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.