Abstract
The investigation depth of transient electromagnetic sensors can be effectively increased by reducing the system noise, which is mainly composed of sensor internal noise, electromagnetic interference (EMI), and environmental noise, etc. A high-sensitivity airborne transient electromagnetic (AEM) sensor with low sensor internal noise and good shielding effectiveness is of great importance for deep penetration. In this article, the design and optimization of such an AEM sensor is described in detail. To reduce sensor internal noise, a noise model with both a damping resistor and a preamplifier is established and analyzed. The results indicate that a sensor with a large diameter, low resonant frequency, and low sampling rate will have lower sensor internal noise. To improve the electromagnetic compatibility of the sensor, an electromagnetic shielding model for a central-tapped coil is established and discussed in detail. Previous studies have shown that unclosed shields with multiple layers and center grounding can effectively suppress EMI and eddy currents. According to these studies, an improved differential AEM sensor is constructed with a diameter, resultant effective area, resonant frequency, and normalized equivalent input noise of 1.1 m, 114 m2, 35.6 kHz, and 13.3 nV/m2, respectively. The accuracy of the noise model and the shielding effectiveness of the sensor have been verified experimentally. The results show a good agreement between calculated and measured results for the sensor internal noise. Additionally, over 20 dB shielding effectiveness is achieved in a complex electromagnetic environment. All of these results show a great improvement in sensor internal noise and shielding effectiveness.
Highlights
Over the last two decades, airborne transient electromagnetic (AEM) systems have become increasingly popular for hydrogeophysical investigations, engineering surveys, unexploded ordnance (UXO) detection, as well as geological mapping and mineral exploration [1,2,3]
Coil diameter D and resonant frequency f 0 corresponding to the area below area below the red satisfy the conditions, which means that awith larger diameter with a lower the red line satisfy theline conditions, which means that a larger diameter a lower resonant frequency resonant frequency can achieve lower sensor internal noise
Coil diameter D and resonant frequency f0 corresponding to the area below the red line satisfy the conditions, which means that a larger diameter with a lower resonant frequency can achieve lower sensor internal noise
Summary
Over the last two decades, airborne transient electromagnetic (AEM) systems have become increasingly popular for hydrogeophysical investigations, engineering surveys, unexploded ordnance (UXO) detection, as well as geological mapping and mineral exploration [1,2,3]. Tumanski et al calculated parameters, such as the resultant area, direct current (DC) resistance, and SNR of an induction coil [10] They considered thermal noise of the coil’s DC resistor as the only noise source and did not take into account the preamplifier noise, which, plays an important role in the sensor internal noise. A new shielded receiver coil with a noise level 2–3 times less than the conventional one has been developed by SkyTEM [17] Problems such as shielding effectiveness optimization, eddy current suppression, and shield grounding are rarely discussed in detail. With the sensor internal noise level determined by penetration depth, the noise model with both induction coil and preamplifier is established and analyzed first. Experiments are conducted to verify the accuracy of the sensor internal noise model and electromagnetic compatibility of the sensor
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