Metadata Report for BODC Series Reference Number 2201191

• 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

Kongsberg Simrad EM12, EM12S and EM12D multibeam echosounders

The EM12 series are low frequency (13 kHz) multibeam echosounders with full ocean depth capability designed for bathymetric surveys. They measure 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 EM12 are transducer arrays (separate for reception and transmission), sea surface sound velocity sensor, preamplifier unit, transceiver unit, bottom detector unit and operator unit. Additional options include acoustic windows (up to icebreaker strength) and a variety of software for processing, plotting and analysing the data.

The basic model is the EM12S which uses a single array of beams with an angular coverage of 90° and a swath width of 2 times water depth. The EM12S-120 is a version of the EM12S with its angular coverage extended to 120° and a swath width of 3.5 times water depth, which translates to a width increase of 4-5 km in 3000-6000 m of water.

The EM12D uses dual arrays, increasing the coverage sector to 150° and the swath width to 7.4 times the water depth. This version has a typical across-track coverage of 20 km from about 2500 m water depth to full ocean depth (an increase in swath width of approximately 10 km in the 3000-6000 m depth range compared with the EM12S). The EM12D also features an extra set of transducers, preamplifier unit, transceiver unit and bottom detector unit.

The EM12 series was based on the EM100, particularly with regards to the use of interferometric signal processing to determine the transducer-to-sea floor distance for each beam, the principles used to compensate for vessel motion and ray bending effects caused by sound speed variations through the water column, and the operator control and data storage systems. The EM12 was replaced in 2000 by the EM120 which was itself superseded in 2008 by the EM122.

Specifications

Further details can be found in the manufacturer's product description or the instrument manual.

RRS Charles Darwin CD168 Cape Verde Bathymetry survey: Originator's Data Processing

Sampling Strategy

Shipboard Simrad EM12 multibeam bathymetry data were acquired as part of a survey undertaken during RRS Charles Darwin cruise CD168 in February 2005. The swath survey was completed around the Cape Verde islands in the Atlantic Ocean.

The main aim of the CD168 survey was to study the influence of landsliding on the Cape Verde islands, in the eastern central Atlantic, in an area where subaerial evidence suggests the occurrence of landslides but where limited previous offshore work had been undertaken. The main objectives were to determine the extent of submarine landslides, the nature of the landslide processes, and the overall influence of landsliding on island evolution.

Further information is available in the cruise report.

An additional short survey of the distal part of the Cape Timiris Channel, at 20° 30’ N and just outside the Mauritanian 200 mile limit, was carried out during the passage between Gran Canaria and the Cape Verdes.

Data Processing

The EM12 instrument was started up soon after leaving Las Palmas at 1300 on the 3rd Febraury 2005 and logging started soon after. The first XBT was used to alter the existing sound velocity profile in the EM12. Upon reaching the Cape Verde working area, the sound velocity probe was lowered near to the sea bed. This sound velocity was used for the rest of the survey with daily XBTs to look for any major changes in the water column.

Neptune software was used to display a real-time seabed image simultaneously to the main lab and bridge. The Neptune software was also used to display planned survey lines overlaid with coverage. The bridge display was connected as a mirrored display via a cat-5 extender. The data were cleaned and processed onboard by the scientific party.

RRS Charles Darwin CD168 Cape Verde Bathymetry survey: Processing by BODC

The multibeam bathymetry data from the CD168 multibeam bathymetry survey around Cape Verde was supplied to BODC as an ASCII file. The dataset was transferred into a common format, a netCDF subset. The following table shows how the variables within the files were mapped to the appropriate BODC parameter codes:

Following transfer, the data are screened visually, using BODC's in-house visualisation software, and any spikes or instrument malfunctions are clearly labelled with quality control flags.

Project Information

No Project Information held for the Series

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: