Gravity field surveying is a passive technique to detect gravity anomalies. Gravity is a global field that always points vertically down, with anomalies caused by very small variations in density of the underlying materials.
A gravity base station measures the rate of a mass falling over a known distance, but it is difficult to measure absolute gravity to the accuracy necessary for interpretation. Instead, gravimeters are used to measure relative gravity with respect to deviations from a fixed base station. A gravimeter consists of a weight offset with a spring stiffened to compensate for local regional gravity conditions. The weight will then move up or down in the presence of a positive or negative anomaly.
Elevation and topography impact gravimeter measurements, and require corrections in order to process the collected data when interpreting density of the underlying materials. For higher elevations, it is necessary to subtract the Bouger correction and add the Free-Air correction, while the inverse applies to lower elevations. Topography corrections are always positive. For airborne surveys, the Eo ̈tvo ̈s correction compensates for the aircraft’s speed and direction at the time of the measurement.
The Earth’s gravity field also has regional variations independent of material properties. Because the Earth is not a perfect sphere, and is instead a flattened spheroid, an observer at the poles is closer to the centre of mass while one at the equator has more mass between them and the center of mass. Add in that the Earth is rotating such that centripetal acceleration is stronger at the equators than the poles, and the net impact is that gravity is approximately +0.53% stronger at the poles than at the equator. When interpreting gravity survey data, latitude corrections may be necessary. Latitude corrections are based on the current geodetic model of the appropriate scale, adding a correction to survey measurements south of the base station, and subtracting a correction for survey measurements north of the base station.
Even base stations vary with time, with cyclic variation from tides that may automatically calculated and corrected, and with instrumental drift caused by spring creep or variations in temperature or pressure that needs to be manually corrected with a drift curve. For surveys that go beyond a local region and latitude corrections become necessary, the survey must overlap base stations, and the base stations tied to the nearest network station where absolute gravity has been accurately measured.
All of these corrections make it relatively difficult to process and interpret gravity data compared to other forms of geophysical surveys. However, surveys can be conducted from almost any platform: ground, air, marine, satellite, and from boreholes.
Time Lapse Microgravity Surveys
Time-lapse microgravity studies are an alternate approach to processing. The same area is surveyed at different times, with only cyclic corrections applied, leaving anomalies from nearby structures and other artifacts intact. The readings are then compared to each other such that any stable anomaly cancels out, leaving only unexplained temporal changes. This can be used to monitor response to fluid injection, reservoir withdrawal, or subsidence.
The response of irradiated rock can be used to interpret the density of that rock. This will be discussed next month.