= Solution
Linearized arrivals contain $\delta t_i=\delta t_0+(\partial T_i/\partial z)\delta z+\cdots$. When the stations have similar depth sensitivities, a depth change shifts nearly all predicted times together and an origin-time change compensates it. Distant stations can also have very weak depth sensitivity. \b[Improve the range of takeoff angles and station distances], especially with nearby or borehole stations, and include S–P or depth-phase differences such as pP–P that cancel origin time while retaining depth information. Accurate phase identification and the <velocity> model remain necessary.
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