Towards a quantitative description of forensic anomaly - a numerical and field study using magnetometry to detect buried weapons

Abstract:

In archeological and other near surface applications of magnetometry, measured anomalies in the total magnetic field or its gradient are related to the presence of buried objects with magnetic contrast. In the case of forensic searches, such objects may be clandestine buried weapons, and finding them can unravel criminal acts or intentions and may ensure justice for victims. So far, the use of magnetometry to detect buried firearms focuses on qualitative analysis for site clearance or selection for detailed investigation and its practical application has been limited to shallow depths (< 0.8 m) and by false positives and negatives. Improving current forensic applications of magnetometry require a more quantitative understanding of the measured magnetics signals. In this study, we extend this quantitative understanding by combining both numerical and field studies to assess the depth limit of investigation and the influence of geometric properties of the buried firearms on measured signals. We simulated the total magnetic field and gradient anomaly responses of buried handguns and rifles at varying depths (0.6 m, 1.2 m and 1.8 m). In computing the magnetization, we assumed a long magnetic dipole for both handguns and rifles and characterized them by their magnetization, length, center, azimuth and plunge. The simulation results were compared with magnetic gradiometer data collected with a Gem Systems GSM-19GW Overhauser magnetometer at a field test site near Toronto, Canada, where six firearms are buried horizontally at depths to 1.8 m. Field magnetometer measurements involved two sensors at 0.25 and 0.8 m elevation (vertical separation of 0.55 m) and a fine measurement grid of 0.25 m ´ 0.1 m. We measured anomalies of +/-20 nT at 0.6 m depth for a rifle and +/-2 nT at 1.8 m depth for a handgun. These anomalies spatially coincide with the locations of weapons, and dipole anomalies align along the orientation of the firearms. Our modeling results show that the magnetic anomaly is enhanced if the buried weapon is tilted. Our study detected buried firearm up to depths of 1.8 m suggesting that gradient magnetometers are useful tools in forensic weapon searches.