We concluded a large-offset seismic experiment in the northwestern Gulf of Mexico using large-capacity air guns and digital ocean-bottom seismographs to determine the velocity structure of the sediments, crust, and upper mantle. Five multiple-detector split-profile lines were shot over an area extending from the shallow midshelf south of Galveston to the continental rise just south of the Sigsbee escarpment. The data were of adequate spatial density and quality to allow combined use of near-vertical reflections, wide-angle reflections, and refractions for interpretation. Several techniques were used to obtain velocity-depth functions: (1) conventional constant-velocity-layer analysis, (2) interval velocities from moveouts of the precritical reflections, (3) analysis in the tau-p domain to determine extremal depth bounds, (4) estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions, and (5) forward modeling using two-dimensional (2-D) ray tracing. A sedimentary sequence of nearly constant thickness (13 to 15 km) covers the study area. Various mobilized salt features exist within the sediments in the slope area, ranging from deeply buried layers and diapirs under the inner slope to shallow, thin, allochthonous bodies under the outer slope. In contrast to the sediments, the crust shows considerable variation in thickness, from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf. The transition under the slope, however, is not monotonic but includes thinning to nearly oceanic thickness under the midslope, possibly suggesting an incipient rift that failed to materialize during the opening of the gulf. ¿ American Geophysical Union 1988