New Technology for a New Century
SHALLOW WATER MULTIBEAM ECHOSOUNDING IN JAPAN HYDROGRAPHIC DEPARTMENT
Yoshihiro MATSUMOTO, Shoichi KOKUTA, Hirokazu MORI and Hiroyuki YAMANO, Japan
Additional paper- not presented at the conference, included only in the proceedings.
Key words: multibeam echosounding, hydrographic survey, bathymetry, nautical chart.
Japan Hydrographic Department is now on the stage that we have to use them for practical hydrographic survey to get valid soundings for nautical chart. For this purpose, guidelines for survey and data processing have been designed mainly by regional HDs. We would like to introduce our hydrographic survey systems and review practical operations of them.
On each 20m-type survey vessel of JHD, a multibeam echosounder SEABAT 8101, RTK-OTF GPS receivers MS750, a motion sensor DMS-10 and a GPS gyro NR230mkII are installed, and data acquisition software HYPACK on a windows PC records data from these devices. A sound velocimeter SVPS is available for sound velocity correction.
Surveys by Multibeam Echosounders
1. Sandy or muddy area
More slanted beams sometimes returns irregular soundings, since they are more refracted by sound velocity distribution, and their footprints become larger. Sandy or muddy area less than 30m depth should be planned to ensonify 100%.
2. Rocky area
In order to distinguish peaks from noises and to ensonify shaded zones behind shallows, swathes from adjacent survey lines should be enough overlapped.
When fish heavens or rocks lie on sandy or muddy shallow water (less than 4m from the sonar head), sometimes soundings are dropped (because of high contrast of reflection intensity) or second echo is recorded. Adjustment of sensitivity could handle this problem, but in worse cases doubled surveys are required: first sensitive for bottom and second for obstructions.
4. Dredged area
In case we miss foreign bodies, adjacent swathes should be enough overlapped. Since dredged face sometimes can shades dredged bottom, we should plan such survey lines that can ensonify the face and inside dredged area. Same way is applicable at wharf frontage.
5. Patch test
In order to obtain accurate soundings, it is required to compensate biases of motion sensor and time latencies. Latency of a positioning device (especially RTK-GPS) is remarkable. When sonar head is permanently installed on the under-hull, changes of these biases are not expected to be large. It is recommended to set a fixed ‘ test area’ around a specific topographic feature and to perform patch test in the same area every time.
As all steps proceed in digital form from survey work to data processing, enormous data files require careful handling: unintended removal or damage of data files is fatal on data processing.
Here we figure out the points to survey efficiently and to obtain valid soundings for nautical chart.
Comparison between SEABAT8101 and conventional single beam echosounder is reported. In this report, soundings by SEABAT8101(processed surface modeling) are compared with single beam soundings along a verification line orthogonal to the previous survey(consequently, along the direction of the swath of SEABAT). This conclusion is that soundings within 65°angle can be adopted.
Yoshihiro Matsumoto has studied geophysics at the Kyoto University first at the Faculty of Science 1991–95 and then volcanology (Master’s degree) at the Division of Earth and Planetary Sciences, Graduate School of Science 1995–97. From 1997 he has worked as Assistant Coastal Surveys Officer at the Coastal Surveys and Cartography Division, Hydrographic Department, Japan Coast Guard mainly engaged in technical support on hydrographic surveys
He has following publications:
He is a member of the The Volcanological Society of Japan and Seismological Society of Japan.
Yoshihiro Matsumoto and Shoichi Kokuta
22 March 2001
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