An optimized and hybrid gating scheme for the suppression of very low-frequency radios in transient electromagnetic systems

• Published in: Geoscientific instrumentation, methods and data systems Vol. 13; no. 1; pp. 27 - 41
• Main Authors: Khare, Smith Kashiram, McLachlan, Paul, Maurya, Pradip Kumar, Larsen, Jakob Juul
• Format: Journal Article
• Language: English
• Published: Gottingen Copernicus GmbH 13.02.2024; Copernicus Publications
• Subjects: Analysis; Communication; Covariance matrix; Electricity; Electromagnetism; Error analysis; Fourier transforms; Freight cars; Gates; Gating; Groundwater; Investigations; Ionosphere; Magnetic fields; Multiple objective analysis; Objective function; Optimization; Particle swarm optimization; Power lines; Radio; Radio communications; Radio signals; Radio transmission; Receivers & amplifiers; Shape optimization; Signal to noise ratio; Standard error; Transmitters; Very Low Frequencies
• ISSN: 2193-0864; 2193-0856; 2193-0864
• DOI: 10.5194/gi-13-27-2024

One of the most widely used approaches for measuring the earth's subsurface resistivity is the transient electromagnetic (TEM) method. However, noise and interference from different sources, e.g., radio communication, the instrument, the atmosphere, and power lines, severely taint these types of signals. In particular, radio transmission in the very low-frequency (VLF) range between 3 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 semi-tapered 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, which are gates with center times spanning 30 to 200 µs , and in the late gates, which are gates with center times spanning 200 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 for synthetic and field data, respectively.