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Swath-bathymetric mapping

  • Gauger, Steffen
  • Kuhn, Gerhard
  • Feigl, Thomas
  • Lemenkova, Polina
Publication Date
Jan 01, 2007
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Objectives The main objective of the bathymetric working group was to perform high resolution multibeam surveys during the entire cruise for geomorphological interpretation, to locate geological sampling sites, to interpret magnetic and gravimetric measurements and to expand the world database for oceanic mapping. Precise depth measurements are the basis for creating high resolution models of the sea surface. The morphology of the seabed, interpreted from bathymetric models, gives information about the geological processes on the earth surface. Methods and equipment The main characteristic of the deep water sounding system Atlas Hydrosweep DS-2 is a coverage angle of up to 120°, which results in a depth profile with a length of 3.4 times the water depth perpendicular to the ship's long axis. Most of the time a coverage angle of 100° was applied. The acoustic signal, generated by the hull mounted transducer, has a frequency of 15.5 kHz and allows measurement up to full ocean depth. Based on the acoustic pulse 240 depth measurements with individual opening angles of about 2.3° (in deep water operation) and an accuracy of approximately 1 % of water depth were derived. In addition, the echo amplitudes were converted to multibeam sidescan (4094 pixels per swath) and angular backscatter data (59 values per swath). For the slant range corrections of the outer sonar beams, CTD (conductivity, temperature, density) profiles, collected on this expedition or on former expeditions of other vessels, were mostly used. Where there was no information about the water properties, the automatic crossfan calibration, which generates a swath in the direction of the ship's long axis and adjusts the vertical position of the outer beams by overlaying with the previous central beams, was used to calculate the mean sound velocity in the water column. To assign the depth measurement to a geographic position, the GPS navigation and the ship's motion data, received from the Trimble MS750 GPS system and the MINS ringlaser gyro respectively, were applied. To prevent the disturbance of marine mammals, the multibeam sonar system was switched off during periods, when there was no scientific necessity for surveying the sea floor and if marine mammals were close to the ship (nearer than 100 m). After updating the multibeam system with the HDBE (High Definition Bearing Estimation) soft beam modus, the source level can be adjusted, so that the acoustic energy transmitted into the water is not higher then needed to obtain high quality measurements. The multibeam sonar was exclusively operated in the source level mode "Maximum Level", where the maximum source level is reduced manually to a minimum, depending on the water depth and hydroacoustic conditions. Besides operating and observing the multibeam sonar system, the data processing was the main part of the work on board. Erroneous depth measurements, caused by hydroacoustic disturbances i.e. because of sea ice, waves or interferences with other sounding systems, need to be cleaned. The depth editing, as well as the cleaning of navigation spikes, was done using the Caris Hips software. Furthermore, data processing includes integration of the ship's navigation into the ESRI database BatGIS, containing most multibeam survey lines of the AWI. The preparation of meta data describing each data set allows data exchange and archiving. The data preparation for terrain modelling includes the projection of geographic coordinates into metric coordinates and the recomputing of depths. In order to make depth data compatible to previous and subsequent measurements, a sound velocity of 1500 ms-1 has been applied.For the interpretation of the sea bottom topography, digital elevation models (DTM's) were calculated out of the edited data and presented in preliminary bathymetric maps, using the Generic Mapping Tool (GMT) and ESRI ArcGIS software. Based on the DTM's (grid spacing up to 50 m in medium water depths), contour line maps with color-coded depth ranges (scales up to 1 : 100000) and additional information like coastlines and surface elevation, sea ice coverage or sampling stations were produced. Using the ArcGIS module ArcScene, virtual flights above the sea floor were prepared in regions of specific interest. These three- dimensional visualisations faciliate the interpretation of morphology and support interdisciplinary work.

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