Using Ground Penetrating Radar to Detect Oil in Ice and Snow

Using Ground Penetrating Radar to Detect Oil in Ice and SnowE. Babcock1, J. Bradford1, H.P. Marshall1, C. Hall2, and D.F. Dickins3

1Department of Geosciences, Boise State University, Boise ID; 2Alaska Clean Seas, Anchorage AK; 3P.Eng., DF Dickins Associates Ltd., La Jolla CA

OverviewGround Penetrating Radar (GPR) theoryConsiderations for detecting oil under ice and snowDemonstrations in controlled environment spill responseFuture work

Brief History of GPR (Olhoeft, 2006)1926: Radar used to sound the depth of an alpine glacier in Austria (Stern, 1929)1958: USAF airplane crashed on Greenland ice sheet as radar energy passes through surface to layers below1960s: GPR used to sound moon during Apollo 171970s: Begin widespread use of GPR as a geotechnical tool1980s: GPR assessed as tool for oil detection under ice(Goodman et al., 1985 and 1987)

3Add refFundamentals of GPRGPR uses electrical energy to interrogate the subsurfaceOperates at radio frequencies10 MHz to 1 GHzTransmit timed pulses of EM energy; measure reflected returns, process data, and display

Annan, 2002.

4Material Electrical Properties in the Arctic Marine Environment MaterialRelative Dielectric PermittivityConductivity (S/m)Velocity (m/ns)Wavelength @ 500 MHzAir100.360 cmSea Water881-5No propagationNo propagationSea Ice4-8.01 - 0.10.134-0.15027 cmSnow1.4 3.10.0000010.25 - 0.168

50 cmOil2-40.00001-0.00050.21242 cm5Emphasize difference between water and oil, as oil replaces water in a small volume under the ice, the reflectivity changes dramatically

Sea ice is strongly anisotropic due to preferential alignment of brine channels. Snow is not homogeneous but is weakly isotropic. These demonstrate some of the challenges we face in oil detectionINSERT PICS

6Fundamentals of GPR: Governing EquationsSimplifying assumptions applied to Maxwells equations result in the wave equation which represents travel of EM energy in the subsurface:

Fundamentals of GPRSensitive to changes in electrical propertiesElectrical permittivity (velocity)Electrical conductivity (attenuation)Contrasts in permittivity can generate changes in reflection strength, or amplitudeConductivity attenuates GPR travelExamples: Ice/salt water interfaceWater/oil contrastGPR for Oil Spill ResponseCan we detect oil under ice and/or snow?What processing do the data require?What resolution can the system provide?What limitations do we experience?What benefits does this technology provide?

System Considerations: Data ProcessingUse standard basic processing stepsTime zero shiftBandpass filterSpherical spreading correctionAttribute analysisInstantaneous phase and frequencyReflection strengthPrevious work with GPR noted potential using attribute analysis to detect oil that was not possible with conventional analysis10System Considerations: Antenna FrequencyFrequency for radar survey is a trade-off Depth of penetrationQuality of resolutionSystem portabilityField testing shows that GPR frequency of 500 MHz is optimal for penetration and resolution of oil under ice11System Considerations: Resolution and DetectionUsing 500 MHz antennasDetect 1-2 cm oil layer in most scenariosResolve 4-5 cm oil layerThin bed analysis problemReflection analysis alone not enough to accurately locate oilPrevious work had indicated attribute analysis as possible solution (Goodman et al., 1985)Consider attributes in conjunction with modeled responseSystem Considerations: Depth vs Resolution

Delete pic and add discussion on using thin bed analysis to get 1-2 cm detectionSystem Considerations: Non-Uniqueness

From Bradford et al., 2008Here emphasize that we can detect down to about 10% change in our reflection strength given usual noise levels and potentially much lower, thus we can see that for these ranges of permittivity and typical arctic snow densities we can detect reflection anomalies of oil in snow.

Direct your attnetion to the plot on the lower right, where you can see the change in reflectivity associated with increasing oil content where oil is present in snow. Depending on snow conditions then we can see that we can detect very low levels of oil content as changes in the reflectivity.14System Considerations:Anisotropy

Data courtesy of Alaska Clean Seas

Control Module (Digital Video Logger)- Sensors and Software PE Prowww.sensoft.ca16

2008 Training on North Slope

Prudhoe Bay, April 2007

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Pulse Ekko Pro GPR500 and 1000 MHz antennasMulti-offset acquisition to determine effective permittivity of icePre- and post- oil emplacement 3D surveying over 20 x 20 m gridLarge scale 2D profilingNorway, 200618GPR for Oil Spill Response: Svalbard

From Bradford et al., 200819Controlled Spill, New Hampshire, 2004,2011-2013Cold Regions Research and Engineering Lab (CRREL), 2011 and 2012Indoor and outdoor testingKnown ice thicknessKnown oil locations500 MHz PE Pro System

9 m x 40 m cold pool7, 2x2 m isolated test cells35 cm ice thickness

20From Bradford et al., 2010GPR for Oil Spill Response: CRREL

From Bradford et al., 200821Time slice on upper right shows high points on oil/ice interface, demonstrates oil detection outside containment cells

Attribute analysis: the containment cell diagrams overlay the corresponding locations on these diagrams with attributesHave anomalies that cover about 80 percent of where we have oil but also get false positives

Also emphasize the blind spots where oil escaped curtainGPR for Oil Spill Response: CRREL 2012

Fix this use background removal tool in matlab to get better combined data!!!!!

Emphasize false positive on lower left and that the high spots are best drill location, could combine with high point analysis22GPR Limitations in the Arctic EnvironmentVariations in sea-ice conductivity and anisotropySnow may generate spurious amplitude anomalies due to water or ice in snowpack: solution is non-uniqueWe can ameliorate these concerns by frequent data truing and cautious interpretation23Conclusions: What Can GPRDo For Us in Arctic Spill Response?

and future research24AcknowledgementsMy advisors John Bradford and HP MarshallCRREL and all the hardworking staff there thanks!Alaska Clean SeasDF Dickins Associates LtdCurrent funding provided byAlaska Clean SeasConoco PhillipsExxonMobilShell OilStatoilReferencesAnnan, A.P. 2005. Ground-Penetrating Radar. In Near Surface Geophysics, Investigations in Geophysics No. 13. Butler, D.K., Ed. Society of Exploration Geophysicists, Tulsa, OK.

Annan, A.P. 2002. GPR History, Trends, and Future Developments. Subsurface Sensing Technologies and Applications, 3(4): 253-271.

Bradford, J.H. and J.C. Deeds. 2006. Ground penetrating radar theory and application of thin-bed offset-dependent reflectivity. Geophysics, 71(3): K47-K57.

Bradford, J.H., D.F. Dickins, and P.J. Brandvik. 2010. Detection of snow covered oil spills on sea ice using ground-penetrating radar: Geophysics, 75, G1-G12, doi:10.1190/1.3312184.

Bradford, J. H., D. F. Dickins, and L. Liberty. 2008. Locating oil spills under sea ice using ground-penetrating radar: The Leading Edge, 27,14241435.

Martinez, A. and A.P. Byrnes. 2001. Modeling Dielectric-constant values of Geologic Materials: An Aid to Ground-Penetrating Radar Data Collection and Interpretation. Current Research in Earth Sciences, Bulletin 247. Online at http://www.kgs.ukans.edu/Current/2001/martinez/martinez1.hmtl

Olhoeft, G.R. 2006. Applications and Frustrations in Using Ground Penetrating Radar. IEEE AESS Systems Magazine, 2: 12-20.Questions?26