GPR pulse excitation of a shallow cylinder: On- and off-axis resonances caused by interference and modal mixing

Abstract

Cylinders simulate common forms of shallow unexploded ordnance. They provide resonances for detection, which we investigate experimentally with a broadband 2350 MHz-centered GPR pulse and short cylinders buried in sand and oriented parallel with the electric field. On-axis resonances from 700 to 5100 MHz result from interference between combinations of direct and reflected waves, and their additive and subtractive mixing with those caused by circumference and length. Off-axis they result from interference between primary and multiply-reflected head and subsurface waves. We verify the resonances with ray arguments. The model dimensions scale to common characteristics of ordnance when profiled at frequencies ten times lower. Off-axis detection translates to distances on the order of a few meters.