Section A
Q1.
(a) Vertical electrical sounding data recorded using Schlumberger electrode configuration shows a three-layer H-type apparent resistivity curve. This data was interpreted using partial curve matching technique. Resistivity and thicknesses of first two layers were estimated as P1 = 100 Ωm; P2 = 10; h1 = 10 m and h2 = 20 m Calculate the effective resistivity and effective thickness of a single homogeneous and isotropic layer obtained by replacing the first two layers into one single layer.
(b) What do you understand by the terms static self-potential and pseudo-static self-potential used in SP log ? Explain positive, negative and zero SP responses in terms of resistivity of formation water and resistivity of mud filtrate.
(c) In time-domain electromagnetic sounding, decay response is recorded on the surface of uniform earth with constant conductivity. Draw neat schematic diagrams showing the characteristics of decay curve as a function of time, in early, intermediate and late time zones. How does decay response vary with increase and decrease of conductivity of uniform half-space?
(d) Illustrate with sketches the nature of Bouguer and free-air gravity anomaly when a structure or region is 100%, 50% and 0% isostatically compensated.
(e) If you took a gravimeter 1 km down a mine in rocks of density 2.4 \text gm/cm^3, how much would be the change in gravity compared to the surface ?
