Ultrasonic and mechanical testing details
Our samples have a maximum ultrasonic velocity of ~6 km/s and considering the testing frequency of 800 kHz, we estimated a maximum wavelength (λM) of 7.5 mm and, to avoid nearfield effects, we prepared cores with a length (L) > 3 λM. Velocities were estimated with the transmission method by measuring the time of travel of the elastic wave along the core plug (Birch, 1960). We corrected the first arrival by the delay introduced by the sample holders that was determined by a standard calibration procedure (e.g., Prelicz, 2005). A pulser-receiver apparatus (JSR Ultrasonics DPR300) generated a negative spike pulse with a typical duration of ~40 ns feeding the source ultrasonic transducer. We used a pulsing rate of 100 pulses/sec (PRF RATE=1), pulse amplitude of ~194 V (PULSE AMPLITUDE = 4, and PULSE ENERGY = HIGH Z 4), and damping of 331 Ohms (DAMPING = 1). In addition, the pulser-receiver produces a trigger signal (5 V in amplitude) to synchronize the pulser and the oscilloscope (Rigol DS1104Z-S) collecting the signals generated by the receiving transducer and amplified by the receiver. The latter has a gain of 66 dB (REL. GAIN = 79), a high-pass filter corner frequency of 1 MHz, and a low-pass filter corner frequency of 3 MHz. Two data transfer switches allow selecting the recording of the VP, VS1 or VS2signal. To improve the signal-to-noise ratio the oscilloscope collects and stacks 1024 signals and transmits the digitized wavelets to a computer via a USB port. Typically, the signal, comprising 1200 samples, is digitized every 0.2 µs or less and saved as a comma-separated-value (CSV) file.
Samples compaction was measured to 1 µm accuracy with a Linear Variable Displacement Transducer connected to the axial piston, whose signal was acquired along with the confining pressure and vertical force.