J.P. Land Associates, Inc.
The Information Potential of Modern Magnetic Surveys
Since magnetic surveys' initial application in petroleum exploration, its resolving power has undergone considerable advances. From what was at first a surface method, an airborne capability was developed that quickly became the dominant magnetic survey mode and the world's prime exploration reconnaissance tool.
Though the majority of magnetic surveys have been accomplished to determine the depth and structural setting of magnetic basement. Beginning with ground surveys in the 1930's, many detailed surveys have been successfully carried out with the objective of developing leads to local sedimentary structure. A number of these are described in the writings of W.P. Jenny.
The Earth's magnetic field is a composite of anomalies of varying frequencies. The highest frequency events of interest are those created by geological conditions in the shallow subsurface. The lowest frequency events are caused by structuring and/or magnetic property contrasts at or beneath the basement surface. Intermediate frequency events are created within the sedimentary section.
There are a number of geological causes for local high frequency distortions of the Earth's magnetic field. The cause perhaps most important to micromagnetics and the detection of local structure is the faulting of the shallow sedimentary formations. Of the sedimentary rock types, sandstones usually have a higher susceptibility than shales which generally are higher than limestones. Faulting puts formations of unlike lithology in contact with one another and, because each formation has its own magnetic susceptibility, such contact gives rise to a minor but measurable anomaly. Basement structure and topography, salt and shale diapirs, igneous plugs, pinnacle reefs all affect subsequent deposition and the stability of overlying formations through such processes as fault reactivation, differential compaction, draping and differential compaction.
Airborne surveys are now routinely flown at a lower ground clearance and tighter grid interval that allows the recording, in great detail, of a greater range of frequencies of the magnetic field, responses created throughout the sedimentary section, top to bottom. Most of the data now being acquired thus has the capacity to allow the interpreter to also resolve anomalies related to sedimentary structure, the very features we hope to focus on when beginning an exploration program.
Structuring the near-surface formations gives rise to a characteristic high- wavenumber magnetic expression. The detail of symmetry of the various elements of such an anomaly, important to their accurate interpretation and definition, is best observed in residual profile form.
With the proper processing and analysis of an area's total spectrum of magnetic responses an interpreter can produce not only a picture of basement structure and topography but also an overlay of the areal distribution of sedimentary magnetic anomalies, the linear anomalies thought to be the expression of deep-seated fracture zones and non-linear anomalies attributable to local structural relief.
It is important for the explorationist to be aware of the focusing power of today's magnetic survey and its ability to rapidly, and inexpensively, produce its unique perspective of a region's structural grain and leads to local structure. Leads that can then be systematically evaluated and defined by more expensive surface methods.
Modern magnetic surveys have the capacity to let us view the distribution and trends of shallow structure relative to deep structure for an expanded perception of the structural setting. It's still a reconnaissance method, but now, one with a greater focusing power and information potential than conventional magnetic surveys of years past.
