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Object-based inversion of crosswell radar tomography data to monitor vegetable oil injection experiments

John W. Lane, Jr.
Office of Ground Water, Branch of Geophysics
U.S. Geological Survey
11 Sherman Place, U-5015
University of Connecticut
Storrs Mansfield, CT 06269

Frederick D. Day-Lewis Dept. of Geology
Bucknell University
Lewisburg, PA 17837
Also at:
Office of Ground Water, Branch of Geophysics
U.S. Geological Survey
11 Sherman Place, U-5015
University of Connecticut
Storrs Mansfield, CT 06269

Roelof J. Versteeg
Idaho National Environmental and Engineering Laboratory
Idaho Falls, ID 83415

Clifton C. Casey
U.S. Navy, Southern Division NAVFAC, North Charleston, DC 29419

Abstract

Crosswell radar methods can be used to dynamically image ground-water flow and mass transport associated with tracer tests, hydraulic tests, and natural physical processes, for improved characterization of preferential flow paths and complex aquifer heterogeneity. Unfortunately, because the raypath coverage of the interwell region is limited by the borehole geometry, the tomographic inverse problem is typically underdetermined, and tomograms may contain artifacts such as spurious blurring or streaking that confuse interpretation.

We implement object-based inversion (using a constrained, non-linear, least-squares algorithm) to improve results from pixel-based inversion approaches that utilize regularization criteria, such as damping or smoothness. Our approach requires pre- and post-injection travel-time data. Parameterization of the image plane comprises a small number of objects rather than a large number of pixels, resulting in an overdetermined problem that reduces the need for prior information. The nature and geometry of the objects are based on hydrologic insight into aquifer characteristics, the nature of the experiment, and the planned use of the geophysical results.

The object-based inversion is demonstrated using synthetic and crosswell radar field data acquired during vegetable-oil injection experiments at a site in Fridley, Minnesota. The region where oil has displaced ground water is discretized as a stack of rectangles of variable horizontal extents. The inversion provides the geometry of the affected region and an estimate of the radar slowness change for each rectangle. Applying petrophysical models to these results and porosity from neutron logs, we estimate the vegetable-oil emulsion saturation in various layers.

Using synthetic- and field-data examples, object-based inversion is shown to be an effective strategy for inverting crosswell radar tomography data acquired to monitor the emplacement of vegetable-oil emulsions. A principal advantage of object-based inversion is that it yields images that hydrologists and engineers can easily interpret and use for model calibration.

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Final copy as submitted to Journal of Environmental & Engineering Geophysics for publication as: Lane, J.W., Jr., Day-Lewis, F.D., Versteeg, R.J., and Casey, C.C., 2004, Object-based inversion of crosswell radar tomography data to monitor vegetable oil injection experiments: Journal of Environmental & Engineering Geophysics, vol.9, issue 2, p. 63-77.