Report of Hydrographic And Oceanographic Researches
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Contents
Entire documents
Original Papers
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GPS/Acoustic seafloor geodetic observation -Progress by the Japan Coast Guard (review)-
, p.1 -
The influence of underwater temperature structure on seafloor positioning
, p.15 -
Observation on Fukutoku-Oka-no-Ba submarine volcano eruption in 2005
, p.31 -
Determination of Marine Geoid model around Japan
, p.39 -
Seasonal Variation of Mean Sea Level and Density of Sea Water in Coastal Waters along the Kuroshio
, p.49 -
Systematic Errors and Corrections of Current Velocity Measured by Shipmounted Three-Beam Type ADCP
, p.61 -
Effects of Ship's Velocity Determined from GPS Data on Current Velocity Measured by Shipmounted ADCP
, p.89
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Abstracts
GPS/Acoustic seafloor geodetic observation -Progress by the Japan Coast Guard (review)-
Progress in the GPS/Acoustic seafloor geodetic observation carried out by the Japan Coast Guard during the 5-year period since 2000 is reviewed, especially focusing on the development of data analysis methods and their results. A precision better than several centimeters has been attained in the seafloor station positioning through tremendous efforts for improving accuracy of component techniques both in software and hardware.
A couple of remarkable results have been presented: an intraplate crustal velocity off Miyagi Prefecture caused by the subduction of the Pacific plate and co-seismic displacements associated with large earthquakes having occurred along the Nankai Trough in 2004 and off Miyagi in 2005. Further issues to be tackled in the future are summarized.
The influence of underwater temperature structure on seafloor positioning
We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. The influence of underwater temperature structure on precision of seafloor positioning was examined.
At sea area off Miyagi Prefecture, underwater temperature fluctuate rapidly with respect to time and space, especially in spring and autumn. This instability deteriorates the precision of measurement results of seafloor positioning.
On the other hand, in summer, underwater temperature structure is stable and measurement results have enough precision. The underwater temperature structure in sea area along the Nankai Trough is more stable than that off Miyagi area. These measurement results of the stations situated along the Nankai Trough show higher precision. These results show that a good undersea condition leads to the good precision of a result.
However, even when the undersea condition is unstable, an enough number of data lead an enough precision of result.
Observation on Fukutoku-Oka-no-Ba submarine volcano eruption in 2005
Fukutoku-Oka-no-Ba had erupted on July 2, 2005, accompanying the discharge of colored waters from the crater. The pumices and pyroclastic materials blew up from the crater of the sea bottom by this eruption, they floated on the surface of the sea with rising the steam. We clarified topography change between the eruptions in 2005 and 1986 by echo sounding investigations of the sea bottom topography, in order to grasp the condition of the sedimentation of the pyroclastic materials around the crater.
After the eruption in 2005, we investigated Fukutoku-Oka-no-Ba using the echo sounder installed on the unmanned ship ¡ÈManbo ¶¡É, and discolored seawater was also sampled. We found two new craters from the topographic change between this investigation and the one in 1999 (after the eruption in 1986), and observed the air bubbles spouted out from the crater.
Determination of Marine Geoid model around Japan
We determined the precise marine geoid model by "remove-restore technique" which is the gravimetric method. The long wavelength reference of CG03 (Forste et al.,2005) which is the degree-360 expansion of the global geopotential model was removed from the local gravity anomalies which contain the ship borne gravity data observed by the Hydrographic and Oceanographic Department. The residual gravity anomalies were converted to geoid undulations using Fourier Stokes Formulas (1D FFT) with the modified kernel.
Comparison of the new marine geoid model with Geographical Survey Institute geoid model (gsigeo 2000 Ver.3.1) gives the standard deviation of 0.178m.
Seasonal Variation of Mean Sea Level and Density of Sea Water in Coastal Waters along the Kuroshio
This note shows the tables and the figures of the seasonal variation of mean sea level at Japanese coast along the Kuroshio Current. Monthly and 15 days running mean data are calculated from the tidal data observed in the period from 1971 to 2004. Characteristics of seasonal variation of MSL along the Kuroshio, such as the primary crest in early October, the secondary crest around summer solstice and a small scale crest / convexity after spring equinox, are described in graphs.
The figures of calculated sea level (CSL) are also described in graphs. The CSL is estimated by the specific volume calculated from temperature and salinity data in the area within 15 to 20 nautical miles along the coastlines around a tidal station. In the almost case of the tidal station on main lands, MSL matches the CSL calculated from the data in the depth of surface to 125 or 150 meters. Differences subtracting CSL from MSL are described in tables. The regional characteristics of the difference are shown in figures.
Systematic Errors and Corrections of Current Velocity Measured by Shipmounted Three-Beam Type ADCP
At present, one of the most widely used instruments to measure current velocity in the ocean is shipmounted ADCP (Acoustic Doppler Current Profiler). The ADCP measures current speed by sending out acoustic pulses downward from transmitters mounted on the ship bottom, and then by processing the Doppler shift of the acoustic signals scattered by particles in the water. Three-beam type ADCP using 3-element transducers has been widely used in Japan including the vessels of Japan Coast Guard, as well as four-beam type.
Although shipmounted ADCPs are very useful for current measurement, non-realistic current data are sometimes obtained mainly due to errors of ship velocity relative to sea water. In this technical article, systematic errors in current measurement by shipmounted three-beam type ADCP and error correction are described in detail.
Effects of Ship's Velocity Determined from GPS Data on Current Velocity Measured by Shipmounted ADCP
It is required to know the ship velocity referred to the earth (ground velocity) in oceanic current measurement using shipmounted ADCPs. At present, the ground velocity of a vessel is usually calculated from the distance between two different points whose positions in latitude and longitude are determined by GPS.
This article discusses potential effects of errors in ship velocities determined by GPS upon current measurement by ADCPs. The mean value of distance estimated by GPS location data is to be always larger than the real distance, on the assumption that the GPS location data show a two dimensional normal distribution around the real position and that the zonal (x-direction) and meridional (y-direction) components of the difference between the real and GPS locations have same standard deviations ( x y). This fact results in overestimation in the ground velocities.
The overestimated error velocity depends not only on the accuracy of GPS but also both on the time interval of velocity calculations and the ground speed of the ship. The error velocity can be suppressed and negligible through averaging procedure for many velocity data, whereas the individual errors may be too big in current measurement. Further investigation on errors of GPS velocities will be necessary for cases of inhomogeneous GPS location data ( x y), which has been reported at some fixed points.
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