Magnetic methods have long played a secondary role in oil exploration, such as helping to define basement structures that control emplacement of hydrocarbon in overlying sedimentary basins. This quadrennium introduced several more direct applications. Near-surface magnetic anomalies often are seen associated with oil-producing regions. To improve our understanding of the sources of these anomalies, Reynolds et al. [1991] conducted a thorough rock magnetic and geochemical analysis of magnetic minerals from three well-known oil-producing areas of the United States. They found widely different petromagnetic settings at each locale, some involving authigenic magnetic minerals directly related to hydrocarbons. They concluded that both abiologic and biologic mechanisms can produce magnetic sulfide minerals (pyrrhotite, greigite, etc.) in some zones of hydrocarbon seepage. Alternatively hydrocarbons can reduce magnetization by replacing detrital magnetic minerals with nonmagnetic sulfide minerals. Thus a direct connection has been established between the presence of hydrocarbon and the nature of near-surface magnetic anomalies in sedimentary basins.
Andrew et al. [1991] made the strong pitch that hydrocarbon exploration should involve a synergistic combination of various kinds of data. They showed correlations between aeromagnetic lineations, Landsat lineaments, and vertical offsets in high-frequency seismic sections from Montana. They interpreted their observations as reflecting conjugate sets of lateral faults that evolved in a wrench system and that apparently have influenced the emplacement of hydrocarbons. Andrew et al. [1991] emphasized that many oil fields have near-surface magnetic anomalies, and elsewhere magnetic anomalies occur over regions with seismic characteristics often indicative of oil resources. For these reasons, magnetic anomalies should play a larger role in identifying exploration targets.
In a similar vein, Sparlin and Lewis [1994] undertook a detailed investigation of a subtle (40 nT) aeromagnetic anomaly situated directly over the Omaha Oil Field in southern Illinois. They conducted a detailed ground magnetic survey of the region and produced a three-dimensional model for the source of the anomaly. They constrained their model with petromagnetic measurements, thin-section analysis, and drill information. They interpreted the anomaly as due to two ultramafic sills emplaced into the sedimentary section, one above the other. Together the sills have served as the structural closure necessary for hydrocarbon accumulation.
Oil was not the only exploration target in magnetic anomaly studies
this quadrennium. Babu et al. [1991], for example,
used magnetic anomalies to define the shape and extent of an aquifer in
Hyaderbad, India. The weathered layer over granitic terranes sometimes
acts as an aquifer, with the base of the weathered layer (roughly 50 m
deep in this locale) corresponding to the base of the aquifer.
Babu et al. [1991] used spectral analysis of magnetic
data to estimate the thickness of the weathered layer, under the
premise that the weathered layer is relatively nonmagnetic with respect
to the underlying, unweathered granite. With many such estimates, they
were able to produce a map showing the topography on the base of the
weathered layer. When compared with eight drill sites, their depth
estimates were accurate to within an rms error of 
10.2 m.