Seismic Tomography of Concrete Structures Phase II: Hardware and Software Developments Phase III: Field Tests at Seminoe Dam

Between 1997 and 2001, the Seismotectonics and Geophysics Group at the Bureau of Reclamation conducted a research project on the application of the seismic tomography method to investigation of large concrete structures. The hardware and software needed to efficiently acquire high-quality seismic tomography data on large concrete structures were developed. This hardware and software, as well as the effectiveness of the seismic tomography method as applied to concrete structures, was evaluated by acquiring and processing seismic tomography data from a concrete arch dam undergoing alkali-aggregate reaction. A total of approximately 13,700 seismic waveforms were acquired across three upstream-downstream cross sections during 9 days of data acquisition. Seismic ray paths ranging from horizontal to approximately 70 degrees from horizontal were obtained through most of each cross section, with excellent signal-to-noise ratio on most seismic traces. The P-wave velocity tomograms computed from these data show velocities ranging from 16,000 ft/s to less than 7,000 ft/s. The areas with the lowest P-wave velocities are interpreted as regions with the most severely deteriorated concrete. The pattern of P-wave velocity variations is consistent with information obtained from core analyses, fracture mapping, and deformation studies, and provides valuable additional information about the spatial distribution of concrete deterioration.