Laserfiche WebLink
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/geotechnical <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 rippabili[y <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 [he 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 [he seismograph via long <br />geophone cables. The seismograph measures the time it takes for the energy to 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 />;;.~~1GF113. ~i`!.~,i}p!}y,ft ~t,"~,i~l r l~' I`;;rc,~ <br />,~'44~,,, i.`: y~ ~, 6~.h AI.f".''( ~.6,y 1x.1' ~ .....; <br />~.',7u'14i~"i' ,~/'t: 1.;~ .. 1'%iltl' ,/ ,' ~~s• ; <br />Iy, . <br />1(~4.. <br /> <br />tr <br />. ~ <br />w <br />~.. <br />a. <br />~., <br />e! <br />,.. Ik <br />) : , ~ ,. <br />~ ' <br />,,i <br />n .I . <br />1• <br /> <br /> <br />.~Y r ~ ~~I r't.. ~~: ~' ~~1~ 1. <br />1'I' v <br /> <br />Fig. 2. Geophone <br />Fig. 3. Shot gather <br />UMS, Inc. Page I 07/09/03 <br />