1. 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

2. Overview Ground Penetrating Radar (GPR) theory
Considerations for detecting oil under ice and snow
Demonstrations in controlled environment spill response
Future work

3. 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

4. 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.

5. Material Electrical Properties in the Arctic Marine Environment
Relative Dielectric Permittivity
Conductivity (S/m)
Velocity (m/ns)
500 MHz
Air 1
0.3
60 cm
Sea Water 88
1-5
No propagation
Sea Ice
4-8
27 cm
Snow
1.4 – 3.1
50 cm
Oil
2-4
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

6. INSERT PICS

7. 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:

8. 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

9. 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?

10. 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

11. 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

12. 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

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

14. 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

15. System Considerations: Anisotropy
Data courtesy of Alaska Clean Seas

16. Control Module (Digital Video Logger)
- Sensors and Software PE Pro

17. 2008 Training on North Slope
Prudhoe Bay, April 2007
2008 Training on North Slope

18. 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

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

20. 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

21. 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

22. 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

23. 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

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

25. 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

26. References
Annan, A.P 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 GPR – History, Trends, and Future Developments. Subsurface Sensing Technologies and Applications, 3(4): Bradford, J.H. and J.C. Deeds 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 Detection of snow covered oil spills on sea ice using ground-penetrating radar: Geophysics, 75, G1-G12, doi: / Bradford, J. H., D. F. Dickins, and L. Liberty Locating oil spills under sea ice using ground-penetrating radar: The Leading Edge, 27,1424–1435. Martinez, A. and A.P. Byrnes 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 Olhoeft, G.R Applications and Frustrations in Using Ground Penetrating Radar. IEEE AESS Systems Magazine, 2:
Questions?