## 1 Introduction  
Adventdalen is located on Svalbard in the Norwegian Arctic, in an area that traditionally has experienced stable subzero winter temperatures and minimal annual rainfall. However, measurements in recent years show that climate is changing rapidly in this region and in the Arctic in general [1]. The IPCC [2] projects that temperatures and precipitation will continue to increase drastically in the years to come, likely contributing to thawing of currently frozen surfaces, including sea ice, glaciers, snow, and fully or partly frozen ground, often referred to as permafrost. Permatfrost is defined as ground that stays at or below \(0\)\({}^{\circ}\)C for at least two years [3], and this purely thermal definition implies that the degree of freezing in permafrost can vary considerably. Methods for monitoring thawing of permafrost are desirable because thawing of currently frozen ground may have large geomorphic consequences [4]. Frozen ground is stiffer and more rigid than unfrozen ground [5], therefore, subsidence and increased avalanche risk are likely consequences of thawing. Additionally, the large currently frozen land areas in the Northern hemisphere hold huge reserves of carbon that may be released to the atmosphere in the thawing process, inducing further climate change [6]. These methods should preferably be non-intrusive due to efficiency and environmental constraints. [PERSON], et al. [7] provide an overview of some previously tested geophysical methods for monitoring thawing of permafrost, based on changes in electric, electromagnetic, elastic, or dielectric properties. All methods show limitations, particularly in areas with saline pore water, see [8] for a good overview. Salt causes freezing point depression and gradual freezing of pore water at subzero temperatures because the salinity of the residual pore water increases as salt is expelled when the water freezes [9]. Typical for saline permafrost is therefore unfrozen water within the permafrost, the amount depending on the salinity. If the salinity varies with depth, the amount of ice also varies with depth leading to an irregularly varying stiffness gradient with depth. For such partially frozen soils, the correlation between temperature and resistivity can be complicated [8; 10], making electric methods difficult to use. Additionally, refraction seismics cannot be used in such environments, because it will not detect softer layers if these are located below stiffer layers. Effective elastic properties of a medium depend on the physical properties and quantity of its constituents, as well as their geometrical distribution, which for permafrost typically means sediment grains, water, ice, and air. Thus, since thawing of a frozen material leads to a change in the ice-to-water ratio, referred to as the ice saturation, thawing will change the effective elastic and seismic properties. Laboratory measurements of the seismic properties of permafrost during the freeze-thaw process [8; 11] support the idea that elastic properties of unconsolidated sediments decrease with increased thawing, however the relation between the degree of thawing and change in elastic properties is not linear. Shear modulus of unconsolidated sediments is particularly affected by variation in ice saturation [12], and since surface wave properties strongly depend on the shear modulus, surface wave methods may be a viable option for detecting thawing in permafrost. Multichannel analysis of surface waves (MASW) was first introduced by [PERSON], et al. [13], and has been extensively used for determining the properties of near-surface sediments through inversion of surface wave properties. However, MASW inversion shows limited success for soils with irregular stiffness gradients, and so more sophisticated methods are needed here.