2007 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.
This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the National Center for Physical Acoustics (NCPA), University of Mississippi. Additional details of research can be found in the report for the in-house project 6408-12130-013-00D, "Development of Acoustic and Seismic Technology to Characterize Soils, Assess Water Content, and Otherwise Reduce Production Costs." A system has been constructed to measure rainfall and the sound speed, nonlinearity parameter, temperature, water content, and water pressure at different soil dpeths. Initial data shows significant weather and seasonal influence on soil properties, reflected by acoustic parameter variations. A strong correlation between soil water potential and the sound speed and nonlinear parameter was found. NCPA analyzed field data from an acoustic technique for characterizing fragipan, a hard subsurface soil layer that inhibits water percolation and plant growth; continued development of a predictive model; and developed a non-linear inversion algorithm. This algorithm used the predictive model to determine soil properties relevant to the acoustic technique. Results indicate that the acoustic signatures correlate well with current technique measurements. Inversion results indicate that the predictive model does not fully account for all soil parameters than influence the acoustic technique. An instrumented raft was used to autonomously collect suspended sediment data during multiple storm events. The calibration jet tank was augmented and laboratory experiments utilizing multiple acoustic frequencies were conducted in order to advance the ability to simultaneously evaluate particle size and concentration without the need for pump sampling. Significant problems in the manufacturing of the final DSP board have delayed deployment of the lower cost DSP system for monitoring sediment. Laboratory measurements of P-wave velocity and absorption obtained for saturated sediment samples containing 5% and 15% clay indicate there are significant differences in acoustic attenuation, phase velocity, and signal velocity. The addition of a pollutant to these samples causes an increase in velocity for the 15% clay sample and a decrease in velocity for the 5% clay sample. Acoustic velocity profiles of accumulated sediment were measured using an acoustic probe in a reservoir and the sediment was mapped with a subbottom profiler. Data from both measurements are being merged to obtain a better estimate of the accumulated sediment volume. Field work was performed to determine the feasibility of using seismic refraction tomography to detect pipes, sink holes, saturated piping zones, or weakenesses in an earthen dam. Seismic P-wave surveys were performed on the crest and upstream side of a dam and seismic velocity tomograms were constructed. The refraction tomogram of the upstream side clearly delineates the foundation trench drain as well as the interface between the compacted dam body and the initial ground surface.