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.