Question

In resistivity measurements for a double-dipole system, the apparent resistivity is NOT affected by:

• A. the electrode spacing
• B. the resistivity of the subsurface
• C. the distance between the centers of the current and potential dipoles
• D. the telluric current

Hint:

In resistivity surveys, apparent resistivity depends on: - Electrode configuration and spacing (affects geometric factor), - Subsurface resistivity distribution, - Signal strength and measurement geometry.
Telluric currents are usually filtered out in active-source methods.

Answer 1

The Correct Option is D

Step 1: Understanding apparent resistivity in a double-dipole system. 
In a double-dipole system, apparent resistivity is calculated using the voltage measured between the potential electrodes, the current injected, and the geometric factor (dependent on electrode spacing and array configuration). The formula is: \[ \rho_a = K \cdot \frac{V}{I} \] where \(K\) is the geometric factor, \(V\) is the measured potential difference, and \(I\) is the injected current. 
Step 2: Analyzing each factor: 
(A) Electrode spacing affects the geometric factor \(K\), so it affects apparent resistivity. 
(B) The true subsurface resistivity influences the voltage measured, affecting apparent resistivity.
(C) The distance between the dipole centers changes the sensitivity and geometry of the measurement, affecting \(K\).
(D) Telluric currents are naturally occurring geoelectric currents in the Earth, but in controlled source resistivity measurements, their influence is negligible or eliminated through signal processing.
Hence, the apparent resistivity is not affected by the telluric current.