Petrophysical Measurements
Petrophysical measurements refer to measuring rocks’ physical and chemical properties in the subsurface in boreholes or cores. In impact crater studies, petrophysical measurements such as porosity, permeability, Young’s modulus, and strength are used to model impact craters. Petrophysical measurements are also used in unravelling fluid flow within hydrothermal systems (Abramov and Kring, 2007; Sanford, 2005 ) to understand the physical and mechanical behaviour (Meillieux et al., 2007 ) and slope stability and landscape evolution assessments for impact craters (Heap et al., 2020 ).
Peck et al. (2004 ) analysed sedimentary cores from Lake Bosumtwi and reported the magnetic hysteresis results. Their results indicate five (5) distinct magnetic zones spanning the 11 m core section representing the last 26000 calendar years. The magnetic zones illustrate climatic variability on various scales indicated by the coercivity of magnetic minerals. Samples taken from drill cores from well LB-07A and LB-08A drilled into the central moat and uplift of the Bosumtwi Impact Crater were characterised for petrophysical investigations. Schell et al. (2007 ) report on the magnetic properties of samples determined in the laboratory. The shape and degree of magnetic anisotropy with magnetic susceptibility were correlated with lithological properties. They could distinguish between lithic breccia, suevite, shale component, and meta-graywacke lithologies.
Ugalde et al. (2007b ) constructed a 3-D model using gravity and seismic data constrained by petrophysical data. Boreholes were drilled within the impact crater in six locations, and post-impact sediments’ average density and thickness were measured. By interpreting wireline logs and televiewer images, Hunze and Wonik(2007 ) aimed to understand the subsurface structure of the crater fill of the Bosumtwi impact structure. They report that the physical properties of breccia, meta-graywackes and slate/phyllites, such as shallow resistivity, p-wave velocity, magnetic susceptibility, and borehole diameter, are useful in differentiating between the subsurface layers. The boreholes were drilled under the Lake Bosumtwi Drilling Project (BCDP) and were supported by the International Continental Scientific Drilling Program (ICDP). The boreholes were drilled with the Global Lake Drilling 800 m system, specifically designed to collect long continuous cores. Borehole geophysical logging, including natural gamma-ray spectrometry (potassium, thorium, and uranium), magnetic susceptibility, electrical resistivity (shallow penetration depth), p-wave velocity, and caliper (borehole diameter) is used to acquire continuous, fine-scale, in situ physical parameters within the borehole which provided continuous lithological and structural data on the impact rocks.
Furthermore, an acoustic televiewer tool was used to investigate fractures, supply information on borehole breakouts, and give information on lithological boundaries, textures, and sedimentary features. Morris et al. (2007 ) attempted to provide validated geophysical signatures of geological materials. Magnetic susceptibility and density measurements made on the BCDP cores show no proof that highly magnetic and dense impact-melt sheets might be the source of the observed magnetic anomalies.