24-hours broadband measurements of elastic waves transmitted through sand soils under dry and saturated conditions to investigate differences in propagation characteristics
Abstract
For more highly porous and unconsolidated media than rock samples, it is unknown how propagation characteristics of transmitted elastic waves vary as they become saturated. Barrière et al. (2012) investigated the characteristics using a sand soil during imbibition/drainage cycles. Nakayama et al. (2017, AGU) investigated the differences in the characteristics for a sand soil under dry and saturated static conditions. However, the measurements were performed for only 1 hour, and they could not obtain reproducible waves throughout the measurements.
In this study, we performed 24-hours measurements in the same dry/saturated situation using the wave consisting of 1-50 kHz components to investigate stability of the system, while Nakayama et al. (2017) used only 1-6 kHz contents. We dropped sand particles (0.2-0.4 mm diameter) in a container (375 mm long, 255 mm wide, and 235 mm high) and made the dry sand soil of 105 mm thickness, whose dry density and porosity were 1449 kg/m3 and 46%, respectively. A shaker as an elastic wave source and three accelerometers were aligned along the vibration axis of the shaker with the same incident angle. We sprinkled tap water until the water level reached the top surface of the sand soil. We designed a linear sweep signal (1-50 kHz) and applied it to the shaker repeatedly. We recorded the transmitted waves at the sampling rate of 204.8 ksps and obtained hourly stacked waveforms. The spectral amplitudes and phases varied over 50% and π rad, respectively, in the frequency band less than 15 kHz, whereas we successfully obtained stable waveforms for 15-50 kHz. For dry and saturated conditions, fluctuations of spectral amplitudes were within 10% and 1% for the last several hours, respectively (Figure 1). Similarly, fluctuations of their phases were within 0.3 rad and 0.03 rad for the last several hours, respectively. The fluctuations of amplitude in the former stage may indicate that coupling between accelerometers and sand had varied within 15 hours. These results suggest that it is necessary to measure them at least 10 hours or confirm the stability continually by real-time monitoring. These stability and fluctuations should be carefully considered when one performs an experiment with water level variations.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H11I1571N
- Keywords:
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- 1835 Hydrogeophysics;
- HYDROLOGYDE: 1859 Rocks: physical properties;
- HYDROLOGYDE: 1865 Soils;
- HYDROLOGYDE: 1875 Vadose zone;
- HYDROLOGY