Recent geologic research on natural fractures challenges assumptions frequently made by geophysicists. Open fractures are not necessarily oriented parallel to the maximum horizontal stress, and fractures do not necessarily close when the fluid pressure within them is reduced. Even in the most mechanically favorable environment, precipitated cements can prop fractures open or seal fractures of any orientation. Fracture sets typically show dispersion in strike, and multiple sets of open fractures can coexist. More importantly, fractures comprise populations that commonly range over orders of magnitude in aperture and length and that occur in nonuniform clusters. Instead of isolated, regularly spaced, large, equally compliant fractures, the Earth presents complex fractal clustering of fractures having a wide range of sizes and variable compliance dictated by natural cements in the fractures and the rock mass. Going beyond anisotropy to document these essential fracture attributes in the interwell space is a key challenge for geophysicists.

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