SCALING AND CONNECTIVITY OF JOINT SYSTEMS IN SANDSTONES FROM WESTERN NORWAY

The scaling properties of a joint system in Devonian sandstones in wes tern Norway have been investigated using seven maps, covering areas Fr om 18 to 720 m across, which were generated by mapping in the field an d from low-level aerial photography taken from different heights. Each map represent a scale 'window' on the fracture population, bounded by resolution al small scales and the sample size at large scales. A pow er-law relationship between fracture trace length and critical observa tion height (maximum height at which a trace can be identified) is der ived and used to create a statistical model for the resolution effects . The model indicates that a continuous smooth curve without a straigh t-line segment on a log-log cumulative frequency distribution plot doe s not necessarily rule out a power law as the underlying population di stribution. Together, the maps indicate a power-law trace-length distr ibution with an exponent of -2.1. This power law may be valid over fou r or more orders of magnitude, with natural lower cut-off of around 1 m. The exponent is significantly different from -2.0 (strictly self-si milar case) and is reflected in a decrease in the abundance of fractur es with length comparable to map size, as map scale decreases. The fra cture trace-length distribution results in a decrease in apparent conn ectivity, with decreasing scale. High resolution (large-scale) maps ar e well connected while the lowest resolution map (smallest scale) is u nconnected. Fractures in the smallest scale map are, however, connecte d by small fractures below the limit of resolution, represented by the largest scale map. The variation in apparent connectivity with scale has implications for fluid flow. When fractures are open to fluid flow the scaling properties of apparent connectivity imply that, beyond a certain scale, the size of fracture controlling flow will be scale-ind ependent. In this fracture system, this appears to occur in sample are as of around 300 m across. (C) 1997 Elsevier Science Ltd.