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 (Fig. 3 below), which is a logical and correct way to define bed thickness in terms of seismic time.

Figure 3. Distinction between one-way and two-way traveltime thicknesses of a thin bed. When describing the time thickness of a bed, it is critical to specify if the time scale is one-way or two-way traveltime, because there is a factor of 2 difference in these two options for defining bed thickness.

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.