Special Gravity Instruments
Published:January 01, 1971
The gravity instruments previously described and referred to in the discussions of measuring and reducing gravity observations are designed for operation on land and on a firm base. Gravity measurements have been extended to the water bottom and to moving ships.
The underwater gravity meter commonly used is basically a land gravity meter in a water-proof shell, and with mechanical and electronic accessories arranged so that the same operations are done remotely through an electric cable to a control box on the ship as would be done if the instrument were manipulated by hand. The instrument is mounted on gimbals within the waterproof case and small motors move the instrument on these gimbals until remote indicators at the control box show that it is level. In some instruments this function is automatic and the leveling motors are operated through a servodevice to keep the instrument level. The position of the beam of the moving system is indicated at the control box and the instrument can be balanced and read in much the same way as it would be manually. All of the operations of leveling, releasing the clamps to free the moving system, balancing the meter, obtaining the gravity value, and clamping the instrument at the end of the observation are carried out from the control box on the surface in a few minutes. The ship is riot anchored but is held within a short horizontal distance from the meter by manipulation of engines and rudders.
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.