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Comparison Of Emi Measurements On Large Projectiles And On Exploded Projectile FragmentsNormal access

Authors: Bruce Barrow and H.H. Nelson
Event name: 16th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Unexploded Ordnance
Publication date: 06 April 2003
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 688.32Kb )

A seeded test site of large inert projectiles was set up on a former artillery range with real
exploded clutter and measured by EM61 and EM61 MkII electromagnetic induction (EMI) sensors. The
major source of non-ordnance signatures was clusters of exploded shell fragments. When fitted to a
simple dipole response model, the ordnance signatures were characterized by a large primary dipole
response and two smaller secondary dipole responses. The ratio of primary to secondary was typically
less than two. The fragment clusters were characterized by two large responses and a much smaller third
response with a primary to secondary ratio greater than two. While only limited MkII data was collected,
comparison of EM61 and MkII signatures over the same object show little difference in signal shape or
signal-to-noise levels. EM63 data was collected with inert rounds and a simulated cluster on a test stand.
This data indicates that the early MkII time gates do not cover the full range of the time decay curves for
large ordnance items. The MkII gates cover only the inverse power law of the decay. After several
milliseconds, the decay rolls over into an exponential decay. In these later time gates, the ratio of
primary to secondary betas grows much larger for the projectiles. It stays constant for the clusters.

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