![height:500px](http://ds.iris.edu/media/product/globalstacks/images/Composite360deg_180min_midsize_1.png)
![height:550px](http://ds.iris.edu/media/product/globalstacks/images/GlobalStack.BHZ.8.sec.180deg.TTcurves.screenshot.png)
https://sites.ualberta.ca/~ygu/courses/geoph624/notes/lecture3-4_2013.pdf