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OxBow Coal Mine Seismic Refraction Survey <br />Geotechnical Investigation Summary of Operations <br />Summary of Operations <br />• Seismic Refraction Survey- OxBow Coal Mine <br />SEISMIC REFRACTION -GENERAL <br />Seismic refraction surveying is a geophysical method traditionally used in engineering/geo[echnical <br />projects. Since the 1980's, seismic refraction surveying has been increasingly used in shallow <br />environmental and engineering site characterization projects. Common applications of seismic refraction <br />surveying include: <br />Profiling top of bedrock <br />Determining depth to bedrock <br />Determining rock rippability <br />Fig. 1. Correlation of time vs. distnnce plot to a geological cross-section. <br />The seismic refraction method involves creating seismic waves at the surface via a hammer, weight drop <br />• or explosives. The seismic waves travel through the subsurface at a velocity dependent on the density of <br />the soil/rock. When the seismic wavefront encounters an interface where seismic velocity drastically <br />increases, a portion of the wave critically refracts at the interface. Due to compressional stresses along the <br />interface boundary, a portion of the wavefront returns to the surface. A series of seismic receivers, called <br />geophones, are laid out along the survey line and receive the reflected wave energy (Figure 2). When <br />seismic survey lines come across a water body such as a small river or lake, hydrophones may be used to <br />continue data collection. The geophones send the wave information back to the seismograph via long <br />geophone cables. The seismograph measures the time it takes for the energy [o travel from the source, <br />refract and return to the surface. The raw seismic record, called a common shot gather, contains all <br />recorded seismic events within the user-specified time window (Figure 3). Only the earliest-arriving <br />energy, called the "first-breaks", are needed in seismic refraction data analysis. <br />..... .~ <br />,YI •'~r%. 1~. 1 k~l~p ref Irll f' rvt l'~~r1~,Y:' <br />~~~~ '~,~i`~ 1L yf•~: <br />. ~: e~ iC~'4 .~ R' .l .~YH.. ~~r. <br />i. .T V <br />,w <br />~ <br />rt12• <br />r <br />.,..r . .~r. -,. <br /> <br /> <br /> <br />, <br />..•,. ~.. <br />„ ,. .. ~ <br />t I <br />` <br />t, t~. <br />. ~Y Y <br />y ,' <br />~~ <br />i <br />a1 ,t~I <br />y <br />. <br />I <br />, <br />. :, <br /> <br />Fig. 2. Geophone <br />Fig. 3. Shot gather <br />UMS, Inc. Page I 07/Oy/03 <br />