1. The use of borehole radar tomography to monitor a steam injection pilot study in a contaminated fractured limestone (Maine, USA) C. Grégoire, J.W. Lane and P.K. Joesten
U.S. Geological Survey, Branch of Geophysics
K.U.Leuven, Department Civil Engineering

2. Outline Steam-enhanced remediation (SER) technique
Modeling: effect of heating on the medium properties
Fluid-temperature logs
Borehole radar tomography
Conclusions

3. Objectives SER pilot project in an old limestone quarry, in Maine, USA
Evaluation of radar methods for monitoring the movement of steam and heat through fractured bedrock

4. SER Technique Remediation of NAPL contaminated sites in porous media
Heating, vaporisation and removal of volatile and semi-volatile contaminants from the vadose and saturated zones
At the Loring AFB, ground water underlying the bedrock is contaminated with chlorinated solvents and petroleum products

5. SER Technique at the Loring AFB
Several injection wells on the Northeast of the site
Number of extraction wells across the entire site Injection at depths from 20 m to the bottoms of the boreholes (30-40m)
Start of injection and extraction September 1, 2002
Injection continued until November 19, 2002
Extraction until November 26, 2002

6. Location of the boreholes at the Loring Air Force Base Site (Maine, USA)

8. Modeling: Effect of heating on the medium properties
Heating effects:
Heating of water present in the fracture system, with eventual replacement by steam - change of fluid permittivity and conductivity
Variation of limestone matrix conductivity
Variation of velocity and attenuation of radar waves

9. Modeling: Temperature effect
Temperature 20ºC to 100ºC: Small increase of velocity
increase of attenuation (porosity 2%)
Steam replaces water: Increase of velocity and
decrease of attenuation (porosity 2%)

10. Modeling: effect of limestone matrix conductivity

11. Modeling: effect of water conductivity

12. Modeling: effect of water permittivity

13. Modeling: results Heating of medium affects (results porosity 2%):
(1) Limestone conductivity increases ( S/m):
v decreases ( m/us) a increases ( dB/m)
(2) Water conductivity increases ( S/m):
v decreases ( m/us) a increases ( dB/m)
(3) Water permittivity decreases (80-50):
v increases ( m/us) a increases ( dB/m)
(1) has the largest effect

14. Field Measurements
Surveys conducted before start of injection (August 2002), 10 days after start of the injection (September 2002) and near the end of the injection (November 2002)
Casing: ~7m steel casing at the top, solid fiberglass casing, capped at the bottom, sealed over the entire length by grout; filled with clean water
Borehole radar reflection
Borehole radar tomography (+ levelrun acquisition)
Fluid-temperature logs before each radar survey
Deviation logs

15. Fluid-temperature logs

16. Fluid-temperature logs
Measure of water temperature in boreholes: for the safety of the equipment and personnel.
September 2002 (10 days after start of steam injection): temperature was unchanged in well JBW-7816; small increases in well JBW-7817A.
November 2002 (near the end of steam injection): increase of a max. of 10°C in well JBW-7816; increase above 40°C in well JBW-7817A.
Larger temperature changes were expected in JBW-7817A because closer to the steam injection well I-4.

17. Borehole radar tomography
Data collected between wells JBW-7816 and JBW-7817A
Identification of velocity and attenuation changes
Acquisition: intervals of 20 cm (after 1 m)
Levelrun acquisition every 20 cm used for data calibration
Inversion of travel-times differences due to the very small changes between the 3 datasets
SIRT algorithm; inversion of levelrun data as starting model

18. Data calibration Calibration of levelrun data Time-zero correction
Amplitude correction due to change in antenna power
Calibration of the tomography data
Deviation in times: correction based on the levelrun data
Amplitudes variation: correction based on the levelrun data

19. Calibration of amplitudes (levelrun)

20. Calibration of traveltimes (tomography data)

21. Inversion of difference time data

22. Inversion of difference attenuation data

23. Results
Increase of slowness ( usec/m) at the end of the injection, at larger depths (below 22 m); velocity decreases
Increase of attenuation (0.7 dB/m) at the end of the injection, at larger depths (below 22 m)
Consistent with the increase of temperature at these depths
Consistent with the modeling (largest effect is due changes in the limestone matrix conductivity because of the low porosity)

24. Conclusion Borehole radar tomography:
Calibration of levelrun and tomography data was necessary.
Inversion of difference time data: small increase of slowness ( usec/m) associated with heating.
Inversion of difference attenuation data: increase of attenuation (0.7 dB/m) associated with heating.
The radar tomography detected thermally induced variations in the fractured limestone.

25. The study was supported by the U. S
The study was supported by the U.S. Geological Survey (USGS) and the KULeuven (University of Leuven, Belgium).
The study was funded by the U.S. Environmental Protection Agency (USEPA) Office of Solid Waste and Emergency Response, Office of Superfund Remediation and Technology Innovation and by the USGS Toxic Substances Hydrology Program.