Landfill mapping using multi-disciplinary geophysical techniques at the U.S. Air Force Academy, Colorado Springs, CO

Abstract

This paper describes a multi-disciplinary geophysical survey conducted over a landfill on the U.S. Air Force Academy grounds near Colorado Springs, Colorado. The landfill is known to contain waste generated during the construction of the Academy and reportedly contains buried steel drums. The purpose of the geophysical surveys was to determine the subsurface distribution of buried metallic objects within the landfill. Different geophysical techniques were evaluated along a test line to determine their relative effectiveness at this site. The geophysical methods included total magnetic field, vertical magnetic gradient, VLF, horizontal and vertical coplanar electromagnetic, GPR and seismic refraction. Magnetic and coplanar electromagnetic (EM) methods were chosen to survey the entire landfill because they easily detected magnetic and conductive sources and have better anomaly resolution than other methods evaluated, as demonstrated by the test line results. In addition, these methods are rapid and cost effective for surveys involving a large number of measurements. Surveys of the landfill identified numerous magnetic and conductive anomalies indicating the presence of buried metallic objects. The vertical gradient and EM measurements indicate that several of the large total field anomalies are produced by groups of smaller objects rather than by single, large buried sources. Many of the smaller anomalies are associated with the position of a recently dismantled railroad track and result from iron and steel parts buried along the abandoned grade. Two long, narrow conductive anomalies were identified by the electromagnetic surveys. These conductive features have no surface expression and apparently run the length of the landfill. The EM data indicates these conductors are narrow and relatively shallow. One conductor is relatively magnetic, the other conductor has no magnetic signature suggesting a different composition. The geophysical surveys determined that large areas of the landfill are relatively free of buried metal due to the lack of observed magnetic or conductive anomalies. The geophysical data also suggests the landfill may be larger than originally thought. Numerous magnetic and conductive responses were observed beyond the eastern edge of the present landfill in an area thought to be natural terrain.