Small-loop electromagnetic induction for environmental studies at industrial plants

The focus of this study is to analyse the reliability of using small-loop frequency-domain electromagnetic induction systems for characterizing buried storage tanks and pipes at industrial plants. As examples, we selected two areas of a chemical plant, one located outdoors and the other inside a room of reduced dimensions. We collected data employing different system orientations and acquisition directions, in order to compare the influence of environmental noise and neighbouring structures on each case. We found that the presence of a metallic gate or other metallic stuff in a neighbouring wall introduces strong distortions in the responses obtained near these objects. The responses decrease when the coils are coplanar with the wall and increase when they are perpendicular to it. Noise levels were higher for the data acquired indoors, but even in this case, we could enhance the signal-to-noise ratios up to very acceptable values by applying a novel spatial filtering technique. This improved the visualization of the anomalies associated with the targets. Finally, we generated pseudo 3D electrical models of the subsoil, by combining the results of the 1D inversions of the filtered data corresponding to the configuration that best evidenced the structures buried on each sector. In both areas, we obtained quite good approximate characterizations of the geometry, conductivity and depth of the detected tanks and pipes, as was later confirmed during remediation works. Remarkably, the model obtained for the area located indoors had enough resolution as to define the existence of two separate, adjacent tanks.