Design of Cable Parallel Air-Core Coil Sensor to Reduce Motion-Induced Noise in Helicopter Transient Electromagnetic System

Motion-induced noise (MIN) caused by the mechanical vibration of the air-core coil sensor (ACS) in the geomagnetic field is an important element of detection method in helicopter transient electromagnetic (TEM) systems. The MIN level heavily influences the detection result when its frequency range covers the TEM signal base frequency (25 Hz). To reduce the MIN, the frequencies between the MIN and TEM signals need to be separated. Accordingly, the mechanical vibration frequency of the ACS must be decreased to a level below 25 Hz. Hence, a cable parallel ACS (CPACS), consisting of the cable parallel structure (CPS) and ACS, is designed for the helicopter TEM system. The CPS is designed to change the natural frequency of the ACS based on the mechanical principle. From a simulation, the natural mechanical frequency range of the ACS is changed to lie within 1.89-3.67 Hz. Post CPACS fabrication tests show that the resulting ACS natural mechanical vibration frequency is approximately 3 Hz. Field experiments implemented to verify the electrical performance of the designed CPACS reveal a MIN frequency of about 2-3 Hz, substantially lower than 25 Hz. The MIN level is reduced from ±22 to ±1 nT/s, further distinguishing anomaly responses during the data collection. In summary, the implementation of the CPACS as the sensor of the TEM system drastically improves the noise reduction performance of the detecting instrument due to its contrivable electrical-mechanical character.