Technical Report NTB 88-32

Crosshole investigationsImplementation and fractional dimension interpretation of sinusoidal tests

The Crosshole Programme was an integrated geophysical and hydrogeological study of a limited volume of rock (known as the Crosshole Site) within the Stripa Mine. Borehole radar, borehole seismic and hydraulic methods were developed for specific application to fractured crystalline rock.

The hydrogeological investigations contained both single borehole and crosshole test techniques. A novel technique, using a sinusoidal variation of pressure, formed the main method of crosshole testing and was assessed during the programme. The strategy of crosshole testing was strongly influenced by the results from the geophysical measurements.

The crosshole sinusoidal testing was carried out using computer-controlled test equipment to generate the sinusoidally varying head in a single zone (the "source") isolated by packers. A second ("receiver") borehole contained a number of straddle intervals and was used to observe the propagation of the sinusoidal signal. The number of positive responses was limited and flow appeared to be concentrated within a few "channels". Analysis was attempted using single fissure, regularly fissured and porous medium models. None gave satisfactory fits to the measured data. A new analysis involving the "dimension" of the flow test has been developed to analyse the results of the crosshole sinusoidal testing. This analysis allows the dimension of the flow to assume non-integer values whereas conventionally the dimension is taken as either one, two, or three: for example, radial flow in a uniform planar fissure would be two dimensional.

The new model is found to give a more consistent description of the test data than the conventional models and suggests a complex pattern of fracture properties within each fracture zone. However, the results presented must be considered as being preliminary since we still have much to learn about how to best apply this model and present the results. Also, it is not yet clear how the derived value of "dimension" can be related to the transport properties of the rock.