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Electrical resistivity method is based on measurement of _____________
Specific resistance
Voltage
Potential drop
Current
Specific resistance
The electrical resistivity method is a non-destructive geophysical technique used in soil investigation that measures the electrical resistivity (or specific resistance) of subsurface soil and rock layers. It determines how strongly soil materials resist the flow of electric current, which varies based on soil composition, moisture content, density, and mineral content. This measurement enables indirect assessment of soil properties and subsurface stratigraphy without drilling.
The electrical resistivity method is a non-destructive geophysical technique used in soil investigation that measures the electrical resistivity (or specific resistance) of subsurface soil and rock layers. It determines how strongly soil materials resist the flow of electric current, which varies based on soil composition, moisture content, density, and mineral content. This measurement enables indirect assessment of soil properties and subsurface stratigraphy without drilling.
╧Б=LRтЛЕAтАЛ тАФ where ╧Б is electrical resistivity, R is resistance, A is cross-sectional area, and L is length
╧БaтАЛ=KтЛЕIVтАЛ тАФ where ╧БaтАЛ is apparent resistivity, K is geometric factor, V is potential difference, and I is injected current
R=A╧БтЛЕLтАЛ тАФ Ohm's law expression for resistivity
When an electric current is injected into the ground through electrodes, it travels through the soil and rock layers. Different soil types have different electrical resistivities depending on their mineral composition, porosity, water saturation, and ion concentration. By measuring the potential difference (voltage drop) between receiving electrodes and knowing the injected current, the apparent resistivity of the subsurface is calculated. This data is then inverted to produce a resistivity profile revealing subsurface layers and anomalies.
Specific resistance (electrical resistivity) is the fundamental property measured, not just voltage, current, or potential drop alone
Electrical resistivity varies significantly with soil type: clay (1тАУ100 ╬й┬╖m), sand (100тАУ1000 ╬й┬╖m), rock (1000тАУ100,000 ╬й┬╖m)
Moisture content strongly influences resistivity; dry soils have much higher resistivity than saturated soils
The method is non-invasive, rapid, and economical compared to drilling and laboratory testing
Apparent resistivity is measured in the field, then inverted to determine true resistivity vs. depth
Non-destructive and non-invasive methodтАФno drilling required
Cost-effective and rapid data acquisition over large areas
Provides continuous subsurface profile with depth resolution
Sensitive to changes in soil moisture, density, and mineral composition
Can detect subsurface cavities, contamination, and groundwater levels
Results are affected by surface conditions, topography, and near-surface variability
Interpretation is non-uniqueтАФdifferent subsurface configurations may yield similar apparent resistivity
Depth of investigation is limited; typically effective to 50тАУ100 m for routine surveys
Accuracy depends on electrode spacing, current injection strength, and soil heterogeneity
Electromagnetic interference and high surface resistivity can affect measurements
Subsurface stratigraphy mapping and soil layer identification
Groundwater exploration and contamination assessment
Detection of cavities, voids, and subsurface anomalies
Foundation site assessment and engineering geology evaluation
Archaeological surveys and mineral exploration
Environmental site characterization and pollution plume delineation
| Feature | Option A | Option B |
|---|---|---|
What is measured directly | undefined | undefined |
Physical basis | undefined | undefined |
Gives soil property info | undefined | undefined |
| Parameter | Detail |
|---|---|
| Standard Resistivity Values (Typical) | Clay: 1тАУ100 ╬й┬╖m | Silt: 10тАУ100 ╬й┬╖m | Sand (dry): 100тАУ1000 ╬й┬╖m | Sand (wet): 50тАУ500 ╬й┬╖m | Gravel: 500тАУ2000 ╬й┬╖m | Bedrock: 1000тАУ100,000 ╬й┬╖m |
| Why Other Options Are Wrong | Option B (Voltage): Voltage depends on both resistivity AND current; voltage alone does not characterize material. Option C (Potential drop): Like voltage, potential drop is an intermediate measurement; resistivity must be calculated from potential drop and current. Option D (Current): Current is the injected source parameter, not a measured soil property. |
| Key Distinction | The method measures RESISTIVITY (a material property independent of geometry), not just resistance (which depends on size and shape). This is why 'specific resistance' (another term for resistivity) is the correct answerтАФit is the intrinsic electrical property of the soil. |
A is correctтАФElectrical resistivity method is fundamentally based on measurement of specific resistance (electrical resistivity), which is the intrinsic property of soil that determines how strongly it resists the flow of electric current.
In GATE/ESE Civil exams, remember that the electrical resistivity method is a geophysical tool used during site investigation (IS 1893, IS 1498). Always distinguish between measured parameters (voltage, current) and derived soil properties (resistivity, stratigraphy). This concept also connects to foundation design, where subsurface soil profiles obtained from resistivity surveys inform bearing capacity and settlement calculations.
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