See user guide.
This module generates a multichannel seismic record from a layered-earth model (*.LYR) by incorporating an impulsive seismic source and multichannel recording configuration. The input model (*.LYR) consists of both elastic and inelastic properties of material assigned into each (laterally homogeneous) layer of a specific thickness (h); the elastic properties include compressional (Vp) and shear-wave (Vs) velocities and density (rho), and the inelastic properties include the attenuation (quality) factors (Qp and Qs) for the P and S waves.
The modeling algorithm is based on the reflectivity method in its original codes developed by Fuchs and Muller (1971), provided by Dr. Klaus Holliger at the Swiss Federal Institute of Technology and Dr. Michael Roth at the Norwegian Seismic Array (NORSAR). Dr. Adam O'Neill at Kyoto University then altered and recompiled the codes to produce a DOS executable ("ss4exe.exe") that has been shared (with a readme file) among researchers in the field of seismology. Special acknowledgments go to all of them, especially to Dr. O'Neill.
The reflectivity modeling provides the most realistic MASW seismic data for the following three reasons. First, in theory, each MASW record represents a 1-D (depth) measurement of subsurface materials and the reflectivity modeling is truly a 1-D algorithm based on the laterally homogeneous medium.
Second, surface waves are considered, from theoretical perspectives, as being generated through a complicated interference of reflections and refractions of (pure and mode converted) P and S waves and the reflectivity method is known to model such wave phenomena in a stratified medium in the most accurate manner at a relatively low computation cost. Lastly, the modeling can incorporate not only elastic (such as Vp, Vs, and rho) but also inelastic (Qp and Qs) properties in the wavefield calculation, and this ability of modeling makes the modeled dispersion image appear closest to the real one observed from the field record.
The user interface to perform modeling has been built based on the documentation provided in the readme file. The interface consists of five (5) main tabs (Figure 1): "Data Acquisition", "Slowness", "Dispersion", "Output", and "Multi-Value Run" tabs. Among them the first tab ("Data Acquisition") is most important. It contains all the acquisition parameters of field geometry (receiver spacing, source offset, type of receiver array, etc.), recording (number of channels, sampling interval, recording time, etc.), and source characteristics (spectral contents, source location, etc.). The "Slowness" tab contains those parameters related to the slowness (or wave number) integration in the reflectivity formulation for which default values are sufficient in most cases. The "Dispersion" tab provides options to generate dispersion images and/or curves as output in addition to the seismic data. The output file name can be specified, as well as the record number for the output seismic record, in the "Output" tab. The "Multi-
Value Run" tab holds those parameters that can execute the modeling multiple times by successively changing one of the parameters in the input model (*.LYR) (for example, by incrementing Vs values in all layers by 50 m/sec each time) or in the source offset (for example, by incrementing X1 by 5-m each time). Output records from this execution are saved in the same file with different record numbers, which are indicative of the value used to generate the particular record.
All parameters previously used (if they exist) are automatically imported as default values at the appearance of the dialog shown in Figure 2. They can also be checked from the display of modeled seismic data by clicking the processing history button as illustrated in Figure 3.