Report on televiewer log and stress measurements in core hole USW-G1, Nevada Test Site, December 13-22, 1981

The operations and the preliminary results of televiewer logging and stress measurements in USW-G1 on Yucca Mountain at the Nevada Test Site, Nevada, carried out between December 13 and December 22, 1981 are described. We anticipate that additional measurements in this area will be made in the future and a more complete interpretation of these data will be attempted. USW-G1 is a core hole drilled on the eastern flank of Yucca Mountain at Nevada coordinates N-770,500, E-561,000. The hole was drilled to evaluate the site as a potential repository for radioactive waste. The work reported here is part of an array of geological, geophysical, and hydrologic studies designed to provide data needed for the evaluation of a potential nuclear waste repository. Information on the state of stress in the rocks is needed for the design and construction of a repository, for the prediction of long-term tectonic stability of the region, and for the evaluation of the hydrologic properties of the site. The stress measurements are made using the hydrofrac method. This method involves isolating a short section of the borehole between two rubber packers and then fracturing the rock in this section with fluid pressure applied through drilling pipe or tubing from the surface. The shape of the pressure-volume time curves can be used to infer the state of stress. An ultrasonic borehole televiewer is used to locate unfractured sections of the borehole suitable for stress measurements. Orientations of the induced fractures are determined from the post-frac televiewer logs or from impression packers that are used when the televiewer records do not show the fractures.