Characterization of buried cables and pipes using electromagnetic induction loop-loop frequency-domain devices

Abstract

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.