RESOLVING A LOW-VELOCITY ZONE WITH SURFACE-WAVE DATA

The main purpose of seismic inversion is the retrieval of seismic velocities and densities in the earth. Inversion of body-wave traveltimes cannot uniquely determine the seismic velocities as a function of depth when a low-velocity layer is present. It is generally assumed that surface waves do not suffer from the same non-uniqueness. The issue is addressed whether it is possible to remove the non-uniqueness of traveltime inversions with a realistic set of surface-wave data. This requires the exact determination of the velocity distribution within and around the low-velocity zone. Waveforms, phase velocities as well as group velocities are investigated qualitatively. Synthetic Love-wave phase and group velocities are actually inverted. Waveforms are shown to be sensitive to the exact velocity distribution of a low-velocity layer, but it is concluded that the removal of the non-uniqueness with the use of waveforms is difficult because the differences of the waveforms are generally small and because complications such as lateral heterogeneity and poorly known source parameters reduce the accuracy of waveform inversions. The inversions of Love phase and group velocities indicate that it is difficult to determine the velocity distribution of a low-velocity layer in a statistically significant way with a realistic set of dispersion data. Group velocities are shown to be more sensitive to the low-velocity structure than phase velocities. Unfortunately, group-velocity data suffers from a practical non-uniqueness because in general only fundamental-mode group velocities can be measured. It is concluded that the non-uniqueness in the detailed structure caused by a low-velocity layer cannot readily be resolved by using surface-wave dispersion data.