Seismic acquisition and analysis in a changing Arctic

Abstract

In the Arctic, temperatures are regularly low enough that water exists in its frozen state, either in the form of ice, snow, or frozen ground. The effective elastic properties of a material depend on the elastic properties of the individual constituents and their geometrical distribution. Since solid ice is stiffer and more rigid than liquid water, effective elastic properties (bulk and shear modulus) increase with increasing degree of freezing; however, the properties strongly depend on the pore ice morphology, so the relationship is not linear. Seismic waves propagate with velocities that depend on the effective elastic properties of the medium they travel in. Hence, seismic waves travel faster in frozen materials than in similar unfrozen materials, and the velocity depth profile in near-surface Arctic rocks is often irregular. Due to this, seismic records from Arctic environments often show dominating and highly dispersive surface waves. The Arctic surface is today warming at the most rapid pace on earth, but knowledge about the following consequences on the physical properties of the subsurface is scarce. How surface temperature is transmitted to depth and how this affects the mechanical properties of the subsurface is uncertain, in particular for areas with saline pore water. Since the stability of near-surface sediments is largely governed by their mechanical properties, understanding how these properties vary in a temporally changing Arctic is vital. The overall objective of this study is to investigate how to safely acquire seismic data for mapping and monitoring of the near-surface sediments in a changing Arctic climate. To address this, we use seismic data acquired on Svalbard in the Norwegian Arctic. We first investigate whether seismic data can be acquired without affecting the vulnerable Arctic animal life (Paper 1), and then investigate how both long- and short-term surface temperature variations affect effective elastic properties (Paper 2) and the seismic wavefield (Paper 3 and Paper 4). We find that seismic experiments of the size considered here are not physically injurious on the hearing of seals, largely due to the dominating low frequencies (<700 Hz) and the rapid decrease in sound level away from the source. Further, we find that effective elastic properties and seismic P- and S-wave velocities of saline unconsolidated permafrost are highly sensitive to variation in ice saturation caused by varying surface temperatures. By using data from synthetic and real active and passive seismic experiments, we demonstrate various time-lapse effects caused by changing surface temperatures. In particular, the surface wave dispersion shows time-lapse effects of both the fundamental and higher-order modes of Rayleigh waves. We also show that rapidly changing surface temperatures correlate with strong transient events in passive seismic data, identified as frost quakes/cryoseisms. Hence, this study demonstrates that seismic may be useful in monitoring thawing effects in the near-surface sediments in the terrestrial Arctic due to climate perturbations. The thesis motivates for further studies on how to use seismic data in monitoring of permafrost degradation.