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<br />t.:a-I:..J--l.::;~ :::I.IIAr.., <br /> <br />t-kl,..W'1 """...~t~....M ,j,1";\",4;I'\,.I,,", <.J.:.;,l,J '~J'l"J <br /> <br />t-'.:J <br /> <br />4 <br /> <br />APPENDIX Methodology Summary <br /> <br />GPR Eauioment and Princioles <br /> <br />GPR is an electromagnetic method that d.!tects interfaces between <br />subsurface materials with differing dielectric constants. The GPR system consists <br />of an antennae, which houses the tr;lnsmitler and receiver; and a profiling recotder, <br />which processes the received signal and produc!!!s a graphic display of the data. <br /> <br />The transmitter radiates repetitive short-duration EM signals into the earth <br />from an antennae moving across the ground surface. Electromagnetic waves are <br />reflected back to the receiver by interfaces betwl~en materials with differing <br />dielectric constants. The intensity of the reflected signal is a function of the contrast <br />in the dielectric constant at the interface, the conductivity of the material which the <br />wave is traveling through, and the frequency of the signal. Subsulface features <br />which may cause such reflections are: 1) natuml geologic conditions such as <br />changes in sediment composition, bedding and c:ementation horizons, voids, and <br />water content: or 2) man-introduced materials or changes to the subsurface such as <br />soil backfill, buried debris, tanks, pipelines, and lJtilities. The profiling recorder <br />receives the signal from the antennae and produces a continuous cross section of <br />the subsurface interface reflections, referred to ;;15 reflectors. <br /> <br />Depth of investigation of the GPR signal i!> highly site specific, and is limited <br />by signal attenuation (absorption) ofthe subsurface materials. Signal attenuation Is <br />dependent upon the electrical conductivity of the subsurface materials, Signal <br />attenuation is greatest in materials with relativel)' high electrical conductivities such <br />as clays and brackish groundwater, and loweslln relatively low conductivity <br />materials such as unsaturated sand or rock. Ma:<imum depth of investigation is also <br />dependent on antennae frequency and generally increases with decreasing <br />frequency; however, the ability to identity smaller features is diminished as <br />frequency decreases. <br /> <br />The various GPR antennas used are internally shielded from abov~round <br />Interference sources, Accordingly, the GPR signal is minimally affected by nearby <br />abov~round conductive objects such as metalrenees, overhead power lines, and <br />vehicles, <br /> <br />A GPR survey ;s performed by towing an cmtennae acroSs the ground along <br />predetermined transect Jines. The antennae is either pulled by a person or towed <br />behind a vehicle. Preliminary GPR transects are performed over random areas of <br />the site to calibrate the GPR equipment and characterize overall site conditions, <br />Based on these preliminary results, the optimum antenna frequency and lime range <br />settings are selected to provide the best combin21tion of depth of investigation and <br />data resolution for the subsurface conditions at this site. <br /> <br />- <br />g <br /> <br />WaueTech <br />