<title>Use of ground-penetrating radar technology in construction of the Los Angeles MetroRail subway system</title>

Abstract

State-of-the-art ground penetrating radar (GPR) technology was used successfully in tunneling through the former L.A. City Oil Field to search for uncharted, abandoned oil wells. A magnetometer probe was previously used for this purpose, because it was felt abandoned oil wells with steel casings may exist ahead of tunneling. These wells were suspected to contain methane gases which could be released into the tunnels. Studies revealed the abandoned wells could be wooden-cased or uncased open holes, indicating they would not be detected using a magnetometer probe. GPR was therefore selected as a geophysical technique more capable of detecting both steel-cased and uncased oil wells. After some initial testing from inside the tunnel, a commercially available GPR system was selected. Procedures were developed for conducting the surveys and evaluating the data profiles for possible oil wells. The profiles were obtained by moving the radar antenna across the smoothed tunnel face. During tunnelling of the oil field area abandoned oil wells were not encountered. However, the GPR surveys did detect anomalous radar reflections that the machine operator was alerted to as possible oil wells. Review of the data indicates that other changes in ground conditions were detected, such as transitions from soft- to hard-ground conditions and zones of oil bearing sands. These results suggest GPR could be useful for other exploratory applications during mining. GPR was also used as an investigative tool to check for possible shallow subsurface voids from the ground surface. Air-filled cavities or voids beneath city streets can sometimes be formed as a result of deeper tunneling-induced ground movements, resulting in dangerous sink-hole forming conditions. The GPR surveys were conducted from the street surface above the tunnels in areas where geotechnical data measured greater ground movements. These surveys helped rule out the possibility of voids beneath the street pavement in an area where over nine inches of ground settlement was measured.