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In November to December 1983, the University of Texas and the U.S. Geological Survey conducted a seismic experiment that attempted to measure crustal and upper mantle depths and velocities along a transect of the Gulf of Mexico south of Galveston, Texas. The transect is composed of 5 along-strike lines spaced to span the continental shelf and slope and reaching the deep basin. Two 2,000-in.³, 2,000-psi air guns were fired simultaneously at 30-sec intervals at 5 knots for a shot spacing of 77 m. The signals were recorded by digital ocean-bottom seismographs with vertical geophones. Four seismographs were placed along each 90-km line. The seismic sections obtained are densely sampled and fully reversed, about half showing seismic arrivals at the full range of the line.

We find that the deep Gulf of Mexico is, as expected, underlain by oceanic crust. On the outer slope, we see deeply penetrating arrivals from below thick salt, and we find that this crust is thicker than the oceanic crust. North of the thick salt, we see the crust thinning to nearly the thickness of oceanic crust. Although this crust may be seismically indistinguishable from oceanic crust, we believe it to be highly extended continental crust. We interpret the two northern lines to show northward thickening, extended continental crust.

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