In their normal geologic application, gravimetric or microgravity surveys, are used for the detection of major subsurface structures such as faults, domes, anticlines, and intrusions.
Gravimetric surveys have been used in engineering studies to detect cavities in limestone and the location of old mine shafts (Ghatge, 1993). Modern instruments are extremely sensitive, however, and the requirement for the precise determination of surface elevations may cause the application of the method to be relatively costly.
Major geologic structures impose a disturbance on the Earth’s gravitational field. The part of the difference between the measured gravity and theoretical gravity, which is purely a result of lateral variations in material density, is known as the Bouguer anomaly. Other factors affecting gravity are latitude, altitude, and topography, and have to be require considered during gravitational measurements to obtain the quantity representing the Bouguer anomaly.
Gravimeters consist of spring-supported pendulums similar in design to a long-period seismograph.
Isogal maps are prepared showing contours of similar values given in milligals (mgal) to illustrate the gravity anomalies. (Note: 1 mgal=0.001 gal; 1 gal=acceleration due to gravity =1 cm/s2.)
Cavity Exploration in Soluble Rock
The bulk density of limestones is about 160 pcf (2.6 g/cm3), and that of soils generally ranges from 100 to 125 pcf (1.6 to 2.0 g/cm3). In karst regions, a gravity-low anomaly may indicate an empty cavity, a cavity filled with low density material, or a change in soil or groundwater conditions. Microgravimetric instruments have been developed in recent years which permit a precision of 0.01 mgal (10 μgal) or better (Greenfield, 1979), equivalent to a change in soil thickness of about 10 in. (24 cm) for a density contrast between soil and rock of 62.4 pcf (1.0 g/cm3). A detected anomaly is then explored with test borings.