The Interpretation of Gravity Results
Published:January 01, 1971
The instruments, field operations, data reduction, and mapping described above produce a gravity map which, ideally and theoretically, is free of planetary effects of the earth as a whole and from the visible topography. If the earth's crust were composed of uniform horizontal sheets with no horizontal variations in density, the gravity map would be featureless. All of the features which it shows have their origin in horizontal discontinuities at any depth from the grass roots down. The objective of gravity interpretation is to deduce the geological character of the subsurface from variations in the gravitational field.
The interpretation is inherently ambiguous. In the first place, there is no single mathematical solution to the determination of the source of a gravity field no matter how precisely that field may be mapped. Figure 36 shows a simple positive gravity anomaly which has been calculated as the gravity effect from a sphere (or point mass) shown as body no. 1. The same anomaly could be accounted for completely by a thin lenticular body. no. 3 in the figure, at a very shallow depth or by an intermediate narrower, and thicker body such as no. 2. There is a “cone of solutions” as indicated by the dotted lines. The spherical mass, no. 1, is the deepest possible solution as any deeper mass would produce a broader anomaly but solutions at all shallower depths to the surface are possible. Also combination solutions are possible for which part of the total mass is in different bodies. The one thing that the several possible solutions have in common is that their total mass (i.e., the product of the volume times the density contrast of the anomalous material) must be the same, the oretically, for each solution.
Figures & Tables
Elementary Gravity and Magnetics for Geologists and Seismologists
The purpose of this work is a general review of the gravity and magnetic nlethodsods of geophysicael xplorationa s applied in the search for petroleum. This material is not designed for the gravity and magnetic specialistb ut rather lo)r the geologistsa nd seismologistwsh o may not have a thorough appreciation of the applications of these metht)ds in the overall expl()ration picture. A subtitlc for this monograph might well be "-l'hc Other Five Percerot." This is because the seismic method and its associated data processing account for sornc 95 percent of the total expenditures Ik)r petroleum exploration geophysicss o that whatever application is made of the gravity and magnetic noethods comes out of the other 5 percent. This does not mean that these methods make a proportionately small contribution to the overall exploration effort. Because of the relatively rapid rate of progress in the field, particularly by airborne magnetics. the total area covered by gravity and magnetic surveys may bc greater than that covered by the much greater seismic expendituresA. s a very rough rule-ofthumb, the relative cost per unit area of magneticg, ravity and seismicf ield work with data processings tand in the ratio of I to 10 to 100. It is the hope and purposeo f this monographth at a better appreciatioonf the valueo f the potential methods and understanding of their applicationsm ay be broughta bouts t) that they can be applied with proper perspective in the overall exploration picture. From the beginning of geophysical exploration in the petroleum industry in the 192()'s, three basic physical principles were used: i.e., the measurement of small variations in the magnetic field, the measurement of small variations in the gravitational field, and the propagation of elastic waves through the earth. These three and only these three physical principles are the basis for practically all of the geophysical work up to the present time. Many other methods have been conceived and tried in the field in a limited way, but none has persisted to the extent that field operations are carried out n a scale at all comparable with that of the three primary methods listed above. The seismic method, of course, usually is much more direct in its relation to the geologyt han the potentialm ethodsR. etlection zones or horizons frequently are directly correlative with geologic strata and give relativelya ccuratem easureosf their depth and form.