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 ).