There is some confusion in the literature and among seismic interpreters when referring to the thickness of a reservoir in time units. This confusion results because many people convert bed thickness from distance units (feet or meters) directly to one-way traveltime across the bed, which is a logical and correct way to define bed thickness in terms of seismic time.

However, all time measurements made from seismic reflection data are two-way traveltime measurements, not one-way traveltime. Thus a bed that is 1.0 ms thick in one-way traveltime has to be expressed as a 2-ms bed when it is imaged with a seismic wavelet because the reflection from the base of the bed involves a downward transmission through the bed (one-way traveltime) and then an up-going reflection return through the bed (two-way traveltime).

Bed thickness can be defined in terms of either one-way or two-way traveltime as long as:

1. The time scale is defined so that there is no confusion, and,

2. Two-way time is always used when bed thickness is being interpreted from seismic reflection data.