Seismic-data processing flows often ignore spatial and temporal variations in the sea surface during marine seismic acquisition by assuming a flat free surface. However, weather patterns during data acquisition can generate rough sea conditions, which can significantly influence seismic full-wavefield source behavior, including ghost reflections and surface-related multiples, by introducing spatial and temporal distortions of the seismic wavelet. To investigate the effects of rough seas on seismic wave propagation, we have developed and solved a new acoustic wave equation using a mimetic finite-difference time-domain (MFDTD) scheme that uses a dynamic (i.e., moving) generalized coordinate system defined to be conformal to the assumed known time-varying free surface. This “sea-surface” coordinate system allows us to model the full dynamic effects associated with this complex boundary condition. Numerical examples demonstrate that the developed MFDTD method can accurately simulate seismic wavefield propagation on a moving mesh for significant wave heights of 5 m and beyond, and it is thus a reliable tool for applications involving modeling, processing, imaging, and inversion of seismic data acquired in rough seas.
The density structure of firn has implications for hydrological and climate modelling and for ice shelf stability. The firn structure can be evaluated from depth models of seismic velocity, widely obtained with Herglotz-Wiechert inversion (HWI), an approach that considers the slowness of refracted seismic arrivals. However, HWI is appropriate only for steady-state firn profiles and the inversion accuracy can be compromised where firn contains ice layers. In these cases, Full Waveform Inversion (FWI) can be more successful than HWI. FWI extends HWI capabilities by considering the full seismic waveform and incorporates reflected arrivals, thus offering a more accurate estimate of a velocity profile. We show the FWI characterisation of the velocity model has an error of only 1.7% for regions (vs. 4.2% with HWI) with an ice slab (20 m thick, 40 m deep) in an otherwise steady-state firn profile.
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