Equivalent yield factors for coupled energy asymptotically approach 100 (i.e., as burial depth increases); indeed an earth-penetrator weapon is effectively fully coupled at a scaled DOB of about 2.3 m/kt1/3. Relative to a contact burst, the ground-shock-coupling factor approaches 50 with increased DOB owing to the surface air-blast contribution.3
For a generic 300 kiloton EPW at 3 meters depth of burst (scaled DOB = 3/(300)1/3 = 0.45), the ground-shock-coupling factor is about 20, which is equivalent to a contact burst of about 6.0 megatons.4 This example illustrates the “efficiency” of an earth-penetrator nuclear weapon in generating comparable levels of damaging ground shock at target depth with significantly lower yield relative to a surface-burst or airburst weapon. As mentioned elsewhere, the coupling factor can be anywhere between 15 and 25, with the greatest uncertainty due to the effect of the radiation from a surface burst, which is sensitive to local conditions.
The following factors influence the uncertainties associated with estimating energy coupling for ground shock.5
Weapon design. Energy deposition into the ground involves two components, which couple differently: debris kinetic energy and x-rays. The physical dimensions and the location of the primary component relative to the secondary also affect energy deposition.