Refraction blind spot

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Problem 11.16

In early refraction exploration for salt domes, a “blind spot” (the region B–A in Figure 11.16a(ii)) was found when the salt dome lay directly on the line between the source and the geophone, that is, arrivals were often too weak to detect. This was called absorption of the wave” by the salt dome. What is the true explanation of the absorption?

Figure 11.15b.  Time-distance curves.

Solution

The vertical section in Figure 11.16a(i) shows rays being bent upon entering and leaving the salt dome. In the plan view of Figure 11.16a(ii), the bending of the rays in the horizontal plane is evident. This spreading of the wave, both vertically and horizontally, but especially the latter, lowers the energy density. Thus, the cause of the low amplitude is the raypath bending.

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