5.0 Discussion
In this study, geophysics played a significant role in recognising the Bosumtwi as a terrestrial impact crater. Twenty-three (23) geophysical papers on the Bosumtwi impact crater were identified from various databases, and 47.8% of these publications were published in 2006 and 2007 attributable to the multidisciplinary International Continental Drilling Program (ICDP), a project that took place in 2004. We also found that 13% of these published scientific articles on the Bosumtwi Impact Crater were written by researchers affiliated with Ghanaian institutions, whereas 87% of the studies were conducted by scientists affiliated with non-Ghanaian institutions. Our findings are significant because they highlight the lack of local financial support for research in Ghana and African countries. The lack of funding for research on the Bosumtwi Impact Crater from Ghana also bears this out. One would have thought that with such a unique location located in the country, adequate funding would have been allocated for research to advance knowledge.
In addition to our main findings, we demonstrate that the geophysical techniques employed so far in studying the Bosumtwi Impact crater include magnetic, gravity, seismic, electrical resistivity, GPR, petrophysical, remote sensing and radiometric methods. A residual negative gravity anomaly is the most conspicuous geophysical signature over impact craters. Gravity measurements at the Bosumtwi impact crater yielded a maximum negative anomaly of 18 mgal over the crater. This was interpreted to be caused by fractured and brecciated rocks in the rim area and below the crater floor, breccias within the crater, and sedimentary and water infilling of the lake. Also, magnetic measurements of core samples, borehole measurements, and magnetic measurements over the crater reveal interesting anomalies. A magnetic low with an amplitude of tens to several hundred nanotesla (nT) is the dominant effect over impact. A large negative anomaly with a minimum value of 55 nT and a less notable positive anomaly to the south of the Bosumtwi impact crater was recorded.
In contrast, weaker negative anomalies were also observed in the southeast and southwest of the crater, which is attributed to magnetised bodies in the central northern area of the lake. Measurements of magnetic susceptibility of the drill cores show mostly paramagnetic values (200–500 × 10–6 SI) throughout the core, except for a few metasediment samples, and correlate positively with natural remanent magnetisation (NRM) and Q values. Seismic refraction and reflection surveys showed the velocity distribution in the Bosumtwi Impact Crater, which expressed how much the shock load has fractured the material. The methods revealed 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. Resistivity measurement at the crater reveals three formations with distinct resistivity signatures: low resistivity regions from the lake’s shore to uphill with resistivities of 64 Ω.m representing lake sediments; moderately high resistivity regions with values between 128 and 200 Ω.m interpreted as impact-related breccias such as dikes, allochthonous or parautochthonous depending on their geometries; and a resistive basement metamorphic formations. The resistivity models were able to determine faults and fractures, as well as the thickness of post-impact lake sediments and breccias.
It is clear from our study that geophysical studies in the Bosumtwi Impact crater have stalled after a high in 2006 and 2007 (i.e., the period immediately after the Lake Bosumtwi Drilling Project in 2004. The Lake Bosumtwi Drilling Project was funded by a consortium of the International Continental Scientific Drilling Program (ICDP), the US National Science Foundation, the Austrian National Science Foundation, the Austrian Academy of Sciences and the Canadian National Science Foundation. Local funding from the Government of Ghana seems to be nonexistent. This study reinforces the overwhelming evidence of chronic internal underfunding of science research in Ghana and Africa (North et al., 2020; Sawyerr, 2004 ).North et al. (2020 ) also identify government spending on research and development as a key reason for low research output from African countries such as Ghana. Over the years, the Government of Ghana has not dedicated resources to investigating the origin, understanding the evolution and encouraging the continuous monitoring of the Bosumtwi Impact Crater. An unintended consequence is that data from geophysical research conducted on the Bosumtwi Impact Crater is unavailable to Ghanaian researchers. For instance, the data gathered during the Lake Bosumtwi Drilling Project should have been in an accessible research repository to Ghanaian researchers, but this is not the case. Original plans to have a museum and research centre at Abonu, the main town in the Bosumtwi Impact Crater enclave, to host some of the petrophysical cores, rocks and results from research conducted have not materialised almost 20 years after the ICDP ended. Although Ghanaian researchers are underrepresented in geophysical research on the Bosumtwi Impact Crater, we do not ascribe the cause to “parachute science” (Harris, 2004; North et al., 2020 ), as evidenced by 19 % of the scientists who worked on Bosumtwi Impact Crater being Ghanaian (Koeberl et al., 2005 ). If anything, the internal evils of lack of resources for research and publications, high teaching loads, and lack of institutional incentives are to blame for the under-representation of African geophysical researchers in studies in the Bosumtwi Impact Crater.
Another significant aspect of our review is the importance of geophysics in understanding impact craters. The geophysical techniques of magnetic, gravity, seismic, electrical resistivity, GPR, petrophysical, remote sensing and radiometric methods were all utilised in the various investigations of the Bosumtwi Impact Crater. Meteorite Impact Craters are pervasive in our solar system, and studying them is useful for understanding the geological processes of other planetary bodies in our solar system, such as Mars, especially in the context of ramped-up exploration of other planetary bodies in the solar system. They are often the most significant modifiers of planetary surfaces. However, our review makes it clear that while geophysical methods did not provide an unambiguous signature for the Bosumtwi Impact crater, they provided constraints for confirming the impact origins. Our review supports French and Koeberl’s (2010) and Koeberl’s (2004) findings.
In our view, a very compelling consequence of the underrepresentation of Ghanaian geophysical researchers in studies on the Bosumtwi Impact Crater is the lack of studies on hazards due to the impact origins of the impact crater. For instance, Jones (1985 ) reported that the earliest mention of Lake Bosumtwi was by a certain Perregaux in 1899, who described inhabitants of the Lake Bosumtwi enclave complaining of bubbles accompanied by sulphurous smell and loud noises in a two-year cycle. Scientists then concluded that this occurrence, along with the crater depression, indicated volcanic origins. Since this report by Jones (1985 ), we have not found any study investigating this phenomenon. This record should be of particular concern to scientists conducting future research since it could have health and safety implications for the inhabitants of the Lake Bosumtwi enclave.
Furthermore, beyond geophysical research for understanding the origin and evolution of the Bosumtwi Impact crater, future research should focus on geological hazards related to impact, which may put the lives and properties of inhabitants of the enclave at risk. In addition, the potential for contamination of the lake through fractured conduits should be investigated. The Bosumtwi enclave is a zone of high agricultural activities, especially cocoa and other crops such as maise, cassava and plantain. Some farmers involved in these agricultural activities use high amounts of pesticides and inorganic fertilisers. The potential for contamination of the lake due to seepage through the fractures is high. Finally, the rapid increase in tourist numbers visiting the lake and the construction of several tourist resorts in the enclave should engender geotechnical research using geophysical methods in the future.