## 4 Discussion and Conclusions  
Both the modeled and the real seismic data reveal that thawing has a significant impact on the seismic wavefield. Seismic properties depend on the ice saturation. Increased surface temperature causes increased heat flux, thawing, and lower seismic velocities of near-surface sediments. The largest change in seismic properties occurs at shallow depths when the surface temperature is high, because this implies the largest heat flux. Since Rayleigh waves are not inherently dispersive, surface wave dispersion means that the elastic properties alter with depth. Correspondingly, thawing or freezing will thus cause temporal changes in the FV-spectra. Figure 7: Top: seismic gathers acquired in Adventdalen in (**a**) October 2013, (**b**) January 2014, and (**c**) August 2014. Bottom: corresponding FV-spectra for (**d**) October 2013, (**e**) January 2014, and (**f**) August 2014. The capital letters G-J identify various wave modes. In the synthetic seismic data, the fundamental mode of surface wave is dominant for all experiments. Since lower frequencies are more influenced by elastic properties of deeper sediments than higher frequencies, the fundamental mode in the FV-spectra reveals that the seismic velocities in the shallow layers decrease from the T0 to the T1 model and, furthermore, from the T1 to the T3 model. This is consistent with the input data. Since the velocity decrease extends to lower frequencies for T3 than for T1, T3 implies deeper thawing than T1 due to the reduced velocities with increased thawing. The real seismic data are, as expected, quite different from the modeled seismic data, since the subsurface model used for the synthetic data was not designed to represent local conditions. Since the inversely dispersive trend in the FV-spectra is the strongest in January, weaker in October, and missing in August, this indicates that the presence of higher modes is related to ice saturation. Temperature data indicate that the upper sediments are most frozen in January when the higher modes are strong, and least frozen in August, when the higher modes are absent. Surface waves are generally attenuated more in unfrozen ground than in frozen ground, which also explains why more low-frequency surface waves are seen in the data from January. We did not consider a varying salinity in the synthetic data example, meaning that seismic velocities here increase steadily with depth. At the study site of the real seismic data, the ground salinity most probably varies with depth, which implies that several velocity depth models are possible. During winter, a stiff upper layer overlies softer sediments, and in this case, strong higher modes are typically generated [14; 45; 46; 47]. This also happens when a low velocity layer is embedded in layers of higher velocities. An analogue to a stiff frozen layer on top of a stack of softer unconsolidated sediments is pavement, which constitutes a thin, very stiff layer overlaying softer sediments. For such velocity depth models, higher order modes of Rayleigh waves have been observed as an inversely dispersive trend in FV-spectra, as well as a large amount of leaky guided waves [48], which are also seen in our synthetic data. During summer, we either have stiffer sediments embedded in softer sediments, or stiffness increasing with depth. For such velocity depth models, higher modes are rarely seen [14; 45]. This supports our observation that the fundamental mode is more dominant in October and August, and higher modes are more dominant in January when the uppermost layer is frozen. Differences in the signatures of the modeled and real seismic data can also be caused by the fact that unlike in the synthetic model, the surface temperature varied frequently between low and high temperatures in the time period between October 2013 and August 2014. Furthermore, variable precipitation throughout the year affects the data. While snow leads to effective insulation of the heat flux, rain alters the moisture content in the uppermost sediments.