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Discussion papers | Copyright
https://doi.org/10.5194/esurf-2018-52
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jul 2018

Research article | 10 Jul 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

Potentials and pitfalls of permafrost active layer monitoring using the HVSR method: A case study in Svalbard

Andreas Köhler1 and Christian Weidle2 Andreas Köhler and Christian Weidle
  • 1Department of Geosciences, University of Oslo, P.O. Box 1047, 0316 Oslo, Norway
  • 2Institute of Geosciences, Christian-Albrechts-Universität zu Kiel, Kiel, Germany

Abstract. Time-lapse monitoring of the sub-surface using ambient seismic noise is a popular method in environmental seismology. We assess the reliability of the Horizontal-to-Vertical Spectral Ratio (HVSR) method for monitoring seasonal permafrost active layer variability in northwest Svalbard. We observe complex HVSR variability between 1 and 50Hz in the record of a temporary seismic deployment covering frozen and thawn soil conditions between April and August 2016. While strong variations are due to changing noise conditions, mainly affected by wind speed and degrading coupling of instruments during melt season, a seasonal trend is observed at some stations that has most likely a sub-surface structural cause. A HVSR peak emerges close to the Nyquist frequency (50Hz) in beginning of June which is then gradually gliding down, reaching frequencies of about 15–25Hz in the end of August. This observation is consistent with HVSR forward-modeling for a set of structural models that simulate different stages of active layer thawing. Our results reveal a number of potential pitfalls when interpreting HVSRs and suggest a careful analysis of temporal variations since HVSR seasonality is not necessarily related to changes in the sub-surface. We compile a list of recommendations for future experiments, including comments on network layouts suitable for array beamforming and waveform correlation methods that can provide essential information on noise source variability. In addition, we investigate if effects of changing noise sources on HVSRs can be avoided by utilizing a directional, narrow-band (4.5Hz) repeating seismic tremor which is observed at the permanent seismic broadband station KBS in the study area. A significant change of the radial component HVSR shape during summer months is observed for all tremors. We show that a thawn active layer with very low seismic velocities would affect Rayleigh wave ellipticities in the tremor frequency band.

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Andreas Köhler and Christian Weidle
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Andreas Köhler and Christian Weidle
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The uppermost part of the permafrost thaws during summer and freezes again during winter. We use a passive seismic method to monitor these changes continuously between April and August 2016 close to Ny Ålesund, Svalbard. Our results reveal some potential pitfalls when interpreting temporal variations. However, we show that after a careful analysis this method has the potential to become a very useful tool for long-term permafrost monitoring, being of special importance in a changing climate.
The uppermost part of the permafrost thaws during summer and freezes again during winter. We use...
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