2008 Annual Report
A laboratory test conducted to study the effect of moisture on the hysteretic nonlinear parameter of soils showed that the nonlinear parameter could reflect changes in soil properties with sensitivity that is orders of magnitude higher than that of sound speed.
A rapid, non-contact, high spatial resolution imaging system is being developed to exploit surface wave propagation to obtain the sound speed profile in soil. Using these results, the sound speed can be used to create water potential and moisture profiles. The system can be extended for 2D surface mapping or 3D soil imaging by using a scanning laser Doppler vibrometer.
The instrumented raft deployed last year to autonomously collect suspended sediment data during storm events suffered severe structural damage during a storm. It was repaired, redeployed, and subsequently hit by lightning, causing massive damage to the electronics, which are now being repaired. The calibration jet tank continued to be used to collect multi-frequency data to improve modeling for evaluation of sediment particle size and concentration. Problems with the fabrication of the digital signal processing (DSP) boards for the lower-cost field unit were identified and new boards have been constructed and are being tested. The unexpected death of a key employee has delayed many of the objectives, most notably the development of the lower-cost DSP system. Work on using acoustics to evaluate fines (less than 100 µm), which was to be examined in the 4th year of Objective 2a, has been advanced to this year.
Laboratory measurements on saturated sediment samples indicate there are significant differences in acoustic attenuation, phase velocity, and signal velocity due to different clay contents and with the addition of a pollutant. The presence of organic matter does not cause a significant change in acoustic response.
Time-lapse seismic surveys were carried out at the USDA ARS Hydraulic Engineering Research Unit (HERU) on an earthen embankment during an internal erosion experiment. Embankment failure due to overtopping or internal erosion is not only dependent on the flood or water flow but is also influenced by the dam composition and construction. Seven successive tomography surveys were carried out on the earthen dam at different stages of breaching. As the breaching progresses the seismic signature of the piping zone changes dramatically.