Review and Conclusions
Published:January 01, 1971
The foregoing text and illustrations have attempted to point out the possibilities and limitations of gravity and magnetic methods in petroleum expioration. Some readers may be more impressed by the limitations, particularly the ambiguity and lack of precise results in terms of form, depth, and geological definition which result from applications of these methods. On the other hand it is hoped that the possibilities of the methods may be better appreciated and that a greater participation of “the other five percent” in the overall petroleum exploration picture may be encouraged.
Since petroleum exploration is now very much dominated by the reflection seismic method it may be well to point out more specifically than is generally appreciated the relations between the potential and the seismic methods and their complementary contributions to overall understanding of the subsurface geology.
The ideal order of application of the three geophysical methods would be first, reconnaissance with airborne magnetics, followed by gravity surveys in a degree of detail depending on the geological objectives and the facility of getting over the surface, and finally by reconnaissance and detailed reflection-seismograph surveys with the detailing being directed to those areas where gravity or magnetics or both point to a feature of particular interest. This idealized overall exploration approach is rarely used, partly because of land or concession or competitive considerations, and partly because of a lack of appreciation of the complementary nature of the different geophysical methods. In many instances the argument is used that an area probably will be covered eventually by the reflection seismograph method and that having
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.