09:30 〜 09:45
[J07-1-05] Short-period ocean fluctuation induced by internal wave and its effect on GNSS/acoustic analysis
In the GNSS/acoustic measurement, monitoring the fluctuation of water column in ocean, hence sound speed structure, is crucial to achieve high precision positioning. In the most case, ocean condition is approximated by time-varying stratified structure, which can be simultaneously solved with horizontal position of the seafloor transponder array using traveltimes at least from three transponders. However, non-negligible amount of horizontal variation of ocean structure often appears, effect of which (~30 cm) is usually cancelled out (~5 cm) by taking very long (~12 hour) time averaging. Kido et al. (2007) proposed that horizontal variation of the ocean structure can be considered employing five or more transponders at once if the structure is expressed by two quantity, i.e., horizontal gradient. However, this hypothesis requires that the variation must has a large spatial scale (> ~2-5km) so that the horizontal variation can be regarded as linear within the extent of acoustic path to seafloor transponders.
We have conducted several times of intensive XBT (eXpendable Bathy-Thermograph) casts in the past surveys, each of which consists of 12 times of profiling at 5-10 minute intervals. After correcting XBT sensor bias both in temperature and fall-rate, fluctuation of vertically integrated sound speed (= nadir total delay) well coincides with the GNSS/acoustic estimate in the 0.1 msec level. At the same time, apparent fluctuation in the horizontal array position is often observed. In the extensive XBT profiling, we found short period vertical oscillation of water column that amounted up to 20 m, which can be interpreted as internal gravity wave passing through the observation site. With density profile in the water column by XCTD cast, we can roughly estimate typical time period of the internal wave and hence the wavelength. This will be key quantity whether the horizontal variation can be approximated by linear structure.
We have conducted several times of intensive XBT (eXpendable Bathy-Thermograph) casts in the past surveys, each of which consists of 12 times of profiling at 5-10 minute intervals. After correcting XBT sensor bias both in temperature and fall-rate, fluctuation of vertically integrated sound speed (= nadir total delay) well coincides with the GNSS/acoustic estimate in the 0.1 msec level. At the same time, apparent fluctuation in the horizontal array position is often observed. In the extensive XBT profiling, we found short period vertical oscillation of water column that amounted up to 20 m, which can be interpreted as internal gravity wave passing through the observation site. With density profile in the water column by XCTD cast, we can roughly estimate typical time period of the internal wave and hence the wavelength. This will be key quantity whether the horizontal variation can be approximated by linear structure.