Abstract
As an electromagnetic field conversion tool in the transient electromagnetic method (TEM), the weak coupling coils reduce the mutual inductance of its transmitter and receiver coils by special structural optimization, so the detection signal can be protruded from the primary field interference generated by the transmitter coil; thus, this kind of coil design can significantly improve the signal-to-noise ratio. However, with the popularity of drag or aerial TEM exploration, the structural stability problem caused by bumps or windage leads to non-negligible primary field leakages, thereby reducing the detection reliability. This paper incorporates the primary field shielding stability as a key indicator of the weak coupling designs and proposes a calibration scheme for this stability assessment, based on which the shielding stability of five typical weak coupling coil designs is quantitatively compared, and the relationship between the primary field density and the shielding stability explored in this study may contribute to the selection and improvement of TEM coils.
Highlights
The transient electromagnetic method (TEM) is an effective non-intrusive geophysical method, which employs loop-source TEM devices in ground exploration [1,2,3,4,5,6,7]
This paper reveals the stability of the weak coupling coil designs on the primary field shielding and quantitatively compares the shielding stability of five popular coil designs, thereby providing solutions for drag or aerial TEM exploration
The shielding of the primary field response helped to improve the signal-to-noise ratio of the loop-source TEM, which popularized the application of the weak coupling coil designs
Summary
The transient electromagnetic method (TEM) is an effective non-intrusive geophysical method, which employs loop-source TEM devices in ground exploration [1,2,3,4,5,6,7]. TEM devices with integrated TX and RX coils in one bracket have become gradually popular in the air borne TEM detection [7] and the drag system [10] In these devices, the relative coil locations can be set to reduce the effect of mutual inductance, thereby a pure secondary field response produced by the underground eddy current can be extracted, as shown by Figure 1. The relative coil locations can be set to reduce the effect of mutual inductance, thereby a pure secondary field response produced by the underground eddy current can be extracted, as shown by Figure 1 These coil arrangements are called the weak coupling coil designs, such as the cross-loop design proposed by [11], the eccentric coils used by the SkyTEM, the opposing coils proposed by [12], the gradient coils [13], and the bucking design [14]. This paper reveals the stability of the weak coupling coil designs on the primary field shielding and quantitatively compares the shielding stability of five popular coil designs, thereby providing solutions for drag or aerial TEM exploration
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