A CASE STUDY ILLUSTRATING SOME LIMITATIONS OF AIRBORNE GEOPHYSICAL INVESTIGATIONS FOR MUNITIONS OF EXPLOSIVE CONCERN

In early 2005, a magnetic and electromagnetic airborne geophysical survey was conducted by Battelle/Oak Ridge National Laboratory over the former multi-range impact area of Fort Ord. Given that the multi-range area has tall vegetation and variable topography, the primary objective of the survey was to use a tool with wide area assessment capabilities to delineate special case areas with high densities of anomalies related to ‘Munitions of Explosive Concern’ (MEC) that might require special treatment (sifting?) or extraordinary amounts of time for excavating individual items. Approximately 25.62 km2 (6332 acres) of the 32.37 km2 (8000 acre) multi-range impact area was surveyed with the airborne magnetics system. The results of the survey included total field and pseudo-vertical-gradient (i.e. computationally-derived) magnetic field maps, sensor altitude maps, analytic signal maps, and an interpretation map delineating areas with higher and medium amplitude magnetic signals. The primary issue of concern is that it is not known how close the survey actually came to achieving the survey objectives, let alone detection limitations, since it is not known exactly what is located in the survey site. The resulting ordnance density interpretations are not reliable because they may not be complete. As part of the system calibration process, multi-altitude surveys (2 m, 4 m, 5.5 m) were performed over the well-documented geophysical prove-out grids developed for the Ordnance Detection and Discrimination Study located in the main survey area. From the calibration results two conclusions can be reached: 1) The flight altitude was too high to detect MEC - the data cannot be used to infer estimates of concentrations that are present or not present; 2) The altitude was low enough to detect something - but what was detected may or may not be indicative of a cluster. Large, single anomalies may be truly single hits or may be part of a much larger cluster. The main conclusion from this survey is that the airborne geophysical data can be used to indicate something is present but cannot be used to indicate whether an area is a special case area or not. Any airborne geophysical survey, and in particular an unexploded ordnance (UXO)-related investigation, must be analyzed with due consideration given to sensor altitude and compared to an area with known parameters.