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 geometry with a characteristic Hurst exponent consistent with the value 0.8 suggested to be universal by several authors. For sandstone, the rugosity spectrum remains independent on the fracturation velocity but displays self-affine characteristics only for characteristic length scales roughly larger than the grain size: furthermore, the corresponding Hurst exponent takes a value 0.47 +/- 0.05 distinctly smaller than the previous one. At characteristic length scales below the grain size, a clear cutoff is observed in the spectrum. This cut-off is shown through numerical simulations to be largely explainable by the faceted shape 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 determined. The differences between the values of Hurst's exponent may reflect the intergranular nature of fracture in the sandstone samples while no effect of structural heterogeneities on fracture propagation is observed in granite.