Enhancement of magnetic signatures of impact structures
Special Paper of the Geological Society of America
Cowan Geodata Services, 12 Edna Road, Dalkeith, WA 6009, Australia; School of Geosciences, University of the Witwatersrand, Johannesburg 2050, South Africa
Aeromagnetic surveys are a useful tool in the detection and analysis of terrestrial impact structures. Although gravity anomalies provide clearer and simpler signatures of impact craters, large regional-scale aeromagnetic surveys are more widely available. A combination of many factors, such as the nature of the target rocks, the impact-related magnetization, and effects of crater fill and post-impact cover results in a great variation of magnetic signatures in the terrestrial impact craters. In crystalline basement targets, the most common signature of a complex impact structure is a magnetic low with a central peak or ring uplift magnetic anomaly. Contributions to the magnetic signature include demagnetization, shock remagnetization, and thermal and chemical remanent magnetization effects. Impact craters in sedimentary targets usually are of small magnetic amplitude, depending on the lithology. The origin of the magnetic signatures in sedimentary targets is not well understood. Enhancement of magnetic signatures of terrestrial impact structures using filtering techniques is an important part of detection and analysis. Derivative and derivative-based (such as sunshading) techniques, along with separation filtering, are probably the most used methods. Here we present our new developments of algorithms for fractional order derivatives and circular shaded relief that have dramatically improved filter results. The fractional derivative order can be varied to optimize the separation of the impact magnetic signature. Given a chosen center location, the circular shaded relief algorithm treats all directions equally, thus preventing fade-out of features subparallel to the shading direction evident in conventional shaded relief. Unlike Hough transform based algorithms, the circular sunshading method is not sensitive to the radius of the circular feature being searched for, and no radius parameter is specified during the data processing We illustrate the new fractional derivative and circular shaded relief algorithms using selected Australian and Canadian impact crater data sets involving both crystalline basement and sedimentary targets. © 2005 Geological Society of America.
aeromagnetic survey; algorithm; crater; crystalline rock; data set; gravity anomaly; magnetic anomaly; magnetization; relief; uplift; Australia; Canada