1. adj. [Formation Evaluation]

Pertaining to the detection of the Earth's gravitational field within a wellbore. Subtle vertical variations of the Earth's gravity field may be detected over the length of a borehole. These depend on the variations in the formation density not only above and below the sensor, but also laterally away from the borehole. Thus, borehole gravity measurements may be used to detect the following phenomena:

• overburden pressure
• lateral formation density changes arising from porosity changes away from the borehole (fracture fields, vugs)
• lateral proximity to lithology changes, such as major faulting or salt intrusions
• time-lapse density measurements to monitor fluid saturation changes during the life of a reservoir.

In borehole gravity measurements, highly accurate formation density measurements, averaged over a large volume, may be made by comparing changes of gravity between measurement stations.

See related terms: borehole gravity meter