A CRUSTAL STRUCTURE REFRACTION SURVEY IN SOUTH TEXAS

Abstract

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 shotpoints is 260.2. km. More than 3,000 lb of explosives were placed in five holes at each shotpoint. The top of the charge was at a depth of 95 ft and the bottom at 250 ft at each shotpoint. 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. Possible geologic identities of the refractors are 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 Paleozoic and Precambrian rocks, the 6.9 km/sec layer is the high‐velocity crustal layer, and the 8.18 km/sec zone represents upper mantle material. The South Texas velocity section appears to correlate well with adjacent measurements in the Gulf of Mexico and in northeastern Arkansas.