Gravity Methods

The following discussion of gravity prospecting was modified from the U.S. Army Corps of Engineers manual "Geophysical Methods for Engineering and Environmental Investigations" edited in part by David Butler and "Introduction to Geophysical Prospecting " by Milton B. Dobrin.

Theory of gravity measurements
In gravity measurements, the quantity actually observed is not the earth's true gravitational attraction but its variation from one point to another. Such lateral differences can be measured with a much greater degree of precision than the total gravitational field, and field instruments are designed to measure differences in gravity rather than its actual magnitude. The gravitational variation observed typically consists of two components: a general, relatively uniform component due to the total earth, and a smaller, more variable component due to lateral density changes in the subsurface. This second component is the gravity anomaly, which gravity prospecting seeks to characterize and to ascribe a source.

Determining the gravity anomaly
Simply put, the gravity anomaly at any point is the difference between the observed value of gravity and the predicted value of gravity based upon the following equation :

g = (9.780318 m/s**2) ((1+0.0053024(sin2F)) - (0.0000059(sin2F)**2))

where g = the acceleration of gravity in m/s**2 and F= the latitude in decimal degrees. Before a valid comparison of the observed and the predicted values can be made several corrections must be introduced to the observed (i.e. measured) value of gravity. The first correction effectively compensates for the fact that the attraction of gravity above sea level decreases with height because distance from the earth's center is increasing. This adjustment is called the free-air correction. The second correction involves subtracting the attraction of the material (assumed to be a horizontal slab) between the elevation of the measurement station and sea level. This change is called the Bouguer correction and effectively places the measured value of gravity at the same elevation as the predicted value of gravity (i.e. sea level). At this stage, a descrepancy between the observed value and the predicted value is termed the simple Bouguer anomaly.

A third correction considers the effect of local topography on the gravity measurement and is typically referred to as the terrain correction. Terrain corrections assign variations in topography around the measurement point to specific zones, with variations in nearer zones evaluated more precisely than variations in farther zones. Once the terrain correction has been applied, any discrepancy between the observed and predicted value is termed the complete Bouguer anomaly.

The size matters
For engineering or environmental applications in which very small anomalies are of greatest interest, there are gravity anomaly trends of many sizes. The larger anomalies will tend to behave as regional variations and the desired smaller magnitude local anomalies will be superimposed on them. A simple method of separating regional anomalies from smaller, microregional variations is to visually smooth contour lines or profiles and subtract this smoother representation from the reduced data. Alternate methods include polynomial fitting, spatial filtering and continuation. No matter what the separation method, the remainder is called the residual anomaly. This regional-residual separation is the most critical portion of the geophysical interpretation.

Microgravity surveys are performed when the scale of the geophysical target is sufficiently small (e.g. 1 -10 meters) so that conventional gravity measurements, such as those made in petroleum exploration, provide inadequate definition. Using very short station spacings (1 to 3 meters), microgravity surveys seek to preserve as much precision as possible in the measurements and analysis so that small objects can be detected.

Interpretation of gravity data
Any valid interpretation of gravity data must overcome two major difficulties. The first is the problem of non-uniqueness; many different earth models can produce the same Bouguer anomaly. For instance, a distribution of small masses at a shallow depth can produce the same effect as a large mass at a greater depth. The second problem, related closely to the first, is the trade-off between density and thickness; if both values are unknown, the ambiguity in any interpretive model will be great. The interpretation of gravity data, therefore, is very dependent upon supplemental data, such as density measurements or known geologic or geometric relationships. The more supplemental data available, the more realistic the interpretation.

Field measurements of gravity
Modern gravity meters have a precision of a few tens of microgals. Even newer, computer-controlled models have irreducible measurement errors of less than 1 or 2 microgals. Thus the instrument is one of the smallest sources of error in a survey.

Other, more important sources of error are the elevation control ( the gravity gradient at the surface is about 2 microgals/centimeter which implies that a centimeter or two of survey precision is required for microgravity surveys) and the roughness (elevation variability) of the surface. The latter is important because of the corrections mentioned previously. A "Bouguer reducing density" must be measured or assumed. This density is multiplied by the elevation differences between stations. For microgravity surveys, the inevitable error in this density (especially from point to point) is a significant factor.

MicroGeophysics Corporation (MGC) has made gravity measurements at spacings of 1 foot, 220 feet and 1 mile. We have made exploration measurements with 250 microgals of uncertainty and 6 milligals of terrain corrections. We have also made microgravity measurements with demonstrated (through repeat measurements) precision of less than 6 microgals.

MicroGeophysics Corporation (MGC) will furnish you with a guide to microgravity measurements if you will furnish us with your E-mail address. We can also respond to a request for a specific calculation of the masking effect of suface terrain on the calculated effect of a specific body. If you will furnish us with the parameters, we will do the math!!

MicroGeophysics Corporation (MGC) will be happy to respond to your questions!!
Phone (1-800-GEOPHYSics) fax (303-424-0807) or E-mail (microgeo@aol.com).