Seismic Methods
The basic concept of seismic methods depends on the propagation of
elastic waves in the Earth. The seismic signal strength and velocity
depend on the elastic properties of the rocks. Even though seismic
methods have been predominantly used in oil and gas exploration
(Mondol et al., 2007 ), they are increasingly used for other
applications. Recent developments in the computing industry have
significantly affected the acquisition,
processing and interpretation of
seismic data. The seismic method has been utilised in detecting
precursors of magma movement and eruption (Chouet and Matoza,
2013 ), in mineral exploration and mine planning (Malehmir et al.,
2012 ), detection of cavities (Grandjean and Leparoux, 2004 ) and
geotechnical investigations (Signanini and Torrese, 2004 ). In
this section, we review the seismic studies in the Bosumtwi Impact
Crater. In impact crater studies, the seismic method can provide
information on the morphology and structure post-impact.
Karp et al. (2002 ) and Scholz et al. (2002 )
investigated the subsurface structure of the Bosumtwi impact crater by
imaging the central uplift at the bottom of the lake and determining the
thickness of the impact-related formations and the post-impact
sediments. They acquired multi-channel seismic (MCS) reflection and
wide-angle data using Ocean-Bottom-Hydrophones (OBH). Source signals
were gotten by firing an airgun. The shots were fired every 25 m with an
airgun at a depth of 2 m. The inverted 2D velocity model showed
indications of a central uplift feature. They interpreted the central
uplift to have a width of ~1.8 km and a height of
120 m. 180 –
300 m thick of post-impact
sediments cover the crater structure.
A vertical seismic profile was acquired in a borehole (LB-08A) to aid in
interpreting seismic reflection and refraction surveys (Schmitt et
al., 2007 ). Schmitt et al. (2007 ) reported two layers of
post-impact sediments (between 73 m to
239 m depth) and hard rock
(between 239 to 451 m) depth. The former has a P-wave velocity of
1520 m/s, whereas the latter’s
P-wave velocity increases by up to 30% to between 2600 to
3340 m/s. Their observations
matched with inversion results from previous seismic surveys. They also
suggested a decreasing density of fractures and microcracks with depth.Scholz et al. (2007 ) report a study using a marine seismic
reflection survey to image the subsurface of the Bosumtwi crater lake.
Multi-channel seismic surveys (MCS) were conducted over eight (8)
profiles in a radial pattern in addition to two other high-resolution
seismic reflection surveys. They observed a buried central uplift and a
section of postimpact lacustrine sediments more than
300 m thick surrounding the
central uplift. They observe a central uplift with a diameter of 1.9 km
and a maximum height of 130 m.Danuor et al. (2013 ) summarised the geophysical
characteristics gleaned over the years from seismic surveys. Their
summary describes the inference of a three-layer model consisting of the
water layer with a velocity of 1.45 km/s and a higher velocity of
between 1.5 km/s to 1.65 km/s interpreted as post-impact sediments and
the crater floor. Habimana et al. (2020 ) deployed seismic
refraction methods to delineate the subsurface structure of the suevites
to the north of the Bosumtwi Impact crater. The investigation aimed to
determine the depth of the suevite body and the p-wave velocity, among
other things. The p-wave
velocities were observed to be between 3 to 3.9 km/s. The suevite
deposits were found to be within 12 m depth.