The Gravity Method
Published:January 01, 1971
All gravity anomalies come from horizontal variations in density. If the earth materials were in layers of horizontally uniform density, there would be no gravity anomalies no matter what the vertical variation in density might be. If layers with different densities are disturbed, anomalies in mass concentration are produced. In Figure 1. let us assume that the layers, 1, 2, 3, and 4 have successively higher density values. d1, d2, d3 and d4. The fiat-lying strata at the margins of the figure will produce no gravity anomaly there because there is no horizontal variation. Over the central part of the figure, these layers are disturbed by a structural uplift which produces the density contrasts indicated by the various hachured areas. In the upper part of the figure, layer 2 is uplifted into the normal area for layer 1 and produces a density contrast d2 — d1 as indicated. Other density contrasts would be produced as indicated on the diagram. In this instance, where each layer is of successively greater density, all the density contrasts are positive and the sum of all of their effects will produce the positive anomaly indicated by the gravity profile.
From this example, it is evident that any disturbance of normally fiat-lying layers of different densities will produce some sort of density contrast which will cause a gravity anomaly. The magnitude and form of the gravity effect depend
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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.