Results of pre‐drilling potential field measurements at the Bosumtwi crater

Abstract

Abstract— Gravity and magnetic measurements were carried out at the Bosumtwi crater to determine the geophysical signature of the crater. Land gravity data was acquired at 163 locations around the structure and on the shore of the lake. The separation between the gravity stations was 500 m for radial profiles, but 700–1000 m along roads and footpaths that ran parallel to the lake's shore. Additionally, a marine gravity survey was carried out along 14 north‐south and 15 east‐west profiles on the lake. Magnetic data was also acquired along 14 north‐south profiles on the lake. In all marine surveys, the line spacing was 800 m, and navigation was provided by a Garmin 235 Echo Sounder/GPS. The gravity signature of the crater is characterized by a negative Bouguer anomaly with an amplitude of about −18 mgal. Using the seismic results as constraints, the gravity model obtained indicates the central uplift at a depth of 250 m. The negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor, the impact breccias within the crater, and the sedimentary and water infilling of the lake. Magnetic modeling yielded a model for the causative body, which is located north of the central uplift: the model has a magnetic susceptibility of 0.03 S.I. and extends from a depth of 250 to 610 m. The causative bodies have been interpreted as impactites.

Integrated 3‐D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana

“…A previous velocity model for the porosity changes inside and outside the central uplift, and the petrophysical logs from the ICDP boreholes for porosity, density, and P-wave velocity (LB-07A and LB-08A). The main outcome of our analysis is that for the first time a 3-D model is constructed over the impact structure, which reveals and allows lateral density variations and refines the previously obtained results from the pre-drilling 2.5-D model of Danuor (2004) and Danuor et al (2007). We were able to generate a sediment thickness grid for the crater, which can be used later for the computation of the actual contribution of the sediments to the gravity signal, or a depth to basement map.…”

“…In 1999, 163 gravity measurements were obtained at points located around the Bosumtwi crater structure to determine the gravity signature of the impact structure itself (Danuor 2004;Danuor et al 2007;J. Pohl, personal communication).…”

“…In 2001, a second gravity survey on a grid of 18 profiles (11 north-south and 7 east-west profiles) was conducted on the lake using a LaCoste-Romberg air-sea gravimeter (Danuor 2004;Danuor et al 2007;J. Pohl, personal communication).…”

“…The measurements were obtained using a motor-driven marine research platform. The distance between the gravity profiles was about 800 m. Navigation was provided using a Garmin 235 Echo Sounder/GPS, which allowed the acquisition of bathymetric data at the same time (Danuor 2004;Danuor et al 2007;J. Pohl, personal communication).…”

“…The equations used for the processing are described in detail in the Appendix. Figure 2a shows the preliminary gravity data that included Bouguer data around the lake and free air data of the lake area (Danuor 2004;Danuor et al 2007;J. Pohl, personal communication).…”