Estimation of physical properties of a rock sample based on a laboratory transmitted wave experiment and 3D numerical simulations

Nana Yoshimitsu, & Takashi Furumura

Published August 15, 2016, SCEC Contribution #6829, 2016 SCEC Annual Meeting Poster #036

Elastic wave transmission during a rock compression test is an efficient way to estimate the characteristics of a sample. Full utilization of transmitted waveform would lead much better understanding of the sample interior, in addition to the utilization of initial motion. To clarify the wave behavior of multiple reflections and conversions in a finite sample, we performed a 3D numerical simulation of wave propagation in a homogeneous laboratory-scaled sample, and obtained good agreement of the waveforms between the simulation and observation. In this study, we introduce a low velocity zone to a model as a fractured zone with time-dependent velocity variation, and compare the waveforms obtained by the simulation and laboratory compression test.

We calculated 3D finite difference method simulation of seismic wave propagation with 4th- and 2nd- order accuracy in space and time referring to the tri-axial compression test of Imahori et al. (2015). The base model had homogeneous cylindrical structure with 100 mm height and 50 mm diameter which grid spacing was 54 and 60 micrometers in horizontal and vertical directions, respectively. Single force was added on the surface of the model to mimic the movement of the transducer. We used estimated P- and S-wave velocity (Vp = 4100 – 5400 m/s; Vs = 2200 – 3100 m/s), rigidity = 2.7 g/cm3, respectively. A fault which tilted 30 degrees from the axis of the cylinder with 0.3 mm width was modeled by referring the X-ray CT image of the recovered granite sample after the tri-axial compression test. The fault zone was regarded as a thin low velocity zone here, where we assumed Vp = 2.7 km/s, Vs = 1.55 km/s, and rigidity of 1.35 g/cm3, respectively. Band-pass filter of 200-400 kHz was applied to the calculated waveforms.

Each phases in the later part of the observed transmitted waves (Imahori et al., 2015) showed different behavior as the loading progress; some phases disappeared and some newly appeared. We applied two pairs of the velocity model to reproduce the waveforms obtained in the earlier stage and the later stage of the loading in the experiment, and both case showed good agreement of the waveforms between the simulation and observation. Newly appeared characteristic phase in the later stage of the loading was well reproduced by the simulation. This agreement suggests the possibility of the interpretation and utilization of the later phases of the transmitted wave.

Key Words
3D finite difference method simulation, laboratory experiment, wave transmission

Citation
Yoshimitsu, N., & Furumura, T. (2016, 08). Estimation of physical properties of a rock sample based on a laboratory transmitted wave experiment and 3D numerical simulations. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)