A couple of years ago I read an explanation of the derivation of interval velocities from sonic logs. This had been conceived by a colleague of mine who had somehow missed the simplicity of the operation by looking at the geophysicist's preferred entity, velocity, rather than the more convenient entity, slowness (or “transit time” of well logs, usually measured in microseconds per foot, and easy to sum to produce interval transit time) favored by petrophysicists. The explanation was more complicated than it would have been had the appropriate average concept been applied consciously. Hence I describe below the simple averages or means that are most commonly invoked, along with some examples in the hope that my thoughts will help clarify the issue of simple averages for other readers as they have done for myself.

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