Australia’s national transport safety investigator, the Australian Transport Safety Bureau (ATSB), is leading a sea floor mapping and underwater search for missing Malaysia Airlines flight MH370 in the southern Indian Ocean. Geoscience Australia is assisting by providing advice, expertise and support regarding the sea floor mapping (bathymetric survey) and the underwater search.
On 8 March 2014, flight MH370, a Boeing 777-200ER registered 9M-MRO, lost contact with Air Traffic Control during a transition of airspace between Malaysia and Vietnam. An analysis of radar data and subsequent satellite communication (SATCOM) system signalling messages placed the aircraft in the Australian search and rescue zone on an arc in the southern part of the Indian Ocean. This arc was considered to be the location where the aircraft’s fuel was exhausted.
Fig. 1: This model of the sea floor terrain was based on coarse pre-existing data, which was derived from satellite gravity measurements and ocean passage soundings. The MH370 bathymetric survey was undertaken to gather more detailed and higher resolution data in preparation for the underwater search phase.
A surface search of probable impact areas along this arc, coordinated by the Australian Maritime Safety Authority, was carried out from 18 March to 28 April 2014. This search effort was undertaken by an international fleet of aircraft and ships with the search areas over this time progressing generally from an initial southwest location along the arc in a north-easterly direction. The location of the search areas was guided by continuing and innovative analysis by a Joint Investigation Team of the flight and satellite-communications data. This analysis was supplemented by other information provided to ATSB during this period. This included possible underwater locator beacon and hydrophone acoustic detections. No debris associated with 9M-MRO was identified either from the surface search, acoustic search or from the ocean floor search in the vicinity of the acoustic detections. The ocean floor search was completed on 28 May 2014.
Refinements to the analysis of both the flight and satellite data have been continuous since the loss of MH370. The analysis has been undertaken by an international team of specialists from the UK, US and Australia working both independently and collaboratively. Other information regarding the performance and operation of the aircraft has also been taken into consideration in the analysis. Using current analyses, the team has been able to reach a consensus in identifying a priority underwater search area for the next phase of the search. The priority area of approximately 60 000 km2 extends along the arc for 650 km in a northeast direction from Broken Ridge. The width of the priority search area is 93 km.
Sea floor mapping and underwater search
The Australian Transport Safety Bureau (ATSB), Australia’s national transport safety investigator, is leading the sea floor mapping and underwater search for missing Malaysia Airlines flight MH370 in the southern Indian Ocean. Geoscience Australia is providing advice, expertise and support to the ATSB in sea floor mapping (bathymetric survey) and the underwater search.
The search comprises two planned phases. Phase one, a bathymetric survey providing a detailed map of the sea floor 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.
Bathymetric survey
Bathymetry is the study and mapping of sea floor topography. It involves obtaining measurements of the ocean depth and is equivalent to mapping topography on land. The bathymetric survey undertaken in the search for MH370 has resulted in a map that charts the contours, depths and hardness of the ocean floor.
Prior to the bathymetric survey, very little was known about the sea floor in the MH370 search area, as few marine surveys have taken place in the area. Existing maps of the sea floor were coarse, having been derived from satellites and only providing a general indication of water depth.
Multibeam Sonar
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.
Fig. 2: The image on the left shows data at around 3400 m resolution (data acquired predominantly by satellite altimetry), while the image on the right shows data with a combination of 250 m and 50 m resolutions (data acquired by bathymetric surveys from a vessel). The higher resolution data on the right more accurately reveals seabed features. This figure is for illustrative purposes only and does not show data from the search area.
In the search area, the water is up to 6000 m 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 (see Fig. 3). 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 (>100 kHz) 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.
The obtained measurements are used to produce a map charting the water depth and hardness of the sea floor. High resolution multibeam data acquisition is time-consuming; it is a little like mowing grass, where bathymetry is mapped line by line.
Sea floor in MH370 search area
Very little is known about the sea floor in the MH370 search area, as few marine surveys have taken place there. What was known, however, was that the search area encompasses the seabed on and around Broken Ridge, an extensive linear, mountainous sea floor structure that once formed the margin between two geological plates. These plates evolved and spread apart between 20- and 100-million years ago, under similar processes found today at spreading plate margins (such as the Mid-Atlantic Ridge).
The sea floor around Broken Ridge still retains many of the large geological structures typical of spreading plate margins where volcanism and tectonic movements produce volcanoes (now extinct), high rugged ridges and deep trenches.
Previous maps of the sea floor in the search area were derived from satellites and only indicated the depth of the ocean at a coarse resolution, not showing the shape of the sea floor in enough detail for safe navigation of underwater vehicles. These coarse maps provide data at a low resolution of approximately 1500 m (per pixel), while the recent multibeam bathymetric survey collected data at 40 to 110 m (per pixel). This newly acquired data is some of the first high resolution data available for these areas.
Fig 3: Ship conducting a bathymetric survey using multibeam sonar. Source: National Oceanic and Atmospheric Administration (NOAA), US Dept. of Commerce. (Image is for illustrative purposes only. NOAA vessels are not involved in the search).
The high resolution bathymetry data collected in the search for MH370 has revealed many finer-scale seabed features for the first time that were not visible in the previous low resolution, satellite-derived bathymetry data. It has also revealed regions of harder and softer sea floor composition (sediment versus rock). This information has been useful in identifying and discriminating certain features, but also in providing a guide to the complexity of the sea floor for the underwater search.
Visualisation of sea floor terrain
Before the underwater search for MH370 could begin, it was necessary to accurately map the sea floor to ensure that the search is undertaken safely and effectively. The survey vessel Fugro Equator and the Chinese survey vessel Zhu Khezhen collaborated on the bathymetric survey, and spent months at sea, scanning the sea floor with multibeam sonar to gather detailed, high-resolution data. That data – which was reviewed, corrected and analysed by experts at Geoscience Australia – revealed many seabed features for the first time.
Three-dimensional models of the sea floor terrain
The three-dimensional models of the sea floor terrain developed from high resolution (90 m grid) bathymetric data from the survey in the southern part of the Indian Ocean can be seen in Fig. 4 a, b and c.
The models show newly discovered sea floor features including:
Seamounts (remnant submarine volcanoes), up to 1400 m high and often forming a semi-linear chain.
Ridges (semi-parallel) up to 300 m high.
Repressions up to 1400 m deep (compared to the surrounding seafloor depths) and often perpendicular to the smaller semi-parallel ridges.
The data also revealed finer-scale seabed features that were not visible in the previous low-resolution, satellite-derived bathymetry data. The identification of these features will assist in navigation during the underwater search.
Over 200 000 km2 of the sea floor were surveyed. The data collected in the bathymetric survey was used to build a comprehensive map of the sea floor in the search area, to be used in navigation for the underwater search. The ATSB also used the data when planning for search timings, methods, procedures, safety precautions and priority areas for each vessel to search. While initial bathymetric survey operations have been completed and phase two has begun, further bathymetric survey work may recommence if the need arises.
Fig. 4 a, b and c: Three-dimensional models of the sea floor terrain in the MH370 search area were developed from high resolution (90 m resolution) data from the bathymetric survey and revealed many seabed features for the first time.
Scientific understanding
The data acquired as part of the bathymetric survey has been collected for the sole purpose of finding the missing plane and to bring closure to the families of those on-board. However, as it is some of the first high resolution data available in these areas, it is of great interest to the scientific community.
For scientists, a greater understanding of deep ocean bathymetry is useful for a range of purposes, including geological interpretation to better understand plate tectonic history; as a baseline product in the creation of hydrodynamic models to gain knowledge on ocean currents and connectivity; the identification of sea floor features; and with the depth of oceans playing a major role in defining the habitat for flora and fauna – to locate areas where unique biological communities may exist.
Conclusion
On completion of the bathymetric survey work in December 2014, the Fugro Equator was mobilised to conduct underwater search operations and in late January 2015 commenced search activities in the defined area. An additional vessel, the Fugro Supporter, is equipped with an autonomous underwater vehicle (AUV) and this is being used to scan portions of the search area that cannot be searched effectively by the equipment on the other search vessels.
The size of the area being investigated, the depth of the water (up to 5000 m) and the extreme weather conditions experienced by vessels and crews are making the search a challenging operation. As of 18 March 2015 around 50% of the priority search area has been searched. Assuming no significant delays with vessels, equipment or from the weather, the current underwater search area may be largely completed around May 2015.
References
[1] Geoscience Australia, www.ga.gov.au
[2] Australian Transport Safety Bureau (ATSB), www.atsb.gov.au
[3] Fugro, www.fugro.com
Contact Geoscience Australia, clientservices@ga.gov.au
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