Short-wavelength low-amplitude SEDimentary MAGnetic (SEDMAG) anomaliers and their validity

What is it about?

Sedimentary Magnetic (SEDMAG) anomalies are typically high-frequency or short wavelength (1 - 5 km) low-amplitude (1 - 20 nT) magnetic anomalies generating in the shallow sedimentary layers as a result of minor concentrations of magnetic minerals, more commonly detrital magnetite which is deposited simultaneously with the sediments that have a nearby source in igneous terrain. Other possible sources of SEDMAG anomalies and micro-anomalies are discussed in Part II of this paper and include diagenetic magnetite and other magnetic minerals produced by chemical changes in the sediments, fault mineralization due to upward-migrating hydrothermal fluids along fractures and fault planes, combustion metamorphism or burning of carbonaceous pyrite-rich and organic-rich outcropping source-rock sediments, related clinkers or burning of coal seams, and diamagnetism of salt and anhydrite. The validation of SEDMAG anomalies is important in any interpretation work. Line-to-line and survey-to-survey repeatability and maximum entropy spectral analysis (MESA) are two of the approaches used to validate the geologic nature of these anomalies. SEDMAG anomalies have been found to be useful in the mapping and study of sedimentary structures (faults, anticlines, synclines, salt and shale diapirs) and stratigraphic trends (unconformities, sand trends and sand channels) in the Gulf of Mexico, North Slope and other sedimentary environments. Furthermore, the large magnetic anomalies of clinkers (burned coal) allow the geophysical delineation of older coal burns and can locate the transition from coal seams, used as an energy resource, to the less valuable clinker, used as an aggregate.

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