Small-loop electromagnetic induction for environmental studies at industrial plants

H. P. MARTINELLI; A. M. OSELLA

JOURNAL OF GEOPHYSICS AND ENGINEERING

IOP PUBLISHING LTD

Año: 2010 vol. 7 p. 91 - 104

1742-2132

Abstract Industrial plants are frequently required to perform analysis of the subsoil looking for evidences of fluid contaminants or unused old objects, which must be removed to avoid environmental damages. In particular, frequency-domain electromagnetic induction systems, composed of two fixed and coplanar small coils, could be adequate for mapping those kinds of targets and hence could be very useful for efficiently planning later remediation strategies. Although, in these environments, it becomes crucial to control the effect of cultural noise, since it can mask the signals produced by the targets. This study is focused on the analysis of the reliability of using this method for characterizing buried storage tanks and pipes in a chemical plant. As example, we selected two different areas: one located outdoors and other located 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. First, from the visualization of data, 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 those objects. An interesting conclusion is that the responses increase when the dipole moments of the coils are coplanar with the wall, and decrease when they are perpendicular to it. As expected, 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. After the filtering, all the detected anomalies associated with the targets become clearer. Finally, we performed 1-D inversions of the filtered data corresponding to the configuration that best evidenced those structures on each sector, and by combining those 1-D results, we generated pseudo 3-D electrical models of the subsoil. In both areas, we obtained quite good approximate characterizations of the targets. In particular, the geometry, conductivity and depth of the detected tanks were fairly well determined, as was later confirmed during remediation works. Remarkably, the inverse model obtained for the area located indoors had enough resolution as to define the existence of two separate, adjacent tanks.