High-resolution Aeromagnetic Interpretation over Sierra and Yoyo Reefs, Northeastern British Columbia
John Peirce, Erwin Ebner, Nathalie Marchand, 1998. "High-resolution Aeromagnetic Interpretation over Sierra and Yoyo Reefs, Northeastern British Columbia", Geologic Applications of Gravity and Magnetics: Case Histories, Richard I. Gibson, Patrick S. Millegan
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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.