2-D Elastic Isotropic Finite Difference Modeling Lab and Polarity Analysis

OBJECTIVE: Model elastic data by a 2-D Finite Difference solution to the isotropic elastic wave equation. Discover how polarity of arrivals is determined by the type of point source and the component you record. Understand implementation of FD algorithm and absorbing BC's.

PROCEDURE:

Load all files into your working directory. Important MATLAB scripts for modeling are (fdps1.m), Ricker wavelet generator (calrick.m), parameter file (param.m), and plotting (plotit.m and plt.m). ). Name each file by their names given above.

In MATLAB, type "fdps1" to generate synthetic data for a 320x120 2-layer model. Display will show you movie of snapshots of Pressure field for a line source below free surface. Examine the fdps1.m code and convince yourself that it honors the FD algorithm.

Examine code to see if it has absorbing boundary conditions at sides/bottom of model. If you are an advanced student you might want to implemment an absorbing boundary by gradual attenuation (weight of 1 at beginning of sponge zone and exp(-ax) at endpoint x0 of sponge zone where exp(-ax0)=.97; maybe make the sponge zone a semi-cylinder with the axis centered at the model's surface.) of particle velocity fields that enter a sponge zone 50 grid points (4-5 wavelengths) from boundary.

The source is a vertical particle velocity point source. Why do the 1st arrivals of the recorded horizontal component seismograms switch polarity on either side of the source location. Why don't you see this switching of polarity on the vertical component phones?

Same question as the previous one, except now replace the vertical particle velocity source with a horizontal particle velocity source. Explain the polarity behavior of the vertical and horizontal particle velocity seismograms.

Identify the direct P and S waves,and the P and S refraction arrivals in the snapshots if they are visible.

Insert an explosive point source that is rich in P wave arrivals and poor in S-wave arrivals. Show results and explain your reasoning.

Insert a shear point source that is rich in S wave arrivals and poor in P-wave arrivals. Show results and explain your reasoning.