In situ seismic measurements in borehole LB‐08A in the Bosumtwi impact structure, Ghana: Preliminary interpretation

Abstract

Abstract-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.

Integrated interpretation of physical properties of rocks of the borehole Yaxcopoil‐1 (Chicxulub impact structure)

“…Investigations performed in recent years at the Vorotilovo scientific deep well drilled in the Puchezh‐Katunk impact structure, Russia [ Popov et al , 1998], and at the Nördlingen‐1973 scientific well drilled in the Ries impact structure, Germany [ Popov et al , 2003a], demonstrated (1) low thermal anisotropy of rocks (brecciated, cataclased, thermally overprinted and shock transformed target rocks) and (2) low values of thermal conductivity, density and velocity in the rocks affected by the shock and postshock thermal conditions [ Schmitt et al , 2007] in comparison with similar rocks outside these impact structures.…”

Abrupt changes in the water balance of tropical West Africa during the late Quaternary

“…Lines labeled TI, TII, TIII, TIVb, and TV are paleolake surfaces discussed in the text. Velocities have been converted into depths as in the work by Schmitt et al [2006]. (b) Enlargement of seismic data showing truncation surfaces associated with estimated lake depths during TI and TII.…”

Initial seismic observations from a deep borehole drilled into the Canadian Shield in northeast Alberta

“…The recorded travel time and known depth level of the receivers provide the interval velocities necessary in deriving a time to depth relationship of the recorded seismic data. However, this is also restricted to the area between the seismic source and borehole location (Schmitt et al 2007). Seismic energy sources positioned within tens of meters of the borehole are considered to be a zero-offset VSP (Stewart 2001).…”

“…#1 and #3 correspond to the change in borehole diameter, whereas #2 corresponds to a known fracture zone that was previously interpreted from the geophysical logs penetration depth of a few centimeters in the borehole. Initial comparison of sonic log velocities to VSP velocities is difficult due to the frequency dispersion resulting from both rock property and scale-dependent effects (Sams et al 1997;Schmitt et al 2007). There is a significant amount of variation in VSP interval velocity as minor fluctuations of even the level of the digitization period of 1 ms in the first arrival pick could easily cause nonphysical variations in the velocities with the depth sampling interval of 2.5 m. This error is reduced using larger intervals in the velocity calculation at the cost of reduced resolution.…”

“…Conventional VSPs are typically acquired at relatively large intervals of 10 m or more primarily of economic cost and the acquisition time required. In this borehole, a frequent depth sampling of 2.5 m was carried out to obtain greater spatial depth resolution on the in situ seismic velocities (Schmitt et al 2007). Waveforms were recorded as functions of time and plotted against depth to produce the vertical profile (Fig.…”