Seismic two-way reflection times have to be converted into depths in order to tie known well control. Two methods of depth conversion are commonly used for areas where velocity variations cause severe problems. The first is the average velocity method, where average velocities are derived from wells or seismic velocities. The second is the “layer cake” method, where interval velocities for each layer are usually obtained from wells. The device for the velocity-lithology correlation is depth-normalization of interval velocities. These depth normalized velocities can be directly related to the lithologic content of layers. For the special case of a two-component layer, depth-normalized interval velocity can be used to determine the approximate percentages of the two lithologic units. Transformation of time into depth involves the use of either average or interval velocities, both of which are measurements of rock properties. Since interval velocity is the more sensitive measurement, normalization for the effects of depth is done in the interval-velocity domain. Depth conversion is therefore directly tied to the velocity-lithology relationship. Cross plots between these two factors and the errors between seismic and well depths are used to quantify the depth conversion accuracy.