Geomagnetic Methods:
Geomagnetic methods have been in existence since the second century BC. They have been deployed in various applications such as locating buried pipes, drums and other objects, archaeological investigations, igneous dykes, and large-scale geological structures such as impact craters. Magnetic anomalies in impact craters are complex due to significant variations in the magnetic properties of rocks (Pilkington and Grieve, 1992 ). In this section on the geomagnetic methods used in the papers retrieved, we focus on both magnetic measurements of core samples, borehole measurements, and magnetic measurements over the crater either through airborne, marine or ground-based measurements.
Plado et al. (2000 ) proposed a magnetic model which showed a circular magnetic halo at the crater rim. Central negative and smaller positive anomalies were observed at the lake’s central north, north and south. Together with weaker negative anomalies observed in the eastern and western parts of the lake, they envelop a central uplift. Their explanation of the magnetic anomalies suggested they were due to several relatively strongly remanently magnetised impact-melt rock or melt-rich suevite bodies. In the study, the magnetic survey was conducted at every ~6.25 m along the flight lines with the survey positions determined using Global Positioning Systems. Data was recorded along 30 profiles with an average length of 22 km. The instrumentation used was a Scintrex CS-2 magnetometer at a resolution of 0.001 nT with a nominal flight altitude of 70 m, flight directions north-south, and line spacing of 500 m. The aeromagnetic survey was conducted in 1997.
Pilkington and Hildebrand (2003 ) used magnetic data to estimate the diameter of central uplift (DCU), the diameter of melt (DM) and the diameter of the innermost slump block (DS). Ugalde et al. (2007c ) reported new experimental methods for conducting magnetic surveys, which were trialled in the Bosumtwi Impact Crater. Due to the existence of previous airborne and marine magnetic surveys in the crater, they attempted to introduce an innovative way of conducting magnetic surveys. They acquired, processed, and interpreted 3-D vector magnetic data. Even though the Earth’s magnetic field is a vector quantity with both amplitude and orientation, currently, magnetic surveys only record the amplitude, i.e., total magnetic intensity. Conducting magnetic surveys with the full magnetic vector provides many advantages for mapping anomalies in the subsurface.
Elbra et al. (2007 ) measured the paleomagnetic properties of drill cores from LB-07A and LB-08A in the Bosumtwi Impact Crater. The measured magnetic susceptibility, the intensity of natural remanent magnetisation (NRM) and the Koenigsberger ratio (Q ratio; representing the ratio of remanent to induced magnetisation) among other petrophysical properties. 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. They inferred that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data show that NRM has shallow normal (and in some cases shallow reversed) polarity, which is consistent with the Lower Jaramillo N-polarity chron direction and is present in ferrimagnetic pyrrhotite.
Kontny et al. (2007 ) investigated the magnetic properties and mineralogy of drilled lithologies to understand and interpret magnetic anomaly patterns. The authors concluded that ferrimagnetic pyrrhotite is the most important carrier of remanent magnetisation. They observed somewhat surprisingly that the prediction of previous researchers of a strong magnetic impact melt body underneath Lake Bosumtwi interpreted from airborne magnetic data and numerical modelling were not confirmed in their study.
Danuor and Menyeh (2007 ) reported on the potential field measurements made in the Bosumtwi impact crater before the International Continental Drilling Program in 2004. Magnetic measurements were made on the lake with two proton precession magnetometers. The profile spacing on the lake was 800 m with station intervals of 10 m. The results revealed a large negative anomaly with a minimum value of 55 nT with less notable positive anomaly to the south. They also report other weaker negative anomalies in the southeast and southwest. The authors attributed the cause of the anomalies to magnetised bodies in the central northern area of the lake. Modelling suggested the most likely source of the anomaly as a body between 250 m and 61 0 m below the lake’s surface.
In order to determine whether the strong magnetic anomalies observed within the Bosumtwi Impact Crater originate from strongly magnetic material in the form of impact melt (Plado et al., 2000 ) or weakly magnetic layers of impactite material overlying the crater floor with the larger magnetic anomalies being attributed to granites and other intrusives in the Proterozoic basement (Ugalde et al., 2007c ), further investigations by Morris et al. (2007 ) focused on gathering additional rock property and borehole information. Magnetic susceptibility measurements were made with the Bartington MS-2 meter with an E probe attachment at ~10 cm intervals on the core. They concluded that there is no evidence of a strongly magnetic impact melt within the layers of the derived sediments in the crater. Danuor et al. (2013 ) summarised the geophysical studies conducted in the Bosumtwi Impact Crater. In contrast, they report the presence of magnetised bodies between 250 m and 610 m below the lake.