Published:January 01, 1971
The early exploration by the petroleum industry was done with a magnetic field balance. This instrument consists essentially of a horizontal magnet supported on a quartz knife edge. The torque due to the reaction of the magnetic poles with the vertical component of the magnetic field is balanced by the weight of the moving system with its center of gravity being adjusted at the correct distance from the knife edge support to achieve this balance. The position of the moving system is indicated by a scale in the eye piece which is reflected from a mirror on the moving system. As commonly adjusted this instrument can measure changes in the magnetic field with an accuracy of 5 to l0 gammas. Such instruments were used very extensively in magnetic prospecting for petroleum in the years from the beginning of such prospecting in the early nineteen twenties until they were largely superseded by airborne instruments beginning shortly after the second World War. In some later instruments, ligaments or fibers are used instead of the rather fragile quartz knife edge bearing. Also electronic instruments, using the principles from the airborne instruments described later, have been used for magnetic surveys on the ground, particularly for very detailed observations.
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.