Detecting anomaly targets using handheld frequency domain electromagnetic system

Abstract In this paper, a handheld frequency domain electromagnetic system CEM-2 is presented for subsurface investigation (within less than 2 m). CEM-2 system employs one set of 3 coplanar coils. The transmitter uses sinusoidal pulse width modulation technology to generate low total harmonic distortion and arbitrary digital signal waveform. It is convenient to match different transmitting coils using the same circuit and system. The signal processing method can reduce some effects caused by unstable factors. Most operations are sum and product operation, contributing to high efficiency. The mounting positions of the metal items (a removable box, pre-amplifier) are selected according to the principle that primary magnetic field has minimum gradient. Experiments were conducted to evaluate the CEM-2 system. The response decreases as the operation frequency increases. The in-phase response changes with frequency faster than quadrature response. When the major axis of a target is in parallel with the system, the response is the strongest while the response is the weakest when the target is perpendicular to the system. This is because the effective conductivity is different due to de-polarization factor under primary field from different incident direction. Furthermore, results in a simulated test site indicate that CEM-2 can give distinct signatures for abnormal targets.

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