Metadata Report for BODC Series Reference Number 1030251
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
Data Description |
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Time Co-ordinates(UT) |
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Parameters |
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Problem Reports
No Problem Report Found in the Database
Data Access Policy
Public domain data
These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.
The recommended acknowledgment is
"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."
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
| Parameter | EM12 | EM12D | EM12S-120 |
|---|---|---|---|
| Frequency | 13 kHz | 13 kHz | 13 kHz |
| Depth coverage | 50-11000 m | 50-11000 m | 50-11000 m |
| Number of beams | 81 | 162 | 81 |
| Angular width | 150° | 150° | 120° |
| Beam width | 1.8x3.5° | 1.8x3.5° | 1.5x3.5° |
| Swath width | up to 3.5 x water depth | up to 7.4 x water depth | up to 3.5 x water depth |
| Accuracy | 50-60 cm or 0.25 % of depth | 50-60 cm or 0.25 % of depth | 50-60 cm or 0.25 % of depth |
| Pulse length | 10 ms for deep water 2 ms for shallow water | 10 ms for deep water 2 ms for shallow water | 10 ms for deep water 2 ms for shallow water |
| Beam spacing | equidistant at 90°, 105°, 120° | equidistant at 98°, 114°, 128°, 140°, 150° | equidistant at 90°, 105°, 120° |
Further details can be found in the manufacturer's product description or the instrument manual.
BODC Data Processing
The data were provided to BODC in as Generic Mapping Tools (GMT) output NetCDF .grd format. In addition to this file, BODC were also provided with the raw xyz file, which is available upon request.
The files provided appeared to have been written with an early version of GMT, using a now deprecated grid file format (cf #10). To solve the issue, the each file was converted to ASCII format from the NetCDF .grd file. The ASCII grid file was then converted back to a GMT NetCDF grid using a current version of GMT. The new file was then transferred, using BODC generated Matlab code, into BODC standard NetCDF format.
During transfer the original variables are mapped to unique BODC parameter codes. The parameter mapping can be seen below.
| Originator's variable | Units | Description | BODC Parameter Code | Units | Comments |
|---|---|---|---|---|---|
| x | Degrees | Longitude | ALONZZ01 | Degrees | Eastern values are positive |
| y | Degrees | Latitude | ALATZZ01 | Degrees | Northern values are positive |
| z | Metres | Sea floor depth | MBAN1234 | Metres | - |
During transfer plots of the original files and of the transferred files were produced. These were visually compared to check that the data has transferred correctly. No problems were detected.
Originator's Data Processing
Bathymetry data were collected by a Simrad EM12 multi-beam echo sounder. Details of sampling are avialble in section 3.8 of the CD174 cruise report .
Project Information
No Project Information held for the Series
Data Activity or Cruise Information
Cruise
| Cruise Name | CD174 |
| Departure Date | 2005-08-11 |
| Arrival Date | 2005-09-08 |
| Principal Scientist(s) | David G Wallis (British Geological Survey Marine Operations) |
| Ship | RRS Charles Darwin |
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:
| Flag | Description |
|---|---|
| Blank | Unqualified |
| < | Below detection limit |
| > | In excess of quoted value |
| A | Taxonomic flag for affinis (aff.) |
| B | Beginning of CTD Down/Up Cast |
| C | Taxonomic flag for confer (cf.) |
| D | Thermometric depth |
| E | End of CTD Down/Up Cast |
| G | Non-taxonomic biological characteristic uncertainty |
| H | Extrapolated value |
| I | Taxonomic flag for single species (sp.) |
| K | Improbable value - unknown quality control source |
| L | Improbable value - originator's quality control |
| M | Improbable value - BODC quality control |
| N | Null value |
| O | Improbable value - user quality control |
| P | Trace/calm |
| Q | Indeterminate |
| R | Replacement value |
| S | Estimated value |
| T | Interpolated value |
| U | Uncalibrated |
| W | Control value |
| X | Excessive difference |
SeaDataNet Quality Control Flags
The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:
| Flag | Description |
|---|---|
| 0 | no quality control |
| 1 | good value |
| 2 | probably good value |
| 3 | probably bad value |
| 4 | bad value |
| 5 | changed value |
| 6 | value below detection |
| 7 | value in excess |
| 8 | interpolated value |
| 9 | missing value |
| A | value phenomenon uncertain |
| B | nominal value |
| Q | value below limit of quantification |
