Reflection polarity of the midcrustal Surrency Bright Spot beneath southeastern Georgia: testing the fluid hypothesis

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. The SBS is 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. The new experiment consisted of four dynamite shots, each recorded at three receiver stations by Seismic Group Recorders (SGR). Comparison of the dynamite records with geophone polarity tests indicate that the SBS is characterized by a positive reflection coefficient at its top. Waveform modeling shows that the SBS is caused by two or more thinly-spaced reflectors. A relatively high-impedance layer about 120 m thick provided 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) -from Authors