Een duidelijke uitleg van het Australische ATSB, de leider van de zoektocht.
Zoektocht naar MH370
The Australian Transport Safety Bureau (ATSB) is leading a seabed mapping and underwater search for missing Malaysia Airlines flight 370 in the southern Indian Ocean. Geoscience Australia is providing advice, expertise and support to the ATSB.
There are two planned phases of the search. Phase one, a bathymetric survey providing a detailed map of the seafloor topography of the search area and phase two, a deep ocean search using scanning equipment or submersible vehicles. The information gained in phase one will be used to build a map of the sea floor in the search area, which will aid navigation during phase two.
Waarom fase 1?
Waarom niet meteen beginnen met zoeken? In fase 2 wordt een apparaat van $ 2 miljoen voortgesleept aan een kabel achter een schip. De sonar zal zo’n 100 meter boven de zeebodem moeten drijven. Als niet bekend is hoe diep het is en hoe de bodem eruit ziet, zal het apparaat tegen onderwater bergen knallen.
Bathymetric survey
Bathymetry is the study and mapping of sea floor topography. It involves obtaining measurements of the depth of the ocean and is equivalent to mapping topography on land. The bathymetric survey will produce a map that charts the contours, depths and hardness of the ocean floor.
Very little is known about the sea floor in the MH370 search area, as few marine surveys have taken place in the area. Previously, coarse maps of the sea floor in this area have been derived from satellites and only indicate the depth of the ocean, which ranges between 1000 and 6000 metres. However, these coarse maps do not show the shape of the sea floor in enough detail for safe navigation, which is required for the second phase of the search.
In order to prepare for these phase two navigation requirements, there is a need for detailed mapping of the sea floor to produce higher resolution grids of data (approximately 45 to 150 meters).
Figure 1: The image on the left shows data at around 3400-metre resolution (data acquired predominantly by satellite altimetry), while the image on the right shows data with a combination of 250-metre and 50-metre resolutions (data acquired by bathymetric surveys from a vessel). The higher resolution data on the right shows underwater features much more clearly. This figure is for illustrative purposes only and does not show data from the search area.
Hoe werkt het?
Multibeam sonar is a widely used tool for mapping the sea floor. It measures the amount of time taken for a sound wave to travel between a ship and the sea floor to calculate the depth (bathymetry). Multibeam sonar uses multiple beams to measure a swath of the seabed under the ship, in contrast to single beam sonar which only maps a point below the ship.
In the search area, the water is up to 6000 metres deep, so the survey relies on acquisition of bathymetry using a multibeam system that can detect the sea floor at great depths.
To acquire the multibeam data needed for the bathymetric survey, a multibeam sonar is mounted on the hull of the survey vessel. The sonar system sends out a pulse of sound, which reflects off the sea floor and returns to the multibeam sonar device. The time of return provides an indication of how deep the water is.
Water salinity, temperature and depth (pressure) impact on how fast sound travels—and noting that these change throughout a water column, signals are corrected for these changes. Different frequencies are used to map different water depths, with higher frequencies (>100kHz) used for shallow water and low frequencies (<30 kHz) for deeper water.
The survey vessel traverses the area of interest at set distances and the multibeam sonar continuously measures both the water depth and sea floor hardness data concurrently.
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