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Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling

Samuël Delefortrie, Daan Hanssens UGent and Philippe De Smedt UGent (2018) Journal of Applied Geophysics. 152. p.17-25
abstract
In this paper, we consider the use of land-based frequency-domain electromagnetics (FDEM) for magnetic susceptibility modelling. FDEM data comprises both out-of-phase and in-phase components, which can be related to the electrical conductivity and magnetic susceptibility of the subsurface. Though applying the FDEM method to obtain information on the subsurface conductivity is well established in various domains (e.g. through the low induction number approximation of subsurface apparent conductivity), the potential for susceptibility mapping is often overlooked. Especially given a subsurface with a low magnetite and maghemite content (e.g. most sedimentary environments), it is generally assumed that susceptibility is negligible. Nonetheless, the heterogeneity of the near surface and the impact of anthropogenic disturbances on the soil can cause sufficient variation in susceptibility for it to be detectable in a repeatable way. Unfortunately, it can be challenging to study the potential for susceptibility mapping due to systematic errors, an often poor low signal-to-noise ratio, and the intricacy of correlating in-phase responses with subsurface susceptibility and conductivity. Alongside use of an accurate forward model – accounting for out-of-phase/in-phase coupling – any attempt at relating the in-phase response with subsurface susceptibility requires overcoming instrument-specific limitations that burden the real-world application of FDEM susceptibility mapping. Firstly, the often erratic and drift-sensitive nature of in-phase responses calls for relative data levelling. In addition, a correction for absolute levelling offsets may be equally necessary: ancillary (subsurface) susceptibility data can be used to assess the importance of absolute in-phase calibration though hereby accurate in-situ data is required. To allow assessing the (importance of) in-phase calibration alongside the potential of FDEM data for susceptibility modelling, we consider an experimental test case whereby the in-phase responses of a multi-receiver FDEM instrument are calibrated through downhole susceptibility data. Our results show that, while it is possible to derive approximate susceptibility profiles from FDEM data, robust quantitative analysis hinges on appropriate calibration of the responses.
Please use this url to cite or link to this publication:
author
organization
year
type
journalArticle
publication status
published
keyword
Electromagnetic induction EMI Loop-loop electromagnetics Levelling Inversion Calibration
journal title
Journal of Applied Geophysics
volume
152
pages
17 - 25
publisher
Elsevier BV
ISSN
0926-9851
DOI
10.1016/j.jappgeo.2018.03.003
UGent publication?
yes
classification
U
copyright statement
I have transferred the copyright for this publication to the publisher
id
8557877
handle
http://hdl.handle.net/1854/LU-8557877
date created
2018-03-30 08:28:25
date last changed
2018-03-30 08:28:34
@article{8557877,
  abstract     = {In this paper, we consider the use of land-based frequency-domain electromagnetics (FDEM) for magnetic susceptibility modelling. FDEM data comprises both out-of-phase and in-phase components, which can be related to the electrical conductivity and magnetic susceptibility of the subsurface. Though applying the FDEM method to obtain information on the subsurface conductivity is well established in various domains (e.g. through the low induction number approximation of subsurface apparent conductivity), the potential for susceptibility mapping is often overlooked. Especially given a subsurface with a low magnetite and maghemite content (e.g. most sedimentary environments), it is generally assumed that susceptibility is negligible. Nonetheless, the heterogeneity of the near surface and the impact of anthropogenic disturbances on the soil can cause sufficient variation in susceptibility for it to be detectable in a repeatable way. Unfortunately, it can be challenging to study the potential for susceptibility mapping due to systematic errors, an often poor low signal-to-noise ratio, and the intricacy of correlating in-phase responses with subsurface susceptibility and conductivity. Alongside use of an accurate forward model -- accounting for out-of-phase/in-phase coupling -- any attempt at relating the in-phase response with subsurface susceptibility requires overcoming instrument-specific limitations that burden the real-world application of FDEM susceptibility mapping. Firstly, the often erratic and drift-sensitive nature of in-phase responses calls for relative data levelling. In addition, a correction for absolute levelling offsets may be equally necessary: ancillary (subsurface) susceptibility data can be used to assess the importance of absolute in-phase calibration though hereby accurate in-situ data is required. To allow assessing the (importance of) in-phase calibration alongside the potential of FDEM data for susceptibility modelling, we consider an experimental test case whereby the in-phase responses of a multi-receiver FDEM instrument are calibrated through downhole susceptibility data. Our results show that, while it is possible to derive approximate susceptibility profiles from FDEM data, robust quantitative analysis hinges on appropriate calibration of the responses.},
  author       = {Delefortrie, Samu{\"e}l and Hanssens, Daan and De Smedt, Philippe},
  issn         = {0926-9851},
  journal      = {Journal of Applied Geophysics},
  keyword      = {Electromagnetic induction EMI Loop-loop electromagnetics Levelling Inversion Calibration},
  pages        = {17--25},
  publisher    = {Elsevier BV},
  title        = {Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling},
  url          = {http://dx.doi.org/10.1016/j.jappgeo.2018.03.003},
  volume       = {152},
  year         = {2018},
}

Chicago
Delefortrie, Samuël, Daan Hanssens, and Philippe De Smedt. 2018. “Low Signal-to-noise FDEM In-phase Data: Practical Potential for Magnetic Susceptibility Modelling.” Journal of Applied Geophysics 152: 17–25.
APA
Delefortrie, S., Hanssens, D., & De Smedt, P. (2018). Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling. Journal of Applied Geophysics, 152, 17–25.
Vancouver
1.
Delefortrie S, Hanssens D, De Smedt P. Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling. Journal of Applied Geophysics. Elsevier BV; 2018;152:17–25.
MLA
Delefortrie, Samuël, Daan Hanssens, and Philippe De Smedt. “Low Signal-to-noise FDEM In-phase Data: Practical Potential for Magnetic Susceptibility Modelling.” Journal of Applied Geophysics 152 (2018): 17–25. Print.