Multi-beam bathymetry

paleoceanic circulation, paleoproductivity, sea-ice distribution and ice sheet dynamics on the surrounding continents in the Pleistocene in a relatively high resolution. Furthermore, the mapping of key lithological layers may allow the correlation paleoenvironmental events across the Arctic Ocean.

In this framework, the geological programme included specific projects, e.g.

mapping sea floor structures along the ship´s track by swath bathymetry and sediment echosounder, re-sampling stations visited during VIII/3 and ARK-IX/4 in 1991 and 1993, respectively, to possibly identify the impact of recent climate change on benthic foraminifer faunas, and collecting surface sediments for calibration of paleoceanographic proxies such as biomarkers, benthic foraminifer faunas, and stable oxygen and carbon isotope composition of planktic and benthic foraminifers. Finally, inorganic geochemistry studies, initiated during ARK-XXIII/3 in 2008 to unravel the origin of manganese-rich layers in the Arctic Ocean, were continued to supplement the records from the East Siberia continental margin with data sets from the various basins visited during the expedition. In the following, we will describe the methods related to work during the expedition.

The geological station work was conducted along the oceanographic transects outside the Russian EEZ. Sites on submarine highs were selected for coring by using information on sea floor and sub-bottom structure from swath bathymetry and sediment echosounding (Hydrosweep and Parasound) to obtain hemipelagic sedimentary records not overprinted by sediment redeposition. Surface and sub-surface sediments were taken by gravity corer, giant box corer, and multicorer.

8.1 Multi-beam bathymetry

Patricia Slabon Alfred-Wegener-Institut Objectives

The main task of the bathymetric work was to conduct multibeam (MB) surveys in support of the geological and oceanographical programs using the new Hydrosweep DS-3 system (Atlas Hydrographic), and monitor the data acquisition to ensure high resolution spatial depth information throughout the expedition. Due to the reduced number of MB-staff, no data processing was carried out. The recorded MB-data is a valuable contribution to datasets of IBCAO (International Bathymetric Chart of the Arctic Ocean) and GEBCO (General Bathymetric Chart of the Ocean).

Another interest was to create a progress report about the new operation software of Hydrosweep DS-3 and the acquisition system HYPACK, which were installed in October 2010. Several open questions remained after the acceptance test and sea trials, which were reviewed during this cruise in detail. All problems concerning data collection and visualization were reported to the corresponding companies. Jointly with the system manufacturer and HYPACK it was tried to solve these problems.

Work at sea

The multibeam survey was started on August 6, 2011 at 9:00am UTC for testing purposes. After solving several technical problems, data acquisition was started in the main research area on August 13 at 8:47am UTC within the Norwegian EEZ

and was continued until September 22 at 7.30pm UTC, before entering the Russian EEZ. No data acquisition was carried out within the Russian EEZ.

At the beginning of the cruise, Hydrosweep (Hydromap Control) caused several system errors that generally were solved by total system shut down and restart.

The HYPACK software caused also problems that forced repeatedly a restart of the system. Apart from all technical problems, HYPACK and Hydrosweep operated relatively stable. Runtime errors, occurring randomly, did not require any repair or a complete reboot of the system.

When leaving the Russian EEZ on September 29 at 3:29pm UTC into international waters and entering the Norwegian EEZ, data acquisition was continued until October 3, 2011 at 9:31am UTC. During the transit to Bremerhaven, heavy sea off the Norwegian coast caused systematic errors and thus poor depth measurements, obviously due to unsatisfactory measurements of ships attitude. These effects were mainly observed in shallow water regions.

In the deep-sea, Hydrosweep was operated in Equal Footprint Beam Spacing mode using a defined number of 345 beams per ping. The used frequency is 15.5 kHz.

The aperture angle of the sonar fan can be selected between several predefined seafloor coverages. During this cruise an opening angle of 100% starboard/100%

portside, depending on the water depth was used. A larger angle of 200%/200%

created low resolution and poor data quality. Only in shallow waters between 100 m and 350 m the wider swath of 200%/200% was chosen. In waters less than 100 m, resulting depths show systematic errors in particular in the outer beams.

In areas with water depths of less than 100 m an angle of 150%/150% was used.

Data acquisition was conducted using HYPACK software. The recorded data is stored in files of 30 minutes time interval in the internal HYPACK raw formats

*.HSX and *.RAW (e.g.: ARK26-3_2011__2291239_0.HSX). In areas exceeding 84°N the universal polar stereographic projection (UPS) was used for visualization, otherwise the UTM projection. As data processing was not carried out, only selected files were checked for correct values of heading and course over ground (CoG) by comparing the MB-data to the D-SHIP Navigation files. Furthermore some files were visualized and checked using CARIS HIPS. For the generation of working maps onboard, few data were exported as ASCII files (longitude, latitude, depth).

During Hydrosweep operation, the actual swath is displayed on the screen. This information is used to find suitable locations especially for the marine geological work in real time. Due to technical problems regarding the import of the SVP (Sound Velocity Profile) into Hydromap Control, correct multibeam depths were not available.

Echo sounders derive the water depth from the travel time of the acoustic signal running from the transducer to the sea floor and back to the receiver. The exact sound velocity in the water column, which depends on pressure, temperature and salinity, is needed. The depth precision can vary strongly due to regional and local variations of the physical parameters in the water column that affect the sound velocity. A well-established technique to derive the water sound velocity is to perform CTD (Conductivity, Temperature and Depth) casts.

The *.cnv files, derived from the CTD values, containing the water depth and the matching sound velocity were processed in the sound velocity profile viewer and stored as *.vel-file for HYPACK/HYSWEEP and without extension for the ATLAS

8.1 Multi-beam bathymetry

SENSOR MANAGER. The ATLAS SENSOR MANAGER reads sound velocity profiles containing up to 128 points and imports the profile into Hydromap Control. 27 CTD-Profiles were processed and used for the water sound velocity correction.

During transits no sound velocity profiles were available. Hence the recorded measurements must be processed using external sound velocity data as available for example from ODV (ocean data view), before using them for mapping purposes.

Preliminary results

During the expedition a nearly continuous recording of data was achieved, except for few gaps caused by unexpected system errors and shutdowns. During 36 days of work in the main search area, a profile length of about 3480 nm (6450 km) was surveyed, generating 7.2 m. pings and 102 bn. beams (before editing). The amount of raw data storage volume is 18.5 GB created by 3778 separate files divided into *.HSX- and *.RAW-files. The observed depths vary between less than 100 meters in the coastal regions of Norway and Greenland and up to 5300 meters in a central valley of the Gakkel Ridge.

Despite the almost permanent ice coverage and the mentioned problems the system worked over periods reliable and provided high quality data. Some disturbances in the data could not be avoided during ice breaking.

The GEBCO_08 - Grid (General Bathymetric Chart of the Oceans) gives an overview on the morphology of the region. These data can be used for general planning. The recorded data differ at several places, due to the low resolution (2 km x 2 km) of the global GEBCO dataset, which is partly based on derived satellite gravity data and sparse bathymetric echo sounder data collected by submarines and icebreakers.

The difference between the GEBCO_08 model and the multibeam data are shown in Fig. 8.1.

Fig. 8.1: Example of differences between bathymetry at the Karasik Seamount from IBCAO (contour lines) and unedited multibeam bathymetry of this cruise (color coded

and shaded).