Schmitt D.R., Milkereit B., Karp T., Scholz C., Danuor S., Meillieux D., Welz M.
Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, AB T6G 2G7, Canada; Department of Physics, University of Toronto, 60 Saint George Street, Toronto, ON M5S 1A7, Canada; Department of Geology, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244, United States; Geophysik GGD, Ehrensteinstrasse 33, 04105 Leipzig, Germany; Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
Schmitt, D.R., Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, AB T6G 2G7, Canada; Milkereit, B., Department of Physics, University of Toronto, 60 Saint George Street, Toronto, ON M5S 1A7, Canada; Karp, T., Department of Geology, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244, United States, Geophysik GGD, Ehrensteinstrasse 33, 04105 Leipzig, Germany; Scholz, C., Department of Geology, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, NY 13244, United States; Danuor, S., Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana; Meillieux, D., Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, AB T6G 2G7, Canada; Welz, M., Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, AB T6G 2G7, Canada
In order to assist in the interpretation of previous seismic refraction and reflection surveys, a vertical seismic profile was acquired in the Lake Bosumtwi (Ghana) hard-rock core hole LB-08A. No seismic reflections are seen in the up-going wave field obtained, and this observation is consistent with the lack of reflectivity observed in the corresponding 2-D surface seismic profile obtained in earlier studies. Direct down-going P-waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open-hole "hard rock" section from a depth of 239 m to 451 m of LB-08A. Analysis of the observed travel times indicates a nearly constant P-wave velocity of 1520 m/s through the soft lacustrine sediments. In the hard-rock, however, the P-wave velocity rapidly increases by nearly 30% from 2600 m/s to 3340 m/s. These values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data. These values are low relative to those expected for the metasedimentary protoliths, an observation that has been made at other young impact structures of similar size. The low velocities, together with the fact that they increase so rapidly, is suggestive of a decreasing density of fractures and microcracks with depth. Consequently, the seismic velocity trend may provide a proxy measure of damage, and hence, the decay of the shock pressure from the impact point. Validation of this requires additional detailed studies of the porosity structure in the core. © The Meteoritical Society, 2007. Printed in USA.
