Case studies in magnetics and ground penetrating radar, Shreveport Convention Center, Shreveport, LA and Yankee Boy Rock Glacier, Ouray, CO

Abstract

The use of geophysical tools such as magnetics and ground penetrating radar are becoming more prevalent in site characterization studies and other geologic research. Two case studies which illustrate this are described here. The first case study is in magnetics. The second case study deals with ground penetrating radar. The re-use of existing sites is growing increasingly important for urban revitalization and prevention of urban sprawl. An extensive magnetic gradiometer survey was conducted using a Geometrics G-858 magnetometer on a previously occupied urban industrial site to locate objects in the subsurface prior to construction of the new Shreveport Convention Center in Shreveport, Louisiana. To map the distribution of surface and buried magnetic material, two cesium vapor total-intensity magnetometers were mounted on an aluminum pole at 2.1m and 1.2m of elevation. The elevation difference between the magnetometers was used to indicate the depth of a buried object by observing the magnetic field "fall-off" between the anomalies generated when magnetic materials were present. A large "fall-off" signifies small near-surface metallic artifacts and a small "fall-off" indicates larger magnetic objects at greater depth. From the data gathered, we were able to identify objects in the subsurface and determine where excavation would be difficult. We were also able to determine areas where caution should be exercised near buried utilities and void spaces. Magnetic surveying has been shown in this case to be a viable technique for urban site characterization prior to redevelopment. The ground penetrating radar (GPR) study was conducted using a PulsEKKO 100 subsurface imaging radar with a 1000 volt pulser and a 25 MHz bi-static antennae arrangement on a temperate rock glacier located near Ouray, CO. A rock glacier is a mass of angular boulders and finer rock material in an ice matrix. The purpose of the study was to image the stratigraphy of the rock glacier and the substrate for implications of similar water-bearing structures on Mars. This joint study was funded by the National Aeronautics and Space Administration (NASA) and performed by the High Alpine Research Program (HARP) of both Texas A&M University and Oklahoma State University.