Distinguishing horizontal/vertical discontinuities

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Problem

Pautsch (1927) showed that a horizontal or vertical interface could give identical first-arrival curves (Figure 6.12a). Add secondary refractions and reflections to show how they can distinguish between the two cases.

Background

At Pautsch’s time only first arrivals were observed and hence interpretation had to be based on them. Today we also observe refraction events (secondary arrivals or secondary refractions) that are not first arrivals (see problem 6.20).

Solution

For the vertical interface in Figure 6.12a(ii) there will be a direct wave plus a reflection directed back toward the source. For the horizontal interface, there is also a reflection and a refraction (head wave), the refraction curve being tangent to the reflection curve at the critical distance. These additional curves distinguish between the two cases. Also, if we move the source, the bend in the curve moves in (ii) but not in (i).

Figure 6.12a.  Models that give identical traveltime curves.
Figure 6.12b.  Models that give identical traveltime curves.

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