Seismic Tomography Applied to Site Characterization

Geotechnical engineers have struggled for years with the difficult task of characterizing geologic conditions that affect the design and stability of various transportation infrastructure, including road cuts, tunnels, and bridge abutments. Commonly, designs are based on data from borings that may or may not reflect the true character or three-dimensional continuity of the materials present and may also not detect the presence of localized structural defects (faults, shear zones, slip planes) or previous excavation activity. Geophysical techniques such as seismic reflection, seismic refraction, and ground penetrating radar have been used with some success to expand the knowledge of subsurface conditions prior to excavation however; these techniques often interfere with the construction process and are time-intensive in terms of data collection and interpretation. To improve the quality, continuity, and tirneliness of geotechnical data for site characterization, NSA Engineering, Inc., has developed a true, three- dimensional tomographic imaging system known as Rock Vision3D^TM. This system is based on the principle that acoustic waves travel through different materials at different speeds and with different attenuation rates. By recording the waveforms from known acoustic sources, three-dimensional tomograms inferring the relative competence of the materials that the waves travel through can be constructed. Although tomographic techniques are not new, advancements in the technology that allow for near-real-time imaging, make tomography a practical tool for geotechnical site characterization, both in the design stages and as excavation proceeds. With the expanded subsurface information that volumetric tomography provides, the risks and expenses associated with change orders are greatly reduced. This publication describes tomographic imaging as implemented in the RockVision3D^TM software package. Included in this discussion are applications of both seismic velocity tomography and seismic reflection tomography systems. Although attenuation tomography also can provide information on subsurface conditions, especially water saturation, we limit our discussion here to velocity and reflection tomography. Several case studies are presented describing applications of volumetric tomographic imaging for highway and tunnel site characterization.