Section A
Q1.
(a) in the Earth. Determine the resistivity of the medium, if an electromagnetic wave of frequency 40 Hz penetrates into the Earth up to a depth of 1006 m. (Consider μ = 4π × 10-7 H/m) 8 Define elevation correction in gravity prospecting. Determine the elevation
(b) correction factor assuming the slab density \rho = 2.65 gm/cc. 8
(c) Show that the Laplace's equation for potential V holds good in an isotropic homogeneous medium when a current of constant frequency (DC) or a very low frequency AC flows in it. 8 Write down the Wyllie's time-average equation. Compute the porosity in a
(d) sandstone reservoir using the above equation considering the following observations: 8 An interval transit time in the sandstone reservoir was measured as
(i) 90 \mus/ft. The acoustic velocity of the rock matrix was 18000 ft/s. (ii) (iii) Fluid transit time was 189 μ s/ft. Define multiples. Distinguish between short-path and long-path multiples with
(e) the help of a schematic diagram for the case of horizontal beds. 8 2.
(a) Explain the static shift phenomenon affecting the magnetotelluric (MT) apparent resistivity data and discuss its effect on MT impedance phase. Define the subsurface dimensionality as a function of MT impedance tensor.
(b) Derive the expressions for potential and electric fields due to a current dipole placed over a homogeneous ground at a distance r from the measuring point. 15 3. Explain the concept of 'zero-length spring'. Discuss the principle of operation of
(a) astatic type of gravimeter. Draw a neat sketch explaining the elements of the Earth's magnetic field.
(b) Express the magnetic potential V at any point on the surface of the Earth in spherical coordinate system, signifying its internal and external origins.
(a) Show the positions of different detectors used in neutron logging tool with 4. respect to a neutron source, making suitable diagrams. Also show the energy range for neutron-thermal neutron log and neutron-epithermal neutron log on the drawing itself. PRO-08-GROCPI54 2
