Abstract

The propagation of compressional, shear, and surface waves was studied along a 3,200 ft profile at a location where a 95-ft-thick surface layer of Austin chalk, with a compressional velocity of about 9,900 ft/sec, overlies a 400-ft section of Eagle Ford shale with a speed of about 6,500 ft/sec. Woodbine sand, with a velocity of about 9,900 ft/sec, underlies the shale.Refracted first arrivals transmitted through the high speed surface layer show an increase of frequency with distance from the shot. A refracted second arrival from the Woodbine decreases in frequency and, after correction for spreading, increases in relative amplitude with distance. This would indicate that the high-speed surface layer acts as a high-pass filter for energy transmitted horizontally and as a low-pass filter for energy transmitted vertically through the layer. Shear waves transmitted through the Austin chalk are also observed.Surface waves consist of two groups of arrivals; a brief train of high-frequency waves (greater than 20 cps) propagated almost entirely in the surface layer is followed by a short train of low frequency waves. Unlike surface waves in most other localities, the two groups show almost no dispersion. The characteristics of both kinds of waves are interpreted qualitatively in terms of the layering.

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