Microgravity measurements detect changes in the earth's gravitational field caused by local changes in the density of the soil and rock or the presence of engineered structures (ASTM 6430-99).   Lateral variations or anomalies in the gravity data can be attributed to lateral variations in the densities of subsurface materials, (e.g. buried channels, caves, cavities, variations in top of rock, mines and tunnels).   Microgravity comes closest of all the geophysical methods to allowing a positive statement regarding the presence or absence of subsurface cavities (rock mass deficiencies) at a site.  Modern gravimeters allow gravity data to be acquired at a rate that is comparable with the other geophysical methods.

Applications

• Identify karst features (caves, voids, sinkholes, weathered zones)
• Map top-of-rock
• Map erosional features (buried channels)
• Characterize stratigraphic thickness and structure
• Identify man-made structures (tunnels and mines)

Advantages

• Provides a means to characterize conditions in geologic and cultural environments, where other geophysical methods may fail
• Data can be interpreted to provide estimates of depth, size, and the nature of the anomaly
• Requires a single operator
• Can be used inside buildings and structures

Limitations

• Irregular topography will produce artifacts in the data unless accounted for in the processing
• Local sources of vibrations, wind, storms, and distant earthquakes can produce interference
• Anomalies are non-unique and models must be constrained with supporting data (a large, deep target can produce the same anomaly as a shallow broad target)