A PRELIMINARY INVESTIGATION OF OPTIMUM MAGNETOMETER TRANSECT SPACING TO LOCATE LEGACY OIL AND GAS WELLS

Legacy oil and gas wells are a potential environmental hazard that may act as a conduit for subsurface fluids such as brines, waste fluids, carbon dioxide, or methane to reach the shallow subsurface or the surface. Many of these wells have been plugged, have surface equipment removed, and have little or no visible trace at the surface, thus making an environmental assessment of these wells difficult. In order to assess large numbers of wells, identifying the typical anomaly size is critical to maximize the transect spacing and therefore minimize magnetometer field work time. Using an Overhauser magnetometer along a set of predefined transects with an initial spacing of 2 m, magnetic anomalies from the metal casing of five abandoned wells were detected. We identified significant variation in the size of the anomalies both horizontally and vertically. The vertical anomaly associated with four of the wells ranged from approximately 1000-4000 nanoteslas (nT), with one well anomaly more than 10,000 nT above background. Horizontal anomalies for the five wells that exceeded 50 nT above the background were all larger than 20 m across. Based on these results, a spacing of 20 m would be sufficient to identify the target location of the anomaly and allow further study.