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| Geophysical Resistivity Survey |
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General Principles:
A host of electrical methods are available for geophysical work. Some of these measure earth’s naturally occurring electric fields, others are based on injection of artificially generated low or high frequency current and radio waves. Out of these the electrical resistivity method is the most suitable for foundation studies and groundwater exploration. It is easy to employ and the equipment is convenient to transport from place to place. Information about aquifers, water tables, salinities, impermeable formations, bedrock depths, etc. can be obtained from such surveys.
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Resistivity Survey:
All methods of exploration geophysics are based on some physical property. Due to the differences in lithological and chemical composition, different rock types and ore deposits have different physical properties. It is the electrical resistivity or its reciprocal electrical conductivity that is used as the physical property in the electrical methods. This physical property has a wide variation. One of the highest resistive rocks is the granite with resistivity values that run into several hundreds and thousands of ohm meters and whereas one of the lowest geological units is clay with values ranging from no more than one ohm meter. Most of the rock formations conduct electricity because of mineralised water flowing through their pores, fractured zones and fissures. Dry rocks and soils are highly resistant. Resistivity of a geological formation, therefore, is a function of porosity, saturation and salinity of the water. Resistivity of water decreases with increasing salt content.
Electrode Arrays:
The electrical impudence of the underlying earth's layers is usually measured by passing a known amount of electricity across two current electrodes staked into the ground at a measured distance and by measuring the voltage generated across two potential electrodes, after nullifying all the naturally occurring potentials. A four electrode Wenner or Schlumberger array, with all the electrodes lying on a straight line, is commonly used for making these measurements. In the Wenner System, the distance between the two current electrodes is three times the distance between the potential electrodes and each time this distance is varied, all the electrodes are shifted to new positions, maintaining the relationship of 3 to 1. In the Schlumberger's configuration, the current electrodes are expected to be at an infinite distance from the potential electrodes and this is practically achieved by keeping the inter potential electrode distance to the minimum possible for the instrument to detect the generated voltage and the current electrodes alone are shifted each time for making a new measurement. The apparent resistivity offered by these layers is calculated using the relationship between the distances of potential and current electrodes and the amperage used to generate the measured voltage difference after nullifying, if any, the natural potential of such materials. Based on parametric soundings at locations where the subsurface lithology is known and by curve matching techniques the thickness and nature of formations in a given area are obtained.
Definition & Formulae:
Resistivity is defined as obstruction to the flow of current in ohms between opposite faces of a metre cube of a material and is measured in ohm-m2/m, or simply ohm-m.
The resistivity r of a material of length L and cross sectional area A having a resistance of R is given by the equation:
r = RA/L
Resistivities of consolidated and unconsolidated rocks range widely, being high for dense impervious rocks, medium for porous rocks containing water, and low for clays and saline water, as indicated below:
Formation Resistivity |
(ohm-m)
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| Compact formation: |
300 -10,000
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Fresh water sandstone and limestone: |
50 - 300 |
Fresh water sand and gravel |
20 - 100 |
Brackishwater sand and gravel |
4 - 20
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Clay and shale |
2 - 10 |
Salt water sand and gravel |
0.1 - 4
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Various arrangements for spacing the electrodes have been standardised. Schlumberger's arrangement is the more common, as it eliminates the errors of surface in-homogeneity and has certain distinct advantages, like speed of operation, less lengths of cable requirement etc., over other methods in the present field conditions.
In the Schlumberger arrangement, as already pointed out, the potential electrodes are kept close together and the apparent resistivity is given by the formula:
ra = (V/I) x P {(AB/2)2-(MN/2)2}/2MN
Where ra = apparent resistivity of the formation in ohm-m
V & I = voltage measured and current injected, respectively
AB/2 = current electrode spacing
MN/2 = potential electrode spacing.
AB/2 must be as much larger as may be practicable and for reliable results it must be at least five to ten times of MN/2.
Soundings and Profiles:
To obtain the nature of soil or rock formations as one expects while drilling through them, a vertical electrical sounding is conducted and whereas to understand or establish lateral changes in them, profiling is done.
In the case of a sounding, the measurement point remains constant, with only the spread of electrodes being increased in several steps. By plotting the calculated resistivity values against the electrode spacing on a double log paper (see Fig: Log Log Graph) and by matching the curve with the theoretical master curves, thicknesses of layers and their true resistivities may be obtained.
In the case of the profile, readings are taken much like in the case of a sounding, but the measuring points are shifted along a desired direction and later the resistivities for different stations are plotted for interpreting lateral changes in the lithology (see Fig: Interpretation of Data)
For a meaningful interpretation of the resistivity data knowledge of local geology is essential.
The ‘DIGIRHO’ digital DC earth resistivity equipment, developed and marketed by NRDCS, has been successfully used for the resistivity investigations over the past three and half decades, all over the country for groundwater and geotechnical studies.
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