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.