  Deep ocean acoustics, in the absence of shipping and wildlife, is driven by
surface processes. Best understood is the signal generated by non-linear
surface wave interactions, the Longuet-Higgins mechanism, which dominates from
0.1 to 10 Hz, and may be significant for another octave. For this source, the
spectral matrix of pressure and vector velocity is derived for points near the
bottom of a deep ocean resting on an elastic half-space. In the absence of a
bottom, the ratios of matrix elements are universal constants. Bottom effects
vitiate the usual "standing wave approximation," but a weaker form of the
approximation is shown to hold, and this is used for numerical calculations. In
the weak standing wave approximation, the ratios of matrix elements are
independent of the surface wave spectrum, but depend on frequency and the
propagation environment. Data from the Hawaii-2 Observatory are in excellent
accord with the theory for frequencies between 0.1 and 1 Hz, less so at higher
frequencies. Insensitivity of the spectral ratios to wind, and presumably
waves, is indeed observed in the data.
