Journal of Earth Sciences and Geotechnical Engineering

GPR Q* Determination from CMP Data as a Means of measuring Frequency Dependent Attenuation

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• Abstract

  Attenuation and velocity in the ground are the major controls on the propagation of GPR signals in the ground and depend on electrical conductivity and dielectric permeability. In addition to these, attenuation of radar signals is frequency dependent; it increases with increase in frequency. The attraction of the GPR method lies in its short wavelength and hence good resolution, i.e. because of the high frequency used; but unfortunately, this makes it prone to attenuation thus limiting depth of penetration of the radar waves. It is therefore necessary that this dependence on frequency be understood, better still, site specific characterization is important. The attenuation coefficient (α, m-1), can be estimated using site specific laboratory or field estimates of electrical conductivity; this may not always be practicable, and hence further means of characterizing attenuation is required. This paper reports on frequency dependent attenuation characterization using the spectral ratios method. The method involves comparing spectra of reflection from successive intervals generated from a CMP section to estimate the GPR quality factor Q* and from there, attenuation. The analysis was applied to two CMP data sets acquired in a limestone quarry, along the quarry floor using 500MHz antennas. The analysis showed that attenuation increases with increase in frequency from 0.2m-1 at 200MHz up to 1.2 m-1 at 800MHz. The analysis serves to improve in – situ characterization of attenuation especially where it is required for further analysis e.g. amplitude variation with offset analysis of GPR data.