COMBINING GEOPHYSICAL TOOLS TO IDENTIFY UNIQUE UNDERGROUND INFRASTRUCTURE AT AN ENVIRONMENTAL REMEDIATION SITE IN NORTHWESTERN ILLINOIS

Geosyntec used electromagnetic conductivity (EM31), magnetometer, and ground penetrating radar (GPR) to locate buried metal, detectable private utilities and underground structures during an environmental redevelopment project. The most significant geophysical target was a 9-foot diameter stone arch stormwater sewer that transects the site, which is expected to be a significant cost driver for remediation and redevelopment. The stormwater sewer was formerly a surface flowing creek that was converted to a stone arch and covered by fill material in the early 1900s. The area overlying the former creek subsequently underwent commercial and industrial development, which resulted in a poor understanding of the true location and dimensions of the stone arch sewer.

An EM31 conductivity meter was used to measure both quadrature (apparent conductivity) and inphase responses, and magnetic measurements were obtained with a cesium vapor total field magnetometer. In areas where the sewer was thought to be located, a GPR survey was completed with a 200MHz antenna cart system. Given the stone arch was covered with less than five feet of material, it was expected that the GPR results would be favorable for identifying the placement of the stone arch.

The geophysical survey yielded both expected and unexpected results. The magnetometer survey identified a few anomalous areas, which were mostly discrete nonlinear zones of interest. The EM31 was effective at delineating numerous linear and discrete anomalies typically found in areas with urban fill; however, unexpectedly the EM31 identified a very strong apparent conductivity response near the suspected location of the stone arch sewer. The results of the GPR survey varied; one survey effectively confirmed the location of the sewer in both the acquired cross-sections and processed plan view plots, but the second survey was less successful and left the exact location of the sewer uncertain. Additional research identified that the stone arch sewer was sealed in the 1920s with sprayed concrete held in place with wire mesh, which was interpreted to be the source of the EM response. As remediation and redevelopment of the site advances, apparent conductivity combined with GPR are proven cost effective tools for detecting the location of the stone arch sewer in other areas not yet studied.