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

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Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates. / Larsen, Jakob Juul; Pedersen, Stine Sogaard; Foged, Nikolaj; Auken, Esben.

I: Geoscientific Instrumentation, Methods and Data Systems, Bind 10, Nr. 1, 04.2021, s. 81-90.

Publikation: Bidrag til tidsskrift/Konferencebidrag i tidsskrift /Bidrag til avisTidsskriftartikelForskningpeer review

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Larsen, Jakob Juul ; Pedersen, Stine Sogaard ; Foged, Nikolaj ; Auken, Esben. / Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates. I: Geoscientific Instrumentation, Methods and Data Systems. 2021 ; Bind 10, Nr. 1. s. 81-90.

Bibtex

@article{48a5234b3f48423e98739c178a09d2e3,
title = "Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates",
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 attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using 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, Den-mark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 his. 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.",
author = "Larsen, {Jakob Juul} and Pedersen, {Stine Sogaard} and Nikolaj Foged and Esben Auken",
year = "2021",
month = apr,
doi = "10.5194/gi-10-81-2021",
language = "English",
volume = "10",
pages = "81--90",
journal = "Geoscientific Instrumentation, Methods and Data Systems",
issn = "2193-0856",
publisher = "Copernicus Publications",
number = "1",

}

RIS

TY - JOUR

T1 - Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates

AU - Larsen, Jakob Juul

AU - Pedersen, Stine Sogaard

AU - Foged, Nikolaj

AU - Auken, Esben

PY - 2021/4

Y1 - 2021/4

N2 - 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 attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using 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, Den-mark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 his. 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.

AB - 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 attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using 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, Den-mark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 his. 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.

U2 - 10.5194/gi-10-81-2021

DO - 10.5194/gi-10-81-2021

M3 - Journal article

VL - 10

SP - 81

EP - 90

JO - Geoscientific Instrumentation, Methods and Data Systems

JF - Geoscientific Instrumentation, Methods and Data Systems

SN - 2193-0856

IS - 1

ER -