The detection and characterization of buried cables and metal pipes has become a key component of field surveys carried out prior to excavation work on construction sites. The very high conductivity and magnetic permeability contrast between any buried cables/pipes compared with the soil makes electromagnetic induction (EMI) instruments very useful for their detection. We have developed a seminumerical method that can be used to model the responses of this type of target. A straight horizontal conductor is equivalent to a series of magnetic dipoles, the magnitude of which can be determined in the spectral domain and then converted back into the spatial domain through the use of an inverse fast Fourier transform. Simulations and case studies allow to establish rules of thumb for the estimation of (1) the nature of the metal: the in-phase response of magnetic cables is of the opposite sign from the conducting ones, (2) the sensitivity to the target characteristic: the influence of the cable/pipe diameter is greater than that of the metal properties, and (3) the depth of the cables. The simulations also underline the role of the coil configuration: Vertical coplanar and perpendicular responses allow a more precise location of the cable/pipe, whereas the horizontal coplanar response is less dependent on the orientation. As ground truth, a known electric cable buried at a depth of 0.5 and 0.002 m in diameter is determined at 0.56 m. The first field test is related to the detection of a buried military cable from World War I, between 2.5 and 3 m below the original ground level. The second field test is related to the detection of a water pipe 0.35 m deep. The modeling technique can be applied to all EMI prospecting methods, and thus it opens the way to the correction of the disturbances generated by cables and pipes.

We have developed an inversion process of electromagnetic induction (EMI) data based on a two-step approach with 1D inversion of the entire studied surface and a fast 3D inversion applied over limited areas. This process is similar to that formerly used in resistivity prospection. For the study of soil (environmental, engineering, or archaeological explorations), low-frequency electromagnetic instruments (referred to as Slingram EMI) have highly useful specificities. They are light, are easy to move in the field, and can simultaneously measure the ground’s electric conductivity and magnetic susceptibility; they have thus been used to map these properties over large surface areas, within relatively short periods of time, and at reasonable expense. The possibility of combining several coil geometries has opened up the potential for multidepth techniques and systematic 1D inversion, which are found to be sufficiently revealing to allow larger portions of surveyed areas to be analyzed. In the “targeted areas” selected for 3D inversion, the geometries of the 3D features and the resistivity and/or susceptibility contrasts are determined. This step is based on the method of moments, where only 3D heterogeneities are meshed, and only a small number of major characteristics, such as contrast, thickness, width, etc., are sought. We first applied this process to synthetic data, then to data acquired at an experimental test site, and finally to field cases. The rapid 3D inversion complements the 1D inversion by solving a series of issues: correction for the apparent anisotropy generated by the instrument configuration, multiarched anomalies, precise location of lateral changes, and determination of the properties contrasts. Our inversion results highlighted the importance of the instrument geometry. We also have determined that apparent magnetic susceptibility data can be more appropriate for the determination of the volume of man-made features and can be highly complementary to conductivity data.