Deep Structure of the Shelf and Slope Northern Gulf of Mexico. Part A.- Expanding Spread Profile (ESP) Experiment
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Four, two-ship expanding spread profiles (ESP's), each 80 km long, were collected along a transect near 94 degrees w longitude extending from the Texas shelf edge to the Sigsbee abyssal plain. The purpose of the work was to determine the deep sediment, basement and sub-basement velocity structure beneath regions masked by salt deposits. ESP's 2 and 3, located along the shelf and upper slope, were sited to avoid the shallower massive salt diapiric structures. ESP 4 was positioned over relatively flat-lying, layered salt deposits on the lower slope. ESP 5 was located just beyond the Sigsbee Escarpment on the abyssal plain. The profiles were acquired at 300 meters shot distance intervals using two, 2000 in3 /2000 psi airguns with Miniranger and LORAN-C navigation and were recorded with 24 and 48 trace multichannel streamer arrays having 50 and 70 meter hydrophone group intervals, respectively. This configuration provided overlapping data trace coverage which was sorted into 50 meter offset "bins" and summed. Offset range-travel time (X-T) plots have been constructed. Ray-tracing and direct Tau-P velocity inversion of the slant-stacked, wide-angle reflection and refraction arrivals suggests that a 5.0 to 6.0 km/sec basement exists beneath the sediments (2.0-4.4 km/sec) along the shelf edge (ESP 2) and upper slope (ESP 3). Beneath the lower slope (ESP 4) a thin salt layer(~ 1-2 km thick) with velocity of 4.0 km/sec appears to be underlain by sedimentary strata (3.8-4.8 km/sec) resting on oceanic type crust having layer velocities of 6.9-7.5 km/sec. A low-velocity layer may be present directly beneath the salt. ESP 5, on the Sigsbee rise shows a thick sedimentary section overlying oceanic type crust with 6.0 km/sec and 6.9 km/sec velocity layers and underlain by Moho with a 8.1 km/sec velocity.