MECHANICAL CONTROLS ON THE SPATIAL DENSITY OF OPENING-MODE FRACTURE NETWORKS

Understanding the controls on fracture density is critical to interpre ting the geologic evolution, permeability, and mechanical properties o f jointed rock. Analyses of fracture trace maps and seismic inversions reveal that near-surface fracture densities are generally less than s imilar to 1-2 over a range in scale of 14 orders of magnitude. The obs erved densities are approximately an order of magnitude greater than t he range typical of deeper (>500 m) formations and may indicate that s urficial controls on fracture growth are different than those at depth . The development of greater densities at the surface may be limited b y the significant decrease in effective elastic stiffness associated w ith fracture densities near unity, As fracture densities approach 1.0, additional strain accommodated by the fractures decreases the driving tension, limiting further fracture growth.