[SSS03-P06] “N”-shaped Y/X coda spectral ratio observed for in-line type OBS of S-net and ETMC
Keywords:S-net, ETMC, DONET, in-line type OBS, peak and notch in Y/X spectral ratio, natural vibration in pressure vessel, coupling with the seafloor
We make a hypothesis that natural vibrations are excited in the pressure vessel due to insufficient coupling of cylindrical shaped pressure vessel placed on the seafloor. The natural vibrations are excited if longitudinal (X component) and transverse (Y component) guided waves incident from both ends of the pressure vessel via connected cables. Depending on the length L of the pressure vessel, these two types of natural vibrations have natural frequencies given by fX=VP/2L and fY=VS/2L, and relationship given by fX/fY=VP/VS is established. Considering that observed notch and peak frequencies correspond to fX and fY, respectively, observed ratio of notch/peak frequencies are around 1.5 to 3 and the ratio seems to be reasonable for VP/VS of pressure vessel. Also, since the length L is 2.3 m and 1.6 m for S-net and ETMC, respectively, fX and fY of ETMC are expected to be 1.4 times higher than that of S-net: it is actually found in the observed spectral ratio. Not only guided waves but also S-coda wave incidents from coupling zone and vibrates the pressure vessel. If coupling is sufficient, energy of S-coda wave will be much stronger than guided waves and natural vibrations of pressure vessel will become invisible. We think this is the reason why DONET and shallow S-net stations do not show “N”-shaped Y/X spectral ratio.
The “N”-shaped Y/X spectral ratio is likely to be specific to in-line type OBSs having cylindrical pressure vessel placed sideways. Enhancing coupling with seafloor by underwater technologies such as installation within groove and burial would be effective to suppress the emergence of natural vibrations in the pressure vessel. Users of in-line type OBS record without sufficient coupling are recommended to correct response functions of X and Y components if necessary, especially when using frequencies above about 3 Hz.