2.0 Geophysical Investigations in Impact Craters
Unlike Earth, impact craters can commonly be recognised from morphological characteristics on the Moon and other planetary bodies that lack an appreciable atmosphere. However, a large portion of the record of impact is modified and partially destroyed due to the extremely dynamic terrestrial geologic environment. The initial morphological components of terrestrial impact craters are swiftly destroyed by erosion, and over time, the geological and structural imprint is also lost. On the other hand, sedimentation obscures craters from view while protecting them. To identify impact craters, a simple workflow usually applied involves detecting crater morphology, geophysical anomalies, evidence for shock metamorphism and the presence of meteorites or geochemical evidence for traces of the meteoritic projectile. Geophysical anomalies are, therefore, an essential process for identifying impact craters (Grieve and Pilkington, 1996 ).
In 1992, Pilkington and Grieve attempted to compile the basic geophysical details of terrestrial impacts. Further reviews on this subject can be found in Grieve and Pilkington (1996 ). The methods commonly used in impact crater investigations are gravity, magnetic, seismic, geoelectrical and Ground Penetrating Radar (Karp et al., 2002; Pilkington and Grieve, 1992; Reimold and Koeberl, 2014 ). The use of these methods depend on the fact that impact craters frequently have a distinct geophysical signature, and changes in the physical characteristics of the near-surface are effective indications of lithological changes.
Simple and complex structures are the two categories of impact structures’ morphology. Positive and negative gravity anomalies represent higher and lower densities, respectively, compared to a baseline value, showing how sensitive gravity readings are to changes in near-surface density. Negative gravity anomalies are often circular and extend to or just beyond the crater rim and are linked to impact craters. Although low-density impact breccias and sedimentary infill may also contribute to the density loss, the target rocks’ fracture and brecciation is the primary reason (Grieve and Pilkington, 1996 ). A magnetic anomaly near zero is a common feature of impact craters, which shows that the impact process has demagnetised the materials there (Pilkington and Hildebrand, 2003; Plado et al., 2000 ). The centre of larger craters may be affected by short-wavelength magnetic anomalies, and if the zone of central uplift is created from magnetic basement material, it will result in a longer-wavelength magnetic anomaly. Short-wavelength anomalies can be caused by various processes, including shock metamorphism, hydrothermal processes, post-impact cooling, melts, and impact breccias. As electric current flows more freely in the salty water that often fills the pore space and fissures, resistivity typically reduces with increased porosity. Thus, at impact craters, resistivity is often higher in deeper portions of the central uplift and lower in less fractured or brecciated rocks, allochthonous deposits, and porous sedimentary infill (Aning et al., 2013 ). Seismic reflection data have been used to image a variety of crater features, including faulted blocks of target rocks that are downthrown and/or rotated in the terrace or megablock zone, coherent reflections in targets rocks that become increasingly disturbed when tracked towards the crater center, and uplifted rocks in the crater center (Karp et al., 2002 ).