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Tomographic Investigations at Landfill 4, Hill Air Force Base, Layton, Utah.

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Technical rept. 3-7 Nov 86,

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In support of soil classification objectives related to an on going study of non-linear soil response, tomographic techniques were developed and tested using seismic data. The data were collected at three locations at Landfill 4, Hill AFB, Utah. The site is a land-fill waste site which had been isolated from the prevailing ground-water flow by a slurry trench. The objective of this study was to identify the effectiveness of geophysical techniques for seismic velocity measurements and tomographicimaging analyses. Seismic data were collected between pairs of 50 ft. deep boreholes on either side of the slurry trench. First break picks were analyzed using an iterative back-projection tomographic technique utilizing curved rays. Initial processing revealed that smearing of velocity anomalies between the boreholes occurred because of non-optimum placement of shots and receivers. Adding reasonable constraints to the inversion, however, showed a low-velocity region, terminating between 35 and 40 ft., at the trench location. Diffraction arrivals travelling around the base of the low-velocity region further constrained its depth. Other minor low-velocity zones, possibly indicating layers with increased silt content, were also indicated by the inversion model. This study showed that cross-borehole tomography can be a useful tool in delineating velocity anomalies roughly perpendicular to paths between the boreholes, but that velocity anomalies parallel to the boreholes will be smeared unless additional constraints are imposed. A modification to the data acquisition technique to help overcome this problem would be to use additional high-energy shot-points on the surface between the boreholes.

Subject Categories:

  • Soil Mechanics
  • Seismic Detection and Detectors
  • Solid Wastes and Pollution and Control

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