Travel Times of Later Phases for Transmitting Waves through a Fracturing Westerly Granite Sample under a Triaxial Compressive Condition

Abstract:

In laboratory, it is well-known that the elastic wave speed varies prior to compression fracture of the rock (e.g., Lockner et al., 1977, JGR). Using an enough number of travel times of elastic wave paths in a sample, we can estimate internal structure of the sample. However, the number of the elastic wave transducers is limited, and only the travel times of the first arrival are available in most experiments. Employing broadband transducers (Yoshimitsu et al., 2014, GRL), later phases become available to be analyzed.

In the present study, we conduct a triaxial compressive test at room temperature under a dry condition and a confining pressure of 50 MPa, using a cylindrical Westerly granite sample of 100 mm long by 50 mm in diameter. Eight transducers are attached on the sample surface. One of the transducers is used as a wave source and voltage steps are repeatedly applied to it. The elastic waves passing through the sample are sensed by the other broadband transducers, and recorded at a sampling rate of 20 Msps.

P-wave speed is estimated from the travel time of the direct P, and Vp/Vs value is assumed to be the √3 to give S-wave speed. We assume that all wave paths never bend except at the top and bottom surface of the sample. We calculate the travel times of later phases reflected at the top and/or bottom surfaces within 3 times.

We collate the calculated travel times with observed waveforms. We can identify the travel time of two phases: single reflection from both top and bottom of the sample. On the other hand, some other observed and calculated phase arrivals do not match with each other. Then, we try to identify some remarkable phases using the calculated travel times of PS and SP converted waves and interfacial waves, taking into consideration of wave speed anisotropy.