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Over the past fifteen years, notable progress has been made in the performance of airborne geophysical systems for mapping and detection of unexploded ordnance in terrestrial and shallow marine environments. For magnetometer systems, the most significant improvements include development of boom-mounted platforms, and implementation of higher sample rates, denser magnetometer arrays, and vertical gradient configurations. Nine magnetometer-based systems are described and their performance summarized. In prototype analyses and recent U.S. Department of Defense Environmental Security Technology Certification Program (ESTCP) assessments using new production systems, the best performance has been achieved with a vertical gradient configuration. As effective as magnetometer systems have proven to be at many sites, they are inadequate at sites where basalts and other ferrous geologic formations or soils produce anomalies that approach or exceed those of target ordnance items. Additionally, magnetometer systems are ineffective where detection of non-ferrous ordnance items is of primary concern. We discuss the development of airborne time-domain electromagnetic systems over the past ten years. Overall, improvements in airborne geophysical systems have led to more consistent detection of smaller ordnance. These trends should continue as additional technological advances are made.

A low altitude helicopter magnetic survey for unexploded ordnance in New Mexico revealed several magnetic anomalies that were most likely induced by lightning strikes. Lightning-strike magnetic anomalies are not necessarily rare, but may be spaced so widely as to make their detection unlikely in a ground survey. Detailed examples are not often reported because ground geophysical surveys may not cover enough area to detect one, and traditional airborne surveys, which do cover large areas, are carried out at an altitude and line spacing which does not appropriately define the unusual shape of the lightning strike anomaly. However, very low-level ([Formula: see text] altitude) airborne magnetic surveys have data densities similar to ground geophysical surveys, yet cover much larger areas. Lightning anomalies appear in magnetic data as radial arms emanating from a central strike point. Each arm has distinct positive and negative lobes. Anomaly amplitudes in the New Mexico survey area ranged from roughly [Formula: see text], and the lightning-strike anomaly density was about one per [Formula: see text]. This is a conservative estimate, as only the most obvious anomalies were counted. The character of the lightning-induced magnetic anomalies changes with an increase in survey altitude or with wider line spacing, making them less distinct from other types of magnetic anomalies.

In January 2008, low-altitude (~1-3 meters above ground level) airborne geophysical surveys were carried out at the Marine Corps Air Ground Combat Center (MCAGCC), near Twentynine Palms, California. The primary goal of the surveys was to assess the viability of airborne magnetic or electromagnetic geophysical surveys at MCAGCC for detection and mapping of unexploded munitions. Due to high magnetic content in the rocks and soils at MCAGCC magnetic methods had been shown in the past to have limited usefulness. The background magnetic conditions made this site a good candidate for the use of a new 8channel Time-domain Electromagnetic system (TEM8) developed by Battelle. The first phase of the project was an assessment of the efficacy of TEM8 and vertical magnetic gradient (VG-22) technologies, based on results from surveys of two 8 hectare areas and a 2 hectare geophysical prove out (GPO) area. This demonstration showed that both VG-22 and TEM8 were useful at this site, but the combination of the two datasets is more effective than either dataset used singularly for target prioritization.

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