Clean Enough For Industry? An Airborne Geophysical Case Study

Data from two airborne geophysical surveys of the Department of Energy’s Oak Ridge Reservation (ORR) were<br>extremely valuable in deciding whether a lOOO-acre (400 hectare) parcel of the ORR should be leased to the City of<br>Oak Ridge for industrial development. Our findings, based on electromagnetic and magnetic data, were<br>incorporated in the federally mandated Environmental Assessment Statement (EAS), and in general supported<br>claims that this land was never used as a hazardous waste disposal site. We estimated the amount of iron required<br>to produce each anomaly using a simple dipole model. All anomalies with equivalent sources greater than<br>approximately 1000 kg of iron were checked in the field, and the source of all but one identified as either a bridge,<br>reinforced concrete debris, or a similarly benign object. Additionally, some smaller anomalies (equivalent sources<br>of roughly 500 kg) have been checked; thus far, these also have innocuous sources. Airborne video proved<br>invaluable in identifying logging equipment as the source of some of these anomalies. Geologic noise may<br>account for some of the remaining anomalies. Naturally occurring accumulations of magnetic minerals in the soil<br>on the ORR have been shown to produce anomalies which, at a sensor height of 30 m, are comparable to the<br>anomaly produced by about 500 kg of iron. By comparison, the electronic noise of the magnetic gradiometer, O.Ol-<br>0.02 nT/m, is equivalent to only about 50-100 kg of iron at a 30 m sensor height. The electromagnetic data,<br>combined with field mapping of karst structures, provided evidence of a northeast-southwest striking conduit<br>spanning the parcel. The possible existence of a karst conduit led the EAS authors to conclude that this is a<br>“sensitive hydrologic setting.” We conclude that aerial geophysics is an extremely cost-effective, and efficient<br>technique for screening large tracts of land for environmental characterization.