Time-distance curves for various situations

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Problem 4.22

The situations shown in Figure 4.22a involve two rock types, the shaded one having the higher velocity [laterally varying according to the shading density in (i) and (j)]. Two dip cases are shown in (c). This figure is adapted from Barton (1929), one of the first publications in English on the seismic method. Sketch time-distance curves in the spaces above 0 [part (a) shows what is expected].

Solution

The time-distance curves are shown in Figure 4.22b. Traveltimes in the updip direction are earlier than those in the downdip direction. Parts (e) and (h) also involve a wide-angle reflection (see problem 6.13), which is not shown because it would be offscale at the top. The headwaves are tangent to diffractions that occur directly over the diffracting points. Curvature of the head-wave curves indicates changes in refractor velocity.

Figure 4.22a.  Various situations for drawing time-distance curves.
Figure 4.22b.  Time-distance curves for Figure 4.22a. Dir indicates a direct wave; , a reflection; , a head-wave; , a diffraction.

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