Velocity and amplitude of P-waves transmitted through fractured zones composed of multiple thin low-velocity layers

Abstract

Fractured zones and faults in rock masses influence the velocity and amplitude of P-waves propagated through them. In the frequency range of seismic exploration, wavelengths of P-waves are comparable to or longer than the thickness of fractures or faults. Fractures can be modeled as thin low-velocity layers in homogeneous high velocity material. We carried out model experiments on P-wave propagation through a low-velocity zone composed of many thin low-velocity layers, with the direction of P-wave propagation normal to the layers. The experimental models were made of aluminum rods and acrylic resin disks: they correspond, respectively, to rock and materials filling fractures such as clay. Next, we numerically simulated one-dimensional wave propagation through the low-velocity zone by using the communication matrix method. Waveforms obtained by this model calculation show good agreement with those obtained by the model experiments. The waveform and the amplitude of the transmitted P-waves vary with the number and the thickness of low-velocity layers. The time-average equation does not provide a good velocity estimation, because the velocity decreases with the increase in the number of low-velocity layers in spite that the total thickness of layers remaining the same. The distribution of low-velocity layers has little effect on velocity and waveform. Effects of low-velocity zones on waveform, amplitude and velocity of the transmitted P-wave vary with frequency of the incident P-wave. We concluded that the wave transmitted through the low-velocity zone was formed by the superposition of the direct wave and the many waves having multiple reflections at the interfaces of the layers in their path.