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In March 1982, 6 seismic refraction lines, 70-90 km (43-56 mi) long, were shot in the southeastern Gulf of Mexico using the advanced University of Texas digital ocean bottom seismometers. Five lines were on the South Florida bank region in water depths of less than 1 km (3,300 ft) and one was in water depth of about 2.4 km (7,900 ft) off the northern coast of Cuba. After data reduction, first arrival picks were made and least squares lines were fitted to the picks to obtain the apparent velocities and intercept times for the layers. Using these values, flat layer crustal models have been initially computed. The 2 most dominant deep refractors have apparent velocities of 5.6-5.9 km/sec (3.5-3.7 mi/sec) and 6.2-6.6 km/sec (3.9-4.1 mi/sec). The top of these refractors varies in depth 2-6 km (6,600-13,000 ft) from the sea surface. They are interpreted to represent the crystalline basement. Basement rocks have been reached at a depth of 3.4 km (11,200 ft) in a well drilled in the Pinellas County arch. In the South Florida bank area, the deepest refractor observed has an apparent velocity of about 7.5 km/sec (4.6 mi/sec) at a depth of about 25 km (15 mi). Absence of any mantle velocity in these long profiles confirms the continental nature of this crust. The only possible mantle arrival (velocity = 8.4 km/sec, 5.2 mi/sec) was observed in the line off the northern coast of Cuba at a depth of about 26 km (16 mi). Similar crustal thickness has been observed in a refraction profile just northwest of this line. This deep crustal structure complements the shallow cru tal structures for this area.
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