35 Surface-Wave Methods for Monitoring Subsurface Properties in Per- mafrost Soils

Permafrost (frozen ground that remains at or below 0◦C for at least two consecutive years) covers 24% of northern circumpolar continents (Zhang et al. 1999). Thawing permafrost in a warming climate can cause hazards of serious concern: (1) Significant amounts of greenhouse gases can be released into the atmosphere due to large-scale microbial decomposition of organic matter that are previously preserved in frozen soils; (2) Thermokarst subsidence in ice-rich permafrost can further intensify the thaw of frozen ground as well as cause severe damage to infrastructure and transportation networks. Both incremental processes (e.g. active layer thickening and talik formation) and transient phenomena (e.g. thaw-induced ground surface settlement and slope instability) affect the rate of permafrost degradation and the detrimental impacts on climate change. Substantial amounts of supercooled water (liquid water that stays unfrozen below 0◦C) exist in permafrost soils. Such unfrozen water not only enables carbon-related microbial activities even in subzero temperatures but also makes the subsurface water/ice content not directly predictable using only temperature information. All these aforementioned issues point to the necessity of permafrost monitoring in various spatial and temporal scales. Thermal monitoring in deep boreholes, despite being one of the most common monitoring approaches, only provides onedimensional plot-scale measurements that do not directly indicate ground ice content. In addition, deep boreholes in permafrost are generally scarce because of the high cost and logistical difficulties, especially in remote Arctic and Subarctic regions. In contrast, geophysical monitoring methods can provide information on subsurface properties over field-scale areas (e.g. hundreds of meters laterally and vertically) at a comparatively low cost and with minimal disturbance to the natural environment. Geophysical monitoring approaches also have the potential for automated acquisition using permanent or semi-permanent sensor networks, a distinct advantage in regions that are difficult to access and in inhospitable environments. Permafrost characterization and monitoring with geophysical methods relies on properties that differ between frozen and unfrozen materials. The most frequently used geophysical properties for studies of periglacial environments include electrical resistivity, dielectric permittivity and seismic velocity. Among other properties of frozen soils, ground ice content is a key factor when estimating risk level of environmental hazard in the context of global warming. Though electrical methods usually require less effort in data acquisition and processing, they often result in difficulties in differentiating between ice and air and are often limited to shallower targets. Seismic methods, on the other hand, do not suffer from such problems and thus are particularly suitable for assessing ground ice content, both as an independent method and a complementary approach to be combined with other geophysical monitoring techniques.