INTRODUCTION

General

Gravity prospecting involves measurements of variations in the gravitational field of the earth. One hopes to locate local masses of greater or lesser density than the surrounding formations and learn something about them from the irregularities in the earth's field. It is not possible, however, to determine a unique source for an observed anomaly. Observations normally are made at the earth's surface, but underground surveys also are carried out occasionally.

Gravity prospecting is used as a reconnaissance tool in oil exploration; although expensive, it is still considerably cheaper than seismic prospecting. Gravity data are also used to provide constraints in seismic interpretation. In mineral exploration, gravity prospecting usually has been employed as a secondary method, although it is used for detailed follow-up of magnetic and electromagnetic anomalies during integrated base-metal surveys. Gravity surveys are sometimes used in engineering (Arzi, 1975) and archaeological studies.

Like magnetics, radioactivity, and some electrical techniques, gravity is a natural-source method. Local variations in the densities of rocks near the surface cause minute changes in the gravity field. Gravity and magnetics techniques often are grouped together as the potential methods, but there are basic differences between them. Gravity is an inherent property of mass, whereas the magnetic state of matter depends on other factors, such as the inducing fields and/or the orientations of magnetic domains.