seismic refraction question

[frazil]

I'm taking a geophysics class and we're doing delay-time seismic refraction right now. The professor gave us this equation:

I don't have in my notes where the tab term comes from. And I can't find anything about it in the book. Can anyone help me?

 

[RIP - VTEnviro]

I don't have in my notes where the tab comes from

A can.

 

[frazil]

thanks vt <_<

I asked someone in class, so I'm all set.