The effect of clay seams in borehole GPR attenuation tomography

Shallow borehole GPR measurements can be of precious aid in geotechnical and environmental studies when the topmost soils show high conductivity and prevent penetration of EM energy from the surface. However, in many sedimentary environments, clay seams or lenses are likely encountered along the borings. The presence of such seams affects the radiation pattern of the antennas, thus corrupting the attenuation tomography results. The influence of small scale clay units on attenuation tomographic results are appraised through numerical modeling and comparison with field data. Borehole GPR measurements were performed to complement the site characterization of a planned expansion of a cement plant. A basic geological model is built from interpretation of these data. Forward modeling is performed with clay lenses of various sizes successively inserted along the transmitting borehole. The results show that the travel times are almost unaffected by the presence of clay. On the other hand, amplitudes are more severely distorted. It is found that the presence of clay can either reduce or amplify the first arrivals, depending on the position of the transmitter and receiver pair relative to the clay seam. The synthetic amplitudes and travel times are inverted, thus allowing the construction of a synthetic conductivity map for the various seam models. These maps show that whatever the size of the lens, the quality of the reconstruction is affected. Therefore, conclusive observations made from attenuation tomography based on amplitude inversion are hazardous in the presence of small scale heterogeneities near the boreholes.