Description

Microgravity is the name that has been adopted for very high-resolution gravity mapping. Microgravity surveys are typically used to detect sub-surface cavities or changes in sub-surface density. With modern equipment and careful field procedures it is now possible to measure gravity changes as small as 1 part in 1,000,000,000. Changes in gravity measured at the earth's surface reflect the underlying geological structure; hence the accurate determination of gravity can lead to an understanding of the ground beneath.

May be useful for location of air-filled voids, or other features resulting in materials of differing densities (e.g. swallow holes in chalk, shallow tunnel workings.

To achieve high accuracy requires very careful measurement technique (no ground vibration/ stable ground surface /no wind/ low temperature drift/good instrument). Variations which are encountered in the search for cavities are typically a few tens to hundreds of microgals. Elevation accuracies need to be to +/- 5 mm or better: this is very difficult to achieve unless ground is relatively firmArtefacts can be caused by:

• Variable rock density
• Shallow in filled depressions with material at lower density
• Poor or absent terrain corrections (i.e. corrections for local small hills or gullies)
• Elevation measurement errors
• Incorrectly applied contouring programmes

Microgravity measurements are unlikely to reveal structures that are not apparent from ordinary data, although very careful measurement techniques may make these structures clearer, and add to the confidence of identification. There are some inflated claims about the microgravity technique which are not apparently justified.