The Use Of Geophysics In Preacquisition Site Assessments (Psa’S) A Case History

Environmental consultants must occasionally perform environmental audits on parcels that<br>have little or no known historic records. In such cases, the accurate evaluation of<br>geophysical data can be invaluable.<br>This paper presents a case study in which a magnetometer survey was used to evaluate a<br>vacant parcel that formerly contained six military helicopter pads and gotentiallg still<br>contained associated underground storage tanks. An in-depth records searc reveale little<br>-about the site, but evaluation of proton magnetometer data indicated a dense, potentially<br>vertical object approximately 15 to 20 feet below the surface near the pad locations.<br>Excavation of the anomaly revealed a concrete vault and a redwood vault connected by a<br>10’ long, 6” diameter metal pipe. The pad and vault were both filled with an assortment of<br>military porcelain, glassware and scrap metal. Unconvinced that such debris caused the<br>anomaly, we scanned the excavation with a metal detector and located an object beneath<br>the concrete pad. Further dewatering and excavation uncovered an 8-inch diameter, 40-<br>foot deep irrigation well remnant of pre-military ranching operations. We concluded that<br>this casing produced the magnetometer anomaly.<br>This study provides a lesson on the necessity of carefully evaluating excavated objects based<br>on the geophysical data before assuming that the objects represent the target anomaly.