24 Oct 2023
 | 24 Oct 2023
Status: this preprint is currently under review for the journal GI.

An Optimized and Hybrid Gating Scheme for the Suppression of Very Low Frequency Radios in Transient Electromagnetic Systems

Smith K. Khare, Paul McLachlan, Pradip Kumar Maurya, and Jakob Juul Larsen

Abstract. One of the most used approaches for measuring the earth's subsurface resistivity is the transient electromagnetic method (TEM). However, noise and interference from different sources, e.g., radio communication, the instrument, the atmosphere, and powerlines severely taint these types of signals. In particular, radio transmission in the very low-frequency (VLF) range between 3 kHz and 30 kHz is one of the most prominent sources of noise. Transient electromagnetic signals are normally gated to increase the signal-to-noise ratio. A precise selection of gate shapes is required to suppress undesired noise while allowing the TEM signal to pass unaltered. We employ the multi-objective particle swarm optimization technique to choose optimal gate shapes and placements by minimizing an objective function composed of standard error bars, the covariance between gates, and the distortion of the gated signal. The proposed method is applied to both fully sampled synthetic TEM data and to boxcar-gated field data. The best output from the search space of gate shapes was found to be a hybrid combination of boxcar and Hamming gates. The effectiveness of hybrid gating over traditional boxcar and semitapered gating is confirmed by an analysis of covariance matrices and error bars. The results show that the developed method effectively suppresses VLF noise in the middle gates, that is, gates with center times spanning  30 µs to 200 µs and in the late gates, that is, gates with center times spanning  200 µs to 1130 µs. The analysis shows that the average improvement in standard errors obtained for the hybrid gating scheme over boxcar gating is 1.719 and 1.717 on synthetic and field data, respectively.

Smith K. Khare et al.

Status: open (until 12 Dec 2023)

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Smith K. Khare et al.

Smith K. Khare et al.


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Short summary
Data recorded with transient electromagnetics are typically gated to improve the signal-to-noise ratio. Gating corresponds to lowpass filtering of data with the shape of the gate giving the frequency response. We show that standard gate shapes can lead to significant correlation between gates caused by distortion from VLF radios. A multi-objective cost function is used to select optimum gate shapes with less influence from radio noise. Performance is demonstrated using synthetic and field data.