Questions
8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip for ONGC/Coal India
Overview
Electrical and electromagnetic methods are non-invasive geophysical techniques that measure the subsurface resistivity and conductivity variations to map geological structures. These methods are critical for mineral exploration and hydrocarbon studies, often appearing in ONGC and Coal India exams due to their practical application in subsurface mapping.
Resistivity Surveys
Resistivity methods utilize current injection through electrodes to determine the apparent resistivity of the ground. Understanding array geometries like Schlumberger and Wenner is essential as they dictate the sensitivity and depth of investigation.
- Apparent Resistivity (ρa) = K * (ΔV/I)
- Geometric factor K for Wenner array: 2πa
- Schlumberger array is preferred for deeper sounding due to higher signal-to-noise ratio
- Ohm's law application in geophysical logging: V = I * R
- Archie's Law: ρ = a * φ^-m * Sw^-n * ρw
IP and SP Methods
Induced Polarization (IP) measures the capacity of the ground to store electrical charge, while Self-Potential (SP) utilizes naturally occurring potentials. These methods are vital for identifying disseminated metallic sulphides and groundwater seepage.
- Chargeability (M) is the time integral of secondary voltage
- SP occurs due to electrochemical gradients and mineral oxidation
- IP is expressed in mV/V or milliseconds
- Percent Frequency Effect (PFE) = ((ρlow - ρhigh) / ρhigh) * 100
- Metal Factor (MF) = (2π * 10^5 * PFE) / ρdc
Electromagnetic (EM) Methods
EM methods depend on electromagnetic induction without requiring direct electrode contact with the ground. TDEM and FDEM are used to measure the conductivity of subsurface media through secondary magnetic fields.
- Skin depth (δ) = 503 * sqrt(ρ / f)
- FDEM utilizes multiple frequencies for varying depth penetration
- TDEM uses transient pulses to map conductivity variation with time
- Phase shift is proportional to conductivity in FDEM
- Secondary field decays exponentially in TDEM as depth increases
Ground Penetrating Radar (GPR)
GPR uses high-frequency radio waves to map shallow subsurface structures and detects buried utilities or archaeological artifacts. It relies on the dielectric permittivity contrast between materials.
- Velocity (v) = c / sqrt(εr)
- Reflection Coefficient (R) = (Z2 - Z1) / (Z2 + Z1)
- GPR frequency range: 10 MHz to 3 GHz
- Resolution increases with higher frequency, but penetration depth decreases
Formula Sheet
ρa = 2πa(ΔV/I) (Wenner)
ρa = (π/2) * ((L^2 - l^2)/2l) * (ΔV/I) (Schlumberger)
ρ = a * φ^-m * Sw^-n * ρw (Archie's Law)
δ = 503 * sqrt(ρ / f) (Skin Depth)
v = c / sqrt(εr) (GPR Velocity)
M = (1/Vp) * ∫(Vs(t) dt) (Chargeability)
Exam Tip
Focus on calculating the Geometric factor K and knowing the direct relationship between frequency/conductivity and skin depth, as these are the most common numerical targets.
Common Mistakes
- Confusing the Geometric Factor (K) formulas for different electrode arrays like Wenner vs. Schlumberger.
- Neglecting the impact of frequency on skin depth in EM surveys, leading to incorrect depth calculations.
- Misinterpreting Archie's law parameters; failing to account for water saturation (Sw) and porosity (φ) correctly.
More Revision Notes
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