Published:January 01, 1971
Each gravity measurement determines, at the station location, the sum of all effects from the grass roots down. A gravity map is almost never a simple picture of a single isolated disturbance but practically always is a combination of relatively sharp anomalies which must be of shallow origin, of anomalies with intermediate dimensions which may be those most probably indicative of geologically interesting sources and from very broad anomalies of a “regional” nature which have their origin far below that of the section within which the geological interest lies. Therefore, gravity interpretation frequently begins with some procedure which separates the anomalies of interest from superficial disturbances on one hand and the smooth, presumably deep “regional” effects on the other. The anomaly separation procedure may consist of the removal of a smooth regional by either of two methods; by an intuitive graphical method or by an analytical method in volving a numerical procedure applied to an array of values usually on a regular grid. The analytical process is basically a filtering process intended to emphasize or enhance certain components of the gravity field and suppress others.
The proponents of these two system have been termed “smoothers” and “gridders.” The “smoother” draws smooth curves on profiles or as contours on a map. These curves represent the component of the gravitational field which is to be removed. This “regional” is subtracted from the observed gravity map and the resulting “residual” contains
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.