Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 5 / a

A gravity anomaly is measured gravity minus a specified reference gravity after the stated reductions. A Bouguer gravity anomaly aims to remove the predictable effects of latitude, observation elevation and surrounding topographic mass, so residual variations can be related to subsurface geophysical density contrasts.

Measure relative gravity with a calibrated gravimeter at stations across and beyond the crater, and survey station positions and elevations accurately, preferably with GNSS tied to a reference datum. Repeat a base station through the day to estimate instrument drift and tie the traverse to a gravity datum. Record station times, because tidal and drift corrections are time dependent.

Apply instrument scale and drift corrections, solid-Earth and relevant ocean-loading tidal corrections, latitude/reference-ellipsoid gravity subtraction, the free-air correction, the Bouguer slab correction for material between station and datum, and a terrain correction for departures from a flat slab. The crater’s strong relief makes the last correction important. A local regional-field/background estimate is also needed if the target is the crater’s residual anomaly. State the reduction density; it is not the inferred breccia density.^[ref] For a fixed land traverse there is no moving-platform Eötvös correction to add.