Cartesian diver observations of double frequency pressure fluctuations in the upper levels of the ocean

Pressure oscillations having a narrow spectrum centered at 0.175 Hz have been measured in the deep sea at depths between 100 and 300 m. At these depths the nonlinear interaction between oppositely directed sea surface gravity waves causes pressure fluctuations, the intensity of which varies inversely with the square of the depth, uninfluenced by microseismic Rayleigh waves propagating on the seafloor below. The observed pressure fluctuations are consistent with the interaction between waves having a unimodel beam pattern whose half beam width is slightly larger than 90 degrees or with a shoreward propagating pencil beam of swells interacting with a small component reflected from the shore.