Search

The article discusses the U.S. Federal Bureau of Investigation (FBI) Laboratory's surveys that aim to gather information about the use of geophysical methods to search for buried or hidden targets of interest to law enforcement. Topics discussed include the use of metal detectors in crime scene investigations, geophysical service providers to support law enforcement, and law enforcement's use of geophysical methods such as ground penetrating radar and magnetometers.

We measured the C- and O-isotopic composition of carbonate minerals in the Waits River Formation, eastern Vermont, to determine the extent of fluid infiltration during regional metamorphism over an approx. 2000 km2 area in a deep-seated (>25 km) Barrovian terrane. From a petrologic study of this terrane, Ferry proposed the existence of a large regional metamorphic hydrothermal system with two first-order features: (1) on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent; (2) on a smaller scale (about 100 km2) flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms. We find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons. The evidence for hydrothermal activity around the plutons is large heavy isotope depletions, up to 6–9‰ in d18Ocarb and d13Ccarb, in diopside zone rocks adjacent to the plutons. These isotopic shifts are greater than can be explained solely by prograde metamorphic reactions. We find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms. First, d18Ocarb and d13Ccarb, within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes, indicating that the rocks equilibrated with a permeating fluid. Second, there are depletions in 18O near the margins of the Waits River Formation which can be interpreted as a dispersed, advective infiltration front displaced toward the antiform axes. These fronts were modelled using Eq. 13 of Bickle and Baker and imply time-integrated fluid of 105–106 cm3/cm2, which are consistent with values derived by Ferry from measured progress of prograde devolatilization reactions.

As a laboratory exercise for a course entitled “Isotope Tracers in the Environment,” we brewed beer with fermentable sugar sources that were from both C-3 and C-4 plants and thus had distinct δ13C values (−25.6‰ and −9.8‰ respectively). We measured the δ13C value of CO2produced throughout the course of beer fermentation and observed a 6.8‰ decrease in the δ13C of the CO2after about 30 hours. This δ13C change is due to exhaustion of the easily fermentable carbohydrates in the corn sugar starting material (glucose) and the later dominance of fermentation of the less easily metabolized sugars maltose and maltotriose found in malted barley extract. Students were instructed to write a “research paper” with this data set. This laboratory introduced students to techniques and concepts used in stable-isotope biogeochemistry.