Consider the refraction wavefront and raypath geometry in Figure 3.4-11a associated with a single-layer nearsurface model. On top, we see a plot of first-breaks. For simplicity, consider a flat surface and flat refractor. For the head wave to form, and thus the refraction to occur, the requirement is that the overburden velocity vw be smaller than the substratum velocity vb.

The traveltime profile depicts the first breaks seen on the shot record in Figure 3.4-11b. - https://wiki.seg.org/wiki/Flat_refractor

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Note added at 31 mins (2019-08-07 09:32:58 GMT)
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4.3.5.2 Travel time picking The P-wave first arrival travel time picks were extracted from the data using the DSI package’s picking tool. Figures 4-34(a) and 4-34(b) show a shot gather with source located at 4,144 ft from the well and the corresponding travel time picks. Indicated on Figure 4-34(b) are turning waves,which are characterized by the change in slope of the first arrivals and change in the vertical component polarity. These turning waves were observed when the source is located 3000 ft and 5000 ft from the well and required new methodologies for the inversion strategy, since current modeling programs do not predict them.

- https://isn.page.link/MFZE - page 115