## 4 Discussion and Conclusions
We also assume a simple horizontally layered geological model, and no lateral soil heterogeneity or anisotropy. Adding complexity to the geological model will certainly affect the seismic data, but will to a lesser degree affect the time-lapse effects in data collected at the same location. The seismic experiments in Adventdalen also included a second seismic data line with the same shot point, but having a 67.5 degree azimuth to the east of the current line. These data display similar FV-spectra and time-lapse effects, indicating gentle variation in the soil properties laterally. The seismic setup of the experiments was not particularly designed for recording surface waves. As full thawing only occurs in the top meter or so in Adventdalen, the receiver interval should in general be minimized to gain more high-frequency data. Although geophone strings were used, the array response retains frequencies up to 100 Hz, which is sufficient for our study. Our results show significant time-lapse effects in both the seismic gathers and the estimated Rayleigh wave dispersion curves, which can be related to thawing of near-surface Arctic sediments. The synthetic seismic data were made with full control over the physical properties of the models, and show distinct time-lapse effects that are due to changes in the thermal regime. The time-lapse study of the real seismic data strongly indicate that low-frequency time-lapse effect are due to changes in the freezing conditions. Analysis of FV-spectra basically provide trends in the stiffness gradient (increasing/decreasing/irregular). However, to quantify exact changes in the mechanical properties of the subsurface due to thawing requires more sophisticated inversion of FV-spectra to S-wavevelocity with depth. Conventional inversion techniques using normal mode methods are not designed for (irregularly) decreasing velocities with depth, or when higher-order modes dominate [45; 49; 50]. This was seen to occur for the real seismic data example, where higher modes became dominant with increasing frequencies. Full-waveform inversion often works for small modal jumps, but fails for large modal jumps, and where the main energy does not return to the fundamental mode at high frequencies [14; 45]. An in-depth discussion or execution of more complex inversion methods is beyond the scope of this study. Our study has been limited to qualitatively assessing the differences between the various seismic gathers and FV-spectra at low frequencies. Our results also highlight the need to know the near-surface soil and pore water properties to estimate the degree of thawing from estimated velocity profiles. The contribution of this study is to demonstrate the potential of using time-lapse surface seismic data in climate monitoring of the terrestrial Arctic. Conceptualization, H.M.S. and T.A.J. ; Data curation, H.M.S., T.A.J. and B.O.R. ; Formal analysis, H.M.S. and B.O.R. ; Funding acquisition, T.A.J. ; Investigation, H.M.S, T.A.J. and B.O.R. ; Methodology, H.M.S, T.A.J. and B.O.R. ; Project administration, T.A.J. ; Software, H.M.S. and B.O.R. ; Supervision, T.A.J. ; Validation, H.M.S., T.A.J. and B.O.R. ; Visualization, H.M.S. and B.O.R. ; Writing--original draft, H.M.S. ; Writing--review and editing, H.M.S., T.A.J. and B.O.R. All authors have read and agreed to the published version of the manuscript. This research is funded by ARCEx partners and the Research Council of Norway (grant number 228107). The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.