The multifold acquisition principle was applied to a borehole radar survey, performed in a granitic site (Grimsel Test Site, Switzerland). Two multifold coverage acquisitions (40‐fold and 20‐fold) were carried out in a subhorizontal borehole. Instrumental drifts (transmission time and sampling frequency fluctuations) were corrected in order to remove shifts observed on CMP gathers and to optimize velocity analysis and trace stacking. Computation of velocity spectra was adapted in order to take into account the features of the medium investigated (homogeneous velocity, various reflector orientations). The NMO velocities were then interpreted as angles between reflectors and the survey line. The processing, based on the computation of several constant velocity stacked sections performed with different NMO velocities, leads to better results than the standard DMO + NMO processing. The signal‐to‐noise ratio of the stacked profile is improved in comparison with the single‐fold section, which results from a standard acquisition. From a practical point of view, the implementation of a multifold radar survey within a borehole is difficult but a greater investigation range is obtained, more reflectors are detected and the mapping of geological discontinuities is improved.
A case history illustrating an innovative application of borehole radar tomography to foundation problems in a karstic area is presented. The principles of the method and the geotechnical interpretation of the results are discussed. It is shown that, although ground conditions may be considered as very unfavourable for reflection radar techniques, good results were achieved in the crosshole mode, giving valuable information on both the geometry and the characteristics of the subsurface.
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