Coaxial EMI Sensor for UXO Detection and Discrimination
Final rept. Aug 2004-Aug 2005
GEOPHEX LTD RALEIGH NC
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The coaxial coil configuration electromagnetic induction EMI sensor is motivated by the potential advantages of the common mode rejection of electromagnetic noise from external sources. The balanced differential receiver gradient measurements reject voltages induced by noise fields that are uniform over distances on the scale of the receiver coil separation, including natural sources such as sferics distant lightning induced, geomagnetic storms sun spot induced, platform motion in the geomagnetic field, as well as man-made sources such as power line fields. The platform motion induced noise has been shown to be particularly problematic for the vehicular towed concentric-coil system GEM-3 in the operational frequencies below 100 Hz. One penalty paid with the coaxial geometry is an increased height of the transmitter coil, reducing the excitation field strength over the target. There is a trade-off between increasing the coil separation to increase the difference signal and reducing the separation to reduce the transmitter-target distance, and the design must provide a good compromise for the anticipated target depth envelope. Also, small separations pose an engineering challenge at achieving adequate bucking receiver coil balance. An existing prototype cart-mounted coaxial sensor was evaluated in order to confirm the noise rejection advantage of the coaxial configuration. Noise associated with platform motion and from external electromagnetic interference is shown to be less, as proposed.
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- Electricity and Magnetism