Deep-water swell distance from period-dependent arrival times ($D=g\Delta t/[4\pi (1/T_2-1/T_1)]$)

For two wave periods emitted together at one location, deep-water group velocity is $gT/(4\pi )$. A distant observer sees the shorter period $T_2<T_1$ later, with $\Delta t=(4\pi D/g)(1/T_2-1/T_1)$. This yields the displayed range estimate. Different emission times, a moving storm, finite-depth dispersion, currents and refraction are possible confounders; the propagation calculation alone does not establish a unique source range without these assumptions.