Reducing Motion-Induced Noise With Mechanically Resonant Coil Sensor in a Rigid Helicopter Transient Electromagnetic System

Abstract

Motion-induced noise (MIN) level is a crucial factor in a rigid helicopter transient electromagnetic (TEM) system, which is limited by two intrinsic constraints. First, the large transient transmitting current generates a disordered electromagnetic field at the sensing area that restricts the configuration of the coil sensor. Paradoxically, the MIN's frequency range is from 10 to 40 Hz, which covers the TEM signal's base frequency (25 Hz) due to the mechanical character of the coil sensor on the platform. Filtering the MIN is very difficult because the MIN and TEM signals are cogenetic from the coil sensor and they have overlapping frequency distributions. An effective way to reduce the MIN is to separate it from the TEM signal in the frequency spectrum. Hence, a mechanically resonant coil sensor configuration is designed in this paper to change the mechanical character of the coil sensor and reconcile the two conflicting aspects. Prospectively, the frequency of the MIN is decreased to 8.6 Hz. After processing the recorded data with the same method as before, the MIN level is reduced to 10 nV/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> from previously 60 nV/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in field exploration. Given the anomaly response over the ore body, the detection resolution is improved. The detection depth can be increased by dozens of meters based on the explanatory layer model.