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Electrical resistivity method is based on measurement of _____________

Specific resistance

Voltage

Potential drop

Current

Correct Answer

⚙️ TE • Technical Geophysical Exploration MethodCivilSoil Mechanics

Option A

Specific resistance

Quick Summary:

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.

⚙️TETechnical SolutionGeophysical Exploration Method

💡 Explanation

🔢 Key Formulas

$\rho = \frac{R \cdot A}{L}$ — where $\rho$ is electrical resistivity, $R$ is resistance, $A$ is cross-sectional area, and $L$ is length

$\rho_a = K \cdot \frac{V}{I}$ — where $\rho_a$ is apparent resistivity, $K$ is geometric factor, $V$ is potential difference, and $I$ is injected current

$R = \frac{\rho \cdot L}{A}$ — Ohm's law expression for resistivity

⚙️ Working Principle

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.

📌 Key Points

▸
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 (1–100 Ω·m), sand (100–1000 Ω·m), rock (1000–100,000 Ω·m)
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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

✅ Advantages

▸
Non-destructive and non-invasive method—no drilling required
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Cost-effective and rapid data acquisition over large areas
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Provides continuous subsurface profile with depth resolution
▸
Sensitive to changes in soil moisture, density, and mineral composition
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Can detect subsurface cavities, contamination, and groundwater levels

❌ Disadvantages / Limitations

▸
Results are affected by surface conditions, topography, and near-surface variability
▸
Interpretation is non-unique—different subsurface configurations may yield similar apparent resistivity
▸
Depth of investigation is limited; typically effective to 50–100 m for routine surveys
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Accuracy depends on electrode spacing, current injection strength, and soil heterogeneity
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Electromagnetic interference and high surface resistivity can affect measurements

🛠️ Applications / Uses

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Subsurface stratigraphy mapping and soil layer identification
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Groundwater exploration and contamination assessment
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Detection of cavities, voids, and subsurface anomalies
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Foundation site assessment and engineering geology evaluation
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Archaeological surveys and mineral exploration
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Environmental site characterization and pollution plume delineation

🔄 Comparison Table

Feature	Option A	Option B
What is measured directly	undefined	undefined
Physical basis	undefined	undefined
Gives soil property info	undefined	undefined

📄 Additional Information

Parameter	Detail
Standard Resistivity Values (Typical)	Clay: 1–100 Ω·m \| Silt: $10^{-100}$ Ω·m \| Sand (dry): 100–1000 Ω·m \| Sand (wet): 50–500 Ω·m \| Gravel: 500–2000 Ω·m \| Bedrock: 1000–100,000 Ω·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 answer—it is the intrinsic electrical property of the soil.

✅

A is correct—Electrical 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.

Core Concepts Used

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Electrical resistivity (specific resistance) as an intrinsic soil material property Relationship between resistivity, resistance, current, and voltage via Ohm's law Apparent resistivity and its inversion to obtain subsurface resistivity profiles Factors affecting soil resistivity: composition, moisture, porosity, and saturation

💡 EXAM TIP

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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