A crustal structure refraction survey in south Texas
Cram, Ira H., Jr
Doctor of Philosophy
On August 11, 1960, sixty oil exploration and research seismograph crews attempted to record two shots detonated in the Texas Gulf Coastal Plain. Shotpoint A was located 16 miles northwest of Cleveland, Texas. Shotpoint B was located 27 miles northwest of Victoria, Texas. The shotpoint line is approximately parallel to the strike of the Cenozoic sediments and the distance between shot points is 260.2 km. More than 3,000 pounds of explosives were placed in five holes at each shot point. The top of the charge was at a depth of 95 feet and the bottom at 250 feet at each shot point. The regional Bouguer anomaly is -25 mgal. The shot instant was keyed to a 140 cps reference signal transmitted by radio station KTRH, Houston, Texas. First and later refracted arrivals were used to determine the velocity structure which was supported by events identified as reflections from the refractors. The results indicate the following thickness-velocity relationships: 2.0 km of 2.3 km/sec material, 5.3 km of 3.94 km/sec material, 12.5 km of 5.38 km/sec material and 13.2 km of 6.92 km/sec material. The Mohorovicic discontinuity is represented by a velocity of 8.18 km/sec and the total crustal thickness is 33.0 km. The geologic identity of the refractors has been interpreted as follows: The 2.3 km/sec and 3.9 km/sec layers represent the Cenozoic and Mesozoic sediments, the 5.4 km/sec layer represents the Paleozoic and Precambrian rocks, the 6.9 km/sec layer represents the transition zone and the 8.18 km/sec zone represents upper mantle material. It is apparent that the Texas Coastal Plain has been growing outward over the oceanic area for long geologic time and that this outward growth has been accompanied by a thickening of the transition zone (6.9 km/sec layer). The behavior of the transition zone is believed to be intimately connected with the regional tectonics of the Texas Coastal Plain-Llano area.