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Airborne EM Data Complement Magnetics in an Unexpected Way

Publisher —
Environmental and Engineering Geophysical Society (EEGS), 2010 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.

Authors —
Les P. Beard*, William E. Doll, Jacob R. Sheehan*, T. Jeffrey Gamey and Jeannemarie Norton, Battelle-Oak Ridge Operations, Oak Ridge, Tennessee; Monika Siwiak, Willy Van Vaerenbergh, AECOM Ltd., Brisbane, Australia.

Expanded Abstract — [pdf] ENV_SAGEEP_AustralianMoDSurvey_LPB_2010-HazardWasteMilitary

In the fall of 2008, a low-altitude airborne geophysical survey was carried out at a military installation in Australia, the primary goal of which was detection and mapping of infrastructure, buried wastes, and other buried and surface metallic objects within a 1738 hectare area. The primary survey system was the Battelle VG-16 vertical magnetic gradient system. Because the base is active, the time frame for conducting the survey was limited to a few weeks. The short time frame and the added expense of an additional system were factors against adding on an electromagnetic survey.

However, the Australian Department of Defence suggested that there might be non-ferrous targets of significance that the magnetic system would not detect. Moreover, the base was located some tens of kilometers from a field of extinct volcanoes, presenting the possibility of magnetic geology in the form of mafic igneous units. Therefore, it was

determined that an airborne transient electro-magnetic system should be included in the project.

The added electromagnetic system proved valuable, but not in the way that was expected. Concentrations of strong anomalies appeared in the magnetic data, the sources of which could be either buried debris or geological. The TEM-8 system was flown over some of the more dense concentrations of magnetic anomalies. In some of these areas the TEM data showed very few anomalies, indicating that either the VG-16 anomalies are associated with magnetic rock types, or that the metallic sources detected by the VG-16 system are too deeply buried to be detected with the TEM-8 system. A few carefully located excavations indicated that the sources of the magnetic anomaly concentrations without associated electromagnetic anomalies were localized concentrations of very magnetic iron-bearing rock.