Progress 09/01/06 to 08/31/10
Outputs
OUTPUTS: This project had three main phases, each marked by a different scale experiment. In the first two years, an experimental tank was designed and constructed, soil was prepared, and multi-frequency ground penetrating radar (GPR) data were acquired in the tank, as described in previous reports. The third year, the tank apparatus was disassembled, equipment for acquiring data in the field was constructed, and the second experiment, a plot-scale infiltration and drying study, was performed using GPR and time domain reflectometry (TDR) techniques. This experiment provided a better understanding of GPR groundwaves in natural soils and allowed testing of some parameters that could not be investigated in the tank. The fourth year, the knowledge gained from the first two experiments was applied in two agricultural field studies where data were acquired using a variety of geophysical techniques including GPR, electrical conductivity, magnetic susceptibility, electrical resistivity, and TDR. For each of the field studies, geophysical data, soil texture, and gravimetric water content measurements were acquired in at least two different field campaigns under relatively dry and under wet soil conditions. Although both field sites had considerable soil texture heterogeneity, one site was primarily composed of fine-grained soils, while the other was primarily coarse-grained; the differing soil textures allow assessment of the differences in geophysical responses under a variety of soil conditions. The geophysical data and soil information were geostatistically analyzed to determine which geophysical method was optimal for soil moisture and texture characterization and to investigate correlations between different types of geophysical data. This project has allowed the PI to teach and mentor fifteen undergraduate students who have participated in this research. While a few students helped primarily with data acquisition, most students were involved in experiment planning, construction of equipment, data processing and interpretation, and dissemination of results. Most students participated in the project for two or more years, while some were involved for the entire duration of the project. These students have matured greatly as scientists over the course of this research; as they gained experience and expertise, they were given greater responsibility, and by the final year of the project, some students were independently planning and implementing different aspects of the experiments. The students are universally positive about their research experience, and several are attending or applying to graduate school to continue research in soil and groundwater issues. Other students have graduated and are starting their careers in environmental or agricultural fields. In addition to helping students who worked directly on this project, this research has also provided teaching opportunities for the public and other undergraduate students, as three public demonstrations of the research were given, and the research was discussed in several classroom lectures. PARTICIPANTS: Katherine Grote (PI): Planned and implemented experiments, processed and interpreted data, authored publications, selected, trained, and supervised undergraduate students working on this project. Cale Anger (undergraduate student): Helped to plan experiments, acquired, processed, and interpreted data, helped with dissemination of results. Anna Baker (undergraduate student): Helped to plan experiments, acquired, processed, and interpreted data, helped with dissemination of results. Anya Benda (undergraduate student): Helped to plan experiments, designed and built equipment, acquired, processed, and interpreted data, helped with dissemination of results. Taylor Crist (undergraduate student): Helped to plan experiments, designed and built equipment, acquired, processed, and interpreted data, helped with dissemination of results. Bryan Hardel (undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. Jacob Heimdahl (undergraduate student): Processed and interpreted data. David Hon (undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. Brian Jordan (undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. Bridget Kelly (undergraduate student): Helped to plan experiments, designed and built equipment, acquired, processed, and interpreted data, helped with dissemination of results. Michael Kristoff (undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. Audrey Mohr (undergraduate student): Helped to plan experiments, designed and built equipment, acquired, processed, and interpreted data, helped with dissemination of results. Troy Moseley(undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. Crystal Nickel (undergraduate student): Helped to plan experiments, designed and built equipment, acquired, processed, and interpreted data, served as assistant coordinator in PI's absence, helped with dissemination of results. Christopher Olson (undergraduate student): Acquired, processed, and interpreted data and designed and built equipment. Herald Schultz (undergraduate student): Acquired, processed, and interpreted data and helped with dissemination of results. The University of Wisconsin-Eau Claire (UWEC) provided financial and in-kind support for this project. UWEC contributed $73,900 to equipment purchases, provided laboratory space at a cost of $24,000, and contributed $30,000 of additional undergraduate student salary so more students could be involved with the project. UWEC also released Dr. Grote from 25% of her teaching duties for two academic years to allow her more time to focus on research. The University of Wisconsin Agricultural Research at Spooner, WI provided a field site and irrigation equipment for some of the research performed in the fourth year of the project. TARGET AUDIENCES: The final target audience, geophysicists and agricultural water managers, have been reached though publication of results in high-impact journals. More information will be available to this audience as other manuscripts describing this research (currently in preparation) are submitted to journals. The other target audience, the 15 undergraduate students who have worked on this project, have benefited greatly from their involvement with this research. The students have learned scientific knowledge specific to this discipline (geophysical theory, data acquisition and processing, soil sampling and processing techniques, movement of water in the vadose zone) and have also developed skills in experiment planning, implementation, revision, and documentation. They also learned how to design and construct equipment, repair equipment in the field, and coordinate research activities within a large team. Since the beginning of the project, the students have grown tremendously as scientists, and all are continuing in careers as scientists, either in graduate school or as employees in government or environmental consulting agencies. This project has also contributed to classroom instruction at the undergraduate level for several different courses on an annual basis. In an upper-division geophysics course, the students were able to use the equipment in supervised laboratory exercises, after classroom instruction in geophysical theory and techniques. In hydrogeology courses, the data acquired as part of this project were used as examples for teaching concepts of water flow in the vadose zone. In introductory environmental geology courses, this project was described during discussions of water scarcity and allocation. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Each of the three experiments described above has resulted in new knowledge that increases the efficacy of GPR techniques for soil moisture and texture characterization. The first experiment, conducted under controlled soil moisture and texture conditions within a large tank, showed that the sampling depth of the GPR groundwave is frequency dependent, with an inverse relationship between sampling depth and frequency. This experiment also showed that the sampling depth is slightly dependent upon soil moisture, but less so than was previously expected from numerical modeling. These experimental results show that multi-frequency GPR groundwave data can be used to estimate soil moisture over different depths (and thus to develop a vertical soil water content profile over a field) and that the effectiveness of this technique does not depend on soil moisture conditions. The second experiment, a plot study of GPR and TDR measurements acquired during infiltration and drying, was partly undertaken to determine if the results of the first experiment, which assumed a sharp vertical transition between dry and saturated soils, were still valid under more natural conditions with a gentle soil moisture gradient. The second experiment showed that the GPR groundwave sampling depth is still frequency dependent under a natural soil moisture gradient, although the absolute sampling depths are somewhat different from those determined in the tank. The second experiment also provides new information on data acquisition methods for GPR groundwaves. Data in this study were acquired at five different common-offset antenna separations, since some numerical models showed that antenna separation affected sampling depth. Our results showed that antenna separation does not significantly influence sampling depth, so whatever antenna separation provides maximum groundwave clarity is appropriate for data acquisition. Secondly, we compiled the data acquired with multiple antenna separations to create variable-offset surveys, which provide a method of estimating the groundwave velocity (and thus soil moisture) without the calibration problems sometimes associated with common-offset data. We also determined that the positioning of the antennas (transmitter preceding or following the receiver) can affect calibration, so a consistent antenna position must always be used. The final experiment, where several different types of geophysical data were acquired, provided information on the optimal techniques for soil texture estimation as well as for water content monitoring. Although data processing and interpretation for this last experiment is ongoing, the results found thus far show that GPR amplitudes do not correlate sufficiently well with electrical conductivity (EC) measurements (from electromagnetic induction) to be used to map EC across a field. However, the results show that GPR travel time measurements provide the best estimates of soil texture and water content. This finding is quite significant, as EC measurements are currently the most popular geophysical technique for mapping soil texture, but GPR groundwave techniques may be more effective.
Publications
- Grote, K., Crist, T. and Nickel, C. (2010), Experimental estimation of the GPR groundwave sampling depth, Water Resour. Res., 46, W10520, doi:10.1029/2009WR008403.
- Grote, K., Anger, C., Kelly, B., Hubbard, S., and Rubin, Y. (2010), Characterization of soil water content variability and soil texture using GPR groundwave techniques, Journal of Environmental and Engineering Geophysics, 15(3), 93-110.
- Crist, T., Benda, A., and Grote, K. (2010), Experimental determination of GPR groundwave sampling depth as a function of data acquisition parameters, EOS. Trans. AGU 91(55), Fall Meet. Suppl., Abstract H13D-0999.
- Mohr, A., Kristoff, M., Crist, T., Benda, A., and Grote, K. (2010), Comparison of Geophysical Techniques for Soil Texture Estimation, EOS. Trans. AGU 91(55), Fall Meet. Suppl., Abstract H13D-1005.
- Nickel, C., Crist, T., Peterson, S., Benda, A., and Grote, K. (2010), Three dimensional estimation of volumetric water content using multi-frequency GPR groundwave data, GSA North-Central/South-Central Joint Annual Meeting, Abstracts with Programs 42(2) Paper 44-8.
- Nickel, C., Crist, T., Peterson, S., and Grote, K. (2010), Three dimensional vadose zone characterization of a Wisconsin orchard using electromagnetic techniques, WI Groundwater Association Annual Conference.
- K. Grote (2009), Experimental estimation of the GPR groundwave sampling depth in variably saturated soils, Bouyoucos Conference on Agricultural Geophysics.
- Moseley, T., Hardel, B., Nickel, C., and Grote, K. (2009), Soil moisture estimation using air-launched GPR techniques, WI Groundwater Association Annual Conference.
- Baker, A., Kelly, B., and Grote, K. (2009), Estimating GPR groundwave penetration depth in variably saturated soils, American Water Resources Association, Wisconsin Chapter.
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Progress 09/01/08 to 08/31/09
Outputs
OUTPUTS: During this year, data acquisition at the laboratory was completed, and two field experiments were performed. In the laboratory, GPR groundwave data were acquired over the third soil type (clayey silt) under dry and saturated conditions. Air-launched GPR data were also acquired to determine whether these techniques could accurately measure soil water content under non-ideal data acquisition conditions. The experimental apparatuses were then disassembled. Processing the laboratory data showed that two goals of the experiment, estimating electrical conductivity (EC) from GPR groundwave amplitudes and evaluating the effects of antenna separation on penetration depth, were not possible using the laboratory data. In the experimental tank, the layered structure of the soil created reflected and refracted waves that frequently arrived at the same time as the groundwave. The superposition of these waves made quantitative use of groundwave amplitudes impossible and limited the number of traces available for antenna separation analysis. To continue to pursue these goals, a new field experiment was performed. In the first field experiment, GPR surveys were acquired over a test area with laterally uniform soil properties during infiltration and drying. TDR probes monitored the vertical water content profile within the test area. A sled system was designed to allow simultaneous acquisition of multi-frequency, constant-offset GPR data. GPR data were acquired in sets, where each set consisted of common-offset data acquired at five antenna separations for each of four frequencies. For each set, the locations of the transmitting antennas across the traverse did not change, thus allowing the GPR data to be analyzed in two ways: as common-offset data with variable antenna separations and as variable-offset data acquired at many locations. GPR data were acquired over relatively dry soil at the beginning of the experiment, then the soil was uniformly saturated using a sprinkler system. GPR data were acquired periodically as the infiltration front moved through the soil and as the soil dried after infiltration. By comparing the TDR measurements at different depths with the GPR data, the penetration depth can be determined as a function of GPR frequency and antenna offset. The second field experiment was performed at a 7 acre field site. Multi-frequency GPR data were simultaneously acquired over four traverses. Electromagnetic data were acquired along these traverses at two depths using a Geonics EM38DD, and vertical profiles of the gravimetric water content and soil texture were acquired at five locations. These data will be used to create a 3-D map of soil water content and to compare maps of EC obtained with the EM38DD and GPR amplitudes. This year primarily focused on experimental design and data acquisition. A large volume of data is ready for processing and analysis. Ideally, data processing would occur simultaneously with data acquisition, but limited temporal availability of laboratory space and favorable weather conditions forced us to focus on data acquisition. The next year will focus on data processing and continued but reduced data acquisition. PARTICIPANTS: Katherine Grote (PI): Dr. Grote was responsible for overseeing all aspects of this project. She recruited four new undergraduate students to work on the project, then trained them in the scientific background of the project and the techniques for data collection and processing. She also trained six undergraduates continuing as members of the research team with new data acquisition and processing methods as the focus of the experiment shifted to field work. Dr. Grote supervised data acquisition, organized the disassembling of equipment after laboratory work was completed, and planned field experiments. She also aided with design and construction of new equipment, selected field sites, designed procedures for calibrating field equipment, and assisted with data acquisition in the field. Dr. Grote processed data and trained undergraduate students in advanced data processing techniques. She prepared two manuscripts that were submitted to journals, helped to prepare four conference presentations, and oversaw student presentations at these conferences. Dr. Grote was also responsible for purchasing supplies, finding vendors, maintaining financial records, and seeking additional funding for student support. Anna Baker, Anya Benda, Taylor Crist, Bryan Hardel, Jacob Heimdahl, Bridget Kelly, Crystal Nickel, Christopher Olson, Shane Peterson, and Herald Schulz are undergraduates who have worked on various aspects of this project in this last year. Some students assisted with experiments performed in the laboratory, including soil preparation, GPR data acquisition, TDR installation and monitoring, and gravimetric water content monitoring. These students also assisted with GPR data processing using seismic data processing techniques. Other students participated in both laboratory work and field experiments. Crist, Kelly, Nickel, and Olson worked to design and construct new equipment that was used in field experiments, and Crist and Nickel worked extensively on experimental design and implementation for one of the field projects. Benda, Crist, Nickel, and Peterson were also very involved in planning, implementing, and processing data for the second field experiment, where all students assisted in data acquisition of GPR, electromagnetic, soil texture, and gravimetric water content data. Most of the students also presented the results of their results at professional conferences or a local undergraduate research event. The University of Wisconsin-Eau Claire (UWEC) provided financial and in-kind support for this project. UWEC provided laboratory space at an annual cost of $12,000 and contributed over $6,600 in additional student salary to help support undergraduate students. TARGET AUDIENCES: Ten undergraduate students have been extensively involved with this project. These students have been trained in the scientific principles and techniques involved in this research and have also learned how to conduct scientific experiments, including planning, implementation, revision, documentation, equipment design and construction, data analysis, and presentation of results. These students have grown as scientists through these experiences. Two of the students who have recently graduated from UWEC have entered graduate school and are studying research topics in the same field as this research. Two other students who are nearing graduation are planning to attend graduate school, while others are considering this option. Other students who have graduated and found employment in industry are using skills with geophysical equipment learned through this research. In addition to helping the students directly involved in this project, this research has also been utilized for classroom instruction. The techniques and theories of the geophysical methods were discussed in the classroom, then students were able to use the geophysical equipment in supervised laboratory exercises. In other classes, data acquired in this experiment were used as examples for teaching concepts of fluid movement through the vadose zone. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The primary outcomes from the third year of this project are a more accurate data processing method for experimental data acquired in the tank, an improved understanding of the role of soil texture in groundwave penetration depth, and a methodology for acquiring simultaneous, multi-frequency GPR data in the field. The new data processing method allows for more accurate evaluation of groundwave data when superposition with reflected and refracted waves is problematic. This method uses data acquired when reflection and refraction waves are present but not superimposed with the groundwave (these conditions are usually met when the overlying soil layer is relatively thick) to identify portions of the groundwave that can be used for velocity estimation when superposition does occur. These portions of the groundwave may be smaller lobes of the groundwave wavelet that arrive earlier in time or segments of the main peak or trough of the groundwave wavelet that do not experience superposition with other waves. Applying this data processing method, data were analyzed for the sand and sandy loam soils under dry and saturated conditions. Comparing the penetration depth estimates for each frequency for these soils showed that soil texture causes variations in the penetration depth of 3 cm or less, indicating that soil texture does not significantly influence the penetration depth. This finding increases the likelihood that multi-frequency GPR groundwaves can be used as an efficient field tool for soil moisture estimation in heterogeneous soil texture conditions. The third larger accomplishment this year was to design and construct a sled system that could be used with a multi-channel adapter to simultaneously acquire multi-frequency GPR data with four pairs of antennas. The sled system designed for this project allows transmitter-receiver pairs to be connected in an "antenna train", so each antenna passes over the same portion of the field as data as the train is pulled along the traverse. The system also allows the transmitting and receiving antennas to be separated a chosen distance, so the separation distance can be increased when estimates of water content from a large area are desired and can be decreased in highly attenuative soils, when the groundwave signal would not be detectable at long separation distances. This adaptation is a significant improvement over currently commercially available sled systems, where the common-offset antenna separation is fixed at whatever distance is considered optimal for reflections. The new sled system also allows the separation distance between antenna pairs to be adjusted, so the starting locations of each antenna pair can be chosen precisely if variable-offset data are desired. This system was used successfully in the small-scale plot experiment and in a larger-scale field study, when it was towed behind a tractor.
Publications
- Grote, K., Crist, T. and Nickel, C. (2009). Experimental Estimation of the GPR Groundwave Sampling Depth. Water Resources Research. (submitted)
- Grote, K., Anger, C., Kelly, B., Hubbard, S., and Rubin, Y. (2009). Geophysical Characterization of Soil Water Content Variability and Soil Texture. J. of Environmental and Engineering Geophysics. (submitted)
- Hardel, B., Kelly, B, and Grote, K. (2008). Estimation of Soil Moisture Content Using Air-Launched GPR Techniques in Variable Soil Conditions, EOS. Trans. AGU 89(53), Fall Meet. Suppl., Abstract H51G-0929.
- Crist, T., Nickel, C., and Grote, K. (2008). Comparison of the Penetration Depth of GPR Groundwaves in Saturated and Dry Soil, EOS. Trans. AGU 89(53), Fall Meet. Suppl., Abstract H51G-0942.
- Moseley, T., Hardel, B., Nickel, C., and Grote, K. (2009). Soil Moisture Estimation Using Air-Launched GPR Techniques, Electronic conference proceedings, Wisconsin Ground Water Association Annual Conference, Milwaukee, WI.
- Baker, A., Kelly, B., and Grote, K. (2009). Estimating GPR Groundwave Penetration Depth in Variably Saturated Soils, Electronic conference proceedings, American Water Resources Association, Wisconsin Chapter, Milwaukee, WI.
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Progress 09/01/07 to 08/31/08
Outputs
OUTPUTS: Most of the activity during the second year of this project has focused on data collection within the experimental tank. The first phase of the experiment (where GPR data were collected over dry sand) was completed, and the second phase (where GRP data would be collected over moist soil) was begun. The experimental plan directed that data would be collected under dry, moist, and saturated conditions. However, after some experimentation, it became obvious that the large volume of soil within the tank could not be kept at a homogenous, intermediate water content for the length of time needed to complete an experiment, as would be needed for "moist" conditions. Therefore, it was decided to collect data under only dry and saturated conditions, as these conditions would provide an upper and lower bound to groundwave penetration depths. Additional laboratory procedures were developed to uniformly saturate the soil and maintain it under saturated conditions for the duration of the experiment. Data collection was then performed for saturated sand. After that phase of the experiment was completed, an organic-rich sandy loam soil was obtained from a local farm field and was oven-dried and pulverized. Data collection using the dry loam proceeded smoothly. Some procedural modifications were implemented for the saturated loam, as the previous method of wetting the soil (mechanically mixing the soil with water until the desired water content was obtained) was problematic in a soil with a significant fine-grained component. A calibrated sprinkler system was developed to add water to each layer of soil within the tank until the desired water content was obtained, as determined by repetitive gravimetric sampling at several locations throughout the tank. The last soil tested was a clayey silt. This soil was dried and pulverized, then data collection was performed. This soil will be tested under saturated conditions using the sprinkling system during the third year of the project. As data were being acquired for each soil texture and moisture condition, data processing and analysis continued. Data collection and processing have required considerably more time than first anticipated, but this has proven advantageous in allowing more undergraduate students to participate in research and receive mentoring through this project. Nine undergraduate students were trained in the theory and techniques of the experiment during the past year, and they then participated in data collection and processing. Three students presented results of this research at a national conference, while three other students presented aspects of the research at a regional conference. The students also gained experience presenting this research at a local undergraduate research event. In addition to the work at the laboratory and data processing, students have also gained experience in selecting and characterizing a field site. A field site near campus has been identified, and two students collected soil samples and electromagnetic data at this site to determine the extent of soil heterogeneity. PARTICIPANTS: Katherine Grote (PI): Dr. Grote was responsible for overseeing all technical and managerial aspects of this project. She recruited nine undergraduate students to work on the project, then trained them in the scientific background of the project and the techniques for data collection and processing. She created data collection plans, scheduled students, and oversaw and aided with data collection and processing. Dr. Grote also implemented techniques to make data collection more efficient and worked closely with students to find solutions to problems. She also worked very closely with students to analyze the data and to prepare presentations for professional conferences. In addition to her work with students, Dr. Grote was also responsible for obtaining materials needed for the experiment, maintaining financial records, and data processing and analysis. Cale Anger, Anna Baker, Taylor Crist, Brian Jordan, Bryan Hardel, Bridget Kelly, Crystal Nickel, Christopher Olson, and Herald Schulz are undergraduates who have worked on various aspects of this project. Each student assisted with soil preparation, including soil drying, pulverizing, mixing, and leveling. All students also collected GPR data, installed TDRs, collected and processed gravimetric water content samples, and monitored moisture content in the tank by analyzing TDR data. Most students processed GPR data using seismic data processing techniques. Each of the students presented the results of this research at a professional conference or local undergraduate research event. Baker, Crist, Kelly, and Olson worked closely with Dr. Grote to develop a calibrated sprinkler system to saturate fine-grained soils. Olson began developing a system to allow GPR data to be acquired at any antenna separation in the field. At the field site, Baker and Crist performed soil sampling and acquired electromagnetic data; they also created a site map using these data. Baker, Crist, Kelly, Hardel, Jordan, Nickel, and Olson also assisted with soil characterization procedures for the soils used or considered for use at the laboratory. The University of Wisconsin-Eau Claire (UWEC) has provided financial and in-kind support for this project. UWEC is providing laboratory space at an annual cost of $12,000 and has contributed $3,900 to equipment costs. UWEC also released the PI from 25% of her teaching duties to allow her to focus more extensively on this research. Finally, UWEC contributed over $6,600 in additional student salary to help support undergraduate students TARGET AUDIENCES: Nine undergraduate students have been extensively involved with this project. These students have been trained in the scientific principles and techniques involved in this research and have also learned how to conduct scientific experiments, including planning, implementation, revision, documentation, analysis, and presentation of results. These students have grown as scientists through these experiences. Several of the students are now considering graduate school so they can continue with research. In addition to serving the nine students who work on this project, this research has also been utilized for classroom instruction. The techniques and theories of the geophysical methods were discussed in the classroom, then students were able to use the geophysical equipment in supervised laboratory exercises. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The primary outcome for the second year of this project has been an improved understanding of the penetration depth of the GPR groundwave as a function of soil water content and soil texture. During this year, multi-frequency GPR data were collected over two soil types: a clean sand and an organic-rich sandy loam. Data were collected over dry sand in the first year, and this year data were collected over wet sand, dry loam, and wet loam. These data show that the GPR penetration depth changes as a function of frequency for each soil, but that the penetration depth does not appear to be highly dependent upon soil water content or texture. While the sensitivity to frequency and the penetration depth for each frequency is in the range of what is predicted by different numerical models, the apparent independence of penetration depth from water content is different from that predicted by theory, as most numerical models show a significant decrease in penetration depth for very wet soils. The experimental results for both of the soils tested show that the penetration depth is approximately equal in wet or dry soils. The unexpectedly high penetration depth in wet soils may be partially caused by preferential channeling of some portion of the GPR energy into the deeper, higher-velocity dry layer. The possible effects of channeling will be investigated during the field portion of this experiment, when soil moisture changes will be more gradational and will be less likely to induce channeling than the artificially sharp interface between the wet and dry soil in the laboratory setting. The apparent independence of penetration depth from soil water content or soil texture increases the likelihood that GPR groundwaves could be used as a convenient field tool, since calibrations for variable soil texture or moisture content seem unnecessary. Data collected this year show a penetration depth of ~12 cm for the 1000 MHz antennas, ~24 cm for the 500 MHz antennas, ~30 cm for the 250 MHz antennas, and ~42 cm for the 100 MHz antennas. Depth estimates for the 250 and 100 MHz antennas are based only on dry and wet sand; data analysis of the loam soils for these frequencies is ongoing. Although the primary focus of this research is to investigate the groundwave penetration depth, air-launched GPR data (where both antennas are held in the air and the reflection amplitude is analyzed) were also acquired for the first few layers that were added to the tank. The air-launched data were analyzed to determine if they would provide accurate water content estimates and if the penetration depth of air-launched data varied with frequency. Only data from the dry sand have been processed thus far, but these data show that the air-launched water content estimates are fairly accurate and that the air-launched penetration depth is between 6 and 9 cm for the three frequencies tested (250, 500, and 1000 MHz.) Thus, a combination of air-launched and ground-coupled GPR data might provide a higher-resolution vertical water content profile than could be achieved using only ground-coupled data.
Publications
- Anger, C., A. Baker, and K. Grote (2007). Experimental Estimation of the Penetration Depth of the GPR Groundwave, EOS. Trans. AGU 88(52), Fall Meet. Suppl., Abstract H23A-1015.
- Kelly, B.B. and K. Grote (2007). Comparison of Soil Moisture Content Estimated with Air-Launched and Ground-Coupled GPR Techniques, EOS. Trans. AGU 88(52), Fall Meet. Suppl., Abstract H23A-1017.
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Progress 09/01/06 to 08/31/07
Outputs
OUTPUTS: Our activity during the first year of this project has focused on preparing a laboratory space, constructing the experimental apparatus, and collecting data for the first phase of the experiment. The experiment was delayed by a few months until a suitable laboratory space was found. Once a laboratory space was obtained, laboratory equipment (industrial oven, soil mixer, etc.) was acquired and modified to allow it to function within this space. Other modifications to the equipment were made to accommodate the soils used in this experiment. A Ground Penetrating Radar (GPR) system was acquired, and seismic analysis software was purchased, since currently available GPR software does not contain the processing techniques needed for this experiment. Additional software was developed to modify the GPR data format for processing with the seismic software. Next, the experimental tank was designed. To determine the dimensions of the tank, we used a finite difference GPR model to
simulate data collection with different GPR frequencies. These models were used to estimate the necessary tank dimensions to avoid interference from the tank sides while providing adequate length for data collection. We also tried to determine the necessary tank dimensions experimentally by acquiring multi-frequency GPR data in traverses leading up to a retaining wall. After analyzing these data, we designed the tank in consultation with geotechnical and material science engineers. Materials for the tank were acquired, and the tank was constructed. Next, fixtures were constructed to aid in placing the soil in the tank, to flatten the soil in the tank without compacting it, and to remotely acquire GPR data within the tank. Soil was prepared for placement in the tank by drying or by wetting and mixing. The texture, homogeneity, and water content of the soil were determined by performing sieve tests, porosity analyses, and gravimetric water content measurements. Lastly, a time domain
reflectometry (TDR) probe and multiplexer system was purchased and installed; software was developed to control the TDR multiplexer data collection, and the TDR probes were calibrated. Although obtaining a laboratory space and acquiring, constructing, and calibrating the equipment took considerably longer than we had anticipated, the laboratory is now finished and data collection is proceeding smoothly. Three undergraduate students helped with constructing the laboratory equipment, and these students were also trained in data collection and analysis. Data collection for the first experiment, to determine the GPR groundwave penetration depth in dry sand, is complete, and data analysis is ongoing. Three undergraduate students are preparing to present the results of this first experiment at a national conference. This research was also publicized more locally in a demonstration event held at the laboratory, where approximately 40 geology students and faculty observed data collection
procedures and preliminary results. This research was also promoted in a video, "The Soil Explorers", produced by the Partners Video Magazine for CREES.
PARTICIPANTS: Katherine Grote (PI): Dr. Grote was responsible for finding a suitable laboratory space and for acquiring the equipment needed for the experiment, including evaluating different equipment options and designing modifications to the equipment as necessary. She designed the experimental tank and the additional structures needed to perform the experiment, and she developed the software needed to collect and process data. She also created procedures for data collection and processing and trained three undergraduate students in these procedures. Dr. Grote supervised these students during the construction of the experimental apparatus and in subsequent data collection, processing, and interpretation. Cale Anger: Mr. Anger was one of three undergraduate students who helped with construction of the experimental apparatus. He designed and built some of the necessary fixtures and contributed extensively to preparing the soil for the first experiment. He also helped with data
collection throughout the first experiment. Anna Baker: Ms. Baker was one of three undergraduate students who helped with construction of the experimental apparatus. She spent considerable time preparing the soil for the first experiment and collecting data during the experiment. She also helped with data processing. Bridget Kelly: Ms. Kelly was one of three undergraduate students who helped with construction of the experimental apparatus. She assisted with soil preparation and data collection, and she has contributed extensively to data processing and interpretation. The University of Wisconsin-Eau Claire (UWEC) has provided financial and in-kind support for this project. UWEC is providing 1,400 square foot of laboratory space and has contributed over $70,000 in equipment costs. UWEC also released the PI from 25% of her teaching responsibilities to allow her to focus more extensively on this research.
TARGET AUDIENCES: Three undergraduate students have received mentoring in experimental planning and design and have been trained in the theory, data collection, data processing, and interpretation of multiple geophysical techniques. These students have grown professionally as they participated in experimental design and implementation and were introduced to scientific material that is much more advanced than what they would learn in undergraduate coursework.
Impacts
The primary outcome for the first year of this project is a change in knowledge of the GPR groundwave penetration depth. Data analysis for the first experiment is ongoing, so only preliminary results are currently available. The objective of the first experiment was to determine the GPR groundwave penetration depth in dry sand, so multi-frequency GPR data were collected as 3-cm layers of dry sand were placed over a basal layer of saturated sand. Analysis of these data showed that the groundwave penetration depth varied as a function of frequency. The 1000 MHz antennas had the shallowest penetration depth, while the 100 MHz antennas had the deepest groundwave penetration. The preliminary penetration depth estimates for each frequency in dry sand are as follows: 1000 MHz, 6 cm; 500 MHz, 9 cm; 250 MHz, 25 cm; and 100 MHz, 36 cm.
Publications
- No publications reported this period
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