The detection of buried firearms remains a critical issue in law enforcement. We assess the suitability of magnetic gradiometers to detect buried rifles and handguns at multiple depths using numerical modeling and field investigations. Our simulation is based on a simple approach to characterize handguns and rifles as long magnetic dipoles with the firearm characterized by its magnetization, length, centre, azimuth and plunge which allows us to calculate their total magnetic field and gradient anomalies. We compare these synthetic data to field gradiometer data collected with a Gem Systems GSM-19GW Overhauser magnetometer at a field site near Toronto, Canada, where six firearms are buried. Our field magnetometer consists of two sensors with a relative vertical sepration of 0.55 m. We measure the largest anomaly (+/-20 nT) for a rifle at 0.6 m depth, and the smallest anomaly (+/-2 nT) for a handgun buried at 1.8 m depth. The measured anomalies spatially coincide with the locations of weapons while dipole anomalies align along the orientation of the firearms. Our modeling results show that vertically buried weapons produce significantly stronger anomalies than horizontal ones, and even slight tilts enhance the anomalies. We recommend a 0.25 m grid spacing to search for weapons using magnetometry. Our study shows that a range of firearms buried up to 1.8 m can be detected, suggesting that gradient magnetometers are useful tools in forensic weapon searches.
Keywords: Controlled geophysical research; Detecting buried weapons; Forensic geophysics; Forensic science; Gradiometry; Magnetic.
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