A small reflection seismic experiment with an explosive source was conducted in southeastern Georgia to determine the polarity of an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), which was found at a depth of approximately 16 km during earlier COCORP profiling in the region. In addition to being very bright, the SBS is notable for being unusually flat and horizontal for about half of its 4 km length. As these characteristics are similar to those of fluid-caused reflections at shallow depths, it has been suggested that the SBS may be caused by in situ midcrustal fluids. If caused by fluid enclosed in fracture porosity in solid rock, the reflection would be expected to exhibit a negative polarity from the top of the porous zone. The vibroseis data, however, gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal. The new experiment consisted of four dynamite shots, each recorded at three receiver stations by Seismic Group Recorders (SGR) borrowed from Amoco Production Company. Comparison of the dynamite records with geophone polarity tests indicate that the SBS is characterized by a positive reflection coefficient at its top. This result itself does not negate the fluid hypothesis-a fluid-fluid interface could cause the positive reflection as well as the 'flat-spot' nature of the reflector.

Waveform modeling shows that the SBS is caused by two or more thinly-spaced reflectors. A relatively high-impedance layer about 120 m thick provides a good match to the observed dynamite data, but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary (0.7 versus 1.0). A ~150 m thick porous zone model also provided a relatively good fit to the observed dynamite data, but in this model the polarity test requires that the initial SBS reflection (of positive polarity) be caused by a fluid-fluid interface within the porous zone and that the top of the porous zone be relatively non-reflective. Though these observations do not preclude the fluid hypothesis, they certainly make the high-impedance model the simpler of the two alternatives presented here. The additional constraints imposed by the modeling also suggest that the SBS is more complex than these simple, two-interface models. In either of these cases wavelet tuning contributes in part to the unusually large amplitude of the SBS reflection. ¿1991 American Geophysical Union