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<br />(0 <br />(Xl <br />r~ cutoff through this material would be extremely expensive, the site became <br />('\.I <br /> <br />much less attractive. <br /> <br />During the course of the study carried out by IECO, the idea was advanced that <br /> <br />possibly the 100-ft terrace gravel. did not cover the entire abutment and that <br /> <br />bedrock actually occurred at higher elevations, specifically to the north and <br /> <br />east. It was decided that it was within the scope of the Yellow Jacket Study <br /> <br />to carry out a cursory seismic refractive survey of the left abutment, as the <br /> <br />result would have a significant effect on the cost estimate for the site. <br /> <br />Field work for this survey was carried out July 27-30, 1982. The following <br /> <br />report briefly describes the seismic refraction method and its limitations, <br /> <br />outlines the program, presents the results of that program, and the conclusions <br /> <br />based on those results. <br /> <br />SEISMIC REFRACTION <br /> <br />Method: The seismic refraction method permits calculation of depth to near- <br />surface soil and rock layers by determining seismic velocity variation be- <br /> <br />tween these layers. The survey is performed by producing a seismic wave and <br /> <br />measuring the time of arrival of the wave at varying distances from the source. <br /> <br />In this survey, the wave was generated by detonating small charges in shallow <br /> <br />holes. <br /> <br />t <br /> <br />Arrivals of resulting compressional waves were detected by 12 geophones placed <br />at intervals of 25 feet along a straight line from the shot point. Electric <br />signals generated at the geophones were transmitted through a coaxial cable to <br />a signal enchancement seismograph (Nimbus ES-1210F). The seismograph contains <br /> <br />>, <br />