Analysis of fracture orientation data from boreholes

Fracture orientation data from boreholes find widespread use in investigati ons of the subsurface. These data invariably are biased, with in situ fract ures at low angles to a borehole being under-represented, Three statistical measures (mean spherical orientation, spherical variance, and moment of in ertia), conventionally used to analyze axial data distributed on a sphere w ithout corrections for borehole bias, are used here to analyze fracture pol e orientations distributed on a hemisphere, with borehole bias being accoun ted for. By assuming a particular model for the in situ distribution of fra ctures, such as a uniform distribution, and then correcting for borehole bi as, one can predict the distribution and statistics of fracture poles a bor ehole survey would yield, The model can then be modified based on the misma tch between observations and predictions. Geologic information on the kinem atics and mechanics of fracturing can be incorporated into the model of the in situ distribution. An analysis of fracture orientation data from a comp lexly fractured granite at the Stripa mine in Sweden shows that this approa ch provides a useful and straightforward way to investigate fracture distri butions in the subsurface. Moment of inertia diagrams and plots of cumulati ve distribution functions for fracture poles greatly augment equal area pro jections in analyzing fracture orientation data from boreholes.