Everhard Muyzert

Wavefield separation using a volume distribution of three component recordings

By making multiple three component (3C) measurements around a volume so that the rate of change of the wavefield in all directions can be estimated, it is possible to calculate the divergence and the curl of the wavefield which in turns corresponds to P- and S-waves. The volume may be of arbitrary shape, but the equations for wavefield separation are simplified if the volume has a known shape such as that of a tetrahedron. The dimensions of the volume should be smaller than approximately one fifth of the minimum wavelength of the recorded wavefield. A field test was carried out in Oman in April 1998. With a lack of distinct P- and S-waves in the data, the technique was instead evaluated on the airwave-induced ground motion. The airwave arrival was found to consist mainly of vertical motion that decays with depth. The wavefield separation technique was successfully tested on this arrival.

The stability of one-dimensional inverse scattering

The stability of one-dimensional inverse scattering is discussed. The analysis is performed analytically using perturbation theory for reflection coefficients that are rational functions of the wavenumber and numerically for realistic reflection coefficients. It is shown that the same instability features occur for real reflections as for rational reflection coefficients. There are three types of error considered which are common in the application of the data. Explicitly examined are an erroneously chosen DC level, an amplitude error and a time error. It is shown that the physical reason for the instabilities of the one-dimensional inverse problem is the lack of sufficient energy penetration in the potential that is examined.

The NARS-DEEP Project

The NARS-DEEP project involves the deployment of broadband seismological stations in Russia, Belarus and the Ukraine. Six stations were installed in 1995, two in 1997, and more stations were to be installed from 1998 onwards. The NARS-DEEP project was initiated as a temporary deployment, but funding from the European Community enabled the stations to become permanent. In this paper we present first results of the NARS-DEEP project. A surface waveform inversion for the upper mantle structure along a profile from Egypt to Spitsbergen shows evidence for strong variations in the shear-velocity structure for the different tectonic units. The most striking features of the model comprise strong lithospheric anisotropy beneath the Eastern Mediterranean and a lithospheric thickness of approximately 200 km beneath the shield areas. The crustal structure beneath the seismic station in St. Petersburg was investigated using the receiver function method. The results of a Monte Carlo inversion for the receiver functions of this station show evidence for a sedimentary layer with a thickness of less than 1 km overlying an upper crustal layer extending to a depth of approximately 16 km. The Moho depth cannot be resolved from the receiver functions of this station.

An alternative parameterisation for surface waves in a transverse isotropic medium

The elastic properties of the Earth can only be retrieved by seismic waves when assumptions about the seismic anisotropy are made. One such an assumption is to assume that the Earth is transverse isotropic. In a transverse isotropic Earth Love waves are sensitive to two independent elastic parameters and Rayleigh waves to four parameters. In addition these waves are sensitive to density. However, resolving four elastic parameters together with density as function of depth from Rayleigh waves phase velocity curves is difficult as the large number of parameters can make the inverse problem easily under-determined. We show that the partial derivatives of fundamental and higher mode Rayleigh wave phase velocities with respect to the parameters of transverse isotropy for periods 20 - T- 200 s are very similar to each other. This means that the inversion of Rayleigh phase velocity data has intrinsic trade-offs. We show that Rayleigh wave data can only resolve three combinations of parameters of the transverse isotropic medium: the S-velocity b , the sum of the horizontal and V wx

S-wave structure of the American Southwest and Mexico from single-station broadband data

We investigate S velocity structure beneath the Southwestern U.S. and Mexico using upper mantle turning S waves and surface waves. The data are tangential component records from Baja California, Mexican and Central American events recorded at broadband station ANMO (Albuquerque, New Mexico). Study geometry allows investigation of the variation of S structure with source-receiver azimuth for sources from Baja to Yucatan. The narrow backazimuthal sector studied comprises a transition from oceanic to tectonic to more ‘continent-like’ structure. We document this transition in the lithosphere and upper mantle to roughly 450 km depth.