Seismic Energy Partitioning and Scattering in Laterally Heterogeneous Ocean Crust

Abstract

Abstract--We present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust. The finite difference formulation is a 2-D solution to the elastic wave equation for heterogeneous media and implicitly calculates P and SV propagation, compressional to shear conversion, interference effects and interface phenomena. Random velocity perturbations with Gaussian and self-similar autocorrelation functions and different correlation lengths (a) are presented which show different characteristics of secondary scattering. Heterogeneities scatter primary energy into secondary body waves and secondary Stoneley waves along the water-solid interface. The presence of a water-solid interface in the model allows for the existence of secondary Stoneley waves which account for much of the seafloor 'noise' seen in the synthetic seismograms for the laterally heterogeneous models.'Random' incoherent secondary scattering generally increases as ka (wavenumber, k, and correlation length, a) approaches one. Deterministic secondary scattering from larger heterogeneities is the dominant effect in the models as ka increases above one. Secondary scattering also shows up as incoherence in the primary traces of the seisograms when compared to the laterally homogeneous case. Cross-correlation analysis of the initial P-diving wave arrival shows that, in general, the correlation between traces decreases as ka approaches one. Also, because many different wave types exist for these marine models, the correlation between traces is range dependent, even for the laterally homogeneous case.

Broadband seismology and noise under the ocean

“…Finite difference models of wave propagation demonstrate that energy readily couples from acoustic waves into short-wavelength modes during interaction with a rough bottom [Dougherty and Stephen, 1991 [Rhines, 1977]. Currents can also be large around bathymetric features such as deep-sea canyons or the tops of seamounts [Brink, 1995].…”

Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings

“…Rather than displaying displacement vectors at each grid point, we display "amplitude density", which is proportional to the square root of the energy density and maintains the sign of the amplitude wave field [Dougherty and Stephen, 1988;Stephen and Swift, 1994]. Snapshots are outputted as compressional and shear energy density at all grid points at various times during the simulation.…”

Bottom Interaction in Ocean Acoustic Propagation

“…Deep seafloor arrivals seem to be an interface wave whose amplitude decays upward into the water column. It appears that the interface wave is excited by secondary scattering from a small seamount (Chapman and Marrett, 2006;Dougherty and Stephen, 1988;Schreiner and Dorman, 1990) but this model is not yet fully developed. Understanding the physical mechanisms responsible for these arrivals will be essential for the proper interpretation of long-range receptions on deep seafloor receivers.…”