Can Seismic Observations of Bed Conditions on Ice Streams Help Constrain Parameters in Ice Flow Models?

Abstract

We investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface‐to‐bed inversion using a Stokes ice flow model. Using high‐resolution measurements along several seismic profiles on Pine Island Glacier (PIG), we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag. However, there is a stronger correlation when comparing average values along the individual profiles. We hypothesize that the correlation appears at the length scales over which basal variations are important to large‐scale ice sheet flow. Although the seismic technique is sensitive to the material properties of the bed, at present there is no clear way of incorporating high‐resolution seismic measurements of bed properties on ice streams into ice flow models. We conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice‐bed interface from acoustic impedance measurements can be of direct use to ice sheet models.

Relevance of Detail in Basal Topography for Basal Slipperiness Inversions: A Case Study on Pine Island Glacier, Antarctica

“…Across all domains the misfit reduces further when using a domain with the bed topography resolved more completely; including more details of the bed allows us to fit to the data better. This is true for results from the inverse model with a whole range of values of the ratio of misfit to regularization in the cost function (the choice of the corresponding parameter, γ , and the robustness of results to its variation is discussed in more detail in Kyrke-Smith et al, 2017). It is encouraging that we see this improved misfit at such local scale; given a model that contains the appropriate physical formulations we would expect a setup more similar to what is producing the observations to allow for a better fit to the surface data.…”

“…, to infer the slipperiness field that allows the best fit of observations to data. As in previous work (e.g., Petra et al, 2012;Kyrke-Smith et al, 2017), this is formulated as a non-linear, least-squares minimization problem of the cost functional:…”

“…The coefficient of this term is several orders of magnitude smaller than that on J reg1 (Table 1) and it therefore does not affect behavior at the boundary. Details of the numerical solution using FEniCS (Logg et al, 2012;Farrell et al, 2013;Alnaes et al, 2015) are given in Kyrke-Smith et al (2017).…”

“…Misfit values are consequently larger when the regularization term is proportionally larger in the cost function. More details are included in Kyrke-Smith et al (2017). Figure 6 shows the L-curves for inversions over both iSTARt1 and iSTARt7 with each different bed topography option.…”

Assimilation of surface observations in a transient marine ice sheet model using an ensemble Kalman filter

“…Most ice flow models are now equipped with variational methods to constrain the basal conditions from surface observations (e.g. MacAyeal, 1993;Vieli and Payne, 2003;Larour et al, 2012;Gillet-Chaulet et al, 2012). However most studies perform "snapshot" calibrations, where the inversion is performed at a unique initial time step.…”