Frequency spectral analysis of GPR data over a crude oil spill

A multi-offset ground penetrating radar (GPR) dataset was acquired by the U.S. Geological Survey (USGS) at a crude oil spill site near Bemidji, Minnesota, USA. The dataset consists of two, parallel profiles, each with 17 transmitter-receiver offsets ranging from 0.60 to 5.15m. One profile was acquired over a known oil pool floating on the water table, and the other profile was acquired over an uncontaminated area. The data appear to be more attenuated, or at least exhibit less reflectivity, in the area over the oil pool. In an attempt to determine the frequency dependence of this apparent attenuation, several attributes of the frequency spectra of the data were analyzed after accounting for the effects on amplitude of the radar system (radiation pattern), changes in antenna-ground coupling, and spherical divergence. The attributes analyzed were amplitude spectra peak frequency, 6 dB down, or half-amplitude, spectrum width, and the low and high frequency slopes between the 3 and 9 dB down points. The most consistent trend was observed for Fourier transformed full traces at offsets 0.81,1.01, and 1.21m which displayed steeper low frequency slopes over the area corresponding to the oil pool. The Fourier-transformed time-windowed traces, where each window was equal to twice the airwave wavelet length, exhibited weakly consistent attribute trends from offset to offset and from window to window. The fact that strong, consistent oil indicators are not seen in this analysis indicates that another mechanism due to the presence of the oil, such as a gradient in the electromagnetic properties, may simply suppress reflections over the contaminated zone.