Preprints
https://doi.org/10.5194/gi-2020-49
https://doi.org/10.5194/gi-2020-49

  28 Dec 2020

28 Dec 2020

Review status: this preprint is currently under review for the journal GI.

Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates

Jakob Juul Larsen1,5, Stine Søgaard Pedersen2,3, Nikolaj Foged2,5, and Esben Auken4 Jakob Juul Larsen et al.
  • 1Department of Engineering, Aarhus University, Finlandsgade 22, 8200 Aarhus N, Denmark
  • 2Hydrogeophysics Group, Department of Geoscience, Aarhus University, C. F. Møllers Alle 4, 8000 Aarhus C, Denmark
  • 3Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
  • 4Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark
  • 5WATEC, Aarhus University Centre for Water Technology, Ny Munkegade 120, 8000 Aarhus C, Denmark

Abstract. The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures but data are inevitably contaminated by noise from various sources. It is common practice to gate signals from TEM systems to reduce the amount of data and improve the signal-to-noise ratio (SNR). Gating acts as a filter, and optimum gating will pass the TEM signal un-attenuated while suppressing noise. In systems based on analog boxcar integrators, the gating corresponds to filtering with a square window. The frequency response of this window shape has large side lobes, which are often insufficient in attenuating noise, e.g., from radio signals in the very low frequency (VLF) 3–30 kHz band. Tapered gates have better side lobe suppression and attenuates noise better but tapering with analog boxcar integrators is difficult. We propose to use many short boxcar gates, denoted sub-gates, and combine the sub-gates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR. The semi-tapering approach is analyzed and tested experimentally on data from a roving TEM system. We quantify the effect of semi-tapered gates by computing an improvement factor as the ratio between the standard error of data measured with boxcar gates and the standard error of data measured with semi-tapered gates. Data from a test survey in Gedved, Denmark with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the ten late-time gates centered between 78.7 μs and 978.1 μs. After inversion of the data, we find that semi-tapering increases the depth of investigation by about 20 % for this specific survey. We conclude that the semi-tapered approach is a viable path towards increasing SNR in TEM systems based on analog boxcar integrators.

Jakob Juul Larsen et al.

 
Status: open (until 07 Feb 2021)
Status: open (until 07 Feb 2021)
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Jakob Juul Larsen et al.

Jakob Juul Larsen et al.

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Short summary
The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures but data are inevitably contaminated by noise from various sources including radio signals in the very low frequency (VLF) 3–30 kHz band. We present an approach where VLF noise is effectively suppressed with a new post-processing scheme where boxcar gates are combined into semi-tapered gates. The result is a 20 % increase of depth of investigation for the presented test survey.