TRAVEL-TIME DEPTH

Echo soundings give us a picture of the earth. A zero-offest section, for example, is a planar display of traces where the horizontal axis runs along the earth's surface and the vertical axis, running down, seems to measure depth, but actually measures the two-way echo delay time. Thus, in practice the vertical axis is almost never depth z; it is the vertical travel time $\tau$.In a constant-velocity earth the time and the depth are related by a simple scale factor, the speed of sound. This is analogous to the way that astronomers measure distances in light-years, always referencing the speed of light. The meaning of the scale factor in seismic imaging is that the $(x, \tau$)-plane has a vertical exaggeration compared to the (x,z)-plane. In reconnaissance work, the vertical is often exaggerated by about a factor of five. By the time prospects have been sufficiently narrowed for a drill site to be selected, the vertical exaggeration factor in use is likely to be about unity (no exaggeration).

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