Interpretation of surface wave spectra in laterally inhomogeneous media

Abstract

SUMMARY Surface waves are used for the investigation of sharp lateral boundaries. The spectra are interpreted in terms of seismic ray theory combined with the period-dependent transmission coefficients. Physical attenuation produces large transmission and reflection losses in this procedure. Some conclusions concerning the territory of the GDR are drawn. While the solution of the inverse problem for surface waves is nowadays routinely handled for lateral homogeneity, there seems to be a lack of satisfying procedures for laterally inhomogeneous media. As long as the deviations from homogeneity are small, surface wave tomography is successfully carried out using modern methods (Nolet 1987). The introduction of sharp boundaries into ray-tracing schemes is possible for body waves as well as surface waves. For the latter, however, this procedure is not trivial, because the reflection and transmission coefficients do not occur as a solution of an algebraic system of equations as for body waves (Malischewsky 1987). The combination of ray theory with the concept of sharp boundaries, with the aim to solve the tomographic problem in 3-D media with discontinuities, is connected with additional difficulties which are the main topic of Malischewsky (1989); in the following a brief outline of this paper is given. Whenever tomography is applied, we can work either in the reflection or in the transmission regime. In the case of surface waves the reflection regime is preferable when the main aim is the detection of unknown discontinuities, because interpretable special wave groups usually occur in the seismogram. In Fig. 1, we show seismograms of a near seismic extent recorded at the stations BerggieShiibel (BRG) and Pritzwalk (PRW) in the GDR. In the PRW record we recognize a Rayleigh wave group between 4 and ,a