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Our seismic measurements employed a variant of conventional resonant bar tests, which allows us to determine the dynamic shear and Young's moduli of a core sample in the sonic frequency range near 1 kHz (Nakagawa, 2011). The basic idea behind this apparatus is that the resonance frequencies of a sample are reduced when sample size and mass are artificially increased by an attached foreign object (e.g., Tittmann, 1977). Once the resonances of the resulting “extended” sample are measured, the seismic properties of the original sample are determined via calibration, modeling, and inversion. In our experiments, a cylindrical rock core (~ 38 mm diameter) is jacketed and placed between a pair of stainless steel rods (Fig. 5.1). The resulting long composite bar is suspended by springs in a tubular aluminum cage, which is inserted into a tubular confining cell (pressure vessel). Longitudinal and torsional vibrations are induced in the bar using piezoelectric sources at one end and measured using accelerometers at the other end. Because the geometry of this apparatus is the same as the conventional Split‐Hopkinson Pressure Bar test apparatus (e.g., Kolsky, 1949), it is called the Split‐Hopkinson Resonant Bar (SHRB).

The longitudinal piezoceramic source is a single disk, and the torsion source is a group of four pie‐shaped laterally polarized shear piezoelectric slabs. Both are made of Type 5600 Navy V piezoceramics (Channel Industries). These sources are driven selectively to introduce a desired mode of vibration in the sample. At the opposite end of the other steel rod, miniature accelerometers (PCB Piezotronics, 352A24) are attached to measure the resulting vibrations. The longitudinal motion is measured by an axial accelerometer, and the torsion motion by a pair of accelerometers oriented in the tangential directions, in diametrically opposing locations. The torsion vibrations are measured by subtracting the output from one of the torsion sensors from the output of the other, resulting in cancellation of electrical noise and unwanted flexural motions contaminating the measurements. During the experiments presented in this paper, the source amplitude was adjusted so that the strains induced in the samples at resonance were in the 10^–6 to 10^–7 range to reduce possible nonlinear effects.

To ensure good mechanical coupling, the surfaces of the steel bars and the sample are polished flat, then a thin (30 μm) lead foil disk is placed on the interfaces. These disks are cut with a cross‐shaped pattern at the center to allow distribution of the pore fluid along the interface. With these preparations, typically 3–4 MPa of effective confining stress is sufficient to reduce the additional compliance introduced by the interface to a negligible level. The jacket is made of heat‐shrink PVC, with a thickness ranging from 150 μm to 500 μm. With appropriately machined smooth sample surfaces and with application of sufficient effective confining stress (>~1 MPa), this results in good seal at the jacket‐sample interface.

Currently, our experiments with the SHRB apply confining stress, using high‐pressure nitrogen gas up to ~35 MPa, and introduce and extract fluids into and from the sample through ports attached to the metal rods. The temperature of the system is controlled using both a fluid‐circulating heating/cooling jacket attached to the exterior wall of the pressure vessel (Temco/Corelab, X‐ray transparent, carbon‐fiber‐wrapped, tubular aluminum cell) and film heaters lining the interior wall of the aluminum suspension cage. (Our past experiments have been conducted at temperatures ranging from −15°C up to 65°C.) During the experiments, changes in the distribution of different fluid phases within the sample can be examined using X‐ray CT (Nakagawa et al., 2013), similar to the experiment previously conducted by Cadoret et al. (1995) during conventional resonant bar tests.

Figure 5.1 Split‐Hopkinson Resonant Bar. A small (typically 2.5 cm–10 cm long, ~3.8 cm dia), jacketed rock core is placed between a pair of metal rods, and one‐dimensional vibrations of the entire rod assembly are examined for the sample's seismic properties. Hydrostatic confining stress is applied by compressed nitrogen gas inside a tubular pressure vessel, and pore fluids are injected and extracted through ports attached to the metal rods. The tubing for the pore fluids is coiled around the rods, effectively reducing the mechanical coupling between the resonant bar and the pressure vessel.