Introduction

Since the discovery of the law of gravitation (Isaac Newton, 1685), the mutual attraction between all masses has been recognized as a universal phenomenon. The phenomenon accounts for the familiar fact that a body when released near the earth will fall with increasing velocity. The rate of increase of velocity is called the gravitational acceleration, or simply gravity, g, which would have a single constant value on the earth's surface if the earth were a perfect sphere of uniform concentric shell structure. In fact, our earth is non-uniform, non-spherical, and rotating, and all these facts contribute to variations in gravity over its surface.

The measurements and analyses of the variation in gravity over the earth's surface have become powerful techniques in the investigation of subsurface structures at various depths. Of prime interest in environmental and engineering studies are the variations in gravity which reflect lateral density contrasts associated with targets at shallow depths. In many cases the density contrasts occur at boundaries between different geological formations, although man-made boundaries such as tunnels and mines also represent contrasts. Gravity variations caused by density inhomogeneities are relatively small (often <10-6 of the earth's gravity), and their accurate measurement is possible only with the aid of extremely sensitive instruments.

As a geophysical technique, the gravity-surveying method has a great deal in common with the magnetic method. Both gravity and magnetic fields are potential fields, and both require fundamentally similar interpretation techniques.