2008 Annual Report
1a.Objectives (from AD-416)
The objective of this cooperative research program is to measure and model the propagation of seismic/acoustic waves in soils to better describe soil physical properties. This will include development of reliable acoustic and seismic technologies for the nondestructive measurement of both soil and crop variables that affect water availability and plant growth in food and fiber production systems. Of particular interest will be the development and measurement of acoustic/seismic coupling in sealing susceptible soils and in characterization of soil compaction. This effort will include development and construction and deployment of prototype instrumentation for measuring rate and total load of sediment discharge and changes in bed topography in alluvial channels. This effort will also include development of acoustic techniques for delineation and characterization of sediment accumulations within flood control structures.
1b.Approach (from AD-416)
Field and laboratory studies will be conducted to measure the acoustic/seismic response of soils in various management conditions and sediment-laden flows in channel systems. Results will be analyzed, evaluated, and compiled with conventional measuring techniques. Laboratory studies will primarily address fundamental aspects of acoustic measurements, improve acoustic techniques and technology, and assist in the interpretation of acquired data. Field studies will be designed to evaluate practical applications of acoustic measurements for a wide range of soil and surface conditions and flow regimes. Instrumentation will be directed toward the design and development of specific monitors and sensors capable of measuring bulk flow, and hydraulic and sediment load properties in alluvial channel systems and of instrumentation for determining the storage capacity and integrity of flood control structures.
A field survey conducted for the past 2.5 years to study seasonal and weather effects on soil properties shows that sound speed reflects variations of soil properties. It was found that water potential is the main factor governing sound speed in soils, moisture plays a minor role, and temperature has a negligible effect. The study showed the feasibility of an acoustic technique for measuring water potential that is more sensitive and operates over a wider range than commercial devices.
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.