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56 Abstract Underground Imaging Technologies (UIT) is developing a technology that makes use of seismic shear waves (S-waves) to image buried utilities. UIT is assisted in this work by their subcontractor, Geomedia Research and Development (GRD). The proposed technique improves on previous technology by using horizontal shear and vertical shear waves to detect the pipe. When compared to longitudinal compressional waves (P-waves) of the same frequency, shear waves have a shorter wavelength. Given that the image resolution is deter- mined by the wavelength, the shorter shear waves will yield improved pipe imaging. With respect to the detection of small objects and utility targets, shear waves offer several dis- tinct advantages over P-waves. S-waves also exhibit a polarization property that is useful. If the S-wave is horizontally polarized, with its primary direc- tion of oscillation parallel to the pipe, then a strong reflection will be returned. This property of S-waves to interact strongly with elongated targets will improve the ability to distinguish buried pipes from background clutter, such as stones or other inclusions. Technology Synopsis and Key Performance Indicators Title: Shear Wave Seismic Reflection Location Provider: Underground Imaging Technology Targets: The gas or fluid in a pipe of any material. Depth range: Up to a maximum of 20 times the pipe diam- eter in homogeneous clay soils. Depth accuracy: At best, 10% of pipe depth in homogeneous clay soils. Location accuracy: At best, 5% of pipe depth in homoge- neous clay soils. Application: At least five S-wave impact sources are recorded into a mobile array of oriented vibration sensors. Two of the five sources are positioned and analyzed with a view toward estimating the large-scale soil shear velocity variations in the survey area. Two shear sources are recorded to use sur- face waves to image near-surface velocity variations. The fifth source is positioned and analyzed to highlight the horizontal shear energy reflected from the gas or fluid in the pipe. Basic principle: The target pipe is viewed as a large coherent scatterer sitting in a sea of small/strong or large/weaker scat- tering objects, such as tree roots, rocks, gopher holes, or filled excavations. The gas or fluid in a pipe will reflect shear- oriented energy when the pipe size is a significant fraction of the shear-energy wavelength. The pipe material will contrib- ute a significant reflection of opposite polarity only if it is extremely strong or thick compared to the wavelength. An S-wave oriented parallel to an empty pipe will give a maxi- mum in reflected energy. Once the reflected energy is identi- fied, the background soil velocity is used to compute a location for the pipe. Limitations: Soils can have highly variable velocities arising from original depositional features, anisotropic minerals and textures, or later excavations and modifications, which can severely limit location accuracies even for strong reflectors. The worst case is that some soil structures may have shadow zones where nothing can be imaged. Depth is expected to be more difficult to resolve than horizontal position because sur- face measurements resolve only horizontal, not vertical, veloc- ities. Higher frequencies capable of imaging smaller pipe attenuate over shorter distances and scatter more strongly before the coherent reflecting pipe can be reached, giving a poor signal-to-noise and image resolution. It is also generally more difficult to both generate and record high frequencies as soils become weaker and more nonlinear. A P P e n d I x d Technical Support Information for Seismic Reflection Technology
57 Additional notes: Previous attempts at locating pipes and tunnels with reflection seismology have identified trade-offs between only a limited set of survey characteristics that influence imaging success. Technically successful surveys use large numbers of sources and receivers with extensive pro- cessing, resulting in a prohibitive time and cost for utility location. Most of these surveys resolve only larger targets by using source-receiver redundancy to compensate for the poor high-frequency response of sources and receivers. The first phase of this study is to develop and test sources and receivers in the appropriate frequency/wavelength ranges that will be used to characterize soil properties and target reflec- tance under field conditions. The results of this phase can then be applied to determine the costâbenefit trade-offs between source-receiver redundancy and survey cost to resolve a given target.
