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Advances in radiological survey capabilities for large sites are discussed.

Radiological Site Characterizations:Gamma Surveys, Gamma/226RaCorrelations, and Related SpatialAnalysis TechniquesRandy Whicker,* Paul Cartier,† Jim Cain,‡ Ken Milmine,§ and Michael Griffin§Abstract: Radiological surveys of a ura-nium mill site in Colorado and several pro-posed uranium recovery sites in Wyomingwere conducted in 2006 and 2007. Advance-ments in Global Positioning System (GPS)-based gamma scanning systems combinedwith gamma/226Ra correlations and Geo-graphic Information Systems (GIS)-basedspatial analysis techniques produced compre-hensive and detailed characterizations of thespatial distributions of gamma exposure ratesand 226Ra concentrations in surface soilsacross extensive study areas. Aside from lim-itations on gamma-based estimates of soil226Ra related to soil heterogeneity or gammashine effects, soil sampling results to dateshow good general agreement between esti-mated and measured values. Spatial charac-terization aspects of the survey approach areclearly more effective than conventional gridsampling methods, particularly for such largesites. Example project applications, data col-lection and analysis methods, challenges en-countered, and resulting mapped estimates ofvarious aspects of these radiological parametersare presented. Health Phys. 95(Supplement 5):S180–S189; 2008

Key words: operational topics; surveys;226Ra; soil

INTRODUCTIONRemediation of uranium min-

ing/milling sites or other siteswhere naturally occurring radio-active materials are present usu-ally requires characterizations ofgamma exposure rates and 226Raconcentrations in soil. Establish-ing pre-operational (background)and post-operational conditionsfor these radiological parametersis important for assessment of ar-eas requiring remediation. Pastapproaches include taking dis-crete gamma measurements andsoil samples across a systematicgrid pattern. A grid sampling ap-proach is indicated by the U.S.Nuclear Regulatory Commission(U.S. NRC) in Regulatory Guide4.14 for uranium mills (U.S. NRC1980), with 40 soil samples col-lected along a radial grid and 80individual discrete gamma mea-surements collected along a sim-ilar pattern.

More recent radiological surveyguidelines found in MARSSIM, theMulti-Agency Radiation Surveyand Site Investigation Manual

(U.S. NRC 2000), also indicate grid-based designs for soil sampling anddirect measurement of radionu-clides in soil, but the number ofsoil samples needed varies accord-ing to statistical requirements andcontinuous gamma scanning(rather than discrete gamma mea-surements) is used to augment thesoil sampling.

At some sites, natural back-ground soil 226Ra concentrationsare quite variable and may exceedlevels commonly used as cleanupcriteria. If such areas are not iden-tified prior to site operations,they can be misidentified duringdecommissioning as contami-nated areas in need of remedia-tion. Improvement in radiologi-cal characterization methods forbackground and potentially im-pacted areas can help improve as-sessment of areas in need of reme-diation and verification of theeffectiveness of that remediation.

Since the above mentionedagency guidance documents werepublished, advanced Global Posi-tioning System (GPS)-basedgamma scanning systems withautomated electronic data collec-tion have been developed andused in the field (Meyer et al.2005a and b; Johnson et al.2006). These systems can recordup to 3,600 individual gammareadings and corresponding GPSmeasurements per hour, providing

* Tetra Tech Inc., 3801 Automation Way, Suite100, Fort Collins, CO 80525; † Terrasat Inc., 1413West 31st Avenue, Anchorage, AK 99501; ‡ CotterCorporation, 0502 County Road 68, Canon City,CO 81212; § Uranium One, 907 North PoplarStreet, Suite 260, Casper, WY 82601.

Randy Whicker holds an MS degree in the radiological health sciences with aspecialization in radiochemistry and 13 years of radiological assessment work includ-ing a combination of applied research and environmental consulting projects. Cur-rently, Randy is an Environmental Health Physicist with the Radiation Protection andMeasurements Group at Tetra Tech, Inc. (Fort Collins, CO), providing expertise inproject planning, experimental design, field sampling, analytical field and laboratorytechniques, radiological measurements, and statistical analysis. His career efforts havesupported entities both public and private in the areas of academic research, environ-mental assessments and remediation, radiation protection, and litigation cases.Randy’s email is [email protected].

Operational Topic

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a detailed record of gamma expo-sure rate conditions acrossscanned areas. Multiple scanningsystems mounted on vehicles canquickly survey large areas andprovide a high spatial density ofmeasurements. This gamma sur-vey technology represents a sub-stantial increase in the amount ofradiological information that canbe efficiently collected relative totechnology available when earlieragency guidance documents werepublished.

Gamma surveys of a uraniummill site in Colorado and severalproposed in-situ recovery (ISR)uranium project areas in Wyo-ming were conducted in 2006and 2007 using multiple GPS-based gamma scanning systemsmounted on off highway vehicles(OHVs). In conjunction withthese surveys, correlations be-tween gamma readings and 226Raconcentrations in surface soils(0 –15 cm) were established.These correlations enabled spatialand statistical information aboutsoil 226Ra concentrations to be ex-tracted from the gamma surveydata to help meet various projectcharacterization objectives. Geo-graphical Information Systems(GIS) software was used for statisti-cal conversion of large survey datasets, interpolation with krigingmethods, field sampling support,special investigation/analysisneeds, and for data presentationpurposes.

The objectives of surveys at theuranium mill site were to developvarious probability-based esti-mates of the areal extent of sur-face soils having 226Ra concentra-tions in excess of pre-specifiedcleanup criteria. At the proposedISR uranium project areas, the objec-tive was to establish pre-operationalbaseline gamma exposure rates andsoil 226Ra concentrations for licens-ing/permitting applications. Theseproject objectives each have impli-cations with respect to eventual sitedecommissioning and terminationof radioactive source materials li-

censes. Continued improvement inmethods to characterize gamma ex-posure rates and soil 226Ra concen-trations at such sites can benefit allstakeholders.

MATERIALS ANDMETHODSGamma surveys

Various automated, GPS-basedscanning system configurationshave been developed for differentsite conditions. For projects dis-cussed in this paper, two YamahaRhino (Yamaha Motor Corp.,6555 Katella Avenue, Cypress, CA90630) OHV-mounted systemswere used (Fig. 1). Given the largesize of these sites, along with oc-casional rugged terrain, tall vege-tation and other obstacles, RhinoOHVs were well suited for theseprojects. Backpack scanning sys-tems were also used in a few smallareas inaccessible to OHVs.

These OHVs are equipped withadjustable outriggers designed tomount three 5 � 5 cm sodiumiodide (NaI) scintillation gammadetectors (Ludlum Model 44-10;Ludlum Measurements, Inc., 501Oak Street, Sweetwater, TX 79556)and paired GPS receivers. Thegamma detectors are coupled toLudlum Model 2350 rate metershoused in a container in the cargobed. Simultaneous GPS andgamma exposure rate data are re-corded every 1–2 s using an on-board PC with special data acquisi-tion software (comReader; TetraTech, 3801 Automation Way, FortCollins, CO 80525).

System configuration involvesabout 2.5 m spacing between de-tectors (measured perpendicularto direction of travel), with eachdetector positioned at either 1 or1.4 m above the ground surface.For many of these projects a de-tector height of 1.4 m was thelowest practical height for thesystem under site conditionsgiven the need for adequate clear-ance of frequently encounteredobstacles such as tall vegetation,ravine crossings, and other fea-tures. As discussed later in thispaper, experimental measure-ments were performed as neededto model approximate equivalentreadings as measured by a high-pressure ionization chamber(HPIC) at 1 m above the groundsurface (Fig. 1).

Based on qualitative field ob-servations of detector responseunder similar measurement ge-ometries, the scanning trackwidth representing each vehicle’slateral range of general scanningsensitivity to elevated planar(non-point) source areas is esti-mated to be about 8 m across,perpendicular to the direction oftravel. Vehicle scanning speedsrange between 3 and 16 km h�1

depending on the roughness ofthe terrain, with a typical averagespeed of 6–10 km h�1.

Data are downloaded daily intoa project database and results areviewed each night with specialfield mapping software (GammaData Map Viewer; Tetra Tech, 3801Automation Way, Fort Collins, CO

Figure 1. Three-detector OHV-mounted scanning systems (left) and static HPIC cross-calibration measurements (right).

The Radiation Safety Journal Vol. 95, suppl 5 November 2008

Operational Radiation Safety S181

80525). This allows scan cover-age assessment and planning ona daily basis and helps to iden-tify any problems with systemsperformance.

For routine scanning acrosslarge areas, a target distance of100 m between vehicles is esti-mated to achieve about 14%ground scanning coverage. Forareas of particular interest,higher-density target coveragescan range from 25–100% but typ-ically involve a vehicle spacing of20 –30 m (35– 45% coverage).Practical considerations such assafety, terrain, and natural ob-structions often dictate actualdistances maintained betweensurvey vehicles.

HPIC/NaI cross-calibration

Gamma exposure rates mea-sured by NaI detectors are onlyrelative measurements as re-sponse characteristics of NaI de-tectors are energy dependent.True gamma exposure rates arebest measured with a less energydependent system such as theHPIC. Depending on the radio-logical characteristics of a givensite, NaI detectors can have mea-surement values significantly dif-ferent from corresponding HPICmeasurement values. NaI detec-tors are typically calibratedagainst a 137Cs source. At photonemission energies near that of137Cs (662 keV), relative detectorresponse is close to 100% (Lud-lum 2006). Under field scanningconditions at uranium recoverysites, a preponderance of lowerphoton energies can be presentdue to primary and secondaryscattered photons from naturallyoccurring terrestrial radionu-clides. At these lower photon en-ergies, response of NaI detectorsrelative to 137Cs is significantlygreater than 100% and NaI detec-tors will overestimate true expo-sure rates. In some locations, ter-restrial concentrations of gammaemitting radionuclides can bevery low and higher-energy cos-

mic sources can dominate detec-tor response resulting in underes-timates of true exposure rates.

NaI systems are useful becausethey can quickly and effectivelydemonstrate relative differences be-tween pre- and post-remediationgamma exposure rate conditions.Unless the same equipment andscanning geometry are used forboth surveys, however, it is nec-essary to normalize the data to acommon basis of comparison.This is the purpose of performingHPIC/NaI cross-calibration mea-surements. Cross-calibration en-sures that the results of futuregamma scans, which may use dif-ferent detectors, detector types, ormeasurement geometries, can bemeaningfully compared againstthe results of pre-operationalgamma surveys. HPIC/NaI cross-calibrations are also necessary incases where external dose assess-ments are part of survey objectives.

To perform HPIC/NaI cross-calibrations, static measurementsare taken at various discrete loca-tions covering a range of expo-sure rates representative of thesite. At each measurement loca-tion, 10–20 individual readingsfrom the HPIC and each OHV-mounted NaI detector are sepa-rately collected and averaged. Apicture of this process is shown inFig. 1 (right). The resulting pairedHPIC/NaI data are analyzed bylinear regression to enable con-version of NaI-based gamma sur-vey data to approximate 1 mHPIC equivalents.

Gamma/226Ra correlations

Depending on the nature andstrength of the relationship be-tween gamma exposure rates andsoil 226Ra concentrations at agiven site, statistical correlationscan be used to estimate approxi-mate soil 226Ra concentrationsacross the entire site based ongamma survey results.

Following methods describedin Johnson et al. (2006), correla-tion soil sampling is conducted as

composite sampling over 10 �10 m plots. Correlation plot loca-tions are selected to be represen-tative of the range of exposurerates found at the site, with addi-tional efforts made to select plotshaving relatively homogeneousgamma readings in the generalarea. Gamma survey maps areused to help determine appropri-ate locations. Within each plot,10 soil sub-samples are collectedto a depth of 15 cm then compos-ited into a single sample to givean average 226Ra concentrationover each 100 m2 plot. Samplesare sent to a qualified laboratoryfor 226Ra analysis.

Each 100 m2 soil sampling plotis also scanned using the sameOHV-mounted systems and de-tector configuration used to scanthe entire study area. The averageNaI gamma reading over eachplot is paired with the corre-sponding average 226Ra concen-tration for statistical regressionanalysis.

RESULTS ANDDISCUSSIONGeneral observations

Radiological survey study areasat individual sites ranged from75–4,358 hectares (185–10,770acres). Scanning rates rangedfrom about 12 to 135 acres h�1

depending on terrain and groundscanning coverage attained. Ingeneral, instrument quality con-trol (QC) charts and field QCcharts for scan systems demon-strated acceptable performance.In cases of unacceptable systemperformance, affected data wereeliminated from the project data-base and the system was not usedagain until the issue was resolved.

Although some cases of unex-pected and problematic resultswere observed during the courseof these projects, supplementaryfield investigations and/or addi-tional data analyses revealed possi-ble explanations and provided a ba-sis for appropriate ways to address

R. Whicker et al. Radiological site characterizations

S182 November 2008

related issues. Final 226Ra estimatesbased on gamma survey datahave thus far generally agreedwell with confirmatory soil sam-pling results.

Uranium mill site surveys

Survey activities at the ura-nium mill site included two sep-arate projects. The first involved a75-hectare portion of the sitescheduled for remedial action.The survey objective was to esti-mate the extent of areas withgreater than 80% statistical prob-ability of having surface soil 226Raconcentrations in excess of the re-spective cleanup criterion of 6 pCig�1 (222 Bq kg�1). Gamma scanresults are shown in Fig. 2 (top).

A GIS-based spatial analysisprogram was used to krig thegamma survey data in order toprovide continuous estimates ofgamma exposure rate readingsacross the study area and betterillustrate spatial distributions(Fig. 2, bottom). Kriging is ageostatistical interpolation proce-dure commonly used in variousearth sciences.

Correlation plot measurementsacross the study area initiallydemonstrated a statistically weaklinear relationship betweengamma reading and 226Ra soilconcentration. Horizontal andvertical heterogeneity in soil226Ra concentrations and/or scat-tered photons reaching thegamma detectors from underly-ing subsurface sources or areasadjacent to the correlation plots(i.e., gamma “shine”) may havebeen contributing factors to thisresult as the outliers all had un-usually low concentration resultsrelative to gamma readings.

To investigate potential rea-sons for weak initial correlationresults, correlation plots were re-scanned using a shielded (colli-mated) gamma detector. Shieldedmeasurements improved the cor-relation and revealed evidencethat 4 of the 14 correlation plotsmay have been significantly af-

fected by gamma shine from ad-jacent areas and/or subsurfacesources. When data from thesepotentially “shine impacted”plots were removed, the statisti-cal strength of the unshieldedcorrelation improved (Fig. 3)with an R-squared value nearly ashigh as the correspondingshielded correlation.

One-tailed upper and lower80% prediction limits for the cor-relation were separately calcu-

lated and plotted along with theregression line (Fig. 3). Gammavalues corresponding to thecleanup criterion for soil 226Raconcentration (6 pCi g�1) at theseprediction limits were used tocreate a soil 226Ra probability mapas shown in Fig. 4. This spatialinformation is being used to helpwith remedial action planning.The small circular omitted portionof the study area represents a linedpond that could not be surveyed.

Figure 2. Gamma scan (top) and kriged mapping results (bottom) for the remedial actionstudy area at the uranium mill site.



The second project at the ura-nium mill site involved a muchlarger portion of the site beyondthe smaller remedial action studyarea. The objective for thisproject was also to estimate theareal extent of soil 226Ra concen-trations exceeding the 6 pCi g�1

cleanup criterion, but in this casethe information was used to de-termine a conservative estimateof the volume of surface soils thatcould potentially require remedia-tion upon site decommissioning.This volume estimate will be usedto update remedial surety bondingand thus a more conservative 95%statistical probability for the esti-mate was needed.

As with the remedial actionsurvey project, initial results ofthe gamma/226Ra correlation de-veloped for the volume studyarea were relatively weak. Again,however, comparisons betweenshielded and unshielded gammadata for correlation plots revealeda few locations where gammashine may have contributed tothis result. When those data wereomitted from the analysis the sta-tistical strength of the regressionimproved (Fig. 5).

The UPL line in Fig. 5 indicatesthat for this study area a gammareading of about 23 �R h�1 has a95% statistical probability of com-pliance with the 6 pCi g�1 criterionfor soil 226Ra. An approximateboundary corresponding to 23 �Rh�1 was drawn on the krigedgamma survey map and confirma-tory soil samples were collectedjust outside this line to verify thereliability of the estimate. Krigedsurvey results with overlays of the95% UPL line and confirmatorysampling results are shown in Fig.6. Areas outside the 95% UPL lineabove 23 �R h�1 were not includedin the volume estimate becausethey are included in remediationplans. Note that the actual regres-sion line in Fig. 5 (rather than theUPL line) predicts that on average,areas with gamma readings of 23

�R h�1 will have corresponding226Ra soil concentrations of about3.2 pCi g�1. This prediction agreeswell with the confirmatory sam-pling results (Fig. 6).

Limitations on spatial andprobabilistic estimates regardingsoil 226Ra concentrations for theuranium mill site study areas in-clude uncertainty due to a lim-ited number of correlation plots,analytical uncertainty in themeasured correlation plot data,and significant potential for esti-mation error in areas where con-siderable gamma shine effects orsoil 226Ra heterogeneity exist. Forareas significantly influenced bythese latter conditions, character-ization using conventional gridsoil sampling approaches wouldlikely prove more effective pro-vided sufficient sampling densitywere used. The data suggest, how-ever, that such areas represent asmall fraction of overall study ar-eas and that the correlationmethod was an effective overallapproach.

An important lesson learnedfrom all project examples pre-sented in this paper is that corre-lation plot selection criteria arevery important. Careful evalua-

tion and planning must be exer-cised when selecting correlationplot locations to ensure that thedata are representative of therange of gamma values found atthe site, and that gamma read-ings in the general vicinity ofeach plot are as homogeneous aspossible. This can be difficult toachieve for locations selected torepresent higher readings as theseareas tend to be small with ahigher degree of small scale spa-tial variability. It is also desirableto try and avoid choosing loca-tions with nearby regions of sig-nificantly higher readings to helpavoid shine issues. A related prob-lem that is more difficult to ad-dress is that it is seldom possibleto predict areas that may be af-fected by shine from shallowlyburied subsurface materials.

Proposed ISR uranium project areasurveys

Because survey objectives atthe various proposed ISR ura-nium project areas in Wyomingwere focused on pre-operationalbaseline characterizations, NaI-based scan data were normalizedto 1 m HPIC readings to approx-imate true gamma exposure rates

y = 0.50x - 8.63

R2 = 0.81

0

5

10

15

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15 20 25 30 35 40 45 50 55 60

Mea

n 22

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80% LPL

Mean Gamma Reading (μR h-1)Figure 3. Correlation results for the remedial action study area at the uranium mill site.


S184 November 2008

and provide a common basis ofcomparison for post-operationalsurveys. Typically, HPIC/NaIcross-calibration curves demon-strated highly significant linearrelationships (Fig. 7, left). As il-lustrated at right in Fig. 7, thenumerical difference betweenNaI readings and HPIC readingswas proportional to the magni-tude of exposure rate being mea-sured (HPIC readings were mod-

eled based on the regressionequation shown at left in Fig. 7,and using a range of hypotheticalNaI readings as the independentvariable).

An example map of krigedHPIC equivalent gamma expo-sure rate survey data for a pro-posed ISR site in Wyoming isshown in Fig. 8. The use of krigedsurvey data overlays on aerialphotos can be an effective way of

illustrating distributional pat-terns of gamma exposure rates orsoil 226Ra concentrations in rela-tion to certain geomorphic fea-tures. Note that the lowestgamma exposure rates at the siteshown in Fig. 8 tend to coincidewith drainage channel basins. Ar-eas of higher gamma readingstend to coincide with areas ofhigher topographical relief suchas ridges or hill tops.

For these proposed ISR sites,cases of apparent spatial relation-ships between geomorphic fea-tures and baseline gamma expo-sure rates are likely related toerosional and depositional pro-cesses that may expose elevateddeposits of terrestrial radionu-clide concentrations at the sur-face, bury such deposits, or grad-ually transport elevated materialsoff site. Sometimes, transitionsbetween areas of consistentlyhigher and lower gamma expo-sure rates are relatively abrupt.Such transitions can occasionallybe associated with visible featureslike changes in slope, rock type,and soil color or texture (Fig. 9).In other cases, there are no obvi-ous features associated with areasof higher or lower readings orwith transition zones.

With respect to gamma-basedestimates of baseline 226Ra concen-trations in surface soils at proposedISR sites, conservative estimationusing statistical prediction limitson correlations was not relevant.Instead, actual regression equa-tions from correlation plot datawere used to provide the averageor “best” statistical estimates ofsoil 226Ra concentrations basedon the gamma survey data.

Relative to the Colorado mill sitesurveys, correlation plot measure-ments for proposed ISR sites inWyoming tended to demonstratestronger statistical relationshipsbetween gamma readings and soil226Ra soil concentrations. In general,fewer cases of unusually low 226Raconcentrations in areas of highgamma readings were observed.

Figure 4. Soil 226Ra probability map for the remedial action study area at the uranium millsite.

y = 0.25x - 2.55

R2 = 0.85

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Mea

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226 R

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Mean Gamma Reading (μR h-1)Figure 5. Gamma/226Ra correlation results for the volume study area.



Again, such cases are likely re-lated to gamma shine from adja-cent areas and/or subsurfacesources and those data were notused for the correlations.

Another notable feature of cor-relation results for the WyomingISR sites was that the data some-times demonstrated nonlinearcharacteristics (Fig. 10). Thisraised the possibility that use ofnonlinear “best fit” models insuch cases could reduce potentialprediction error for soil 226Ra esti-mates based on gamma surveydata.

Reasons for apparent nonlin-earity observed in correlationdata from some sites appear to be

related to a kind of threshold ef-fect in the relationship betweendetector response and the ratio ofterrestrial to cosmic sources ofgamma radiation. Cosmic sourcescan dominate detector responseuntil terrestrial sources becomeconcentrated enough to have sig-nificant correlative impact onreadings. This idea is consistentwith a comparison of observedcorrelation data between varioussites.

Sites with higher minimummeasured soil 226Ra concentra-tions (e.g., 4–5 pCi g�1) tended toexhibit linear correlation charac-teristics. Sites with lower mini-mum measured soil 226Ra concen-

trations (e.g., 1 pCi g�1) tended toexhibit nonlinear correlationcharacteristics, with relatively lit-tle change in 226Ra concentrationover the lower range of measuredgamma values until a kind ofthreshold is reached and 226Ra be-gins to increase with increasinggamma readings.

Reasons for this threshold ef-fect are likely partially related tothose mentioned in the earlierdiscussion of differences betweenNaI detector and HPIC readings.At a given site, cosmic sources arerelatively constant and variationsin NaI readings are due to varia-tions in terrestrial radionuclideconcentrations. When terrestrial226Ra sources begin to exceedabout 1 pCi g�1 at these sites, agreater percentage of lower en-ergy photons interact with theNaI detectors and relative re-sponse appears to cross a thresh-old between underprediction andoverprediction of true exposurerates. As gamma readings in-crease above this threshold, amore linear correlative relation-ship between 226Ra and gammareadings becomes apparent.

Despite the potential explana-tions above for an apparentthreshold effect, both linear andnonlinear models were used toconvert gamma survey data toestimates of 226Ra concentrationsin surface soils. Both data setswere kriged and mapped to helpassess which model at each site isbest supported by subsequent ra-dial grid soil sampling results(U.S. NRC Regulatory Guide 4.14soil sampling protocols are alsobeing implemented as part ofbaseline studies at these sites).This type of confirmation sam-pling can also help to assess therepresentativeness of correlationplot sampling locations.

Spatial differences in the distri-butions of estimated soil 226Raconcentrations based on linearand nonlinear models for a pro-posed ISR site are shown in Fig.11. In terms of remedial issues, the

Figure 6. Gamma survey results for the volume study area showing approximate regionswith gamma readings above and below 23 �R h�1, the gamma value with a 95% statisticalprobability of compliance with the 226Ra cleanup criterion. Confirmatory soil samplinglocations and annotated 226Ra results (pCi g�1, in parentheses) are also shown.

0

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Figure 7. Example HPIC/NaI cross-calibration curve (left) and corresponding modeleddifferences between NaI and HPIC readings (right) for a proposed ISR uranium site inWyoming.


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implications of which predictivemodel is used are quite apparentat this particular site. Regardlessof what model is ultimately used,it is unlikely that areas with ele-vated radiological baseline condi-tions would be adequately char-acterized based solely on gridsampling as indicated by currentlyapplicable regulatory guidelines.These elevated areas are generallydownwind of the proposed plantlocation and often fall just out-side of respective radial grid sam-pling locations as indicated inRegulatory Guide 4.14. This ob-servation highlights a key advan-tage of using GPS-based, high-density gamma scanning andcorrelation techniques to charac-terize entire sites.

Available data to date have en-abled one proposed ISR site to beevaluated with respect to whichtype of predictive model is moststrongly supported by confirma-tory soil sampling results. Overall,a nonlinear model predicted soil226Ra concentrations at this sitemore accurately than a linearmodel. Nonlinear modeling esti-mates and actual soil sampling re-sults are shown in Fig. 12. Optimalspatial detail at individual sam-pling locations is not resolved inthis figure but locally enlargedviews of the data indicate that

Figure 8. Kriged 1 m HPIC equivalent gamma survey map of a proposed 1,618 hectare(4,000 acre) ISR uranium project area in Wyoming.

Figure 9. Visible, geomorphic boundary delineating abrupt transition in gamma expo-sure rates.

y = 0.39x - 8.10R2 = 0.84

02468101214161820

10 20 30 40 50 60

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y = 0.012x2 - 0.45x + 5.24R2 = 0.94

02468101214161820

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i g-1

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Mean Gamma Reading (μR h-1)Figure 10. Comparison of linear (left) and nonlinear (right) models fitted to combined gamma/226Ra correlation plot data from two nearbyISR sites in Wyoming.



differences between modeled andmeasured values are generally lessthan �1 pCi g�1, not greatly differ-ent from analytical uncertaintiesreported by the laboratory (whichranged up to �0.6 pCi g�1). Asmentioned, however, not allsites demonstrate nonlinearcorrelation characteristics andcorrelation data need to be ade-quately representative to have

the best chance of choosing theappropriate model.

Finally, caution must be exer-cised with respect to extrapolat-ing predictive models beyond therange of measured correlationdata. In these studies, predictiondata outside this range weresometimes artificially truncatedto avoid such extrapolation, de-pending on the nature of the cor-

relation and respective potentialto significantly impact kriging re-sults. In all cases, the validity ofgamma-based estimates of 226Raare limited to the range of mea-sured correlation data and be-yond that range only generalqualitative statements such as“less than” or “greater than” arejustified. Furthermore, limita-tions mentioned earlier for ura-nium mill site estimates also ap-ply to estimates developed for theproposed ISR uranium projectarea studies.

CONCLUSIONAlthough gamma/226Ra correla-

tion techniques are not new, theGPS-based scanning systems usedfor these projects involve morerecent technology that canquickly and efficiently collectlarge amounts of informationabout the spatial distribution ofterrestrial sources of gamma radi-ation across extensive areas.Mapped data presentations andconfirmatory soil sampling re-sults suggest that high-densitygamma scanning combined withcorrelation techniques was an ef-fective overall survey approachfor these projects and representsgeneral improvement in charac-terization capabilities for largesites.

Figure 11. Comparison of continuously estimated soil 226Ra concentrations based on linear (left) and nonlinear (right) models fitted togamma/226Ra correlation plot data for a proposed ISR site in Wyoming.

Figure 12. Comparison of continuous estimates of soil 226Ra concentrations predictedwith a nonlinear model vs. actual soil sampling results at a proposed ISR site in Wyoming.


S188 November 2008

Limitations on correlation-based 226Ra estimates includepotential prediction error in areaswith significant heterogeneity insoil 226Ra concentrations, gammashine effects, or areas beyond therange of measured correlationdata. Poor correlation results canresult from insufficient samplesize, inadequate representative-ness of correlation plot locations,soil 226Ra heterogeneity, orgamma shine. Nonlinearity incorrelation characteristics can re-sult at sites where pervasively low226Ra concentrations are reflectedin the measured correlation data,possibly due to a threshold effectbetween detector response andthe ratio of terrestrial to cosmicgamma sources.

Integrating a full range of GISspatial analysis capabilities intothis radiological survey approach

allows various and sometimessubtle types of information con-tained in the survey data to besuccessfully identified, inter-preted, and assessed with respectto project objectives. Kriging re-sults displayed on topographicalcontour maps or aerial photoscan provide detailed and highly in-formative characterizations of vari-ous radiological parameters acrossentire sites. This information canhave important implications withrespect to site decommissioning andlicense termination.

REFERENCESJohnson JA, Meyer HR, Vidyasagar M. Char-

acterization of surface soils at a formeruranium mill. Health Phys 90(Suppl1):S29–S32; 2006.

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