GEOPHYSICAL SIGNATURE OF METEORITE IMPACT CRATERS – FIRST AND SECOND ORDER FOOTPRINTS

Through the integration of seismic and potential field data, remote sensing, exploration drilling and numerical modeling, we can constrain the size, shape and morphology of most terrestrial impact craters. New petrophysical and potential field data from the mid-sized Wanapitei, Bosumtwi and Monturaqi structures, as well as seismic data from Ries, Sudbury and Bosumtwi demonstrate the common geophysical signatures of impacts: pronounced gravity lows, prominent magnetic anomalies and often reflective target stratigraphy.

Target rocks are subjected to high pressure and temperature conditions during impact, resulting in fracturing, stress-induced shearing and mixing of materials. There is typically an exponential decay in both porosity and fracture density as radial distance from the crater center increases. Fracture porosity will enhance the first-order gravity low associated with impact structures, and serve to reduce seismic parameters in the second order (velocities and densities) by increasing total porosity out to a limit where impact damage is negligible (Polanskey and Ahrens, 1990; Xia and Ahrens, 2001). In seismic profiles, areas of high brecciation appear transparent; footwall and basement structures in particular show no traceable horizons despite the sometimes large vertical contrasts observed in petrophysical logs. Analysis of physical property logs indicate that these structures have small scale lengths that describe the high degree of mixing and heterogeneity, resulting in only small amounts of seismic scattering. As a consequence of this mixing, pre-impact lithologies are typically disrupted in the vicinity of impact structures, giving rise to characteristic seismic profiles such as those over the Ries, Sudbury and Chicxulub impact structures.

Polanskey, C.A. and Ahrens, T.J. (1990) Impact spallation experiments: fracture patterns and spall velocities, Icarus, 87, p.140-155.

Xia, K. and Ahrens, T.J. (2001) Impact induced damage beneath craters, Geophys. Res. Letters, 28(18), p.3525-3527.