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.