FIGURE 4.1 Equivalent yield factors for total coupled energy and ground-shock-coupled energy normalized to a contact burst. Positive numbers signify below ground. (See Appendix C.)

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/kt^1/3. 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.