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Surveying

Data Acquisition Systems

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.

Multibeam Echosounder (MBES)

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.

View Multibeam echosounder simulation by RealSim Movies.

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:

Shaded Relief from Galway Bay
Backscatter from the Celtic Sea off the Waterford coast
Shaded Relief from Galway Bay
Backscatter from the Celtic Sea off the Waterford coast

 

 

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.

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Single Beam Echosounder (SBES)

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.

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Shallow Seismic Pinger/Sub Bottom Profiler

These systems operate in a similiar way to SBES, but at lower frequencies.


A pinger system transmits a single frequency (~4 kHz) while the chirp system transmits a sweep of frequencies (e.g. 2-7 kHz) in a single pulse.

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.Depths of up to 30 m
can be achieved in soft sediments.

The image above is a SBP line from the inner part of Galway Bay. The image spans 9 km from southwest to northeast. Horizontal scale lines are 10 m apart. The top of bedrock is clearly visible as the base reflector over most of the area. Bedrock varies between approximately 6 and 20 m beneath seabed. It is gently dipping. The bedrock is overlain by a soft layer of sediment. The seabed shoals up toward the northeast.

Image of a pinger seismic section of symmetrical sand waves.

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Seismic Sparker

The Geo-Spark 200 sparker is another piece of equipment used for sub-seabed investigations where deeper penetration is required or coarse/compacted sediments precludes mapping. Operating at lower
frequencies (500 – 2000 Hz) the unit is towed behind the vessel, transmitting a more powerful pulse of sound into the seabed.

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Side Scan Sonar

When high resolution information on specific seabed features is required (e.g. habitat mapping or wreck investigations), a side scan sonar is used. Towed behind the vessel and in close proximity to the seabed, it transmits high frequency sound pulses that map the seabed either side of the unit.

INFOMAR uses the Edgetech Side Scan Sonar (SSS) which 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.

The back deck of the Celtic Voyager

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Magnetometer

A magnetometer detects changes in the magnetic field that varies over a wide area according to the geology or locally over ferrous objects such as shipwrecks. Its cylindrical shape and positive buoyancy are specifically designed to reduce risk of snags with moored gear and equipment losses.


    

A Marine Magnetics SeaSPY Magnetometer with a 200m tow cable

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Light Detection and Ranging (LiDAR)

This method of surveying emits two laser light beams from a sensor on-board an airplane. The red beam reaches the water surface and bounces back, while the green beam penetrates the water hits the seabed and bounces back. The difference in time between the two beams returning allows the water depth to be calculated. Digital photographs can also be taken using a downward looking geo-referenced digital camera.


Water depths between 0 and 70 metres can be surveyed. In Ireland the typical depth penetration is 15 metres. This may vary if sediment or biological material is present in the water.

Fenit Harbour. Lidar shaded relief on left and Lidar aerial photos on right.

 

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Vessels | Acquisition | Tides | Referencing | Sound Velocity | Ground Truthing | Survey Progress

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