Travel Time Tomography Using Frequency Responses Collected By Cwr Experimentation

Abstract

Detecting and imaging dense non-aqueous phase liquids (DNAPLs) in the subsurface is a challenging problem that is of interest to the Department of Energy, Department of Defense, and several local and state agencies. Current DNAPL detection techniques, such as Direct Push Probe Technologies (DPT) and In-Situ Tracers (IST) have risks and limitations. Cross-well radar (CWR) is a radar-based geophysical technique with low invasiveness for real time monitoring of DNAPLs. This technique uses electromagnetic waves transmitted and received through antennas in the subsurface. The computerized tomography is applied to a pilot-scale experimental facility constructed by the authors (referred as SoilBED) in this research. Cross-tomography data at multiple depths and locations were collected to simulate a 1/100 scaled contaminated soil problem. The experimental results are converted to the time domain signals. The resulting signal travel times are compared with the simulated results by FDTD. The transformed signals are used to prepare signal intensity and travel time tomograms of the SoilBED cross-sectional slices, for background and scattered field to study the signature of the scatterers. The results are able to visualize the dielectric objects in the saturated soil. Introduction Cross-well radar is a potential technique for DNAPL detection. Most of the research and field works done in Cross-well radar and other radar based geophysical techniques deal with a single frequency. Selecting one single frequency may be effective for one non-dispersive medium, but perhaps not for other non-dispersive or most dispersive media. This research covers a broadband of frequency from 400 MHz to 2.2 GHz. Having broadband frequency data helps to either use the entire frequency range and work in frequency domain or select a single frequency or narrower frequency range that seems to have enough detection potential out of the entire observed frequency response. It is important to consider cost and practicality of the system. Establishing practical and cost effective systems require strong theoretical and laboratory background conditions. Therefore, both theoretical and experimental aspects of the research are studied in parallel, to evaluate the feasibility of DNAPL detection using CWR and to study wave propagation behavior and forward problem. Collecting repeatable and reproducible frequency response data is a very challenging process. This difficultly achieved data can be manipulated first or directly used towards image processing. As any research subject, conducting both experimental and theoretical approaches is the most desirable way to advance fundamental knowledge in the field of interest. Frequency response and arrival time can be converted to each other. Thus, frequency domain results can be transformed to time domain and used towards techniques such as travel time tomography and vice versa. The experimental frequency domain results can be converted to CTF (Channel Transfer Functions) and compared to the simulated frequency domain results by the first order Born approximation (Farid 2004, Farid et al. 2005, and Zhan 2005). The FDTD simulation code (Farid 2002 and 2003) is available as the theoretical tool to perform the comparison in time domain and study travel time. If the experimental results are converted to time domain and transformed to travel time, they can be compared to the FDTD simulated results. Travel Time Computed by FDTD To establish a better knowledge of wave propagation in soil, both dry and saturated backgrounds are studied. The sandy soil in the SoilBED facility with 3.88% moisture content is referred to as dry soil.