Geophysical Methods

Barry J. AllredSoil Drainage Research, U.S. Department of Agriculture—Agricultural ResearchService (USDA-ARS), Columbus, Ohio, U.S.A.

Viacheslav I. AdamchukBioresource Engineering, McGill University, Anne-de-Bellevue, Canada

Raphael A. Viscarra RosselBruce E. Butler Laboratory, CSIRO Land and Water Flagship, Black Mountain,Queensland, Australia

Jim DoolittleNational Soil Survey Center, U.S. Department of Agriculture—Natural ResourcesConservation Center (USDA-NRCC), Newtown Square, Pennsylvania, U.S.A.

Robert S. FreelandBiosystems Engineering and Soil Science Department, University of Tennessee,Knoxville, Tennessee, U.S.A.

Katherine R. GroteGeosciences and Geological and Petroleum Engineering Department, MissouriUniversity of Science and Technology, Rolla, Missouri, U.S.A.

Dennis L. CorwinU.S. Soil Salinity Laboratory, U.S. Department of Agriculture—Agricultural ResearchService (USDA-ARS), Riverside, California, U.S.A.

AbstractNear-surface geophysical methods have become an important tool for agriculture. Geophysical investiga-tions for agriculture are most often focused on the first 2 meters directly beneath the ground surface, whichincludes the crop root zone and all, or at least most of the soil profile. Resistivity, electromagnetic induction,and ground-penetrating radar are the three geophysical methods most commonly employed for agriculturalsoil investigations; however, optical reflectance and γ-ray spectroscopy are increasingly becoming morewidely utilized. Temporal and spatial variation of conditions and properties in the soil profile are importantconsiderations when conducting a geophysical survey within an agricultural setting. Geophysical methodshave been applied to soil surveys, precision farming, soil water content measurement, and soil salinitymonitoring; with new agricultural geophysics applications continuing to evolve. Future development ofmulti-senor platforms and the use of unmanned aerial vehicles will dramatically improve geophysical soilinvestigation capabilities with respect to field accessibility and data interpretation.

INTRODUCTION

Agricultural geophysics as described in this entry involvesthe application of physical quantity measurement techni-ques to provide information on conditions or featureswithin the soil environment. With the exception of boreholegeophysical methods and soil probes with electrical con-ductivity, optical, and penetration resistance sensors, thesetechniques are generally non-invasive with physical quan-tities determined from measurements made mostly on or

directly above the ground. Agricultural geophysics tendsto be heavily focused on a 2 m zone directly beneath theground surface, which includes the crop root zone and all,or at least most, of the soil profile.[1] Complexities encoun-tered with agricultural geophysics include transient soiltemperature and moisture conditions, which can apprecia-bly alter, over a period of days or even hours, the values ofmeasured soil physical quantities (especially electrical con-ductivity and dielectric constant (K)). Additionally, physi-cal quantities measured in the soil environment with

Encyclopedia of Soil Science, Third Edition DOI: 10.1081/E-ESS3-120053754Copyright © 2016 by Taylor & Francis. All rights reserved. 1

geophysical methods often exhibit substantial variabilityover very short horizontal and vertical distances. The threegeophysical methods most commonly used for agriculturalpurposes are resistivity, electromagnetic induction (EMI),and ground-penetrating radar (GPR). However, opticalreflectance and gamma (γ)-ray spectroscopy are gainingmore widespread utilization. All five of these employedagricultural geophysics methods are summarized in thenext section of this entry.

GEOPHYSICAL METHODS

Resistivity

The resistivity method, employed in its most conventionalform, uses an external power source to supply electricalcurrent between two “current” electrodes inserted at theground surface. The propagation of current in the subsur-face is three-dimensional, and so too is the associated elec-tric field. Information on the electric field is obtained bymeasuring the voltage between a second pair of “potential”electrodes also inserted at the ground surface. The twocurrent and two potential electrodes together comprisea single four-electrode array. The magnitude of the currentapplied and the measured voltage are then used in con-junction with data on electrode spacing and arrangementto determine an apparent soil electrical conductivity(ECa), for a bulk volume of soil. The current and potentialelectrodes are often arranged inline, and the depth ofinvestigation increases with increased length of the elec-trode array.

Continuous measurement galvanic contact resistivitysystems integrated with global positioning system (GPS)receivers have been developed in the United State andEurope specifically for farm field soil investigations(Fig. 1). These resistivity systems can have more thanone four-electrode array providing shallow investigationdepths of 0.3–2 m, with short time or distance intervalsbetween the continuously collected discrete ECa mea-surements. The location for each ECa measurement isdetermined accurately by GPS. For the system shownin Fig. 1, steel coulters (disks) that cut through the soil

surface are utilized as current and potential electrodes.Maps of ECa spatial patterns across a farm field aregenerated with resistivity survey data to gain insighton lateral variations in soil properties/conditions, suchas water content, soil texture, organic matter content,and so on.

Electromagnetic Induction

Electromagnetic induction (EMI) methods also measurethe ECa for a bulk volume of soil directly beneath thesurface. An instrument called a ground conductivitymeter (GCM) is commonly employed for relatively shal-low EMI investigations (Fig. 2). In operation, an alter-nating electrical current is passed through one of twosmall electric wire coils spaced a set distance apart andhoused within the GCM, which itself is positioned at, ora short distance above, the ground surface. The appliedcurrent produces an electromagnetic field around the“transmitting” coil, with a portion of the electromagneticfield extending into the subsurface. This electromagneticfield, called the primary field, induces an alternating

Fig. 1 Example of a continuousmeasurement galvanic contactresistivity system, Veris 3100Soil EC Mapping System (VerisTechnologies, Salina, Kansas,United States; left) and close-upof steel coulters used for currentand potential electrodes by theVeris 3100 Soil EC MappingSystem (right).

Fig. 2 EMI DUALEM-21 S GCM (DUALEM, Milton, Ontario,Canada) measuring soil electrical conductivity at a golf coursegreen.

2 Geophysical Methods

electrical current within the ground, in turn producing asecondary electromagnetic field. This secondary fieldextends back to the surface and the air above. The sec-ond wire coil acts as a receiver measuring the resultantamplitude and phase components of both the primaryand secondary fields. The amplitude and phase differ-ences between the primary and secondary fields are thenused, along with the intercoil spacing, to calculate an“apparent” value of ECa for a bulk volume of soil. Pro-vided low induction number conditions are satisfied, theEMI depth of investigation is dependent on the spacingdistance between the electric wire coils within the GCMand the orientation (vertical, horizontal, perpendicular)of the electric wire coils.[2] As with resistivity methods,maps of ECa spatial patterns across a farm field aregenerated with EMI data to gain insight on lateral var-iations in soil properties/conditions, such as water con-tent, soil texture, organic matter content, etc.

Ground-Penetrating Radar

A ground-penetrating radar (GPR) system directs an elec-tromagnetic radio energy (radar) pulse into the subsurface,followed by measurement of the elapsed time taken by theradar signal, as it travels downward from the transmittingantenna, partially reflects off subsurface boundaries orobjects, and is eventually returned to the surface, where itis picked up by a receiving antenna. Reflections from dif-ferent depths produce a signal trace, which is a function ofradar wave amplitude vs. time. Radar waves that travelalong direct and refracted paths through both air andground from the transmitting antenna to the receivingantenna are also included as part of the signal trace.Antenna frequency, soil moisture conditions, clay content,

salinity, and the amount of iron oxide present all have asubstantial influence on the distance beneath the surface towhich the radar signal penetrates.[3] The K of a materialgoverns the velocity for the radar signal traveling throughthat material. Differences in K across a subsurface dis-continuity feature control the amount of reflected radarenergy, and hence radar wave amplitude, returning to thesurface. GPS technologies are often integrated with GPRsystems to obtain accurate and precise geographic posi-tions of GPR measurements and to improve the efficiencyof GPR field surveys (Fig. 3). As an end product, radarsignal amplitude (energy) data are displayed as two-dimensional depth sections or aerial maps to gain insighton belowground conditions or to provide information onthe positions and character of subsurface features. TheGPR data collected can be used to measure soil volumetricwater content (VWC), determine soil horizon depths/thicknesses, and map buried agricultural drainage pipesystems.

Optical Reflectance (UV/VIS/NIR/MIR)

Optical sensors are used to determine the soil’s ability toreflect light in different parts of the electromagneticspectrum. The light can be from the sun or from anartificial source. Proximal optical sensors are fundamen-tally the same as optical remote sensors; the advantageof proximal sensors is that they can provide measure-ments at either the soil surface or within the soil. Theseproximal optical sensors can be used for on-the-spot oron-the-go soil measurements (Fig. 4). Optical sensingsystems cover the ultraviolet (UV; 100–400 nm), visible(VIS; 400–700 nm), near infrared (NIR; 700–2500 nm),and mid-infrared (MIR; 2500–25,000 nm) portions ofthe electromagnetic spectrum. Typically, instrumentsused for soil measurements include their own lightsource (e.g., halogen light bulb or light-emitting diode).Photodiodes or array detectors are used to estimate theintensity of reflected light and relate this measure to thelight reflected from a given set of standards. Both sourceand reflected light can be transmitted through the air,via fiber optics, or when feasible, through a contact win-dow fabricated from highly resistive material, such as sap-phire or quartz.

Measurements obtained using optical sensors can berelated to a number of soil attributes, such as soil mineralcomposition, clay content, soil color, moisture, organiccarbon, pH, and cation exchange capacity (CEC). Measure-ments can be viewed as direct, when relationships are basedon a physical phenomenon that affects light reflectance in aspecific part of the spectrum (e.g., predicting soil mineral-ogy or water content using characteristic water absorptionbands); or indirect, when the relationships are deterministicfor a finite domain and the combined effects of several soilattributes can be related to a given soil characteristic (e.g.,predicting soil organic matter). Sensor calibration strategies

Fig. 3 GPR SIR 3000 system (Geophysical Survey Systems,Inc., Nashua, New Hampshire, United States) integrated withTopcon HiPer XT Real Time Kinematic—GPS (RTK-GPS)receiver (Topcon Positioning Systems, Inc., Livermore, Califor-nia, United States).

Geophysical Methods 3

ranged from a simple linear regression to multivariatemethods, chemometrics, and data mining. Although someof these models may be applied to large geographic areas,most are linked to a specific range of soil and environ-ments. UV radiation has also been used in combinationwith visible and infrared spectra to characterize iron oxidesand organic matter.

γ-Ray Spectroscopy

γ rays contain a very large amount of energy and are themost penetrating radiation from natural or artificial sources.γ-ray spectrometers measure the distribution of the intensityof γ radiation versus the energy of each photon. Sensorsmay be either active or passive. Active γ-ray sensors use aradioactive source (e.g., 137Cs) to emit photons of energythat can then be detected using a γ-ray spectrometer. Pas-sive γ-ray sensors measure the energy of photons emittedfrom naturally occurring radioactive isotopes of the ele-ment from which they originate. As shown in Fig. 5,on-the-go measurement of soil elemental isotopes can beperformed by installing a γ-ray sensor on a vehicle. Datainterpretation may include analysis of measures related tothe isotopes of potassium, thorium, and uranium, the totalisotope count, or the entire energy spectrum. While shownto be a useful tool for predicting soil properties in differentsoil landscapes, a significant amount of preprocessing isoften required to reveal relationships between the γ-rayspectra and the soil data. The γ-ray sensor signal can be

related to soil mineralogy, particle size distribution, and theeffects of attenuating materials such as water and bulkdensity.

Inelastic neutron scattering (INS) spectroscopy relies onthe detection of γ rays that are emitted following the captureand reemission of fast neutrons, as the sample is bombardedwith neutrons from a pulsed neutron generator. The emittedγ rays are characteristic of the excited nuclide, and the γ-rayintensity is directly related to the elemental content of thesample. A thorough review of these methods and otherproximal soil sensing methods can be found in the workof Viscarra Rossel et al.[4]

APPLICATIONS TO SOIL SCIENCE

U.S. Department of Agriculture (USDA)/ NaturalResources Conservation Center (NRCS)Soil Surveys

Since the 1970s, GPR and EMI have been used by theNational Cooperative Soil Survey (NCSS) as qualitycontrol tools to investigate, map, and interpret soils.GPR has been used principally, as shown in Fig. 6, todocument the presence, depth, lateral extent, and vari-ability of diagnostic subsurface horizons that are used toclassify soils, and to name, characterize, and improvethe interpretations of soil map units.[5] GPR has alsobeen used as a research tool to assess spatial and

Fig. 5 The Mole γ-ray sensor(The Soil Company, Groningen,Netherlands) operated by Practi-cal Precision, Inc. (Tavistock,Ontario, Canada; left), and anenergy/intensity relationshipused to define reported measure-ments (right).

Fig. 4 Veris® P4000 (VerisTechnologies, Inc.) probeequipped with two spectrometerscovering VIS and NIR parts ofthe spectrum as well as electricalconductivity and insertion loadsensors operated by McGill Uni-versity (Ste-Anne-de-Bellevue,Quebec, Canada; left), and exam-ples of soil spectra obtained bythis instrument at different depthsor locations (right).

4 Geophysical Methods

temporal variations in soil properties. With the nearcompletion of soil mapping in the United States, GPRis being increasingly used to support USDA conserva-tion programs that help sustain agricultural productivityand environmental quality, address issues of soil healthand soil functions, and improve the utility of soil surveydata for modeling dynamic soil-hydrologic conditionsand functionality at different scales.

The NCSS has used EMI to indirectly measure and mapthe spatial and temporal variability of soil properties at fieldscales. Initially used to assess soil salinity, EMI is used tomap soil types; refine soil boundaries; identify contrastingsoil components within soil map unit delineations; charac-terize soil water content and flow patterns; assess variationsin soil texture, CEC, ionic composition, calcium carbonatecontent, organic carbon content, plant available nutrients,pH, and bulk density; and determine the depth to subsur-face horizons, stratigraphic layers or bedrock, among otheruses.[6] As a tool for high-intensity soil surveys, EMI hasbeen used to assist site-specific management, characterize

variability in soil physiochemical properties, and direct soilsampling.

Soil surveys require periodic maintenance and updating,as land use change and new demands require additionalsoil properties to be identified and interpreted. Itis anticipated that, as soil surveys evolve, a greaterunderstanding of the complexity and interaction ofsoil structures, processes, and functions will be neededat increasingly higher levels of resolution. In fulfillingthis need, both EMI and GPR should find greater usein field-scaled research projects on representativesoilscapes.

Precision Farming

The agricultural production practice, called precision farm-ing, employs spatially mapped field variables that provideassistance to producers for cropping decisions. For exam-ple, on-board computers can reference digital maps, direct-ing automated field machinery to vary spatially the

Fig. 7 Soil ECa and crop yield com-parison for a 3 hectare field in northwestOhio. The ECa map (left) was obtainedwith a Veris 3100 Soil ECMapping Sys-tem, and values are in millisiemens/meter. The soybean yield map (right) hasvalues in kilograms/hectare. The spatialcorrelation coefficient (r) between ECa

and soybean yield is �0.51.

Fig. 6 Variations in the depth to shale bedrockare used to distinguish areas of shallow Weikert,moderately deep Berks and very deep Rushtownsoils along this surface normalized GPR profilefrom the Northern Appalachian Ridges and ValleyRegion of central Pennsylvania.

Geophysical Methods 5

application rates of fertilizers, pesticides, and seeds. Asprecision agriculture technologies evolve, higher spatialresolution surveys of the near subsurface will be requiredbecause some soil properties, such as soil moisture andorganic matter, continually change.

GPSs were a major technology trigger for precisionfarming during the first decade of the 21st century. GPSis a common farm tool, and it provides accurate positioningfor automated field-machine guidance and speed controland supplies the real-time positioning required for the pre-cise placement of seeds, nutrients, and pesticides, alongwith the geospatial mapping of harvested yields. Globalpositioning integrated with massive, yet inexpensive, datastorage and computational power allows for large acreagesto be geospatially mapped by farmers at very highresolutions.

The relatively low-sampling data density of both on-the-go resistivity and EMI instruments, when coupledwith GPS positioning, supplies an effective solution forspatially mapping any soil characteristic that can bedelineated by relative resistivity/conductivity boundaryshifts. For these sensors, the low-data density character-istics are their greatest advantage, as this easily allowsfor the generation of detailed spatial field maps for verylarge acreages with little visual interpretation require-ments. This simplicity was a trigger for their rapid adop-tion by farmers and farm consultants. Field maps of ECa

from resistivity or EMI surveys, often show significantcorrelation with crop yield maps (Fig. 7). Consequently,ECa maps can be employed to separate the field intodifferent management zones and thereby increase cropproduction.

GPR is another geophysical tool showing great prom-ise in precision agriculture. At the small plot scale,researchers have shown GPR’s abilities to spatially mapnumerous soil properties essential to agricultural produc-tion. Almost any near-surface soil characteristic bound-ary that can be spatially delineated by sharp shifts in Kcan be mapped. However, the extreme amount of datagenerated and its variability when deployed over largeacreages are too time-consuming and challenging forvisual interpretations. The data computational require-ments and survey data storage capacity of GPR surveyswere once significant limiting factors, but they are nolonger. However, efficient automated processing of theGPR data sets is lacking for large watershed-scale sur-veys having naturally occurring soils. In contrast withother geophysical sensors having lower data quantityrequirements, this geophysical technology has notgained a following by producers using precision agricul-ture technologies.

Unmanned aerial vehicles (UAVs) coupled withremote geophysical sensors are projected by someexperts to revolutionize precision agriculture. Rapiddelivery of essential, time-critical information at verylow cost is the advantage of the UAV over ground-

based geophysical surveys. Aerial field surveys willbecome automated, inexpensive, and on-demand. Forexample, miniaturized and lightweight EMI sensors arebeing introduced for UAV surveys over large areas. Thelow-flying UAV will closely examine crops, pastures,and timberlands. As they should be able to detect infes-tations at the very outset, farmers will be able to focusprecisely on containment. Cattlemen will be able toremotely monitor livestock health and pinpoint bothstrays and their predators while they “ride herd” over-head using UAV thermal imaging. Georeferenced nutri-ent and yield maps will become commonplace,advancing precision agriculture using geophysical appli-cations even further.

Soil Water Content Measurement

GPR can be used to estimate the VWC of soil or rock,as electromagnetic velocity (ν) primarily depends on thevolume of water in the pore space of earthen materials.Velocity is only slightly affected by mineralogy, grainsize, or temperature, so measurements of ν can be con-verted easily to VWC. Velocity is usually converted toK before calculating VWC; several petrophysical rela-tionships based either on empirical data or volumetricmixing models are available in the literature to convertK to VWC.[7]

The three most common techniques for estimating VWCfrom GPR data are reflected waves, groundwaves, and air-launched methods.[8] For reflected waves, the travel time to

Fig. 8 GPR groundwave data acquired over an infiltrated areasurrounded by dryer soil. Wetter soils have lower velocities andlonger travel times, as observed over the infiltrated zone.

6 Geophysical Methods

a subsurface interface can be measured while the GPRantennas are pulled in the common-offset mode along atraverse. If the depth to the interface is known, the traveltime is used to estimate ν, which can then be related toVWC. If the depth to the interface is not known, ν can beobtained by performing a variable-offset survey, which pro-vides information on both ν and depth to an interface at onelocation. Reflections from a continuous subsurface inter-face can be used to measure VWC to greater depths (i.e.,the root zone) and to understand VWC variability across afield. If a continuous interface at a known depth is notavailable, reflections from isolated subsurface objects orburied pipes that create a reflection hyperbola in the GPRrecord can be used to estimate ν using reflection hyperbolaanalysis.

GPR groundwave techniques can be used to esti-mate VWC by measuring the travel time of this wave.Groundwaves travel in the shallow subsurface (0 cm to*30 cm) directly between the transmitting and receiv-ing antennas; by noting the antenna separation and thetime needed for energy to travel between antennas, ν canbe calculated (Fig. 8). Groundwaves do not require areflective interface, and so can be used in many near-surface soil environments to provide continuous VWCmeasurements.

Air-launched GPR techniques use the magnitude ofthe reflection from the ground surface to estimate K.Air-launched data can be acquired and processedquickly, but have a sampling depth of less than 5 cm,and the accuracy of the data greatly diminished byvegetation, uneven soil surfaces, and vertical variationsin water content, this technique can be limited in itsapplications.

Soil Salinity Monitoring

Soil salinity refers to the concentration of dissolved inor-ganic solutes in the soil solution, consisting of four majorcations (Na+, K+, Mg2+, and Ca2+) and five major anions(Cl−, HCO3

−, NO3−, SO4

2−, and CO32−). Soil salinity

reduces plant growth and, in severe cases, causes crop fail-ure by limiting plant water uptake due to an osmotic effectmaking it more difficult for the plant to extract water, byspecific-ion toxicity, or by upsetting the nutritional balanceof plants.

In the laboratory, soil salinity is most commonly deter-mined from the measurement of the electrical conductivityof the solution extract of a saturated soil paste (ECe), whichis proportional to the concentration of ions in the solution.However, because salinity is a highly spatially and tempo-rally variable soil property, the use of ECe to measure salin-ity at field scales is impractical due to the need for hundredsor even thousands of soil samples. The use of ECe to mea-sure salinity at field scales is only practical when soil sam-pling is directed using correlated spatial information.Geospatial measurements of ECa using geophysical techni-ques (i.e., electrical resistivity, EMI, or time domain reflec-tometry) are sources of spatial information used to directsoil sampling to characterize field-scale soil salinityvariation.

ECa measures the conductance through not only the soilsolution but also through the solid soil particles and viaexchangeable cations that exist at the solid–liquid interfaceof clay minerals. The interpretation of ECa measurements isnot trivial due to the complexity of current flow in the bulksoil through the three conductance pathways–liquid, solid,and solid–liquid interface. Because of the three pathways of

Fig. 9 Schematic of ECa-directed soilsampling for mapping the spatial vari-ability of soil salinity.

Geophysical Methods 7

conductance, the ECa measurement is influenced by severalsoil properties–soil salinity, texture, water content, bulkdensity, and temperature. Detailed protocols for conductingan ECa survey to characterize the spatial variability of soilsalinity with ECa-directed soil sampling are provided byCorwin and Lesch.[9,10] A schematic of ECa-directed soilsampling is shown in Fig. 9.

CONCLUSION

Near-surface geophysical methods have become anincreasingly important tool for soil investigations in agri-cultural settings. The methods predominantly employedare resistivity, EMI, and GPR; however, optical reflec-tance and γ-ray spectroscopy are beginning to find morewidespread utilization. Furthermore, research indicatesthat other geophysical methods, such as cosmic-ray neu-tron probes, seismic, self-potential, magnetometry,nuclear magnetic resonance, etc., all exhibit potentialfor measurement of soil properties or conditions. Theapplication to soil surveys, precision farming, soil watercontent measurement, and soil salinity monitoring hasbeen described in this entry, but geophysical methodshave also been used for soil investigations in forestedareas, confined animal feeding facilities, and golfcourses, with new applications continuing to evolve.Development of multisenor platforms and sensorsmounted on UAVs will dramatically improve geophys-ical soil investigation capabilities with respect to fieldaccessibility and data interpretation. Consequently, thefuture of near-surface geophysics in soil science appearsvery promising.

REFERENCES

1. Allred, B.J.; Ehsani, M.R.; Daniels, J.J. General considera-tions for geophysical methods applied to agriculture. InHandbook of Agricultural Geophysics; Allred, B.J., Daniels,J.J., Ehsani, M.R., Eds.; CRC Press LLC: Boca Raton,2008; 3–16.

2. McNeill, J.D. Electromagnetic Terrain Conductivity Mea-surement at Low Induction Numbers – Technical Note TN-6; Geonics Limited: Mississauga, 1980; 15 pp.

3. Conyers, L.B. Ground-Penetrating Radar; AltaMira Press:Walnut Creek, 2004; 201 pp.

4. Viscarra Rossel, R.A.; Adamchuk, V.; Sudduth, K.; McKenzie,N.; Lobsey, C. Proximal soil sensing: An effective approachfor soil measurements in space and time. In Advances inAgronomy 113; Sparks, D.L., Ed.; Academic Press: SanDiego, 2011; 237–282.

5. Doolittle, J.A.; Butnor, J.R. Soils, peatlands, and biomoni-toring. In Ground Penetrating Radar: Theory and Applica-tions; Jol, H.M., Ed.; Elsevier: Amsterdam, 2008; 179–202.

6. Doolittle, J.A.; Brevik, E.C. The use of electromagneticinduction techniques in soils studies. Geoderma 2014,223–225, 33–45.

7. Jol, H.M. Ground Penetrating Radar: Theory and Applica-tions; Elsevier Science: Amsterdam, 2009; 524 pp.

8. Huisman, J.A.; Hubbard, S.S.; Redman, J.D.; Annan, A.P.Measuring soil water content with ground-penetrating radar:A review. Vadose Zone J. 2003, 2, 476–491.

9. Corwin, D.L.; Lesch, S.M. Characterizing soil spatial vari-ability with apparent soil electrical conductivity: I. Surveyprotocols. Comput. Electron. Agr. 2005, 46 (1–3), 103–133.

10. Corwin, D.L.; Lesch, S.M. Prorocols and guidelines forfield-scale measurement of soil salinity distribution withECa-directed soil sampling. J. Environ. Eng. Geoph. 2013,18 (1), 1–25.

8 Geophysical Methods