Observations of joint persistence and connectivity are made by comparison of digital borehole wall images of fractures, fluid conductivity logs and hydraulic injections test results. The fractures were found to be generally impersistent across vertical boreholes about 8 m apart. Many hydraulic connections were found in the same volume of rock. Direct connections through single fractures seem to be rare and connectivity appears to be controlled by fracture networks, even over small volumes.
Joint persistence and connectivity have strong influences on the hydraulic and mechanical behavior of rockmasses. Interpretation and joint system modeling using estimates of persistence has been described extensively by Dershowitz and Einstein (1988). Persistence and connectivity are difficult to measure however, and often the only way to estimate these parameters is by observation of outcrops or excavation surfaces. In this paper we describe observations of persistence and connectivity made using high resolution digital images of borehole walls combined with fluid conductivity logging (Tsang, 1990) and injection tests. The borehole wall images were obtained using a relatively new instrument called the Borehole Scanner System (BSS). Conductive fractures were identified using the BSS images and the fluid conductivity results in three boreholes. The orientation, aperture and aperture anisotropies of conductive fractures were extracted from the BSS images. The orientation of the conductive fractures were used to project joint intersections with other boreholes. Since the three borehole studied are fairly close to one another, it was expected that these joints would be persistent enough to make connections across the boreholes and provide direct paths for fluid flow across the boreholes. Similarity of apertures and aperture anisotropies were expected to further enforce the persistency argument enjoints with the same orientation that project towards each other based on orientation. These connections in turn were expected to explain the hydraulic pressure response connections between packed of sections of the boreholes observed from the injection test results. This paper describes results made from making these observations at the Lawrence Berkeley Lab's Raymond field site (Karasaki et. al., 1994).
The BSS consists of a probe, depth encoder, winch, controller, TV monitor and a VTR unit. The watertight probe houses a white light source and a magnetic compass at the bottom . A mirror rotating at 3000 rpm sits directly above the lamp, and reflects light from the lamp onto the borehole wall through a glass window. The light reflected off the borehole wall directed into a photoelectric transformer. The photoelectric transformer measures the intensity of the incoming light in the red, blue and green wavelength bands and converts the intensities into digital form. The digital data from the photoelectric transformer is passed to an azimuth gauge which marks the point in the data stream corresponding to north. The data then passes through an amplifier to the controller at the surface where it is stored on a digital tape. The entire borehole wall is scanned along a spiral path in this manner as the winch lowers the probe.