Abstract Analysis of the Project Magnet aeromagnetic data acquired by the U. S. Navy in 1969 permits us to predict a new sedimentary basin, Heuksan Basin, south of the known Gunsan Basin in Block II, offshore South Korea. The basin appears to consist of three subbasins trending north-northwest'south-southeast. The results of our analysis provide not only an independent assessment of the Gunsan Basin, but also new important information on the tectonic origin and mechanism for the two basins as well as for the entire region. The basin-forming tectonic style is interpreted as rhombochasm associated with double-overstepped left-lateral wrench faults. From magnetic evidence, a few northeast-southwest-trending major onshore faults are extended to the study area. We also interpreted the faults to be left-lateral wrenches. This new gross structural style is consistent with the results of recent Yeongdong Basin analysis (Lee, 1990). The senses of fault movement also are supported by paleomagnetic evidence that the Philippine Sea has experienced an 80 clockwise rotation since the Eocene. Based on a 2.5-D model study, the probable maximum thickness of the sediments in the Gunsan Basin is approximately 7500 m. We believe that the new Heuksan Basin was left unidentified because a high-velocity layer may overlie the basin. Because the overall structural configuration of the Heuksan Basin appears to be favorable for hydrocarbon accumulation, a detailed airborne magnetic survey is recommended in the area, to verify the magnetic expressions of both this thick basin and the tectonic style. A detailed marine gravity survey acquired in conjunction with a highenergy source seismic survey also is recommended, to delineate the sedimentary section and to acquire data supplemental to the magnetics.

Abstract There has been a sudden resurgence of aeromagnetic data acquisition in western Canada since 1994 because the new high-resolution aeromagnetic (HRAM) surveys are now able to map intrasedimentary faults and fractures on a regional scale, whereas previously, aeromagnetic data were used only to delineate basement features. This increase in resolution has been achieved primarily because of more precise navigation and positioning of data using the Global Positioning System (GPS), and also because of flying in controlled drape mode close to the ground to enhance shallow sources. Better instrumentation in the aircraft and improved software for analysis and visualization also have helped. In 1994, the first HRAM survey in western Canada was flown for Focus Seismic Corp. by Questor Surveys, Ltd., over the Sierra and Yoyo Reefs in northeastern British Columbia. Although these Devonian reefs are well known and have been producing gas for years, the area is also one of active exploration for smaller pinnacles and carbonate bank plays. This paper discusses many of the concepts applicable to designing, flying, processing, and interpreting an HRAM survey, using the Sierra survey to demonstrate the points. Particular emphasis is placed on editing the data culturally, choosing a line spacing appropriate to the complexity of the expected results, and the value of depth analysis on the profiles to resolve subtle magnetic anomalies associated with faults and fractures. Such linear features can be mapped on adjacent profiles and traced for many kilometers. The consistency of the fault interpretations from profile to profile is an essential factor in sorting out geologically significant features from mathematical artifacts. We hypothesize that the cause of these fault-related intrasedimentary anomalies is magnetization in the plane of the faults or fractures. Recent work (Pierce et al., 1998) has demonstrated the importance of the sulfur geochemistry in catalysing reactions involving iron-bearing minerals in fractures in shales. Specifically, authigenic macroscopic pyrrhotite has been observed in shales at shallow depth. Pyrrhotite is important magnetically because it has a similar susceptibility to that of magnetite. Pyrite, which often is considered nonmagnetic but actually has a small susceptibility, also may contribute significantly to fracture magnetization when the surrounding sedimentary section is nonmagnetic. This approach to magnetic interpretation allows one to map regional faulting patterns over large areas at relatively low cost. The final product, a magnetic structural-grain map, allows one to define exploration fairways and to position seismic programs more efficiently, and it usually reveals several new exploration leads.

Abstract Total magnetic intensity anomaly expressions are derived for four simple sources: the point pole, line of poles, point dipole and line of dipoles. Type curves are presented for the point pole and dipole. For all cases, factors are calculated which may be multiplied into the half-maximum distances on the anomaly profiles to yield depth estimates. These methods serve as a first approximation in the interpretation of aeromagnetic data, but their limitations must be kept in mind. Two examples of the application of the methods are given.

Abstract Quantitative interpretation of aeromagnetic anomalies has been based for the most part on the use of physical models in which the direction of magnetization is parallel to that of the earth’s field, al though laboratory results show that this assumption is invalid for many rock units. Magnetic fields have been calculated for inclinations of magnetization significantly different from that of the earth’s present field. The calculations are for a rectangular flat-topped mass with finite and infinite vertical sides and a 75 degree inclination of the earth’s field. Several significant empirical relationships between the physical model and the computed fields have been obtained. For low dips of magnetization, the maximum and minimum points, rather than the points of inflection, mark the edges of the rock masses. The dip of the magnetization vector may be estimated from the ratio between the maximum and minimum anomaly amplitudes. The depth calculation techniques described in Geological Society of America Memoir 47 are based on the assumption of induced polarization, but these same empirical rules can be applied equally well to the total intensity field when remanent magnetization is present. This probably explains the success of the Memoir methods when applied to observed aeromagnetic anomalies over sedimentary basins