The main purpose of this paper is to determine the minimum depth value beyond which it is possible to state that borehole-radar data are not significantly affected by the lateral wave and the wave reflected at the ground-air interface. This depth value is of paramount importance for cross-borehole electromagnetic probing. Three methods of solving the problem in question are presented: a numerical method, a graphical method and an analytical method. Five practical examples illustrate the use of the methods. Thus the advantages of the proposed methods over the classical way of solving the same problem are stressed. Thanks to the new methods, it is possible to avoid the unfruitful tasks of collecting and processing data that will be discarded later, when it is discovered that they are significantly degraded by the ground-air interface. The costs of the unproductive tasks may be high in geotomographic electromagnetic surveys owing to the large amount of data they typically involve. Practical examples show that the proposed methods make easier the planning of such surveys and the interpretation of geotomographs. The cause of artifacts in a geotomograph cannot be ascribed to the ground-air interface when the methods indicate that its influence over the borehole-radar data is negligible. The plane of incidence contains both boreholes. An electric dipole, placed in a borehole, can emit transverse magnetic (TM) waves. An axial slot on a conducting cylinder can emit transverse electric (TE) waves from a borehole. The proposed methods apply to both these cases.
Geoprocessing; Interface; Borehole; Polarimetry; Georadar
