Effects of fractures on seismic-wave velocity and attenuation

Abstract

S U M M A R YThe effects of fractures on the seismic velocity and attenuation of a rock are investigated using theoretical results and experimental data. Fractures in a rock mass influence the traveltimes and amplitudes of seismic waves that have propagated through them. The displacement discontinuity model, recently employed in fracture investigations, is modified to describe the effect of fractures on seismic-wave velocity and attenuation. This new model, the modified displacement discontinuity model (MDD), is formulated in a way analogous to transmission-line analysis. The fractures are treated as transmission lines for the passage of seismic waves. The M D D takes into consideration realistic fracture parameters which include the fracture length, the fractional area of a fracture surface in contact, and the nature of the infilling material. A single fracture of varying geometric and material properties is shown to affect dramatically the transmission properties of a propagating waveform, and hence the seismic velocity and attenuation. These effects have been shown to result in a frequency-dependent velocity and attenuation. The sensitivity of the fracture parameters to seismic-wave velocity and attenuation was investigated and interesting results were obtained. Fracture parameters used in designing experimental models consisting of synthetically manufactured cracks were fed into the M D D and a well-known crack model, Hudson's model, for comparison. Velocities as a function of the incident-wave angle were obtained from both numerical models and were compared with the results from the experimental modelling. For P waves, the M D D model results show better agreement with those of the experimental model for all crack densities investigated than those from Hudson's model.

Magmatic processes in the Alaska subduction zone by combined 3‐D b value imaging and targeted seismic tomography

“…Subduction processes most easily reactivate existing fissures, though because of bending, new cracks and fissures are created as well [Ranero et al, 2003]. With increasing fracturing, seismic velocities (V P and V S ) decrease [Boadu and Long, 1996] and b values become higher [Wang et al, 2000]. Circulating fluids in such a fracture system might affect shear wave velocity more strongly than P wave velocity.…”

High-resolution shear-wave seismic reflection as a tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany

“…2 to 3 m at 50 m depth. Boadu and Long (1996) found that seismic amplitudes and phases change at fractures, depending on the fracture parameters such as fracture length and fracture spacing. Only part of the energy is reflected at the fault itself and the rest is either scattered or guided by the fracture.…”

Review article "Remarks on factors influencing shear wave velocities and their role in evaluating susceptibilities to earthquake-triggered slope instability: case study for the Campania area (Italy)"

“…Gaviglio, 1989;Watanabe and Sassa, 1995;Kahraman, 2002) dealt with the study of propagation velocity of longitudinal waves in fractured rocks, as they are more easily detectable than transversal waves. Boadu and Long (1996) proposed a model of fracture called MDD (Modified Displacement Discontinuity) to study the propagation of both types of seismic waves (P and S) in fractured media. Laboratory studies based on this model (Leucci and De Giorgi, 2003) established empirical relationships between parameters related to the degree of fracturing of a rock formation and the propagation velocity of both P and S waves.…”