## 3 Results
### Real Seismic Data Example
Figure 6 shows the location of the study site, and a picture of the  
Figure 5: The array response of geophone strings with eight geophones where the length and group intervals are 2.5 m. The colors represent the gain of using the array relative to using a single geophone. Using geophone groups significantly downgrades the quality of the surface wave data at high frequencies, but affects in a minor way the amplitudes at low frequencies, except for at very low phase velocities. seismic spread in October 2013. As the picture and topographic map reveals, the ground below the seismic line was flat, and thus no topography effects were assumed. Since we lack information about the exact near-surface soil properties and the corresponding elastic properties at the study site, it is not possible to compute theoretical modal curves for these data. However, since we know that salinity is assumed to increase with depth in this area, this implies that the upper sediments freeze at a higher temperature than the deeper sediments due to the freezing point depression in saline pore water. When temperatures decrease gradually with depth, like in the synthetic example, a few possible scenarios are likely: the first is that during summer, we have an irregular velocity gradient in Adventdalen: a thawed upper low velocity layer above a largely frozen low-salinity and high velocity layer, above a less frozen high-salinity and lower velocity layer. A second possibility is that temperatures are high enough during summer so that the low-salinity layer completely thaws, which implies an increasing velocity gradient with depth: a thawed low-salinity and low velocity layer above a partly frozen high-salinity and higher velocity layer. This implies a similar velocity gradient as seen for the synthetic data example. During winter, it is likely that the uppermost low velocity layer disappears, i.e., velocity decreases with depth. Figure 7a-c show seismic gathers obtained from the three seismic experiments. The data from January are clearly of different nature to the other two gathers, while the differences between the data from October and August are more subtle. A weak event (G) can be seen in all gathers. This is most likely a refraction or reflection from the underlying stiff, consolidated sediments at approximately 65 m depth. The ringing nature of this event seen in the gathers from October and August is probably due to related multiples. Event H in the January data is the Rayleigh wave caused by the frozen, stiff near-surface sediments. The air wave is defined by event I in all gathers. The lack of distinct Rayleigh waves in the data from October and August reveals that the surface sediments are soft. As noted by [44], there could be a strong coupling between the air wave and Rayleigh waves when the upper soil layer supports surface waves with a phase velocity identical to that of the air wave. This requires a soil layer  
Figure 6: The location of the real seismic experiments in Adventdalen on Svalbard in the Norwegian Arctic is marked with a red dot in the small topographic map. The picture shows the study site in October 2013. Gimballed geophone strings are laid out with a 2.5 m group interval. Maps courtesy of Norwegian Polar Institute. with an S-wave velocity less than 330 m/s at the surface and increasing with depth. The surface waves in the August and October data make a monochromatic wave train (J), which is further indicative of a group velocity lower than phase velocity. This is typical for Rayleigh waves when V\({}_{\rm s}\) is increasing with depth (i.e., phase velocity decreasing with increasing frequency). Figure 7d-f show the corresponding FV-spectra for the three seismic experiments. The energy peak in all spectra is related to the air wave, with a velocity of about 330 m/s, but other events are also seen to vary between the different spectra. The most prominent events are observed in the data from January, where there are several strong amplitude events at increasing phase velocities with increasing frequency.