Location: Watershed Physical Processes Research
2022 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
This is the final report for project 6060-13000-027-000D, “Utilizing Acoustic and Geophysics Technology to Assess and Monitor Watersheds in the United States” which ended in 03/10/2022. This project has been replaced by new project #6060-13000-031-000D, “Acoustic and Geophysical Methods for Multi-Scale Measurements of Soil and Water Resources” which started 03/02/2022.
Progress on Objective 1: A laboratory measurement system that combined the high-frequency method and the seismoelectric technique was built to explore the soil profile under controlled hydraulic conditions in a water tank. This method consisted of an optical vibration detector controlled by a stepper motor. An electrode array was placed on the soil surface to record electrical signals induced by seismic waves. A number of time domain reflectometers sensors were buried at different depths to measure water contents. The tank has the capability to control water level. After the completion of the measurement system, many efforts have been made to record the seismoelectric signals. However, it was found that the seemingly well-received signals were later identified as fake signals, which are known as cross-talk. It was then concluded that the seismic-electrical signals for unsaturated soil are so weak that they are essentially non-detectable. Based on this conclusion, the task was terminated.
Progress has been made on the geophysical characterization and investigation of the internal structures of earthen dams. Multiple geophysical methods have been successfully applied to investigate soil pipe formations, void formations under concrete spillway slabs, and slide failures on dams across Mississippi. Significant progress was also made in establishing correlations between soil geophysical and geotechnical properties using an artificial neural network (ANN). These correlations, developed by conducting laboratory geophysical and geotechnical tests (including hydrological and physical properties) on remolded soil samples, lead to an improved interpretation of geophysical data.
Progress on Objective 2: Acoustic measurement systems to quantify sediment flux in fluvial environments were broken into two categories: the use of passive audible-range acoustics to monitor coarse bedload transport and the use of active ultrasonic acoustics to monitor suspended fine particles. For the passive monitoring of coarse bedload transport, hydrophones and data acquisition hardware were deployed in three gravel-bed river systems alongside physical sampling. In addition, several types of experiments were conducted attempting to account for the noise due to flowing water. Analysis of the data from these experiments provided no universal method for converting acoustic signals from gravel motion into sediment load for the field deployments. For the active acoustic monitoring of suspended fines particles, the Single Frequency Acoustic Attenuation System was deployed in the Goodwin Creek Experimental Watershed in Panola County, Mississippi alongside physical pump sampling. The system was used to collect data for 16 flow events between November 2019 and March 2020. The sediment loads measured by the Single Frequency Acoustic Attenuation System compared well with the sediment concentration measured by the physical pump samples.
Accomplishments
1. A surface wave-based soil profiler for near surface soil exploration was developed. In agricultural applications and farmland soil management, it is important to have a non-invasive tool to measure soil mechanical and hydrologic properties, map, and monitor their temporal and spatial variations. To achieve this goal, an acoustic surface wave technique, the high-frequency multichannel analysis of surface wave method, has been developed, by ARS researchers in Oxford, Mississippi, to noninvasively measure soil profiles, i.e., the shear wave velocity as a function of depth, up to 2 meters below the surface. This technique is based on the well-established relationship among the acoustic velocity and soil mechanical and hydrologic properties. Over the last five years, this technique has been applied to many agricultural applications, including (1) near surface soil profiling, (2) a long-term-survey for studying weather and seasonal effects, (3) a short-term monitoring of rainfall events, (4) detecting fragipan layers, (5) studying surface sealing/crusting, and (6) a farmland compaction study. These studies demonstrated that the technique is an effective non-invasive tool to measure soil subsurface mechanical and hydrological properties. An on-going research goal is to evaluate the performance of irrigation using the technique. This technique will be further improved to develop a portable device that can be employed in the field quickly. With the translation and commercialization of the device, the potential users could be soil scientists, famers, environment researchers, and military personnel.
2. Geophysical integrity assessment of earthen dams and Levees. A thorough investigation of the current state of the nation's aging earthen dams and levees is required for an adequate rehabilitation or removal plan. A sudden failure of earthen dams and levees due to subsurface problems could lead to catastrophic consequences. Implementation of multiple surface-based non-invasive geophysical methods for the integrity assessment of earthen dams and levees is being used to address this problem. In order to develop better methods for assessing earthen embankment integrity, ARS researchers in Oxford, Mississippi, conducted studies at Carroll County Dam, Big Sand Watershed Structure Y-32-12, and Town Creek Watershed Structure 19. Results showed that geophysical methods provide spatial information on the internal structure of earthen embankments that are not typically observed through visual or traditional geotechnical approaches. In a broader context, these geophysical methods can target and complement invasive geotechnical methods leading to an economical early mitigation action.
3. Agrogeophysical methods for mapping internal soil pipes on agricultural watersheds. Internal soil pipes directly contribute to the total soil loss from agricultural fields, threatening agricultural sustainability. However, the amount of soil erosion caused by subsurface processes has often been overlooked. Locating, measuring, and mapping internal soil pipes and their networks is a methodological problem in soil science, and an advance in this area is vital to assessing the total soil loss in agricultural fields. To address this problem, ARS researchers in Oxford, Mississippi, conducted surface-based, non-invasive, and expedient geophysical measurements on a soil pipe-affected agricultural field in the Goodwin Creek experimental watershed in Panola County, Mississippi. The study showed that geophysical measurements such as ground-penetrating radar (GPR) are effective indicators of soil pipe formations and provide the depth and pathway of soil pipe networks. Electromagnetic induction (EMI) methods can quickly cover large agricultural areas and provide zoning information with varying degrees of soil pipe susceptibility. In addition, this study showed that an optimized and cost-effective EMI survey parameter can be selected by applying an artificial neural network. Findings from this research may be useful to land managers, farmers, and engineers who need to evaluate land for susceptibility to erosion.
4. Forecasting geotechnical parameters from geotechnical properties using artificial neural network (ANN) models. A better interpretation of geophysical methods requires an improved understanding of the correlation between soil geophysical and geotechnical properties. Therefore, translating geophysical parameters to engineering parameters is needed to facilitate the use of geophysical data in the broader scientific community. To identify and study such correlations, an approach of using an artificial neural network (ANN) on data from laboratory geophysical and geotechnical measurements on remolded soil samples was performed by ARS researchers in Oxford, Mississippi. The geophysical and geotechnical measurements were conducted on multiple soil samples with varying clay, sand, and silt proportions commonly used for earthen dam construction. This study developed ANN models that predict correlations between geophysical and geotechnical parameters with better accuracy than traditional regression analysis to allow prediction of geophysical parameters from geotechnical parameters. The sensitivity of these predictions to changes in the geophysical parameters was also investigated. Findings from this research can help assess the strength of earthen structures based on soil properties. Soil scientists and geotechnical engineers can use this methodology to determine geotechnical parameters for their scientific studies and engineering designs.
5. Development of technology for detecting gravel movement in rivers. Gravel bedload in rivers is difficult and expensive to measure, but it is necessary information for hydraulic engineers and river managers. In order to investigate the potential for using the sound made by gravel impacts in a river, hydrophones and data acquisition hardware were deployed in three gravel-bed river systems: Halfmoon Creek near Leadville, Colorado, the Elwha River near Port Angeles, Washington, and the Trinity River near Weaverville, California. Each deployment was conducted in conjunction with physical sampling of the bedload in transport. It was found that times of increased bedload were generally accompanied by increases in the amplitude of sound, but ambient sounds and interference from turbulence on the hydrophone mounting structure reduced the quality of the data. ARS researchers in Oxford, Mississippi, concluded that the method may be best suited to identify times when bedload is moving rather than the quantity of transport. This technology is useful for river managers who need to determine the onset and end of gravel transport automatically in remote locations on streams and rivers.
6. Deployment of Single Frequency Acoustic Attenuation System at Goodwin Creek Watershed. Monitoring the suspended sediment that is transported in rivers and streams can be difficult and expensive and results in very temporally coarse measurements. The modified acoustic attenuation unit, which can be remotely deployed, collects near-continuous acoustic data that can be used to estimate the concentration of suspended sediment. ARS researchers in Oxford, Mississippi, installed the system on a concrete structure at the Goodwin Creek Watershed alongside a dedicated pump sampling line. The high temporal resolution of the Single Frequency Acoustic Attenuation System provided valuable sediment transport data that cannot be provided by traditional pump samplers. The successful data collection effort resulted in continuous fine sediment concentration data across several storm hydrographs. The increased temporal resolution, along with the automated nature of the measurements, are a significant improvement on traditional sampling methods that require personnel to be on site during storm events. The technology may be used for measuring fine sediment concentration in research efforts and eventually for monitoring fine sediment concentration in streams for agencies such as state departments of environmental quality or the U.S. Geological Survey.
7. Investigation of the effect of algae on acoustic attenuation. Algae in the water column can interfere with acoustic signal propagation, making acoustic measurement of fine sediments in the water column ineffective in some locations. During a multi-year study on the Middle Rio Grande near San Acacia, New Mexico, an unexpected daily pattern was found in acoustic attenuation signals. There was no way to account for the pattern based on expected trends in sediment concentration, river stage or discharge. In the absence of a hydraulic explanation for the periodic fluctuations in acoustic signal, ARS researchers in Oxford, Mississippi, hypothesized that biological factors, such as population variation or movement of algae, could have been the cause. This prompted a supplemental experiment at the University of Mississippi Biological Field Station in conjunction with Agricultural Research Service studying algal growth and activity. It was found that algae do affect acoustic signal propagation, and more work is under way to determine the effects can be mitigated and if acoustic signal can be used to measure algal biomass. This information is useful for river managers and hydraulic engineers who need suspended sediment data and is potentially useful to biologists and ecologists who need to measure algal biomass.
