Marine Survey
The CHERISH project collects a range of marine 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 around Ireland and Wales, CHERISH uses; the Kongsberg Simrad EM2040, EM2040P and Reson T20P multibeam echosounders (MBES). These have 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 between 200-400 kHz 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. Multibeam systems emit sound waves in a fan shape beneath a ship’s hull. The amount of time it takes for the sound waves to bounce off the seabed and return to a receiver is used to determine water depth. Multibeam systems use beamforming to extract directional information from the returning soundwaves, producing a swath of depth readings from a single ping. In order to determine the transmit and receive angle of each beam, a multibeam echosounder requires accurate measurement of the motion of the sonar relative to a Cartesian coordinate system. The measured values are typically heave, pitch, roll, yaw, and heading.
A Multibeam Echosounder’s main function is to use acoustic energy to calculate depth. However, Multibeam Systems such as the Kongsberg EM2040 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.
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.
Shallow Seismic Pinger/Sub Bottom Profiler (SBP)
These systems operate in a similar 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.
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.
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 on either side of the unit.
CHERISH 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 CHERISH project uses SSS to acquire good images of wrecks that have been identified on the MBES.
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.
Acoustic Doppler Current Profiler (ADCP)
The CHERISH project is planning to acquire a vessel mounted ADCP unit which will enable the team to measure water current velocities over a range of depths in order to understand the processes affecting certain submerged sites. ADCP units utilise the Doppler effect of sound waves scattered back from particles within the water column.
Sound Velocity Profiler (SVP)
A sound velocity profiler is an instrument which is deployed overboard and descends to the sea floor. Its purpose is to measure the speed of sound throughout the water column. A number of factors affect the speed of sound in the water column such as; temperature, pressure, sediments, fresh water influx / mixing water bodies and salinity. Refraction appears in the data if insufficient SVPs are obtained during survey operations.
Ground Truthing
This involves the direct verification of the interpretation of sonar and other indirectly obtained data. The following are some of the techniques used:
- direct measurement of water depth
- direct sampling of the seabed and subsequent biological, geological and chemical analysis
- deployment of video cameras to inspect objects and features that are indicated by indirect scanning methods.
GNSS
We use an Aplannix POS MV system (Position and Orientation System for Marine Vessels) which consists of antennae, a motion reference unit (MRU) and a topside unit. This gives us real time GNSS position information and allows us correct for heave, pitch, roll, yaw and heading.
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