S. O. Krøgli, H. Dypvik, B. Etzelmüller
Department of Geosciences, University of Oslo
P. O. Box 1047, NO-0316 Oslo, Norway
Telephone: 0047 22856656
Fax: 0047 22854215
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Impact cratering is a common geological process in the Solar System and most planetary bodies display geomorphologies strongly influenced by impacts (Lowman 1997). Fresh impact craters are normally characterized by a circular morphology (Melosh 1989). This surface expression is modified on Earth by active geological processes. The variation of terrestrial impact structure expressions suggests a simple characteristic to use in automatic detection, usually the circular shape. Automatic techniques may detect candidates in regional data, but field and laboratory analysis are required to possibly confirm an impact origin by finding shock metamorphic effects or traces of meteorites (Koeberl 2004).
A first approach to detect candidates was conducted comparing typical impact crater morphologies and topography (Krøgli et al. 2007). Size-dependency scaling characteristics, e.g. relations of crater diameter, crater floor diameter and crater depth, have been established for heavily cratered areas like the Moon (Pike 1977). On Earth the catalog presently consists of 176 proven impact structures (Earth Impact Database 2009). Despite the low number, size-dependencies have also been established for terrestrial impact structures (e.g. Grieve and Pesonen 1992). To search crater-like circular depressions Krøgli et al. (2007) calculated correlations between circular templates, based on terrestrial and lunar size relations, and digital elevation models.
The geophysical properties of impacted target areas may also change during impact and can be found as anomalies in e.g. gravity and magnetic potential field data. Fracturing and brecciation of target rocks and the presence of low-density sedimentary infill cause a circular gravity low, while a central uplift of heavier rocks from deeper crustal levels may cause a circular gravity high (e.g. Grieve and Pilkington 1996). There has not been found a one to one relationship between shapes of magnetic anomalies and impact structures, but circularity may often be present (French 1998). An algorithm that detects circular orientations of slope values has been constructed to search impact structure candidates, treating regional gravity and aeromagnetic data as surface models. The algorithm, that also works on DEMs, examines only the outline of possible circular features.
Both methods (template matching and circular oriented slope values) detected features with different degrees of circularity. The number of detected features depends on the choice of threshold, but is usually large and requires further manual or automatic analysis to refine the number before field investigations. Results can be compared to maps of e.g. geology and drainage patterns and to additional methods and data (e.g. multispectral images). An approach to reduce the number of candidates is presented here as a filter technique, removing candidates from symmetry measurements.