GPR attenuation tomography for detecting DNAPLs

Using the ground penetrating radar (GPR) to detect organic contaminants in the geological formations is a relatively new technique for environmental site characterization. We present here an attenuation tomography method that could be used to image subsurface organic contaminants like DNAPLs. This method is based on centroid frequency shift when the electromagnetic (EM) waves propagate in subsurface media. In most natural materials, EM wave attenuation increases with frequency. As a result, the centroid of the signal's spectrum experiences a downshift during propagation. This downshift is proportional to a path integral through the attenuation distribution and can be applied to the GPR data to tomographically reconstruct the attenuation distribution. The frequency shift method is applicable to any data when the signal bandwidth is broad enough and the attenuation is high enough to cause noticeable losses of high frequencies during propagation. In comparison to some other methods of estimating attenuation the frequency shift method is relatively insensitive to geometric spreading, reflection effect, source and receiver coupling, radiation patterns, and instrument responses. High attenuation has been regarded as the major hurdle for using GPR in areas with high electric conductivity, this method provides an alternative view to the attenuation.