Gravity Method
Gravity measurements are one of the most important techniques for identifying and confirming impact structures on Earth, unlike volcanic craters (Pilkington and Grieve, 1992 ). In 1960, the first gravity measurements in the Lake Bosumtwi area were taken. The findings primarily reflected regional gravity field patterns (Jones, 1985 ) and could not disclose anything about the impact-formed crater structure. However, 163 gravity measurements were taken around the Bosumtwi Crater complex in 1999 to ascertain the gravitational signature of the impact structure (Danuor and Menyeh, 2007 ). Before the Lake Bosumtwi Drilling Project in 2004, Danuor and Menyeh (2007 ) reported on the Bosumtwi Impact Crater from the results of potential field measurements. In order to determine the impact-related crater structure, the authors used gravity and magnetic measurements. A 2.5-D gravity model was created for a south-north profile across and through the lake’s centre, with a half-strike length of 1 km and the profile assumed to be perpendicular to the strike. They observed that the central zone of the lower boundary of layer three (breccia) is uplifted at a depth of about 780 m, indicating the impact’s depth extent.
Similarly, Ugalde et al. (2007b ) constructed a 3-D gravity model from gravity data acquired over the lake and in its surroundings between 1999 and 2001. The model integrates gravity, petrophysics, and seismic data. The model’s purpose was to determine the extent of fracturing due to the impact. Danuor et al. (2013) used the gravity method with magnetic and wide-angle seismic reflection and refraction studies to obtain information on the impact-related anomalies. Gravity measurements 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. A central uplift was clearly shown. It was also observed that the central zone of the lower boundary of layer three at a depth of about 780 m is uplifted.