Wavefield Inversion of Surface Waves for Delineating Seismic Structure in Saline Permafrost: A Case History from the Barrow Peninsula, AK

Abstract:

Seismic investigations of permafrost are essential in cold-region applications including static corrections for seismic exploration and site characterization for infrastructure development. Surface-wave methods are advantageous because their applicability does not require regular velocity gradients. But distinct challenges also exist: The irregular velocity variations in permafrost, combined with the marked velocity contrasts between frozen and unfrozen ground, often yield complicated dispersion spectra in which higher-order and leaky modes are dominant. Owing to the difficulties in retrieving dispersion curves from such spectra, dispersion-curved-based inversion methods become inapplicable.

Here we present a case study of using wavefield inversion of surface waves to infer the permafrost structure on the Barrow Peninsula of the Alaskan Arctic Coastal Plain. In May of 2014, we conducted an active multichannel surface-wave survey along a 4300-m (2.7-mi) NE-SW trending transect that extended from the coastal to the interior areas of the peninsula. We acquired surface-wave supergathers—each covering a distance of 147 meters—from four nearly equidistantly distributed subsections of the transect. The dispersion spectra show dominant higher-order and leaky modes, as well as inversely dispersive trends (i.e., phase velocities increase with increasing frequencies). Preliminary results reveal a “sandwich” velocity structure, in which a pronounced low-velocity layer (with S-wave velocity reductions up to ~45%–60%; tens of meters thick; overlain by 3–4 m of high-velocity strata) is embedded within high-velocity strata, and the low-velocity layer itself contains irregular velocity gradients. Considering the low ground temperatures of –10 °C to –8 °C, this low-velocity feature is likely to be an embedded saline layer that is only partially frozen due to freezing-point depression of dissolved salts. Because saline permafrost is particularly sensitive to thermal disturbances, its extensive presence on the Barrow Peninsula may impact the stability of the permafrost system in a warming climate. This case study exemplifies the complexity of permafrost structures, and the surface-wave-based wavefield inversion is effective for delineating seismic structures of permafrost.