Potential Field Techniques
Microgravity Profiling
Detail
Microgravity profiling is a passive technique that involves highly accurate (to within 1 part in 109 (1mgal)) measurement of relative changes in the Earth's gravitational field. Subtle localised changes in gravity result from variations in the density of subsurface materials or the presence of buried engineering structures. As a consequence the method can be used to successfully locate voids (tunnels/mineshafts or solution features) or dense structures such as underground storage tanks (USTs) and dense mineralised (usually base metal) orebodies.
A variety of other factors affect the amplitude of the gravitational field measured at a particular location and time and in order to identify the often subtle anomalies that are of interest in engineering and environmental studies, careful correction of the raw data is required. The effects of instrument drift and earth tides are overcome by repeat readings at a fixed base station throughout the survey in order to build up a drift curve. Drift corrections are then calculated based on the closure error between the first and last base reading in each loop. Tidal effects can alternatively be calculated using computer based programs or published tables of tidal gravity corrections and removed from the data prior to the drift correction.In order to account for the increase in the acceleration due to gravity towards the poles a correction for latitude is applied using the 1967 International Gravity Formula. The latitude correction is made by subtracting the theoretical gravity, calculated for each station location, from the measured value. The Free Air Anomaly (FAA) takes account of the elevation of the station above the spheroid and requires accurate topographic levelling of all the survey stations. The Bouguer Anomaly is calculated from Microgravity meters essentially comprise highly sensitive temperature stabilised spring balances. The spring normally comprises either a fine metal (helically wound) or a quartz rod. GSI (UK) Ltd. commonly use both semi-automated digital (Edcon Super-G and Scintrex CG3 and CG3-M) as well as analogue (Lacoste-Romberg Model D & G) gravimeters.
