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Q31. Index of Alteration (IA) = 100*[Al2O3]/([Al2O3]+[Na2O]+[K2O]). Correct statement:
During weathering, mobile cations (Na, K) are leached out, leaving immobile Al. This drives the ratio towards 100.
Q32. Which one of the following mineral assemblages is stable under eclogite facies conditions?
Eclogite facies is high-pressure/high-temperature and is defined by the absence of Plagioclase feldspar. The typical assemblage is Garnet + Omphacite (Cpx).
Q33. Which of the following physical properties of rocks has the widest range of variation?
Electrical resistivity in rocks can vary by many orders of magnitude (from 10^-8 to 10^14 Ohm-m) depending on porosity and fluid content.
Q34. In electromagnetic (EM) sounding, the depth of investigation [?] with increasing frequency.
EM depth of investigation (Skin Depth) is inversely proportional to the square root of frequency. Higher frequencies penetrate less.
Q35. The most abundant mineral in the Earth's crust is:
Feldspars (including Plagioclase and K-Feldspar) are the most abundant mineral group in the Earth's crust, making up about 60% of it.
Q36. Which one of the following is the Poisson’s ratio for an incompressible fluid?
For an incompressible material, the bulk modulus is infinite. The relationship K = E / (3(1-2v)) implies that for K to be infinite, (1-2v) must be zero, so v (Poisson's ratio) = 0.5.
Q37. If the dissociation constant of pure natural water at 50°C is 10^-13.10, the pH of the water will be:
For neutral water, pH is half of pKw. At 50°C, pKw is 13.10, so the neutral pH is 13.10 / 2 = 6.55.
Q38. Which one of the following statements is CORRECT regarding shoreline movement?
Forced regression occurs when relative sea level falls, forcing the shoreline to move seaward. Transgression is landward movement.
Q39. Which one of the following rocks has the highest magnetic susceptibility value?
Gabbro is a mafic igneous rock often containing magnetite, giving it much higher susceptibility than sedimentary or metamorphic rocks like limestone/quartzite.
Q40. Which of the following logging techniques is best suited to estimate the shaliness of hydrocarbon reservoirs?
Gamma Ray logs measure natural radioactivity. Shales are naturally radioactive (due to K, U, Th), so high Gamma readings indicate shale.