Anisotropic reflection and transmission calculations with application to a crustal seismic survey from the East Greenland Shelf

Abstract

A three‐component refraction data set recorded over the East Greenland Shelf contained two anomalous shear wave arrivals. The direct‐S phase emerged at the receiver as an apparent SH wave even though the airgun source was purely compressional. We have concluded that this arrival is the result of anisotropy in the upper crust. By employing a number of fairly simple techniques we were able to highlight a second, delayed quasi‐shear arrival and interpret shear wave splitting. Modeling of the polarization, amplitude, and delay time of direct‐S suggested that the anisotropy is due to aligned subvertical fractures perpendicular to the local seafloor spreading direction. The second shear arrival of interest is a P to S conversion from the Moho (PmS). The relative strength of this arrival suggests that the Moho is a sharp transition in this region. PmS also contained a significant transverse component but attempts to identify shear wave splitting were inconclusive. However, modeling revealed that the transverse PmS displacement may be due to anisotropy in the upper mantle (alignment of olivine crystals with the spreading direction), which causes the converted S reflection to have a significant SH (transverse) component. An understanding of, and ability to model, anisotropic reflection and transmission effects are essential to these interpretations. Therefore we also outline an efficient method to obtain all of the reflection and transmission coefficients at boundaries between anisotropic media. This has been incorporated into a new ray tracing package, A‐TRAK, based on asymptotic ray theory, capable of modeling three‐dimensional inhomogeneous media separated by curved interfaces.

Modelling converted seismic waveforms in isotropic and anisotropic 1-D gradients: discontinuous versus continuous gradient representations

“…Thus a more complete evaluation of the reflection and transmission (R/T) properties is needed. Over the past several decades significant contributions have been made in the evaluation of R/T coefficients for isotropic (e.g., Gilbert & Backus, 1983;Molotkov et al, 1976;Novotný et al, 1980;Kennett, 1983) and anisotropic (e.g., Garmany, 1983;Fryer & Frazer, 1984;Guest et al, 1993) layered media (see Kennett, 2001a). In most of these approaches, the solution to the R/T response involves using a local plane-wave and planeboundary approximation (see Hudson, 1980;Kennett, 1983).…”

“…From here on, the lateral coordinates and slownesses are denoted by Greek subscripts (e.g., xα and pα). After Fourier transformation and using a modified form introduced by Woodhouse (1973), the equation of motion 3.2 is written in matrix form (Guest et al, 1993) …”

Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

“…Reflected seismic transit times (e.g., Wang & Tsvankin, 2013) and amplitudes carry valuable information about the underlying elasticity, and its directional dependency is the key to the understanding of elastic anisotropy (e.g., Ekanem et al, 2013;Far et al, 2013;Guest et al, 1993;Hall & Kendall, 2003;Landrø & Tsvankin, 2006;Sayers & Dean, 2001).…”

A one-way wave equation for modelling variations in seismic waveforms due to elastic anisotropy

“…Interference between the anisotropic quasi‐shear (qS) waves is the most diagnostic wave‐propagation effect. Sometimes two distinct shear waves are apparent (Winterstein & De 2001; Guest et al 1993). If they are not apparent one can infer only that the two shear‐wave speeds are close together in the particular propagation direction sampled.…”