mapping shallow underground features that influence site-specific agricultural production

Ž .Journal of Applied Geophysics 40 1998 19–27

Mapping shallow underground features that influence site-specificagricultural production

Robert S. Freeland a,), Ronald E. Yoder a, John T. Ammons b

a Dept. of Agricultural Engineering, Tennessee Agricultural Experiment Station, The UniÕersity of Tennessee, KnoxÕille, TN37901-1071, USA

b Dept. of Plant and Soil Science, Tennessee Agricultural Experiment Station, The UniÕersity of Tennessee, KnoxÕille, TN37901-1071, USA

Received 5 February 1997; accepted 12 March 1998

Abstract

Ž .Modern agricultural production practices are rapidly evolving in the United States of America USA . These newproduction practices present significant applications for nonintrusive subsurface imaging. One such imaging technology isGPR, and it is now being incorporated within site-specific agriculture in the detection of soil horizons, perched waterŽ .episaturation , fragipans, hydrological preferential flow paths, and soil compaction. These features traditionally have been

Ž .mapped by soil scientists using intrusive measurements e.g., soil augers, soil pits, coring tools . Rather than developing atool for soil mapping, our studies are targeting the identification, dimensioning, and position of subsurface features thatdirectly influence agricultural productivity. It is foreseen that this information will allow for an increase in agriculturalefficiency through infield machinery automation, and it will also greatly enhance development of highly efficient cropproduction strategies. The field sensing methodologies that we have developed using existing geophysical technologies arehighly dependent upon both the soil and site characteristics due to seasonal variations. The GPR applications presentedherein were conducted primarily in a region of loess soil that extends east of the Mississippi River into western Tennessee.GPR studies were also conducted in central Tennessee on the Cumberland Plateau within a region of shallow, sandy loamsoils. Additional studies were conducted on the karst area of central Kentucky. Although targeting site-specific agriculture,our results and procedures may benefit the traditional users of GPR technology. We suggest that large-scale agricultural

Ž .applications of the technology would be enhanced by integrating global positioning GPS technology in future hardware andsoftware products. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: agriculture; agrochemicals; bedrock; ground-penetrating radar; horizons; soils

1. Introduction

Agricultural productivity is highly correlatedto the physical characteristics of the soil. As aroutine practice, a farmer or consultant approxi-

) Corresponding author. E-mail: [email protected].

mates the overall physical characteristics offields from soil surveys that provide general soilseries information. The resolutions of the mapsare determined by a soil specialist who hassurveyed the overall terrain and has taken man-ual samples using a soil probe to provide gen-eral soil classifications. Soil fertility, organic

0926-9851r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 98 00014-7

( )R.S. Freeland et al.rJournal of Applied Geophysics 40 1998 19–2720

matter, and acidity values are determined on afield-by-field basis, and are typically obtainedfrom random ‘grab’ samples that are submittedperiodically for laboratory analysis.

Because the three-dimensional distributionsof the physical characteristics beneath mostagricultural terrains were formed by nature, thevariability of the subsurface is often muchgreater than what manual approximations mightsuggest. For most agricultural sites, detailedthree-dimensional soil morphologies are largelyunknown on a micro scale. Therefore, unless itis apparent from direct or indirect observations,subsurface terrain variability is largely disre-garded during traditional cropping strategies.

A new agricultural production practice is nowrapidly evolving that is termed site-specificagriculture. This production practice is based onGPS, digital mapping, and field-machinery au-tomatic control systems. Access to highly-de-tailed three-dimensional spatial data pertainingto a site’s soil morphology is highly desirablefor these and other site-specific agriculture ap-plications.

By using GPS technology, farmers are begin-ning to adopt site-specific agricultural produc-tion technologies that address field variabilityon a precise, microscale. By referencing de-tailed crop yield and historical patterns of weedinfestations, farmers are adapting their farmingmethods toward increasing future harvest yieldsand reducing pest infestations.

Because real-time field positioning data arenow easily obtained using mobile-mounted GPSreceivers, GPR technology has the potential toprovide detailed data that will help compensatefor subterranean variations at very fine spatialincrements. GPS receivers supply previouslyunattainable geospatial accuracy for infield op-erations at a nominal cost. Examples of currentautomated field operations are precision seedand fertilizer placement, pesticide application,

Žand localized harvest yield mapping Kvien et.al., 1995 .

Geophysical near-surface surveying has beenproven as a very efficient and effective tool for

the nonintrusive gathering of continuous agri-cultural field data on many soil types, soil mois-ture conditions, and underground topographiesŽDoolittle and Asmussen, 1992; Doolittle andCollins, 1995; Johnson et al., 1979; Saarenketo,

.1996; Smith et al., 1992; Truman et al., 1988 .Emerging geophysical technologies offer greatpotential for significant contributions to produc-tion agriculture. As such, a collaborative, multi-discipline research team from southeastern USALand-Grant state universities and the United

Ž .States Department of Agriculture USDA ispresently conducting studies of noninvasive re-connaissance of cropland soil and its near-surface hydrogeological features.

As an integral tool of precision farming, agri-cultural-specific geophysical survey methods arebeing developed to provide an efficient, eco-

Ž .nomical, and rapid means of 1 identifyingŽ .areas of excessive soil compaction, 2 dimen-

sioning soil horizon thickness and bedrockŽ .depth, and 3 assessing soil hydrological prop-

erties over vast agricultural production regions.Ultimately, these data will be converted intothree-dimensional digital maps for precisionfarming integration. Reported herein are ourteam’s short-term successful applications, todate, and our proposed future investigations inaccomplishing the above objectives.

2. Equipment and methods

Using commercially-available GPR equip-1 Ž .ment GSSI, Model SIR-10A , geophysical

data are being obtained at seven agriculturalŽ .sites in the southeastern USA Fig. 1 . Five sites

are near Memphis, TN, on deep loess soils.Another research site is on the CumberlandPlateau of Tennessee, a locale having shallow,

1 Mention of a commercial product or trade name doesnot imply an endorsement or a performance guarantee bythe authors or their respective organizations.

( )R.S. Freeland et al.rJournal of Applied Geophysics 40 1998 19–27 21

Ž . Ž . Ž . Ž .Fig. 1. Site locations of GPR studies: 1 Cumberland Plateau, 2 central Kentucky, 3 northwest Mississippi, 4 southwestŽ . Ž . Ž .Tennessee, 5 west Tennessee, 6 northwest Tennessee, and 7 southwest Kentucky.

sandy soil. A seventh site is on karst nearLexington, KY.

Unless otherwise noted, data are presentedwithout post-processing or image enhancement,using only the equipment settings that pertain tothe survey and the antenna model being used.Gain settings are at either three or five setpoints, filling 80% of the waveform’s nominalrange by depth during infield calibrations. Typi-cally, the equipment is calibrated at least twiceper day of surveying. Optimum equipment set-tings are not static for shallow explorations dueto the seasonal variations of temperature andmoisture within the rhizosphere, and the dailycyclic variation of soil temperature.

Depending on a site’s characteristics and thesurvey’s targets, we typically selected two an-tenna models from our set of three antennas.For our survey locales, a 900-MHz antenna

provided the highest horizontal resolution todepths approaching 1 m in sandy soils, and itsuse was ideal for the detection of increased soilcompaction from infield machinery trafficking.A 500-MHz antenna produced moderate hori-zontal resolution to depths of approximately 2m, and was used primarily for mapping sand-stone bedrock beneath sandy loam soil. A 300-MHz antenna penetrated to depths of approxi-mately 7 m in loess. The 300-MHz antennagenerated the lowest horizontal resolution; how-ever, its horizontal resolution often approxi-mated the traditional soil horizon differentia-

Ž .tions used by soil scientists Fig. 2 .At all GPR survey sites, a soil morphologist

manually classified the soil horizons from anŽ .adjacent soil pit or from infield auger samples

for subsequent GPR ‘groundtruthing’ and cali-bration. Core samples were also taken for per-


Ž .Fig. 2. Profile of Site 3 obtained using a 300-MHz antenna with soil classification overlay 100 ns range .

cent sand, clay, silt, and moisture profiling bydepth. Targeting soil features that typically gen-erate stronger reflections, sharp breaks in plot-ted relative percent changes by depth of the four

Žconstituents primarily moisture and to a lesser.extent clay greatly assisted in image interpreta-

Ž .tions Fig. 3 .

3. Successful applications

3.1. Soil horizons and bedrock

In west Tennessee near the City of MemphisŽAmes Plantation Water Quality Project, Han-

.cock Site , three major parent material se-quences are identified within a depth of 4 mfrom the surface. Below the surface continuingfor about 2 m is loess, which is grayish-brownin color and principally silt. This fertile andhighly-erodible soil was wind-blown onto thesite, having been deposited atop a stream terrace

Ž .alluvium a paleosol or ancient soil during thepast four glacial and interglacial periods. Thealluvium is approximately 1-m thick, is reddishin color, and has a high clay content. In turn,the alluvium overlays the Coastal Plain, whichbegins at approximately 3 m below the surface.The Coastal Plain is a Tertiary formation of


Ž .Fig. 3. Core sample Site 4 analyzed by depth assisted image interpretations by targeting sharp relative changes in physicalproperties, which typically corresponded to high reflectivity.

Ž .high sand content. Ammons et al. 1994 de-scribed the soil morphology at this site in detail.

The 300-MHz antenna is routinely used atthis site and on other similar deep soil sitesnearby. We can detect the horizon interfacesbetween the upper three parent materials easily.Presently we are topographically mapping thepaleosol, the earlier-formed soil whose topogra-phy is buried by the loess approximately 2 m

Ž .beneath the surface Fig. 4 .Both on the shallow, sandy soils of Ten-

Ž .nessee’s Cumberland Plateau Hamlett, 1996Žand on karst near Lexington, KY Freeland et

.al., 1996 , GPR technology has shown to bevery proficient in mapping bedrock depth, form-ing sinkholes, soil horizon thickness, mudstone,

Ž .weathered sandstone, the plow layer Ap , andbreaks in soil horizons having sharp relative

Ž .changes in clayrmoisture content Fig. 4 . Thesoil morphology of the Cumberland Plateau,due to its high sand content and undulatingbedrock topography, is one of the best-suitedterrains for GPR profiling of the entire stateŽ .Fig. 5 .

3.2. Soil hydrology-fingering

Perhaps one of the most useful applicationsof GPR is hydrogeological investigations.Within the deep stratified soils at sites nearMemphis, TN, we are observing defined colum-nar images that are highly suggestive of verticalmoisture movement throughout the lower pro-files. A dynamic process, these nonstationary,

Ž .columnar images or fingers have been ob-served slowly to form and disappear over 24-hintervals. At the west Tennessee site, the fingersbegin to form at a depth equal to the interfacebetween the fine clay soil of the Stream TerraceAlluvium and the coarser sand of the Coastal

Ž .Plain Fig. 6 .Soil cores obtained at this site also provide

strong supporting physical evidence of the verti-cal movement of moisture. Upon close examina-tion, core samples reveal very defined paths ofvertical macro pores. Also supporting the GPR

Ž .observation of vertical i.e., columnar moisturemovement at survey sites are the laboratoryinvestigations of and theoretical discussions by


Ž . Ž .Fig. 4. Profile of west Tennessee loess soil Site 4 using a 300-MHz antenna 160 ns range .

Ž .Hill and Parlange 1972 and Baker and HillelŽ .1991 . These studies suggested a phenomenon,

Žcommonly called ‘fingering’ a partial-volume

.flow or wetting-front instability , that occurswhenever a saturated, finer-textured soil over-lays and drains into a drier, coarser-textured

Ž . Ž .Fig. 5. Radar profile obtained on the Cumberland Plateau Site 1 using a 500-MHZ antenna Freeland et al., 1996 . TheŽ .artifact is a small wire laying on the surface 60 ns range .


Ž .Fig. 6. Radar profile obtained in west Tennessee Site 4 using a 300-MHZ antenna, and enhanced using the HilbertŽ .Transform 160 ns range .

soil. Moisture within an upper profile accumu-lates and channels, dropping slowly downwardin steps defined by soil layers into the lowerprofiles as vertical wetting columns. Thesecolumns form and disappear dynamically. At asite in northern Mississippi, we observed anelevated, highly-reflective profile immediatelysurrounded by these ‘draining’ areas that doesnot exhibit the fingering phenomena. This isstrongly indicative of perched water.

4. Future investigations

( )4.1. Perched water episaturation

Perched water occurs within localized areawhere a horizontal restricting layer prevents thetypical downward migration of water into theregion’s watertable. The horizontal restrictinglayer may be a fragipan, a high clay soil hori-zon, flat boulders, bedrock, etc. Not only doeslocalized, saturated soil have a detrimental im-pact on crop yields, it is also theorized thatperched water is a major contributor to the

offsite horizontal transport of agricultural chem-icals.

4.2. Soil compaction

In cooperation with The University of Ken-tucky Agricultural Engineering Department,studies are being conducted to map regions ofextreme soil compaction due to machinery wheeltraffic across row-crop fields. Subsurface hardpans of compressed soil caused by machinerytraffic on certain soil types greatly impede rootdevelopment and subsequent crop yield. Period-ically removing the hard pans through deeptillage is energy intensive. Soil compaction mapswould significantly reduce energy requirementsby only targeting regions of severely compactedsoil for deep tillage.

When using a 300-MHz antenna, crossingseverely compacted soil produces a distortedsignal from the upper profiles. At present, ob-taining absolute quantitative compaction valuesby radar image alone is not feasible. However,three-dimensional maps of relative changes havebeen found to reveal previous traffic patterns.


Shallow, lightly compacted soil within theŽupper meter profile can be differentiated rela-

.tively from its immediate horizontal surround-ings within a three-dimensional image using a900-MHz antenna. Thus, trafficking routes caus-ing compressed soil within fields are mappable.

5. Discussion

Near-surface, nonintrusive survey technolo-gies have a significant potential for major con-tributions to production agriculture. However,

Žpresent limitations of the technology other than.equipment and software cost are its data stor-

age requirement and the effort required to set uptraditional survey transects and grids as refer-

Ž .ence points i.e., markers . A typical survey daywill generate several gigabytes, and will oftenrequire significant effort to establish a surveygrid prior to using the GPR equipment. Contin-ual technological advancement should help alle-viate these limitations. Incorporation of GPStechnology within both GPR hardware and soft-ware products would be of great benefit to theend user.

Familiarity with a site’s physical character-istics, proper equipment calibration and setup,and understanding antenna model performancesat differing locales are necessary for accurate,post-survey image interpretations. Survey tech-niques developed for one site are often notapplicable to other sites. As such, advancedtraining, research, and continued equipment andmethodology developments specific to agricul-tural applications over the wide range of soilsare needed. In addition, novices of geophysicstend to have great difficulty visualizing reflec-tive images in raw formats; thus, enhancedthree-dimensional data presentations of reflec-tion images should be continually advanced. Weare presently investigating the use of RADAN

Ž . Ž3D GSSI and SLICER Fortner Research,.LLC for 3D presentation.

For our seven agricultural sites, numerousgraduate students have aided in the acquisition

of field data. Individuals who have traditionaladvanced soil classification skills, are proficientin GPR operation and image interpretation, andare familiar with a survey site’s soil morphol-ogy often do not require ‘groundtruthing’ fromsoil pits or cores in order to classify the site’sGPR image during post-processing. Rather thansupplying quantitative measurements, many soilphysical parameters that are measurable by GPRŽ .i.e., soil compaction and soil moisture can beinferred only as relative values from imageinterpretations.

Most importantly for agricultural applicationsin the loess regions of west Tennessee, the twomajor crop limiting subsurface features that oc-cur in these areas are the depth of the loesslying above Coast Plain Sediments and the depthto a fragipan. We have found both features arereadily sensed using GPR.

6. Conclusions

Ground probing radar is a very efficient andeffective tool for the nonintrusive gathering ofcontinuous agricultural field data on many soiltypes, soil conditions, and underground topogra-phies. Successful short-term applications andrealizable goals are mapping soil horizons,perched water, fingering, and soil compaction.Present limitations of the technology includelarge data storage requirements, survey training,and image interpretation and presentation.Rather than supplying quantitative measure-ments, some soil physical parameters that are

Žmeasurable by GPR i.e., soil compaction and.soil moisture can be inferred only as relative

values from image interpretations.

Acknowledgements

Funding for this research was provided, inpart, by USDA-CSRS Southern Regional Pro-ject S-252, Engineering Principles for Conserva-tion Cropping Systems.


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mapping shallow underground features that influence site-specific agricultural production

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