Effect of fracture scale length and aperture on seismic wave propagation : an experimental study

In this paper, we investigate the effects of fracture scale length and aperture on seismic wave propagation through seismic physical modelling. The physical models are constructed from a solid background of epoxy resin with inclusions of silicon rubber chips which come with different radius and thickness to simulate fractures with different scale length and aperture. The chips embedded in each model are of the same radius and thickness, and the fracture density is kept constant for all models in order to understand the effects of the scale length and aperture. P and S waves that propagate parallel and perpendicular to the fractures are then recorded using a pulse transmission method. The experimental results show that given the same fracture density the changing of radius has an only minor effect on the P-wave velocity and amplitude, and there are also little effects on the shear-wave amplitudes. The main observable effect is an increase of the slow shear-wave velocity with radius, leading to a decrease in shear-wave splitting with radius. The changing of fracture thickness has also little effects on the shear-wave amplitude except an obvious decrease in the slow shear-wave velocity, leading to an increase of shear-wave splitting with thickness. However, the increasing in fracture thickness induced a strong attenuation in the P-wave, in particularly for P-wave propagating perpendicular to the fracture. These findings may be useful for differentiating the effects of thin microcracks and large open fractures.