Seismic Characterization of Subsea Permafrost: Interpretative Advantages of Using All Seismic Arrivals.

Abstract:

Extensive areas of offshore permafrost occur beneath the continental shelf of the Arctic Ocean. In response to natural and anthropogenic stresses, permafrost degrades increasing the risk of seafloor failures and releasing ice-trapped greenhouse gases and contaminants into the environment. Permafrost, depending on its state, may contain unfrozen pore fluids in addition to ice. Changes in permafrost conditions affect the elasticity of the medium and therefore can be detected by seismic methods. Traditionally, refraction and reflection methods have been used to study permafrost. Here, all seismic arrivals including refractions, reflections (primaries and multiples), diffractions and guided waves, are analyzed in combination with viscoelastic forward modeling to characterize changing permafrost conditions from data collected on the continental shelf of the Canadian Beaufort Sea. Results show that changing conditions along the top of the permafrost affect energy partitioning, as the various arrivals contained in the seismic data present distinct wave properties. For discontinuous permafrost, important variations of amplitude and frequency are observed. The presence of coherent arrivals having distinct signatures suggests the coexistence of changing permafrost conditions. This is supported by velocity surveys conducted in two wells located ~4 km apart where permafrost thickness is ~310 m. At well M-13, permafrost is faster, the top permafrost is very reflective and generates high amplitude multiples. At well I-44, permafrost is slower and seismic data are rich in low frequencies. Viscoelastic modeling suggests that increased attenuation is induced by permafrost degradation as the pore filling changes from solid to mixed-phase. Moreover, guided waves are also recorded at the location where permafrost has a slower velocity, increasing the low frequency content of the data. Additionally, diffractions suggest a patchy permafrost distribution as this arrival is generally produced at the termination of a reflector. The analytical scheme shows that seismic methods are very effective to characterize the state of the subsea permafrost especially when information provided by various arrivals are integrated in a complementary manner rather than limiting the analysis to one type of arrival in particular.