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Offset vertical radar profiling

Alan Witten, School of Geology and Geophysics, University of Oklahoma, Norman, OK

John Lane, U.S. Geological survey, Office of Ground Water, Branch of Geophysics, Storrs, CT

Introduction

Microbial digestion of organic contaminants is an in situ means for bio remediation that is currently under development and evaluation. Vegetable oil has been proposed as an agent to stimulate the natural microbial community. This system becomes anaerobic as the vegetable oil is consumed providing the necessary conditions for metabolizing volatile organic compounds (VOCs). In order for this process to be effective in remediating groundwater contamination, the vegetable oil must be introduced into contamination either directly or by creating a "curtain" down gradient.

There are several ways in which such a remediation can fail. One of these is that the vegetable oil stimulant does reach the desired area. As part of a larger program to evaluate the efficacy of the above-cited form of in situ bio remediation, geophysical techniques were employed to remotely characterize the spatial distribution of vegetable oil injected into a shallow unconfined area. The site is a riverfront park on the eastern bank of the Mississippi River and immediately down gradient of known VOC contamination. Approximately 100 m from the river a closely space cluster of wells was developed. Three inline wells were used for vegetable oil injection and approximately 13,700 liters of vegetable/groundwater mix was introduced into these wells over a 3 m screened interval at a depth of approximately 8 m.

As part of this project, downhole radar, cross-well radar tomography and vertical radar profiling (VRP) data sets were acquired for subsequent interpretation and processing. Here, the VRP aspect of the study is presented. In VRP, a receiving antenna is moved vertically in a well for an array of transmitting antenna positions arranged along a line on the ground surface radially outward from the receiver well. Many of the wells in the cluster were used for this purpose in the study. Unlike the more traditional reflection-based radar data, VRP is almost impossible to directly interpret. The are a number ways to reconstruct images of spatial variations in wave speed or attenuation and the one selected for implementation here is diffraction tomography (Devaney, 1984). The following section presents an overview of the diffraction tomography algorithm for VRP, followed by a description of data acquisition, processing steps and the imaging results.


Final copy as submitted to Leading Edge for publication as: Witten, A., and Lane, J.W., Jr., 2003, Offset vertical radar profiling: Leading Edge, v. 22, no. 11, p. 1070-1076.


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