Example Rayleigh Wave Processing: Manual Interpretation (FwdR1.m)

The output dispersion file bvax.his can be read by program FwdR1.m to manually invert the Rayleigh wave dispersion problem. The default install location for the Octave program is /usr/local/share/octave/site-m. The initial model sets the number of control points in the model, shear wave velocities, and depth points. The initial model is placed in a text file, for example call it model.txt.

Below is an example for a 5 point Vs profile. The first row is just the number of control points, 5 in this case. The next row are the shear wave velocities at each depth point given in the 3rd row. The units are meters/second and meters.


{fwdr1}
5
125. 153. 217. 165. 387.
0 1.6 3.2 6.6 16.
fwdr1


The Octave procedure requires Fortran subroutine, rwv.f and files wrapper.cpp, build_disper_oct, to be located in the working directory or path. The Octave program will check to see if the ELF shared object, disper.oct, is compiled and in the directory or path. If not, it will automatically compile and link this subroutine which speeds up the computations. The script, build_disper_oct must be executable. The program FwdR1.m prompts the user with a number of setup GUI's after the initial question for the name of the model text file. The first asks for the compressional velocity setting (constant ratio or fixed value). If you choose constant ration of Vp/Vs, then you are prompted for Poisson Ratio, Grain Density, Porosity and Degree of water saturation. Edit if needed and then click OK. A message will display the Vp/Vs ratio and density that will be used.

If data are present in the form of a bvax.his file, those will be plotted blue with error bars and the model dispersion will be plotted in red. One is then prompted to continue or quit. If you continue, click Yes and the current model will be displayed as a list of entry boxes. Make changes as desired to bring the red curve closer to the blue curve. The Nlayer value is displayed, but if you change that, it will be ignored. To change the number of control points, you will need to start fresh with a new model.txt file. Once you have a satisfying fit, click No to exit the loop and then save your plots. The title of the Figure 1 plot has the current model. NOTE: Rayleigh wave dispersion is primarily sensitive to shear velocity, Vs, but compressional velocity Vp and density will have a noticeable effect. If you change the Vp and Density assumption, then you will likely make an adjustment to Vs and depth.

Figure 46: Manual modeling with FwdR1.m, final trial (A) dispersion and (B) soil profile. Vs30 is in the title bar of (B) assuming parameters remain constant down to 30 meters.
\includegraphics[scale=.85]{FigureMM}