistivity field work:-
The necessary components for making Resistivity measurements include a power source, meters for measuring
current and voltage, cables and reels. The power may be dc or low frequency ac, preferably less than 60 Hz.
Instrument:- The instrument for measuring subsurface Resistivity is called Tetrameters. The current supplied
into the ground and the potential difference between the potential electrodes is fed into this instrument. The instrument
then displays the V/I ratio, i.e., the ground resistance. When this ratio is multiplied by a constant it gives the apparent
Resistivity of the subsurface formation.
Two terrameters have been used :
1.
SAS 300
2.
SAS 1000
Both the meters uses a.c of low frequency preferably 60 Hz .The importance
about this meters is that they uses square a.c wave pulse which has advantages over the sinusoidal pulse
and over d.c meters.
In SAS 300 frequency can be set by using a switch and cycles can also be
set to 4,8,16 32 by an indicator switch, these cycles takes the average values in each step thus giving almost accurate
value of V/I in ohm or in mohm. Moreover there is an additional port which can be used for booster.
In SAS 1000 all the adjustment relating to frequency, cycles, voltages etc can be set
using keypad, adjustments and data collected can be saved in the memory and displayed through a LCD screen. The output gives
direct apparent Resistivity thus it is the advanced version of SAS 300.
Advantages:
1.The d.c meters uses direct current which have definite polarity at the electrodes due to which the neighboring mass surrounding
the electrodes accumulates +ve or-ve charges .This give rise to a localized current and the cumulative effect are recorded
by the meter.
2. The sinusoidal wave as we know, the amplitude of the pulse varies linearly with time from 0 to peak to 0 again and so on,
but in the case of square pulse
It has a definite values of peak amplitude and zero only.
Site selection:
Selection of site for conduction survey should be such that it serves the
purpose. In case, geophysical anomaly falls at a point, which is not approachable for drilling, its extension should be identified/demarcated
by observing additional profile.
If the profile is near a concrete structure like road or building or bridge,
the survey site should be located in such a way that no potential electrode position falls within10m of the structure. That
is, both the electrodes should be perceptibly on apparently homogeneous ground through out the MN range.
Processing of Data:
Sounding curves obtained by Schlumberger configuration are generally discontinuous,
with upward of downward shifting of curve segments, because of the shifting of potential electrodes. There are two types of
displacements on account of the shifting of the potential electrodes (MN), viz., (a) due to the change in geometric position
of the potential electrodes in relation to current electrodes and (b) due to the lateral inhomogeneities near the potential
electrodes. The geometric shifting of a curve segment should be in a prescribed manner if there is no lateral inhomogeneity.
Sounding curve can be smoothened by shifting the curve-segments up or down depending on the type of curve (ascending or descending).
Conventional shifting of curve depends on relative resistivities of the layer sequence. When potential electrode spacing is
increased, depth of investigation is some what reduced and therefore theoretically the apparent resistivity should tend towards
the preceding value while the ascending curve should show a downward shift, the descending curve should shift upward. The
correct magnitude or downward shifting is ascertained. In some of the curves, it may be difficult to decide which segment
is to be shifted, the first one or the subsequent one. Difficulty in shifting of curve segment can be overcome by observing
the trends of nearby soundings. Shifting of curve-segments due to lateral surface inhomogeneities near potential electrodes
can be differentiated from the normal potential electrode displacements.
Inhomogeneities near current electrodes can also be recognized by distortion
in sounding curve. A sharp curvature of maximum value in a sounding curve does not indicate a resistive layer of regional
extent but a lateral inhmogeneity. Curves presence of lithological contact of varied resitivity, i.e., sounding profile is
expanded across strike of a bed or a fault and can be smoothen distorted curve and identify which current electrode has caused
the sifting.
Location of field work:
The fields chosen for my groundwater prospecting work were two villages namely Manklawas and Manaklao.
Schlumberger array was applied in vertical sounding method .Data were acquired and they were processed and interpreted by
software as well manually.
Field
Data:-
Sounding no-1
|
SERIAL NO. |
AB/2 |
MN/2 |
CONSTANT
(m) |
V/I (W) |
r (W- m) |
|
1 |
2.0 |
0.5 |
11.8 |
10.41 |
122.8 |
|
2 |
3.0 |
0.5 |
27.5 |
4.84 |
133.4 |
|
3 |
4.0 |
0.5 |
49.4 |
2.77 |
136.9 |
|
3A |
4.0 |
1.0 |
23.6 |
6.06 |
161.2 |
|
4 |
6.0 |
1.0 |
55.0 |
2.59 |
142.5 |
|
5 |
8.0 |
1.0 |
98.9 |
1.9 |
138.8 |
|
6 |
10.0 |
1.0 |
155.5 |
0.843 |
139.0 |
|
6A |
10.0 |
2.0 |
75.4 |
1.728 |
130.0 |
|
7 |
15.0 |
2.0 |
173.5 |
0.586 |
101.7 |
|
8 |
20.0 |
2.0 |
311.0 |
0.229 |
71.0 |
|
8A |
20.0 |
5.0 |
117.8 |
0.657 |
77.4 |
|
9 |
25.0 |
5.0 |
188.5 |
0.311 |
58.6 |
|
10 |
30.0 |
5.0 |
274.8 |
0.1535 |
42.2 |
|
11 |
40.0 |
5.0 |
494.7 |
0.0539 |
26.7 |
|
12 |
50.0 |
5.0 |
777.4 |
0.0263 |
20.4 |
|
12A |
50.0 |
10.0 |
376.9 |
0.0558 |
21.0 |
|
13 |
60.0 |
10.0 |
549.7 |
0.0329 |
17.8 |
|
14 |
80.0 |
10.0 |
989.4 |
0.0143 |
14.1 |
|
15 |
100.0 |
10.0 |
1534.8 |
0.0083 |
12.7 |
|
15A |
100.0 |
20.0 |
753.8 |
0.0193 |
14.5 |
|
16 |
120.0 |
20.0 |
1099.3 |
0.0074 |
8.1 |
|
17 |
160.0 |
20.0 |
1978.8 |
0.0132 |
26.1 |
|
18 |
200.0 |
20.0 |
3109.6 |
0.0075 |
23.3 |
Interpretation:
Quality interpretation of sounding curves includes, visual inspection to
identify the type of curves and demarcate areas with similar types of curves such as,
i)
Ascending or A-type: r1<r2<r3
ii) Descending
or Q-type: r1>r2>r3
iii) Minimum
or Bowl or H-type: r1>r2<r3
iv) Maximum
or Bell or K-type: r1<r2>r3.
The above of type curves for various combinations of 3-layer or multi-layered sub-surface Resistivity variations.
Quantitative interpretation of Resistivity sounding data is initially made on empirical or semi-empirical methods. The field
curves plotted on double-log transparent paper are smoothened and visually matched with a variety of 2-layer and 3-layer theoretical
master curves along with auxiliary point charts. It is a graphical method involving a sequence of partial curve matching.
In curve matching technique , correct estimation of Resistivity and thickness of top layer is very important , as subsequent
layer parameters are controlled by the “first cross”(ρ1 and h1)obtained matching the first segment of the
curve.
Curve matching technique is effective for 3 to 4 geoelectrical layering , as the curves are can be interpreted by partial
curve matching or complete curve matching. When layering exceeds, errors are introduced in partial curve matching and other
computer based technique should be attempted.
Now a days, with the advent of modern soft wares, the interpretation of sounding curves are done on the software, namely RESIXP.
In this software, the sounding data are fed and it automatically gives the depth of different subsurface layers, their thicknesses
and their respective apparent resistivities. With the help of this software it is possible to interpret up to 4 as well as
5 layers which is definitely more effective than manual interpretation.
Interpretation of above data:
· Manual interpretation:
The acquired data is plotted on a double log paper and a smooth curve is drawn and after proper curve matching using three
layer master curve the following output in terms of Resistivity and depth is obtained.
TABLE 1
|
RESISTIVITY (Ω-m) |
DEPTH (m) |
|
ρ1 = 120.8 |
2.6 |
|
ρ2 = 181.2 |
5.2 |
|
ρ3 = 18.12 |
41.6 |
|
ρ4 = 181.2 |
|
· Interpretation using RESIXP:
The software used is RESIXP. After closely scrutinizing the subsurface the following data of Resistivity and depth is obtained.
Percentage error 3.24%
TABLE 2
|
RESISTIVITY (Ω-m) |
DEPTH (m) |
|
ρ1 = 123.6 |
1.98 |
|
ρ2 = 193.0 |
4.9 |
|
ρ3 = 34.67 |
16.95 |
|
ρ4 = 8.18 |
68.78 |
|
ρ5 = 1151.4 |
- |
Inference: In both the interpretation, the Resistivity of the third
layer suggest the presence of slightly saline water at a depth of around 17 to 70m. The resistivity value of the third layer
extracted manually suggest the presence of saline water between 6 to 84m but the same layer on closely scrutinizing
gets divided into two layers by the software of resistivity 34.67Ω-m and 8.18Ω-m respectively.
Since the software has the ability to give the information about additional
layers together with its Resistivity and the percentage of error calculated being 3.24 it can be assumed that third layer
is containing slightly saline water at a depth of around 30-35m.
Advantages:
The electrical resistivity method is cost-effective and employs non-destructive field technique.
It is effective in assessing quality of ground water and therefore enables location saline/fresh ground
water interface, or saline water pockets.
Resistivity contrast associated with presence or absence ground water gives geometry of aquifer and
zones favorable for ground water accumulation.
It gives lithologic information, depth to resistive bedrock, direction of ground water movements orientation
of fracture zones, faults, paleo-channels, cavities in limestone etc.
The method can be used for specific environmental application like ariel extent of ground water pollution,
zones suitable for artificial ground water recharge, protection of aquifer from contamination, soil salinity mapping, reclamation
of coastal saline aquifers and monitoring the quality.
Th method can be used to assess the hydraulic parameters by drawing analogy between the Ohm’s
law and Darcy’s law. The correlation between hydraulic conductivity and aquifer resistivity and transmissivity and transverse
resistance can be established.
Disadvantages:
Overlapping resistivity ranges and a very wide range of resistivity makes it difficult to characterize
ground water targets by their resistivities unless standardized locally.
Accuracy and resolution decrease with increasing depth of target and decreasing contrast in resistivity.
In its predictive application, like other geophysical methods based on potential theory, it lacks in
uniqueness in subsurface model solution because a geoelectric layer is discernible through product or ratio of its resistivity
and thickness and moreover the resistivity is function both lithological and groundwater characteristics. Ignorance these
aspects often the cause of bore hole failure following the geophysical investigation.
Limitations:
Presence of very high or very low rsistivity surface soil affects the interpretation. While the former
increases the contact resistance, the latter masks t signals coming from deeper layers. It is a serious problem in resistivity
surveys. Conductive top layer grabs a big share from input signal going into the subsurface as well as output signal coming
from the deeper zones. Deployment of a sensitive instrument would help to some extent in recording smaller potential values
accurately.
The response being dependant of two parameters- the geometry (that is in the sounding he layer thickness)
and resistivity of the target (layer), there is no unique solution and a number of equivalent models are found. While conducting
soundings on a multi-layered earth, it is observed that the parameters intermediate layers could be altered to a certain extent,
keeping either ratio of thickness of resistivity or the product of thickness and resistivity constant. This would not produce
any appreciable/detectable change (within the accuracy of the observation) in the shape of resistivity sounding curves. This
phenomenon is known as equivalence. The effect is pronounced if the layer are thin it can bot be resolve by a single technique
and requires support of independent information for fixing either of the parameters interpreted or obtaining the same parameters
through joint interpretation with other techniques.
The response is dependent of the effective presence, i.e. the depth of burial and resistivity contrast
of the target. Thin layers or layers with less resistivity contrast with the surrounding are suppressed.
In a layer sequence, the interfaces between successive layer having monotonously increasing or decreasing
order of Resistivity, can not be distinguished accurately, particularly at depths, because of transition in Resistivity.
Dipping layers distorts the measurements and also produces ambiguity. The presence of inhomogeneties
either at the potential or current electrodes produces distorted or shifted curves. Sometimes these curves can not be interpreted
fruitfully.
The Resistivity of the top (first) layer and the bottom (last) layer must be accurately known or objectively
assumed because these invariably affect evaluation of the intervening geoelectric layers
