Abstract

Pressure amplitudes were determined for various kinds of seismic signals observed on special test records obtained during field tests conducted along a 14,000-ft seismic lines in Eugene Island Block 184, offshore Louisiana. Vibrators attached to a Seismograph Service Corp. (SSC) boat generated swept-frequency and monofrequency signals. Signals from detectors on a streamer cable towed by the boat were recorded by an SSC recording system. Signals from a vertical spread of detectors were recorded by a DFS/9000 recorder on the Transco 184 platform centrally located in the test area. Location of the boat was determined by analysis of time relations of signals from responders located at established positions some distance from the test area. Clock times from manually referenced timing code generators were recorded by both the SSC and DFS recorders to permit synchronization between separately recorded signals.The signals analyzed were separated into three classes: S 1 includes direct and refracted waves; S 2 consists of primary reflections; and S 3 includes signals diffracted from scatterers. The average level of first-arrival signal S 1 and reflected signal S 2 for frequency sets 25, 40, 42.2, 50, and 70.4 Hz in the range of 1414 and 2143 ft, which encompasses streamer cable single-detector groups, is 337 and 29.6 microbars, respectively. The amplitude of signals S 3 , believed to be diffracted from the contact between key reflectors and a salt dome, ranges from 13 to 20 microbars and is 10 to 100 times the amplitudes of towing and ambient noise, respectively. The observed decay of first-arrival signal amplitude is approximately proportional to the square root of range distance, or about 2 dB/1000 ft. The observed decay of reflected signal amplitude with range distance is approximately 1 dB/1000 ft.

First Page Preview

First page PDF preview
You do not currently have access to this article.