EXPERIMENTAL-ANALYSIS OF FRACTURE RUGOSITY IN GRANULAR AND COMPACT ROCKS

We compare high resolution roughness measurements on fractured granite , basalt and sandstone samples. For granite and basalt, the fractures display over the full experimental length scale range a self-affine ge ometry with a characteristic Hurst exponent consistent with the value 0.8 suggested to be universal by several authors. For sandstone, the r ugosity spectrum remains independent on the fracturation velocity but displays self-affine characteristics only for characteristic length sc ales roughly larger than the grain size: furthermore, the correspondin g Hurst exponent takes a value 0.47 +/- 0.05 distinctly smaller than t he previous one. At characteristic length scales below the grain size, a clear cutoff is observed in the spectrum. This cut-off is shown thr ough numerical simulations to be largely explainable by the faceted sh ape of the sand grains and to a minor degree by the finite size of the profilometer sensor tip. None of these factors seems to influence the low frequency part of the spectrum front which Hurst's exponent is de termined. The differences between the values of Hurst's exponent may r eflect the intergranular nature of fracture in the sandstone samples w hile no effect of structural heterogeneities on fracture propagation i s observed in granite.