CSIR NET June 2016 Q92: Geophysics
Observed gravity varies with both elevation (free-air effect, ~0.3086 mgal/m) and latitude (normal gravity increases toward the poles, with gradient ≈0.811 sin(2φ) mgal/km at mid-latitudes).
At 45°N, the latitude gravity gradient is at its maximum (~0.811 mgal/km, since sin(2×45°)=1). Over the 20 km separation between A and B, the expected increase in normal gravity purely due to moving north is 0.811 × 20 ≈ 16.2 mgal -- which exactly matches the observed 16.2 mgal difference. Since the entire observed gravity increase is accounted for by the latitude effect alone, there is no residual difference attributable to elevation (free-air effect), meaning B is at the same elevation as A. Option 1 is correct.
1) Correct — the latitude correction alone fully explains the 16.2 mgal difference, leaving no elevation-driven (free-air) contribution. 2), 3), 4) Incorrect — these would require a residual gravity difference after accounting for the latitude gradient, but here the latitude effect exactly matches the observed value, leaving no room for an elevation difference.