Overpressure

The term Overpressure (Δp) is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in various circumstances:

• In engineering: the pressure difference over the wall thickness of a pressure vessel. It is the overpressure, not the absolute pressure, which causes mechanical yield and, if the overpressure exceeds the vessel's capacity, ultimately the failure of the vessel.
• In geology: the pressure regime in a stratigraphic unit that exhibits higher-than-hydrostatic pressure in its pore structure. This phenomenon is the primary cause of "oil gushers". This is described below.
• In military terminology, the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by sonic boom or by explosion, and the resulting overpressure receives particular attention when measuring the effects of nuclear weapons or thermobaric bombs.