Location: Watershed Physical Processes Research
2018 Annual Report
Objectives
1. Develop acoustic and orthogonal geophysical methods to characterize and monitor surface and sub-surface soil properties and processes that contribute to water driven erosion and transport of soil and to assess the potential for dam and levee failures.
1.A-1. Develop seismic instrumentation and methods for characterizing subsurface soil mechanical and hydraulic properties in the vadose zone.
1.A-2. Develop a combined seismoelectric technique and high frequency-MASW (HF-MASW) method to measure subsurface soil hydraulic properties.
1.B-1. Geophysical monitoring and surveying of dams, levees and streambanks within the agricultural watershed.
1.B-2. Conduct laboratory studies to investigate the correlation between geophysical properties and the physical state of a soil.
1.B-3. Investigate wind induced ground surface vibration as a source for measuring the mechanical properties of the ground.
2. Develop and deploy acoustic measurement systems across a watershed to provide improved data collection of sediment flux for decision makers.
2.A. Interpreting the acoustic environment of natural fresh-water gravel-bed channels for use in monitoring bedload flux.
2.B. Advance the application of multiple acoustic surrogate techniques to monitor suspended sediment transport.
Approach
There is a continuing need for better methods to non-invasively measure sediment transport and soil properties in situ. Furthermore, the Nation’s aging dams need to be assessed for structural integrity. Acoustic and orthogonal geophysical techniques will be developed for measuring the mechanical response of soil to remedial measures for upland erosion, autonomous monitoring of sediment transport in streams, and imaging the internal structure of earthen dam and levees. Shear wave propagation can be used to map spatial distributions of subsurface soil mechanical and hydraulic properties, and field experiments will be used evaluate their use for detecting compaction and the extent of plow-pans. A modified shear wave acquisition system will be developed to measure temporal changes in the shear wave velocity profile to infer variations in water potential and water content. The results will be correlated with information from time domain reflectometers (TDR) buried in the test site at different depths to measure water content, a tensiometer to measure water potential, and a rain gauge to measure precipitation. In exploratory work, a laboratory study will be conducted under controlled conditions to establish a relationship between seismoelectric signals and soil hydraulic properties. We will investigate the use of wind-induced vibrations to determine mechanical properties of soil. The method does not need high-energy acoustic/seismic signals, making it suitable for remote field sites. We will perform geophysical site characterization at dams or levees showing signs of internal erosion or seepage during visual inspection. The same procedures will be applied to groundwater recharge zones and streambanks. In order to facilitate the integration of geophysical and geotechnical information, laboratory measurements of compressional and shear wave velocities and electrical resistivity will be conducted on synthetic, remolded soils and field cores. Acoustic methods can be used to improve the accuracy and effectiveness of sediment monitoring programs, but they are in need of continued development. Multiple acoustic methods will be deployed across a watershed to improve the integration of technologies and interpretation of acoustic data. The movement of coarse particles along the stream bed is particularly difficult to measure. Sound generated by coarse particle movement in streams will be used to improve the measurement of bed load transport. The focus will be on separating the sound made by moving particles from other sounds, such as bubbles and other extraneous environmental noise. Through collaborative efforts with soil scientists, hydrologists, and agricultural engineers, the new measurement technology will facilitate more comprehensive studies on sources of sediment, sediment transport and deposition in streams and lakes, and stability analysis of earthen dams and stream embankments.
Progress Report
Construction of a multi-channel data acquisition system for high-frequency multi-channel analysis of surface wave (HF-MASW) consisting of a geophone array with 40 geophones that measures surface vibrations, and a time domain reflectometers (TDR) array that measures water content at different depths. A rain gauge measures precipitation. This system is monitoring the instantaneous responses of soil profiles to rain events. Several enhanced techniques have been developed to improve the accuracy of the soil profile measurements.
An enhanced HF-MASW method was developed to study compaction effects on a farmland. The soils were compacted using a John Deere 7320 tractor. Two-dimensional HF-MASW surveys were conducted and soil profile images in terms of the shear wave velocity were obtained and compared between the compacted and non-compacted soils. The influence of compaction and the formation of plowpan can be determined from the shear wave velocity image. It was found that the compaction causes a significant increase in the S-wave velocity in the top 20 cm soil. The compaction can affect soil properties down to 60 cm deep where a plowpan is formed. The study demonstrates the capability of the HF-MASW method to noninvasively assess compaction effects.
An LDV-based system consisting of a non-contact optical surface vibration sensor and a stepper motor controlled transition frame allowing for scanning the soil surface at very high horizontal resolution was developed. Using a phase locking-in technique, very high frequency components of surface waves can be detected which results in very high vertical resolution. Using this system, a surface sealing/crusting test is planned for summer of 2018.
Seismic refraction and electrical resistivity measurements were conducted at Carroll County Dam, a high hazard dam located in northwest Mississippi. The dam was affected by moderate seepage and sand boil formations. Sand boils developed despite the dam retaining very little water and it raised concern about the dam integrity and the existence of seepage through the dam structure. Results from the application of geophysical surveys indicated that seepage is taking place through a small lens of silty sand (higher porosity) imbedded within a clay layer that was not properly sealed during preparation of the base of the dam. Subsequent discussions with the Mississippi Department of Environmental Quality Dam Safety Division, a plan is underway to return to Carroll County dam and conduct a high resolution three-dimensional electrical resistivity tomography (3D ERT) in the vicinity of the sand boils as well as test the use of ground penetrating radar (GPR) to investigate possible voids associated with the sand boils. Discussions are also ongoing for geophysical surveying at another dam location.
Progress has been made on establishing relationships between geophysical observations and geotechnical properties. Laboratory tests were conducted to measure electrical resistivity and seismic velocity on synthetic and remolded soil samples of different soil types. It was found that changes in electrical resistivity follow the general trend of theory and are more sensitive to changes in saturation than changes in porosity. Seismic velocity measurements did not show a specific trend. Comparing the different soil samples, the changes in both compressional and shear wave velocities were not significant.
Laboratory electrical resistivity and seismic velocity measurements were also conducted on synthetic loam soils to quantify the influence of the percentage of kaolinite and montmorillonite (bentonite in this case) in the clay fraction. Even though all the samples used were classified as loam based on the USDA soil texture classification, results from this study indicated that the mechanical behavior of soils in not unique to soil type. The soil samples showed that maximum dry density decreases as the bentonite percentage increases. Changes in electrical resistivity follow the general trend of theory and are more sensitive to changes in saturation than changes in porosity. Seismic velocity measurements do not show a specific trend.
Measuring the mechanical properties of soils requires some type of source. Wind induced ground vibrations is being investigated as a possible source. A Master’s thesis was published describing wind induced ground vibrations which include: the dynamic response of the ground and the inhomogeneity and anisotropy of the ground. The dynamic response of the ground under the influence of a harmonic vertical surface load was obtained through both an analytical solution and finite element modeling. The two approaches are in a good agreement. The response function for different types of non-uniform grounds due to a vertical surface load was developed using finite element modeling in COMSOL. The predictions of the wind induced ground deformation indicate that higher frequencies are sensitive to the upper layers whereas the lower frequencies are sensitive to the deeper layers. Our theoretical formulation of wind induced ground vibrations requires a description of: the wind pressure source, the coupling of the wind at the ground surface, and the mechanical response of the ground. Measurements of the ground response function due to a mechanical source were conducted at additional sites. However, the use of these results for predicting the wind induced ground vibration was marginal. Additional measurements will be conducted to better understand the coupling of wind pressure to ground vibrations.
Progress was made on Objective 2 related to measurement methods for monitoring sediment flux. Progress with respect to monitoring bedload flux (Objective 2A), multiple experiments have been conducted in flumes at the National Sedimentation Laboratory with the goal of identifying and quantifying the acoustic signal recorded due to the flow of water around the recording instrument. In addition, a set of experiments were conducted with a moving hydrophone instead of stationary hydrophones so that the hydrophones moved with the flow of water - thus greatly reducing the background noise. Analysis techniques have been developed to differentiate noise constituents in acoustic data. Individual constituents will be identified manually and investigated for characteristic acoustic signatures that may be used for identification of the noise source.
Methods for testing acoustic propagation in the field have been determined in tanks and flumes. A Benthowave BII-8030 underwater transmitter is capable of reliably producing arbitrary sounds at sufficient amplitude to travel large distances underwater. Laboratory tests have been conducted to fully verify the properties and limitations of this acoustic source. Sound is detected by two HTI 96-MIN hydrophones and collected by a Zoom H4N wave recorder. This data collection system is portable and robust allowing for measurements to be taken at a variety of desired locations within a river system. Experiments are being planned to further investigate the propagation of acoustic waves in a variety of settings with different physical properties.
Progress in the area of surrogate techniques to monitor suspended sediment transport include a preliminary long-term deployment in Goodwin Creek watershed near Batesville, Mississippi incorporating acoustic signals and co-located temperature measurements. Also an installation of a long-term monitoring system on the Rio Grande Floodway at San Acacia, New Mexico, was completed in conjunction with researchers from the Bureau of Reclamation (BoR) and United States Geological Survey (USGS).
A prototype bracket has been modified to accept an orthogonal transducer that will allow for simultaneous backscatter measurements in concert with the attenuation measurements. During the summer, a second long-term deployment will be installed on the Rio Grande Floodway at San Acacia, New Mexico in conjunction with researchers from the BoR and USGS.
Accomplishments
1. Surface based vibration sensors to map the degree of soil compaction as a function of depth. The University of Mississippi in collaboration with ARS researchers at Oxford, Mississippi, have developed a method using only surface based vibration sensors to map the degree of soil compaction as a function of depth. Soil compaction induced by the usage of agricultural machinery can reduce crop yields in field areas where this effect becomes pronounced. The assessment and delineation of compacted soils and plow plans are needed to define tillage protocols, fertilizer applications, and irrigation schedules within a soil management plan. The method sends vibrations into the soil and records these vibrations at various distances from the vibration source using ground surface sensors. Theses vibrations get modified as they propagate through the soil and are used to produces images of the soil profile in terms of soil stiffness. The instrumentation developed during this research can produce the higher frequency vibrations required for imaging the soil profile with appropriate resolution. Furthermore, it was found that compaction causes significant changes in our measured property, the shear wave velocity. This accomplishment provides agricultural engineers and soil scientist with a non-destructive field mapping tool for compaction assessment and soil imaging. Use of the spatial distribution compacted soils could allow for more judicious use of fertilizers and water resources with increase crop yields.
