In this report, we investigate the characteristics between surface waves and pressure waves in detail on the basis of weakly nonlinear directional wave theory, and propose a new method to recover surface wave information from subsurface pressure record. The proposed method is examined for field wave data obtained at Hitachinaka Port (water depth is 30m) and Nakagusuku Bay (water depth is 50m). The validity of the method is discussed with the following conclusions.
1) The frequency response function from water pressure to wave elevation shows peculiarity in its lower and higher frequency ranges in dependance on the angular spreading function of directional spectrum. This peculiarity can be successfully explained on the basis of weakly nonlinear directional wave theory.
2) The wider the angular spreading function becomes, the greater the difference grows between the frequency response function of linear wave theory and that of weakly nonlinear directional wave theory.
3) The proposed method is quite reliable in the wide range of wave height and period,i.e., from calm sea conditions to severe stormconditions.