A Dual-Air-Core Coil Sensor With Wide Bandwidth and Large Effective Area in Electromagnetic Exploration System

Air-core coil sensors are widely used in geophysical electromagnetic exploration because of their ease of production, high sensitivity, and broadband. In order to increase the bandwidth of the air-core coil without reducing the number of turns, a new type of dual-air-core coil sensor was proposed for the shallow exploration in the electromagnetic method. The design principle of the dual-air-core coil sensor was introduced, consisting of a low-bandwidth coil and a high-bandwidth coil. The output voltage of the high-frequency coil is used as a linear reference to obtain the amplitude-frequency response data of the low-frequency coil, and the fitting model is constructed to extract the equivalent inductance and distributed capacitance of the low-frequency coil, so as to realize the correction of the system response. In addition, the crosstalk model between air-core coils was established, and a method to suppress crosstalk was proposed. In order to prove the feasibility of the dual-air-core coil sensor, in-laboratory and field test platforms were set up. The experimental results show that the crosstalk between the coils can be eliminated by adjusting the relative position of the coil. The fitting method can effectively increase the bandwidth of the low-bandwidth coil, so that it exceeds the high-frequency coil used, and it is shown to have better performance. The effective area is 59.5 m
²
, the amplitude bandwidth of −3 dB is 200 kHz, the phase bandwidth with the error less than ±5° is 220 kHz, and the noise level is 18 fT
$\cdot $
Hz
$^{-1/2}$
at 1 kHz.