Meteoritics & Planetary Science, Volume 42, Number 4-5 (2007)http://hdl.handle.net/10150/6412492024-03-29T08:14:37Z2024-03-29T08:14:37ZIntegrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, GhanaUgalde, H.Danuor, S. K.Milkereit, B.http://hdl.handle.net/10150/6567132021-02-16T02:12:23Z2007-01-01T00:00:00ZIntegrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana
Ugalde, H.; Danuor, S. K.; Milkereit, B.
The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling.
2007-01-01T00:00:00ZImpactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact craterL'Heureux, E.Milkereit, B.http://hdl.handle.net/10150/6567122021-02-16T02:11:52Z2007-01-01T00:00:00ZImpactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater
L'Heureux, E.; Milkereit, B.
The recent drilling of the Bosumtwi impact structure, Ghana, has provided a unique opportunity to study the petrophysical properties of a young, well-preserved impact crater. The damage induced by impact results in extensive fracturing and mixing of target materials. We discuss here a means of using sonic velocity and density logs from two boreholes through the Bosumtwi crater fill and basement to estimate the degree of heterogeneity and fracturing within the impacted target, in order to understand the discrepancy between the large impedances derived from the log data and the nonreflective zone of impactites observed in seismic sections. Based on an analysis of the stochastic fluctuations in the log data, the Bosumtwi impactites are characterized by vertical scale lengths of 23 m. From the resolution of the seismic data over the crater, horizontal scale lengths are estimated at <12 m. The impactites therefore fall within the quasi‐homogeneous scattering regime, i.e., seismic energy will propagate through the medium with little disruption. Scale lengths as small as these are observed in the fractured basement rocks of impact structures, whereas non‐impact related crystalline environments are characterized by scale lengths an order of magnitude larger. Assuming that the high‐frequency fluctuations observed in the log data are more sensitive to fracture distribution than petrology, this suggests that the small scale lengths observed within impact structures are characteristic of impact‐induced damage, and could be used to estimate the extent of fracturing undergone by the rocks at any depth below an impact structure.
2007-01-01T00:00:00ZCharacterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibilitySchell, C.Schleifer, N.Elbra, T.http://hdl.handle.net/10150/6567112021-02-16T02:11:14Z2007-01-01T00:00:00ZCharacterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility
Schell, C.; Schleifer, N.; Elbra, T.
Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rockchanging processes, e.g., deformation or sedimentation. To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite. The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), ntermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.
2007-01-01T00:00:00ZThe Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?Ugalde, H.Morris, W. A.Pesonen, L. J.Danuor, S. K.http://hdl.handle.net/10150/6567142021-02-16T02:13:01Z2007-01-01T00:00:00ZThe Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?
Ugalde, H.; Morris, W. A.; Pesonen, L. J.; Danuor, S. K.
The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.
2007-01-01T00:00:00Z