When we think of geophysical interpretation, seismic interpretation often comes to mind immediately. However, many other geophysical methods are used in geophysical interpretation, and we briefly consider two of them here — the gravity and magnetics methods. These are often termed potential-field methods because they deal with force fields derivable from potential energy functions. Potential fields have been used in prospecting for minerals in the mining industry and as reconnaissance tools in the petroleum industry.
Nettleton (1971, p. ix) terms the use of gravity and magnetics methods as “the other five percent,” because the seismic method accounts for 95% of expenditures in geophysical exploration for petroleum. However, this does not mean that potential-field methods make an inconsequential contribution. Although these methods have less resolving power than seismic prospecting methods, they are generally less expensive. As a rule of thumb, Nettleton gives comparative costs of magnetic, gravity, and seismic data as 1:10:100.
Gravity exploration is sensitive to the anomalous mass of a subsurface body, whereas magnetic surveys are sensitive to magnetic susceptibility of rock bodies. Gravity exploration uses gravimeters, and magnetic surveys use magnetometers. Good descriptions of these instruments are given in Telford et al. (1976). Gravimeters and magnetometers can obtain measurements in the air, on sea, on land, and inside the borehole.
Potential-field methods are useful in many aspects of exploring for petroleum traps, including analysis of source-rock deposition, source maturation, hydrocarbon migration, seals, and timing of emplacement of each component of the petroleum system. These
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Fundamentals of Geophysical Interpretation
Fundamentals of Geophysical Interpretation, SEG Geophysical Monograph Series No. 13, is a practical handbook for the petroleum geophysicist. Fundamental concepts are explained using heuristic descriptions of seismic modeling, deconvolution, depth migration, and tomography. Pitfalls in processing and contouring are described briefly. Applications include petroleum exploration of carbonate reefs, salt intrusions, and overthrust faults. The book includes past, present, and possible future developments in time-lapse seismology, borehole geophysics, multicomponent seismology, and integrated reservoir characterization.