A REINTERPRETATION OF THE 1970, HALES, ONSHORE-OFFSHORE REFRACTION LINE IN THE NORTHERN GULF OF MEXICO

This project is a re-interpretation of seismic refraction data from the Northern Gulf of Mexico and the Gulf Coast Plain that was collected and analyzed by Hales (1970) and then reinterpreted Gurrola (1985). The data set is a north-south trending line extending 350 km along the Texas Louisiana border and extending offshore to the Sigsbee Escarpment. The shots were all offshore and included 21 one ton shots and almost a hundred 100 to 150 pound shots. The smaller shots were only recorded to distance of up to 90 km but the larger shots were recorded to 750 km offset. The technology available to Hales in 1970 enabled a 2-D model to be developed by connecting a series of traditional 1-D to 1.5-D models. Gurrola later used 2-D tracing to re-interpret these data. We will be reinterpreting this data set using modern raytracing and finite element modeling (forward and inverse) that allows 2-D velocity gradients.

The current model by Gurrola 1985 found the basement to be only about 3 km deep at the Sabine Uplift sloping to a depth of 13 km just past the coast line and continues to deepen only slightly more southward. The continental crust-basement is thinned to about 16 km beneath the continental shelf. The Moho beneath the continental shelf has a velocity of 7.4 km/sec at a depth of about 30 km. A normal mantle velocities (8 km/sec) are found at 46 km beneath the continental shelf. The Moho beneath the Sabine Uplift is 40 km deep and has a velocity of 8.0 km/sec. The seismic data indicated that the 6 km/sec layer shallowed to about 8 km toward the Sigsbee Escarpment. Gurrola interpreted this as a continental fragment. Since basement is denser than sediment, it was necessary to deepen the Moho to more than 35 km to model the gravity data. More recent models of the Gulf of Mexico have found the salt to be much thicker near the Sigsbee Escarpment than assumed in our previous model. It is, therefore, likely that the shallowing high velocities in the refraction data are due to a salt body. The low density of the salt would not require a deep crustal root. We are modifying the old model using modern forward and inverse seismic modeling techniques to incorporate information from other newer models of the region.