Barry J. Allred

Handbook of agricultural geophysics

Agricultural Geophysics Overview General Considerations for Geophysical Methods Applied to Agriculture, Allred et al. Past, Present, and Future Trends of Soil Electrical Conductivity Measurement Using Geophysical Methods, Corwin History of Ground-Penetrating Radar Applications in Agriculture, Collins Agricultural Geophysics Measurements and Methods Theoretical Insight on the Measurement of Soil Electrical Conductivity, Corwin et al. Resistivity Methods, Allred et al. Electromagnetic Induction Methods, Daniels et al. Ground Penetrating Radar Methods, Daniels et al. Magnetometry, Self-Potential, and Seismic: Additional Geophysical Methods Having Potentially Significant Future Use in Agriculture, Allred et al. The Global Positioning System and Geographic Information Systems Integration of the Global Positioning System (GPS) to Agricultural Geophysics, Grejner-Brzezinska Integration of Geographic Information Systems (GIS) to Agricultural Geophysics, Merry Agricultural Geophysics Application Examples Resistivity and Electromagnetic Induction Case Histories Introduction to Resistivity and Electromagnetic Case Histories, Allred and Ehsani. Apparent Electrical Conductivity for Delineating Spatial Variability in Soil Properties, Wienhold and Doran Dependence of Soil Apparent Electrical Conductivity (ECa) Upon Soil Texture and Ignition Loss at Various Depths in Two Morainic Loam Soils in SE Norway, Korsaeth Relations Between a Commercial Soil Survey Map Based on Soil Apparent Electrical Conductivity (ECa) and Measured Soil Properties on a Morainic Soil in SE Norway, Korsaeth et al. Mapping Pesticide Partition Coefficients by Electromagnetic Induction, Jaynes Can Apparent Soil Electrical Conductivity Be Used to Map Soil-Herbicide Partition Coefficients? A Case Study in Three Colorado Fields, Shaner et al. Delineating Site-Specific Management Units Using Geospatial Apparent Soil Electrical Conductivity (ECa) Measurements, Corwin et al. Mapping of Soil Drainage Classes Using Topographical Data and Soil Electrical Conductivity, Kravchenko Productivity Zones Based on Bulk Soil Electrical Conductivity: Applications for Dryland Agriculture and Research, Johnson et al. A Four Year Summary of the Use of Soil Conductivity as a Measure of Soil and Crop Status, Eigenberg Mapping of Apparent Soil Electrical Conductivity (ECa) Helps Establish Research Plots at a New Research and Extension Center, Farahani et al. Mapping Golf-Course Features with Electromagnetic Induction, with Examples from Dublin, Ohio, Taylor Comparison of Geoelectrical Instruments in Different Soilscapes, Gebbers and Luck Ground Penetrating Radar Case Histories Introduction to Ground Penetrating Radar Case Histories, Allred. Ground Penetrating Radar Surveys Across a Prototype Surface Barrier to Determine Temporal and Spatial Variations in Soil Moisture, Clement and Ward Soil Water Content Measurement Using the Ground- Penetrating Radar Surface Reflectivity Method, Redman Assessing Spatial and Temporal Soil Water Content Variations with Ground Penetrating Radar, Lennartz et al. Ground Penetrating Radar Mapping of Near-Surface Preferential Flow, Freeland An Application of Ground Penetrating Radar in Golf Course Management, Boniak et al. Ground Penetrating Radar Investigation of a Golf Course Green: Computer Processing and Field Survey Set-Up Considerations, Allred et al. Agricultural Drainage Pipe Detection Using Ground Penetrating Radar, Allred and Daniels Using Ground Penetrating Radar to Estimate Tree Root Mass: Comparing Results from Two Florida Surveys, Butnor at al. Glossary Index

Hydrogeophysical Case Studies in the Vadose Zone

The focus of this chapter is the characterization of the vadose zone, or the unsaturated section of the subsurface, using hydrogeophysical techniques. The regions of water saturation as they relate to the physical properties are shown for reference in Figure 14.1. Characterization below the water table (in the saturated section) is described in Chapter 13 of this volume and will not be discussed in detail here. From a physical properties perspective, the zones of variable saturation above the water table are transitional, and depend upon the soil or rock type and the lateral heterogeneity of the materials. In the vertical direction, the boundaries between all of these zones are dependent upon the types of soil, regolith, or rock that are present; the current and historical climatic conditions; and the regional and local geomorphology of the site. These same factors affect the heterogeneity of the vadose zone in the horizontal (lateral) direction and generally compound the problems of defining the different regions of moisture in the vadose zone.

DETECTION OF BURIED AGRICULTURAL DRAINAGE PIPE WITH GEOPHYSICAL METHODS

One of the more frustrating problems confronting farmers and land improvement contractors in the Midwest United States involves locating buried agricultural drainage pipes. Enhancing the efficiency of soil water removal on land already containing a subsurface drainage system typically involves installing new drain lines between the old ones. However, before this approach can be attempted, the older drain lines need to be located. Conventional geophysical methods have the potential to provide a solution to this problem. Therefore, in order to determine a better way to detect buried drainage pipe, the abilities of four near-surface geophysical methods were investigated, including geomagnetic surveying, electromagnetic induction, resistivity, and ground penetrating radar (GPR). Of these four, only GPR proved capable of finding agricultural drainage pipe. Furthermore, GPR grid surveys were conducted in southwest, central, and northwest Ohio at 11 test plots containing subsurface drainage systems, and in regard to locating the total amount of pipe present at each site, this technology was shown to have an average effectiveness of 81% (100% of the pipe was found at six sites, 90% at one site, 75% at two sites, 50% at one site, and 0% at one site.) GPR proved, on the whole, to be successful in finding clay tile and corrugated nplastic tubing drainage pipe down to depths of approximately 1 m (3 ft) within a variety of different soil materials. Consequently, although more research is certainly warranted, ground penetrating radar methods appear to have excellent potential with respect to agricultural drainage pipe detection.

ANIONIC SURFACTANT TRANSPORT CHARACTERISTICS IN UNSATURATED SOIL

Surfactants have potential use with respect to in situ removal of organic contaminants from soil. The efficiency and effectiveness of using surfactants for this purpose may depend on their mobility under unsaturated flow conditions. For this reason, transient horizontal unsaturated column tests were used to study anionic surfactant transport characteristics in a loamy soil. Two commercial anionic surfactants, an alkyl ether sulfate (AES) and a linear alkylbenzene sulfonate (LAS), were tested. For each surfactant, the concentration and moisture content profiles plotted versus the Boltzmann transform (column distance/(test time) 0.5 ) exhibited similarity between experiments having the same boundary conditions but different time durations. Concentration profile similarity is an indication that soil/surfactant interactions were reversible and equilibrium conditions quickly achieved for AES and LAS during testing. Penetration of the AES and LAS concentration fronts were, respectively, one-half and one-fifth the advance of the wetting front, indicating a high degree of sorption.Where present at substantial concentrations, both surfactants reduced soil moisture diffusivity values significantly.

IMPORTANT CONSIDERATIONS FOR LOCATING BURIED AGRICULTURAL DRAINAGE PIPE USING GROUND PENETRATING RADAR

Enhancing the efficiency of soil water removal on land already containing a subsurface drainage system typically ninvolves installing new drain lines between the old ones. However, the older drainage pipes need to be located before this napproach can be attempted. In ongoing research, a near-surface geophysical method, ground penetrating radar (GPR), has nbeen successful in locating on average 72% of the total amount of drainage pipe present at 13 test plots in southwest, central, nand northwest Ohio. The effective use of GPR for drainage pipe detection requires careful consideration of computer nprocessing procedures, equipment parameters, site conditions, and field operations, all of which were thoroughly investigated nin this study. nApplication of a signal saturation correction filter along with a spreading and exponential compensation gain function nwere the computer processing steps most helpful for enhancing the drainage pipe response exhibited within GPR images of nthe soil profile. GPR amplitude maps that show the overall subsurface drainage pipe system required additional computer nprocessing, which included 2-D migration, signal trace enveloping, and in some cases, a high frequency noise filter and a nspatial background subtraction filter. Equipment parameter test results indicate that a 250-MHz antenna frequency worked nbest, and that data quality is good over a range of spatial sampling intervals and signal trace stacking. In regard to the site nconditions present, shallow hydrology, soil texture, and drainage pipe orientation all substantially influence the GPR nresponse. Additionally, drainage pipe that are as small as 5 cm (2 in.) in diameter can be detected. However, the fired clay nor plastic material of which the drainage pipe is comprised does not appear to have much of an impact. Finally, with respect nto GPR field operations, bidirectional surveys offer the best chance for finding all the buried drainage pipe possible, and for ndisplaying a subsurface drainage system on an amplitude map, the narrower the spacing between GPR measurement lines, nthe better the result. Although it is important to note that the amplitude maps generated with a wider spacing between GPR nmeasurement lines, still provided plenty of useful data on drainage pipe location. The information supplied by this study can nbe employed to formulate guidelines that will enhance the potential of success for using ground penetrating radar in locating nburied agricultural drainage pipe.

COMPARISON OF ELECTROMAGNETIC INDUCTION, CAPACITIVELY-COUPLED RESISTIVITY, AND GALVANIC CONTACT RESISTIVITY METHODS FOR SOIL ELECTRICAL CONDUCTIVITY MEASUREMENT

In situ measurement of apparent soil electrical conductivity (ECa) is an important precision agriculture tool useful nfor determining spatial changes in soil properties. Three near-surface geophysical methods are available for rapid, ncontinuous measurement of ECa in agricultural fields. These methods are electromagnetic induction (EMI), capacitively ncoupled resistivity (CCR), and galvanic contact resistivity (GCR). Acceptance for using geophysical methods to gauge spatial nchanges in soil properties hinges to a significant degree on there being consistency of the measured ECa spatial pattern nbetween geophysical methods. Testing of all three methods was conducted on two adjacent test plots having fine-grained soils nand during two time periods with dissimilar shallow hydrologic conditions. Different operational modes for each of the three ngeophysical methods were evaluated, including three primary electromagnetic field frequencies (8190, 14610, and 20010 Hz) nused for the EMI method, four spacing distances (0.625, 1.25, 2.5, and 5.0 m) between the two dipoles employed with the CCR nmethod, and two different Schlumberger electrode array lengths (0.7 and 2.1 m) utilized for the GCR method. Therefore, a ntotal of nine geophysical method - operational mode combinations were tested. nBased on spatial correlation analysis, the areal ECa patterns measured by the nine method - operational mode ncombinations showed substantial similarity to each other, with one exception. The exception was the short electrode array nmode of the GCR method that when paired with the various modes of the other two geophysical methods, exhibited an average ncorrelation coefficient, r, that ranged between only 0.30 and 0.45. All other average r values for pairs of different geophysical nmethod - operational mode combinations ranged between 0.62 and 0.97. Spatial correlation coefficients, for both test plots, nbetween the same method - operational mode combination, but at two different times in which hydrologic conditions varied, nranged from 0.55 to 0.95 for eight of the nine method - operational mode combinations, with the GCR short electrode array nhaving values of 0.32 and 0.58. Regarding the test plot EC average or median, there were substantial differences in values nobtained by the three geophysical methods. Electromagnetic vertical sounding measurements along with results obtained by nCCR, and GCR surveying, when combined, indicate that for both test plots, from the surface to a depth of a little over 2 m, nsoil electrical conductivity generally increased first and then decreased. Most importantly, although the measured ECa nmagnitudes vary between the three geophysical methods, results show that EMI, CCR, and GCR all provide useful and nconsistent information on soil electrical conductivity spatial patterns.

Location of agricultural drainage pipes and assessment of agricultural drainage pipe conditions using ground penetrating radar.

Methods are needed to not only locate buried agricultural drainage pipe, but to also determine if the pipes are functioning properly with respect to water delivery. The primary focus of this research project was to confirm the ability of ground penetrating radar (GPR) to locate buried drainage pipe, and then determine if GPR provides insight into drain line water conveyance functionality. Ground penetrating radar surveys using 250 MHz transmitter/ receiver antennas were conducted at a specially designed test plot under drained, moderately wet soil conditions (water table below drain lines) and undrained, extremely wet soil conditions (water table above drain lines). The test plot contained four drain lines: one a clay tile to corrugated plastic tubing (CPT) drain line that was completely open to flow; one comprised of CPT with an isolated obstruction near the midpoint, completely preventing through-flow of water; one comprised of CPT but filled with soil; and one comprised of CPT but severed near its midpoint, producing a partial obstruction to water flow. Subsequent GPR computer modeling simulations were employed to assist with interpretation of the GPR field data. Results of the GPR field surveys indicate that given suitable shallow hydrologic conditions, GPR not only finds drainage pipes, but can also determine the position along a drain line where there is an isolated obstruction that completely blocks water flow. However, results show that a partial pipe obstruction is difficult to locate using GPR. Surprisingly, the soil-filled drain line was clearly detectable under both soil hydrologic conditions tested. The GPR computer modeling simulations indicate that soil had likely settled within the pipe, and that the GPR responses obtained at the test plot for the soil-filled pipe were responses representative of a pipe that was in fact only partially filled with soil. Overall, these research results provide valuable information for those contemplating the use of GPR to locate agricultural drainage pipes and then determine their functionality.

Golf Course Applications of Near-Surface Geophysical Methods: A Case Study

As of the year 2000, there were over 15,000 golf course facilities in the U.S.A. alone. The upkeep of these facilities requires continual maintenance and occasional remodeling. The superintendents and architects responsible for the maintenance and remodeling efforts need non-destructive tools for obtaining information on shallow subsurface features within parts of the golf course, particularly tees and greens. The subsurface features of importance include, but are not limited to, constructed soil layer characteristics and drainage system infrastructure. Near-surface geophysical methods can potentially provide a non-destructive means for golf course superintendents and architects to obtain the shallow subsurface information required to address their maintenance and remodeling concerns. This case study assessment of near-surface geophysical methods in regard to golf course applications focused on electromagnetic induction (EMI) and ground penetrating radar (GPR) techniques. The investigation employed two different EMI ground conductivity meters. Two GPR systems were also tested including the evaluation of antenna center frequencies ranging from 250 to 1,000 MHz. The study incorporated three separate sites. Measurements with both EMI and GPR were collected on a tee and a green at the Muirfield Village Golf Club in Dublin, Ohio, U.S.A. and on a practice green at the Golf Club of Dublin in Dublin, Ohio, U.S.A. GPR was also tested on a golf course green at the Guelph Turfgrass Institute & Environmental Research Centre in Guelph, Ontario, Canada. Results indicate that use of the appropriate EMI equipment can provide information on spatial changes of shallow apparent electrical conductivity (ECa) within golf course green constructed soil layers. This ECa data could potentially be employed to gauge constructed soil layer conditions, including wetness, salinity, etc., within different areas of a green. GPR proved quite capable of obtaining useful information on the golf course tee and greens that were studied, at least in regard to constructed soil layer thicknesses/depths or their areal extent and in locating the subsurface drainage pipe systems present. For the GPR center antenna frequencies evaluated, ranging from 250 to 1,000 MHz, all seemed to work relatively well for mapping tee and green constructed soil layer areal extent and drainage pipe locations. The higher frequency 900 and 1,000 MHz antennas appeared to work best for resolving thicknesses/depths of constructed soil layers within the tee and greens investigated. In addition, computer modeling of synthetic GPR profiles provide valuable insight and help considerably with data interpretation. While more research is certainly warranted, near-surface geophysical methods, especially EMI and GPR, appear to show promise with respect to acquiring the data needed in golf course maintenance and remodeling applications.

EFFECTS OF NITRATE CONCENTRATION AND IONIC STRENGTH ON NITRATE ANION EXCLUSION UNDER UNSATURATED FLOW CONDITIONS

Transient unsaturated horizontal column experiments were conducted with a computer-controlled syringe pump to assess the impacts of nitrate (NO3−) concentration and solution ionic strength on anion exclusion processes that affect NO3− transport through soil. A loam soil was used in all the column experiments. Duplicate tests were conducted with seven different injection solutions applied at the inlets of relatively dry soil columns (initial volumetric water content averaged 0.018). The injection solutions contained either dissolved potassium nitrate (KNO3) alone or a combination of dissolved KNO3 and calcium chloride (KNO3 + CaCl2). These seven solutions allowed for the evaluation of the anion exclusion effects of four different NO3−-N concentrations (50, 200, 1000, and 2690 mg L−1) and four different solution ionic strengths (0.0036, 0.0142, 0.0714, and 0.1929 M) applied at the column inlet to be evaluated. Soil water content and soil solution NO3−-N concentration profiles were compared between tests with different injection solutions to quantify the effects of NO3− concentration and ionic strength on NO3− anion exclusion. Anion exclusion was exhibited in the NO3−-N concentration profiles for all experiments carried out in this investigation. Specifically, NO3−-N concentrations at the column inlet were 5% to 26% less than the injected NO3−-N concentrations, and NO3−-N concentrations at the wetting front were greater than injected NO3−-N concentrations by factors of 1.1 to 2.6. Considering results from tests conducted with both the KNO3 and KNO3 + CaCl2 injection solutions, it is apparent that ionic strength governs the magnitude of the NO3− anion exclusion effect to a far greater extent than the NO3−-N concentration. A strong logarithmic relationship (R2 ranged from 0.9474 to 0.9850) was found to exist between the injection solution ionic strength and various column inlet or wetting front parameters used to quantify the anion exclusion effect. Consequently, under unsaturated conditions, the anion exclusion process influencing NO3− mobility in the soil profile will be more affected by the valences and amounts of all anions and cations present in the soil solution and not so much by just the NO3− concentration.

NITRATE MOBILITY UNDER UNSATURATED FLOW CONDITIONS IN FOUR INITIALLY DRY SOILS

Solving environmental problems associated with nitrate (NO3−) requires a better understanding of how NO3− moves through the soil profile. Transient unsaturated horizontal column experiments were conducted to assess processes affecting soil NO3− transport. Duplicate tests were conducted on four soils having different physicochemical and mineralogical properties. In each test, a 200 mg/L NO3−-nitrogen (NO3−-N) solution was applied at the inlet of the relatively dry soil columns, and the value of sorptivity kept constant at 0.0073 cm/sec0.5. Comparison of corresponding soil water content and soil solution NO3−-N concentration profiles from the column tests clearly indicated anion exclusion to be an important process impacting NO3− mobility under unsaturated flow conditions. Evidence of anion exclusion for all four soils included soil solution NO3−-N concentrations near the inlet that were 13% to 21% less than the concentration (200 mg/L NO3−-N) injected at the inlet. Further evidence of anion exclusion included peak soil solution NO3−-N concentrations up to twice the injected concentration near the wetting front for three of the four soils. The fourth soil, possibly because of a combination of dispersion processes, low pH, and the mixture of clay minerals present, behaved somewhat differently than the other soils by having a peak soil solution NO3−-N concentration above 200 mg/L located approximately halfway between the column inlet and wetting front. Overall, this research indicated that anion exclusion can be a key process affecting NO3− mobility in a variety of soil environments.