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Abstract
An air-coil sensor (ACS) is a type of induction magnetometer used as a transducer to measure the variations of a magnetic field. This device is widely applied in helicopter transient electromagnetic method (TEM) exploration. Most helicopter TEM explorations generate common-mode noise and require extreme ACS specifications, both of which inevitably challenge geophysical explorations. This study proposes a differential air-core coil combined with a differential pre-amplifier to reduce the common-mode noise induced in exploration surveys. To satisfy the stringent performance requirements, including the geometric parameters and electrical specifications, the physical calculations in theory and the equivalent schematic of an ACS with noise location are investigated, respectively. The theory calculation and experimental result for the optimized ACS are then compared on the basis of a differential structure. Correspondingly, an ACS is constructed with a mass, resultant effective area, 3 dB bandwidth, signal-to-noise ratio, and normalized equivalent input noise of 2.5 kg, 5.5 m2 (diameter is 0.5 m), 71 kHz, 20 (the varying magnetic field strength is 1 nT/s), and 5.43 nV/m2, respectively. These data are superior to those of the traditional induction sensor 3D-3. Finally, a field experiment is performed with a fabricated sensor to show a valid measurement of the time-varying magnetic field of a helicopter TEM system based on the designed ACS.
Highlights
The helicopter transient electromagnetic method (TEM) is a popular geophysical method that is mainly used by three commercially operational companies: Aeroquest Ltd. (AeroTEM, Mississauga, ON, Canada), Geotech Ltd. (VTEM, Aurora, Canada), and THEM Geophysics Inc. (THEM, Gatineau, QC, Canada) [1]
An air-coil sensor (ACS), which is well known as the search coil magnetometer of a helicopter TEM system, is one of the most important sensors used to measure the variations of a magnetic field [2,3,4]
A custom ACS with optimized specifications for helicopter TEM exploration should be developed. This manuscript (a) presents an optimized ACS combined with differential air-core coils and pre-amplifiers to reduce the common-mode noise; (b) investigates the physical calculation of air-core coil in theory and the electrical equivalent schematic with noise source to overcome the stringent performance requirements; (c) analyzes the technical specifications of an ACS, such as mass, size, bandwidth, equivalent input noise (EIN), and signal-to-noise ratio (SNR); (d) presents optimization research for ACS specifications to meet the stringent requirements for helicopter TEM application; and (e) presents performance tests and field experiments on an ACS in comparison with an AeroTEM system to verify the valid measurement of the time-varying magnetic field of the optimized ACS
Summary
The helicopter transient electromagnetic method (TEM) is a popular geophysical method that is mainly used by three commercially operational companies: Aeroquest Ltd. (AeroTEM, Mississauga, ON, Canada), Geotech Ltd. (VTEM, Aurora, Canada), and THEM Geophysics Inc. (THEM, Gatineau, QC, Canada) [1]. A custom ACS with optimized specifications for helicopter TEM exploration should be developed This manuscript (a) presents an optimized ACS combined with differential air-core coils and pre-amplifiers to reduce the common-mode noise; (b) investigates the physical calculation of air-core coil in theory and the electrical equivalent schematic with noise source to overcome the stringent performance requirements; (c) analyzes the technical specifications of an ACS, such as mass, size, bandwidth, EIN, and SNR; (d) presents optimization research for ACS specifications to meet the stringent requirements for helicopter TEM application; and (e) presents performance tests and field experiments on an ACS in comparison with an AeroTEM system to verify the valid measurement of the time-varying magnetic field of the optimized ACS.
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