Using Ground Penetrating Radar to Detect Oil in Ice and Snow E. 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

Overview Ground Penetrating Radar (GPR) theory Considerations for detecting oil under ice and snow Demonstrations in controlled environment spill response Future 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 below 1960s: GPR used to sound moon during Apollo 17 1970s: Begin widespread use of GPR as a geotechnical tool 1980s: GPR assessed as tool for oil detection under ice(Goodman et al., 1985 and 1987) Add ref

Fundamentals of GPR GPR uses electrical energy to interrogate the subsurface Operates at radio frequencies 10 MHz to 1 GHz Transmit timed pulses of EM energy; measure reflected returns, process data, and display Annan, 2002.

Material Electrical Properties in the Arctic Marine Environment Relative Dielectric Permittivity Conductivity (S/m) Velocity (m/ns) Wavelength @ 500 MHz Air 1 0.3 60 cm Sea Water 88 1-5 No propagation Sea Ice 4-8 .01 - 0.1 0.134-0.150 27 cm Snow 1.4 – 3.1 0.000001 0.25 - 0.168 50 cm Oil 2-4 0.00001-0.0005 0.212 42 cm Emphasize 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 detection

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Fundamentals of GPR: Governing Equations Simplifying assumptions applied to Maxwell’s equations result in the wave equation which represents travel of EM energy in the subsurface:

Fundamentals of GPR Sensitive to changes in electrical properties Electrical permittivity (velocity) Electrical conductivity (attenuation) Contrasts in permittivity can generate changes in reflection strength, or amplitude Conductivity attenuates GPR travel Examples: Ice/salt water interface Water/oil contrast

GPR for Oil Spill Response Can 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 Processing Use standard basic processing steps Time zero shift Bandpass filter Spherical spreading correction Attribute analysis Instantaneous phase and frequency Reflection strength Previous work with GPR noted potential using attribute analysis to detect oil that was not possible with conventional analysis

System Considerations: Antenna Frequency Frequency for radar survey is a trade-off Depth of penetration Quality of resolution System portability Field testing shows that GPR frequency of 500 MHz is optimal for penetration and resolution of oil under ice

System Considerations: Resolution and Detection Using 500 MHz antennas Detect 1-2 cm oil layer in most scenarios Resolve 4-5 cm oil layer Thin bed analysis problem Reflection analysis alone not enough to accurately locate oil Previous work had indicated attribute analysis as possible solution (Goodman et al., 1985) Consider attributes in conjunction with modeled response

System Considerations: Depth vs Resolution Delete pic and add discussion on using thin bed analysis to get 1-2 cm detection

System Considerations: Non-Uniqueness Here 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. From Bradford et al., 2008

System Considerations: Anisotropy Data courtesy of Alaska Clean Seas

Control Module (Digital Video Logger) - Sensors and Software PE Pro www.sensoft.ca

2008 Training on North Slope Prudhoe Bay, April 2007 2008 Training on North Slope

Norway, 2006 Pulse Ekko Pro GPR 500 and 1000 MHz antennas Multi-offset acquisition to determine effective permittivity of ice Pre- and post- oil emplacement 3D surveying over 20 x 20 m grid Large scale 2D profiling

GPR for Oil Spill Response: Svalbard From Bradford et al., 2008

Controlled Spill, New Hampshire, 2004,2011-2013 Cold Regions Research and Engineering Lab (CRREL), 2011 and 2012 Indoor and outdoor testing Known ice thickness Known oil locations 500 MHz PE Pro System 9 m x 40 m cold pool 7, 2x2 m isolated test cells 35 cm ice thickness

GPR for Oil Spill Response: CRREL From Bradford et al., 2010 Time 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 attributes Have anomalies that cover about 80 percent of where we have oil but also get false positives Also emphasize the blind spots where oil escaped curtain From Bradford et al., 2008

GPR 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” analysis

GPR Limitations in the Arctic Environment Variations in sea-ice conductivity and anisotropy Snow may generate spurious amplitude anomalies due to water or ice in snowpack: solution is non-unique We can ameliorate these concerns by frequent data truing and cautious interpretation

Conclusions: What Can GPR Do For Us in Arctic Spill Response? …and future research

Acknowledgements My advisors John Bradford and HP Marshall CRREL and all the hardworking staff there – thanks! Alaska Clean Seas DF Dickins Associates Ltd Current funding provided by Conoco Phillips ExxonMobil Shell Oil Statoil

References Annan, 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, 27,1424–1435. 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?