aus dem MARUM und dem Fachbereich Geowissenschaften der ...

MARIA S. MERIAN-Berichte, Cruise 20, Leg 4, Bridgetown – Freeport, 15.3.2012 – 7.4.2012
5.2.2 USBL POSIDONIA
IXSEA’s POSIDONIA 6000 is an ultra-short baseline (USBL) underwater navigation that was
used during ROV dives and for one of the box corer stations. The moon-pool mounted antennas
require a calibration and a proper SVP. Most recent calibrations for the system were done in
2009 (MSM13-4) and 2010 (MSM15-2). Although the SEAPATH 200 is delivering DGPS and
motion sensor data, just GPS-information is used for the USBL because POSIDONIA 6000 has
its own motion sensor. The motion sensor, IXSEAS OCTANS in a wet pot, is mounted directly
above the four antennas. SVP has been updated for every area of investigation or whenever
changes in surface sound velocity appear to be higher than 3 m s -1 (see Appendix 1 for settings
of POSIDONIA during MSM 20-4).
5.2.3 Multibeam Echosounder (MBES)
Seabed mapping during MSM20-4 was performed with two devices, the ship’s hull-mounted
KONGSBERG EM120 deep-water MBES (12kHz) and the moon-pool mounted KONGSBERG
EM1002 that is a shallow- to medium-water MBES (95kHz). The EM120 has a depth range of
20 to 11,000 m, while the depth range of the EM1002 is 2 to 1,000 m but achieves a much higher
depth resolution of 2-8 cm, depending on the pulse lengths (0.2-2 ms).
On R/V MARIA S. MERIAN, the EM120's footprint of a single beam is limited to 2° by 2°.
The angular coverage sector is up to 150°. EM120 has 191 beams per ping, while the EM1002
has 111 beams per ping. Achievable swath width on a flat bottom is up to 5 times the water
depth dependent on the character of the seafloor. The angular coverage sector and beam pointing
angles are set to vary automatically with depth according to achievable coverage. This
maximizes the number of usable beams. The beam spacing is equidistant to equiangular. All
settings applied for the two MBES systems during MSM 20-4 are listed in Appendix 1. For
reasonable hydroacoustic recording a proper SVP is essential. Thus, several CTD’s were taken
during the cruise (see Chapter 5.3). In every survey region at least 1-2 SVP’s were calculated
based on the SEABIRD CTD measurements (Fig. 5.2). The SVPs have been applied to EM120
and EM1002 right after the data were collected. The SVPs show a wide variability over the
entire water column, documenting that the precision of the seabed bathymetry map relies on a
SVP in each survey area.
Fig. 5.2
Sound velocity profiles (SVP) calculated from CTD data.
11