A new nondestructive seismic tomography system called RockVision3D TM was used to image the volume extending between Alcove #8 and Niche #3 of the Exploratory Studies Facility (ESF), Yucca Mountain, Nevada. This work was performed as part of the infiltration tests for the Enhanced Characterization of the Repository Block (ECRB). The scope and objective of this reported work were to use 2-D and 3-D velocity tomographic images to assess the structural state of the rock mass prior to infiltrating water from Alcove #8 into Niche #3. The results of the cross-borehole seismic investigation, combined with published geotechnical and geologic information on the Topopah Spring Tuff formation, identified three distinct velocity zones: (1) Upper non-lithophysal zone; (2) intermediate or transition zone, 7 m (23 ft) thick; and (3) middle non-lithophysal zone. This paper describes system components, test configurations and procedures, and data analysis and interpretation. Results, conclusions, and recommendations are also presented.
A new high-tech nondestructive tomographic imaging system called RockVision3D TM, developed by NSA Engineering, is a complete tomography package, from signal collection to CAD output display. The system has been used successfully in geological and geotechnical site investigations throughout highway, bridge, foundation, sinkhole, slope, and tunneling and mining construction industries (Hanna et al. 2000; Nell et al. 1999; Hanna et al. 1998).
This technology involves surrounding a 2-D or 3D region of interest with source and receiver boreholes, collecting seismic signal data through the plane or volume over a network of "ray paths," and reconstructing an image of the structure based on signal arrival time computations (Rock et al. 1997). Full waveform shear, or S-wave, or compressional P-wave signals are recorded using a sampling rate and duration appropriate for the signal source utilized, the rock materials anticipated, and the physical scale of the study.
Any ground condition or material property that exhibits a velocity contrast can be readily imaged. For example, areas of higher velocity generally correspond to areas of more competent or consolidated material (rock, soil, etc.) whereas areas of low velocity represent areas of less consolidated or medium-to-soft material (Westman et al. 1996; Nur 1987; Yu 1992).
The system software combines wavefront and curved ray theories to produce a 3-D attenuation or velocity tomogram of the volume or region to be imaged. In a uniform rock or other low-velocity contrast medium, the velocity and attenuation ray paths are generally straight. However, the ray paths are not normally straight, but rather bend (refract, curved ray) depending on the physical properties of the medium or the velocity contrasts between various material units (Rock et al. 1997). For example, in karstic material, the seismic energy will not pass through voids and cavities, but will bend or travel around them. If a cavity is filled with water, clay, or sand, a portion of the energy will travel through it at a different velocity and attenuation level (Hanna et al. 2000).
This paper describes the application of the RockVision3D TM system using the cross-borehole velocity method to image the zone between Alcove #8 and Niche #3 of the ESF.
The investigation was performed as part of the infiltration experiment in the ECRB. The scope and objectives of this reported work were to use 2-D and 3-D velocity tomographic images to (1) assess the structures and lithologic features within the volume extending between Alcove #8 and Niche #3; (2) provide information on the structural state of the rock mass as interred by th