“N”-shaped Y/X coda amplitude spectral ratio observed for in-line type OBS networks: interpretation based on the natural vibration of the pressure vessel

Abstract:

We detected anomalous peak and notch in the amplitude spectral ratio of Y (perpendicular to cable axis) and X (along cable axis) components of coda wave in frequencies about 5 - 10 Hz and 10 - 20 Hz, respectively, for considerable number of OBSs belonging to in-line type networks of S-net and ETMC deployed near the Japan Trench and the Sagami Trough, respectively. This distinct peak and notch are characterized by "N"-shape, which is most distinct for pair of X and Y components among various orthogonal horizontal component pairs. This "N"-shape is not formed for S-net OBSs buried in the seafloor sediments. Seismograms of DONET, a node-type OBS network deployed near the Nankai Trough, are not characterized by such "N"-shape regardless of buried or non-buried. We think origin of the "N"-shaped Y/X amplitude spectral ratio is not horizontal anisotropy below the subsurface medium, but is due to specificity of response functions of X and Y components for the in-line type OBS pressure vessel.

We model the "N"-shaped Y/X amplitude spectral ratio by natural vibrations of cylindrical pressure vessel, which are excited when coupling between the pressure vessel and the seafloor is insufficient. We suppose that S-coda signal contains both S-waves directly incident to the pressure vessel and guided waves propagate through the cables and pressure vessel. As the coupling becomes weak, less S-wave can transmit into the pressure vessel and guided waves become dominant in S-coda time window. The guided waves may excite longitudinal (X-comp.), tortional (Y-comp.), and bending (Y-comp.) motions as natural vibrations when incident to the pressure vessel. By a simple elastic theory and specification of the deployed pressure vessel, we evaluate the natural frequency of the longitudinal mode can be 1.4 to 3.8 times higher than that of the torsional and bending modes. This natural vibration model reproduces the observed "N"-shape well and explains why the "N"-shape becomes distinct as the coupling decreases.

Enhancing coupling of the pressure vessel with the seafloor by burial would be effective to suppress excitation of the natural vibrations. We recommend users of in-line type OBS records to carefully examine if there are anomalous responses in the X and Y components, especially when using frequencies above about 3 Hz.