Full-wavefield inversion of surface waves for mapping embedded low-velocity zones in permafrost

Abstract

Surface waves are advantageous for mapping seismic structures of permafrost, in which irregular velocity gradients are common and thus the effectiveness of refraction methods are limited. Nevertheless, the complex velocity structures that are common in permafrost environments often yield unusual dispersion spectra, in which higher-order and leaky modes are dominant. Such unusual dispersion spectra were prevalent in the multichannel surface-wave data acquired from our permafrost study site at Barrow, Alaska. Owing to the difficulties in picking and identifying dispersion curves from these dispersion spectra, conventional surface-wave inversion methods become problematic to apply. To overcome these difficulties, we adopted a full-wavefield method to invert for velocity models that can best fit the dispersion spectra instead of the dispersion curves. The inferred velocity models were consistent with collocated electric resistivity results and with subsequent confirmation cores, which indicated the reliability of the recovered seismic structures. The results revealed embedded low-velocity zones underlying the ice-rich permafrost at our study site -an unexpected feature considering the low ground temperatures of −10°C to −8°C. The low velocities in these zones (∼70%-80% lower than the overlying ice-rich permafrost) were most likely caused by saline pore-waters that prevent the ground from freezing, and the resultant velocity structures are vivid examples of complex subsurface properties in permafrost terrain. We determined that full-wavefield inversion of surface waves, although carrying higher computational costs than conventional methods, can be an effective tool for delineating the seismic structures of permafrost.

A Review of Geophysical Methods for Soil Structure Characterization

“…The principles of MASW methods have been successfully used for in situ studies, for example, the methodology employed by Ryden and Park () to determine the thickness and shear velocities of pavement layers. These concepts have already been used in other systems with similar characteristics (e.g., permafrost study by Dou & Ajo‐Franklin, ). An interesting study for soil structure was presented by Mandal et al () for linking soil strength with P wave velocities.…”

Coincident aboveground and belowground autonomous monitoring to quantify covariability in permafrost, soil, and vegetation properties in Arctic tundra

“…Site core samples at 3-6 m depth [O'Sullivan, 1966;Brown, 1969;Sellmann et al, 1975] have shown salinity values up to 2 times seawater salinity [O'Sullivan, 1966]. Bulk electrical resistivity imaging, seismic, and analysis of core samples at the study site [Dou and Ajo-Franklin, 2014;Dafflon et al, 2016] suggested that the partially unfrozen saline permafrost is spatially extensive and heterogeneous.…”

Skeletonized wave equation of surface wave dispersion inversion

“…Inverting surface waves for the S-wave velocity model fall into two categories: 1) the classical method of inverting dispersion curves (Evison et al, 1959;Park et al, 1998;Xia et al, 2004) for a 1D layered medium, and 2) waveform inversion (Groos et al, 2014;Solano et al, 2014;Dou and Ajo-Franklin, 2014) for 2D and 3D media. The classical method accurately inverts for a 1D S-wave velocity model, but becomes less accurate with increasing lateral heterogeneity in the subsurface velocity model.…”