The principle aims of INFOMAR as a marine mapping project is to collect a range of geophysical datasets that determine the bathymetry (water depth) of the survey area and the nature of the sediments on and below the seabed. This is achieved using a range of instruments outlined below.
For the shallow water surveys of the bays around Ireland, INFORMAR uses the Kongsberg Simrad 1002 and 3002D multibeam echosounder (MBES). This has been found to give a satisfactory balance of data quality and intensity along with efficient area coverage.
At a basic level, the hull-mounted MBES transducers emit sound around 95 kHz (EM1002 system) or 200 kHz (EM3002 system) that travels down through the water column. As it is a high frequency sound wave, when it reaches the seabed most is reflected back towards the surface where sensors record the returning sound wave.
A Multibeam Echosounder’s main function is to use acoustic energy to calculate depth. However, Multibeam Systems such as the Kongsberg EM3002 also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.
Output data from the MBES is used in the production of shaded relief, bathymetric contour, backscatter and seabed classification charts. It also forms the basis for graphic fly-throughs of the bays generated using Fledermaus.
Images generated from MBES data:
Flythrough movie showing an example of a multibeam echosounder dataset from an area of seabed mapped off the coast of Cork and Kerry, southwest Ireland.
The Kongsberg Simrad EA400 Single Beam Echosounder (SBES) works on a similar principle as the MBES. However in the case of SBES acoustic energy is directed straight down from the hull of the vessel as opposed to the swath of beams seen in MBES. This means that water depth measurements can only be made along the ships track with single beam. As a result, the output data from SBES is in profile form compared to the area coverage from MBES.
Operating at a lower frequency (3.5 kHz) than the MBES, the energy from the SES Probe 500 pinger is both reflected from and penetrates through the seabed. The sound that penetrates through the seafloor may be reflected due to density changes within the sediments. The result is a series of sound waves returning to the vessel at slightly different times depending on how deep they penetrated through the sediment before returning. These are displayed in the pinger output as a series of layers than can be interpreted to reveal past sedimentation patterns for the area. Penetration and so data quality of the pinger is dependent on sediment type (good through sands, poor through gravels and bedrock) and gas content (poor through gaseous sediments).
Image of pinger data from north of Lambay Island off the Dublin coast showing an infilled channel-like structure beneath a relatively flat seabed.
The Geo-Spark 200 sparker is another piece of equipment used to penetrate the seabed to investigate sub-seafloor geology. The sparker can operate at a range of energy levels and at frequencies between 500 – 2000 Hz that a result can produce high resolution profiles depending on project standards and requirements.
The Edgetech Side Scan Sonar (SSS) is another sonar system that allows images of the seabed to be generated. In contrast to the pinger system, SSS uses sound waves directed perpendicular to the direction of travel to ‘see’ the seafloor on either side of the towed fish. The result is an image where the central area beneath the fish is blanked out by the returning sound. Moving away from this centre line, objects and features on the seabed are picked up to produce relatively detailed images of the seafloor. The INFORMAR project uses SSS to acquire good images of wrecks that have been identified on the MBES.
SSS image of a wreck lying on the seafloor of the Irish Sea off Dalkey Island, Dublin.
Some shallow areas within the bays are not safe to survey using boats so another method of airborne LiDAR carried out by Australian company TENIX LADS is used. The basic principle behind this method is to use laser pulses from the airplane to determine the distance from the sea surface and seabed. The difference between the two beams allows the water depth to be calculated. Water depths between 0 and 70 metres can be surveyed but in Ireland the typical depth penetration is 15 metres. This may vary if sediment or biological material is present in the water.
Images from successful LiDAR surveys of two INFOMAR bays with spectacular imagery of the drumlin swarm in Clew Bay, Co. Mayo with some drowned by rising sea-levels after the end of the last glaciation (top) and the shallow areas of Tralee Bay, Co. Kerry (bottom).
|Geological Survey of Ireland
Beggars Bush, Haddington Road
|Marine Institute Headquarters,