The need to image shallow targets (<4m) is becoming more and more necessary as cities andrnother man made buried structures become more aged. Among the targets that require locatingrnare tunnels, abandoned mines, Pipes, and shallow soil stratigraphy.rnNormal applications of shallow geophysical methods such as ground penetrating radar (GPR)rncannot achieve significant penetration (more that a couple feet) in situations where surficialrnmaterials are highly conductive and/or cluttered with shallow pipes, wires, or rebar. Resistivity isrnoften confounded by linear conductive objects, as is EM. Seismic methods have previouslyrnlacked both resolution and were susceptible to high levels of urban noise,rnA high-resolution shear wave reflection seismic method with sufficient resolution to identifyrnvoids (or pipes) on the order of 1-foot diameter has been developed. Key to the application ofrnthis method is a patented vibratory source capable of putting shear wave signals into the groundrnat frequencies ranging from 30-4000Hz at a peak force of 200 pounds. This electromechanicalrnshear or compressional wave source (a micro-vibrator) overcomes many of the problemsrnencountered with seismic methods in urban environments.rnSurveys performed to date with the micro-vibrator have recovered up to 600 Hz reflected shearrnwave signals from depths of 10m and 300Hz from depths of 10+ m. Given the typically lowrnshear wave propagation velocities of shear waves in soils (< 300m/s) wavelengths of less thanrn0.5 m have been recovered, yielding detection resolutions of 0.3 m or smaller. The device hasrnextremely high noise rejection capabilities due to the vibroseis cross correlation, the unit is smallrnenough to be used inside buildings, and the unit is capable of producing energy up to 4 KHz.rnThis paper discusses the principles of high-resolution shear wave acoustic methods for subsurfacernimaging, and the vibratory source. Two field applications are also discussed:rn1. Void Detection: A major oil company sought to develop a surface geophysical method tornlocate voids on and around storm sewers in various refineries. The target size wasrnspecified as a 1-foot diameter void located on a 3-foot diameter sewer pipe. Both shearrnwave reflection seismic and a guided wave "refraction" amplitude analysis techniquernwere utilized on a test sewer constructed specifically for the evaluation of geophysicalrntechniques.
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