Published:January 01, 1971
It is not intended to describe the physical details or operation of the considerable number of gravity field instruments which have been developed over the years. Some twenty or more different land gravity meters have had very extensive use in the field.
A body on the surface of the earth has a “weight” which results from the gravitational attraction of the entire earth. If the body is allowed to fall, it is accelerated by this weight. The unit of acceleration is the gal, named after Galileo, and one gal is simply one cm/sec2. The average acceleration on the earth's surface is about 980 cm/sec2 = 980 gals (but increases by about 1/2 percent from equator to pole).
In geophysical prospecting we are interested in “anomalies” in the gravitational field produced by variation in density as defined in the previous chapter. These are only very small fractions of the earth's total field, and a smaller unit is needed. The commonly used unit is the miilligal or mgal which is .001 gal. Anomalies from local geologic structures are commonly of the order of one to ten mgals.
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.