Metadata Report for BODC Series Reference Number 1171313

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

Instrument Description

A Kongsberg Simrad EM 120 multibeam echosounder was used throughout the cruise. The nominal sonar frequency was 12 kHz with an angular coverage sector of up to 150 and 191 beams. The EM 120 can map a swath width of about 4 times the water depth. The angular coverage sector and beam pointing angles were set to vary automatically with depth according to achievable coverage, maximising the number of usable beams.

Kongsberg EM120 Multibeam Echosounder

The EM120 is a low frequency (12 kHz) multibeam echosounder with full ocean depth capability designed for bathymetric surveys. It measures water depth by monitoring the travel time of an acoustic signal that is transmitted from the ship, reflected off the seabed and received back at the ship.

The main system units of the EM120 are transducer arrays (separate for reception and transmission), preamplifier unit, transceiver unit and operator unit. Sub-bottom profiling capability is an optional extra. For both transmit and receive arrays standard beamwidth is 1° or 2°, and 4° beamwidth is available for the receive array.

The system has 191 beams with pointing angles automatically adjusted according to achievable coverage or operator defined limits. The beam spacing is normally equidistant, corresponding to 1% of depth at 90° angular coverage, 2% at 120° and 3% at 140°. The transmit fan is split into several individual sectors, each of which is corrected independently for vessel roll, pitch and yaw, which places all soundings on a "best fit" to a line perpendicular to the survey line.

The EM120 supersedes the EM12 and was itself superseded by the EM122 in 2008.

Specifications

Further details can be found in the manufacturer's specification sheet.

BODC Processing

Processed files in both ASCII and GMT grid formats were provided by the originator. The GMT grid file was used by BODC as the ASCII file did not span the whole data area. The data from the grid file was converted into the BODC internal format using established BODC data banking procedures. All bathymetry values of zero where set to the absent data value. The following table shows how the variables within the grid file were mapped to appropriate BODC parameter codes:

Originator's Data Processing

Sampling Strategy

The Kongsberg Simrad EM 120 multibeam echosounder fitted to the James Clark Ross was used to support a variety of scientific activities which took place during JR211, including selecting sites for sediment coring, CTD sampling, OBS deployment and defining the seismic reflection tracks.

The system corrected for the ship's motion by steering the beams so that they reflect of the correct part of the seafloor. A total of 669 lines were acquired, during a period of 26 days in a near continuous fashion, covering a total of 4700.5 km of tracks.

Data processing

The CARAIBES (CARtography Adapted to Imagery and Bathymetry of Sonars and multibeam echosounders) seabed mapping software from IFREMER was used during the cruise to process bathymetry from multibeam data. Pre-processing included importing the data from the Kongsberg Simrad EM 120 multibeam echosounder system (xx.raw.all files) to CARAIBES (xx.raw.mbb files) line by line. After the importation of the data, quality control of each line was carried out by looking at the navigation file extracted from the raw data and at a rough grid of the unprocessed data. Once the navigation and bathymetry files were considered to be of good quality, the following processing flow was applied: 1) Invalidation of the incoherent values, interactively invalidating georeferenced bathymetry data using a mesh. 2) Generation of a Digital Terrain Model (DTM) from the soundings included in each bathymetry file. The interpolation method, used to compute values at DTM nodes (regular grid in X and Y of cartographic projection), was an assignment to the 4 nearest nodes. The grid spacing was chosen depending on the water depth and the swath width which varied approximately from 10x10 m for shallow water depths less than 500 m; 15x15 m for water depths between 600 and 1000 m and 20x20 m grid for water depths greater than 1000 m. After the processing of each line they were converted into ArcGIS format (xx. t and xx.hdr), GMT grid format and ASCII xyz format.

Project Information

Dynamics of gas hydrates in polar environments, International Polar Year (IPY) - Gas Hydrates project document

IPY - Gas Hydrates aims to determine how much gas hydrate is present in marine sediments in the Arctic today and how it might be affected by changes in climate in the future.

Gas hydrates are naturally-occuring ice-like crystals that form at high pressure and low temperature in marine sediments at water depths greater than 300 m whenever there is sufficient methane and pore water. Gas hydrates are now known to be widespread around the world and are often underlain by potentially vast fields of free gas. Together the gas hydrate and underlying free gas reservoirs comprise almost half of the Earth's organic carbon. In the Arctic, gas hydrate is widespread, trapped within marine sediments and permafrost. The polar regions of the Earth are highly sensitive to the effects of global change, and climatic warming in particular could cause widespread dissociation (breakdown) of gas hydrate and subsequent release of methane - a highly potent greenhouse gas - into the atmosphere. Numerical models predict that this reservoir is highly mobile and that escaping gas has a significant potential to accelerate climate change releasing as much as 2000 Gt of methane over a short period of time. As methane is a potent greenhouse gas it would course further global warming. Arctic gas hydrates are most vulnerable to future climate change because (1) it is predicted that temperatures will increase faster in the Arctic than in low latitudes (2) the intercept of the gas hydrate stability zone with the seabed is within the reach of fast warming surface waters and (3) the water column above the vulnerable zone of gas hydrates is smaller than in warmer oceans facilitating more efficient transport of greenhouse gases to the atmosphere.

The key objective of this project is to quantify the mobility of Arctic marine gas hydrate reservoirs.

The hypotheses tested:
1. The quantity of gas hydrate-bound methane in the climatically-sensitive part of the gas hydrate stability zone is significant in terms of its possible effect on climate.
2. Some gas hydrate has moved out of its stability field as a result of post-glacial warming, producing methane gas on dissociation, and current venting is the result of this change in pressure and temperature conditions.
3. Future bottom-warming will induce gas hydrate dissociation on the Svalbard margins.

IPY - Gas hydrates includes seismic surveys, multibeam bathymetry data, sidescan sonar imagery, 2D seismic reflection profiling, seafloor photography, bottom water and atmospheric methane sampling, ocean bottom seismometers, sediment cores and isotope analysis of both seawater and Arctic air samples.

IPY - Gas hydrates is funded by the UK's Natural Environment Research Council and carried out as part of the International Polar Year programme.

For more information please see the official project website.

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: