Magnetic and GPR surveys of a former munitions foundry site at the Denver Federal Center

We made magnetometer and ground penetrating radar (GPR) surveys over part of the foundation of a World War II-era foundry located on the Denver Federal Center. The site-contains a number of highly magnetic source bodies, concrete foundation walls, and underground openings, buried under a clay cap. The cap is several feet thick and has a conductivity of about 35 mS/m, making the features underneath it a poor target for conventional GPR. Indeed, the raw data look unlike typical GPR data, but rather show reverberation (?) bands under sidewalks and other shallow buried sources. Using a newly-written computer package, we made plan maps of the GPR response at different time slices. The sliced GPR data did not outline buried foundry foundations, as we had hoped it might. The resulting plan maps of the sliced data show sidewalks and other blobby features, some of which correspond to magnetometer highs. STUDY AREA The study area (“DOP Foundry”) is a grassy rectangular lot measuring 43.2 meters in the nominal east-west direction by 87.5 meters in the nominal north-south direction. The lot is located east of Building 20, Denver Federal Center (DFC). It is bounded on all sides by a 1.6-2.0 meter sidewalk, and is crossed by other sidewalks forming a curvy X-shaped pattern. Campbell and Eckhart (1999) described this study area and its history, but mistakenly believed an underground bunker was installed there. We are now fairly sure that there was no such bunker. Rather, Kaiser Industries built, but may have never operated, a foundry for 8” artillery shells on the study area during late stages of World War II. A contemporary air photo (Fig. 1 of Wright and others, 2000, this volume) shows that this foundry building covered a huge footprint and towered above the 2 stories of all but the bay of Building 20 to the west. The foundry was moved to Anchorage, Alaska, at the very close of the War. Air photos taken just after the War show an empty field where the foundry had been. The present study area is a central subportion of the ground inside the original foundry building. A number of two-loop frequency-domain EM surveys of this study area (Eckhart, 1998; Campbell and E&hart, 1999) showed many buried features there, which probably reflect buried cooling/settling tanks and underground manways beneath the ground level of the foundry. In this report, we extend these earlier EM findings by showing the results of magnetic and ground penetrating radar (GPR) surveys of what has now become a quite useful geophysical test area for us.