The gradients vary as a function of depositional history, compaction, mineralogy, depth of burial, and many other conditions.
Normal pressure has a pore pressure gradient equal to that of a water column.
Where flow of pore fluids is restricted is considered abnormally pressured or geo-pressured. The relationship between seismic interval velocity and pore pressure gradients used for the prediction of pore pressure gradients in areas where direct measurements are impractical. This is the relationship that is used in pore pressure prediction to predict the magnitude of the pore pressure gradient as a function of depth. Pore pressure gradients are considered from well data including mud logs. Mud logs provide measurements of pore pressure per unit of depth.