One of the most exciting parts of this profession is the fast-moving nature of technological developments. Technologies used to survey the seabed need to be specially designed to withstand the challenging marine environment. More and more is becoming possible at sea and each new technology provides a valuable opportunity to test it in practice!! This part of the blog will explore some of the latest developments.
Emily’s Pinnacles are impressive hard coral formations found in Bermuda and provide the building blocks for the reef. Darwin classified three main types of reefs – barrier reefs, fringing reefs and atolls, with others being patch reef and pinnacle reefs. A pinnacle reef occur when a patch reef occurs at an open shelf, rather than at an atoll.
Here, Google have partnered with The Catlin Seaview Survey, a major scientific study of the world’s reefs, to make these amazing images available to millions of people through the Street View feature of Google Maps. The Catlin Seaview Survey used a specially designed underwater camera, the SVII, to capture these photos. For further views please see the Google Street view Ocean gallery. For more information about the Catlin Seaview Survey please view their website. Explore it here, with brain corals below:
Brain coral is a common name given to corals in the family Faviidae so called due to their generally spheroid shape and grooved surface which resembles a brain. Each head of coral is formed by a colony of genetically identical polyps which secrete a hard skeleton of calcium carbonate; this makes them important coral reef builders like other stony corals in the order Scleractinia. The corals reefs of Bermuda have been especially vulnerable to coral bleaching. Bleaching occurs when the conditions necessary to sustain the coral’s zooxanthellae cannot be maintained and is a generalized stress response of corals.
Multibeam backscatter is the reflectivity measurement, where as the sidescan sonar imagery is the actual intensity of the return signal. The Sidescan sonar towing configuration provides greater maneuverability, as the depth of the tow-fish above the seafloor can be adjusted, in view of the swath width. For example, the sidescan imagery is less prone to be affected by the slope of the seafloor as it can be positioned, where as the multibeam can only receive the backscatter intensity as it reaches the survey vessel.
The footprint size at the outer beam of the sidescan sonar is larger than at the nadir beam as slant range is greater in the far range; subsequently increasing the two way travel time of the acoustic signal. A larger footprint has a greater uncertainty of detecting the first return, as well as a lower resolution. Hence, the theoretical maximum speed at which the survey vessel should go can be calculated, to ensure not only 100% coverage of the along track beam footprint, but also optimize the footprint size for all the beams. Therefore speed is an important factor to consider when planning a survey.
Thus practically, gaps in the multibeam data sets are present if the survey speed is too fast, as the vessel would have moved away before the acoustic return can reach the receiver. On the other hand, multibeam surveying uses expensive resources, such as ship fuel and a very slow survey speed would use the resources with a limited efficiency, resulting in a smaller area ensonified in the survey. Additional effects are also present for the sidescan sonar as the system comprises of a towed fish. Slow speeds may result in the fish having a decreased momentum/tension in the cable, changing the position of the fish relative to the vessel.
The difference between multibeam positioning and the sidescan sonar positioning is that the sidescan towed fish is found behind the ship, and hence this needs to be corrected for, in relation to the on board DGPS navigation system. This can be done most simply by doing trigonometric calculations, based on the length of the cable out (lay back) and the depth of the fish above the seafloor, as well as accounting for the heading. Base line acoustic positioning systems may also be used, with an acoustic signal being sent by the fish to the vessel, or by triangulation with transducers being based on the seafloor. Similarly the position of the multibeam transducer and receiver on the vessel also needs to be added to a vessel configuration file during processing. Furthermore, a time-lag correction needs to be applied, ensuring the correct position is recorded, with synchronized times of the navigation and the satellite.
If a survey was taken at another time of the year then different weather conditions affecting the movement of the survey vessel in the x,y and z directions would be recorded in the raw data acquired. Hence, attitude sensors on the vessel need to be used to correct the data to different types of movement, or the roll, pitch, heave and yaw. The pitch is a measure of the rotation of the survey vessel in the x axis; the roll is the rotation in the y axis and heave in the z axis. The yaw is the offset between the survey lines. A consequence of more turbulent weather conditions, which has a greater impact on the sidescan system, is the increased presence of air bubbles. This affects the way sound propagates in the surface mixed layer and hence can cause artefacts in the data. Bubbles may also originate from the propellers of other vessels which may be present at other times of survey.
The sea floor composition and the angle of incidence primarily cause variation in backscatter intensity, according to Lambert’s law. The roughness and hardness or acoustic impedance of the sea floor are two key parameters which vary with geological and biological characteristics. These parameters have varying contributions to the backscatter intensity, depending on the angle of incidence. Hence the energy of the ping which is reflected or absorbed is affected by the sediment geotechnical properties, as well as by the grazing angle. Additionally, scattering by targets (e.g. fish, zooplankton, submarines) in the water column can alter the backscatter intensity. Other factors to consider affecting the strength of the backscatter are the depth of the water column and the initial energy of the acoustic signal, as transmission loss occurs in the water column. Here is an example of multibeam backscatter acquired using Reson 7125 system.
Update: learn more about multibeam backscatter at the GeoHab Backscatter Working Group (BSWG) backscatter report.
The oceans contain a huge amount of energy. Changes in salinity, thermal gradients, tidal currents or ocean waves can be used to generate electricity using a range of different technologies. These could provide reliable, sustainable and cost-competitive energy. Capturing ocean energy could have substantial benefits.
The energy in the ocean waves is a form of concentrated solar energy that is transferred through complex wind-wave interactions. The effects of earth’s temperature variation due to solar heating, combined with a multitude of atmospheric phenomena, generate wind currents in global scale. Ocean wave generation, propagation and direction are directly related to these wind currents. On the other hand, ocean tides are cyclic variations in seawater elevation and flow velocity as a direct result of the earth’s motion with respect to the moon and the sun and the interaction of their gravitational forces. A number of phenomena relating to earth rotational tilt, rate of spinning, and interaction among gravitational and rotational forces cause the tide conditions to vary significantly over time. Tide conditions are more apparent in coastal areas where constrained channels augment the water flow and increase the energy density.
The development of wave and tidal resources as a source of energy is the subject of growing international investigation. Ireland’s offshore renewable energy resources have significant development potential and are considered as being among the best in the world, with the practicable wave energy resource estimated at more than 6000MW.
At present there is no well-established wave energy industry anywhere in the world. Ireland has the potential to become a world-leading developer and manufacturer of the technologies that will enable the harnessing of ocean energy resources. To achieve this, the Marine Institute is working with Sustainable Energy Authority Ireland to implement a National Strategy for Ocean Energy. The main objectives are the creation of a centre of excellence in OE technology and the stimulation of a world-class industry cluster and the connection of 500MW of ocean energy by 2020.
Finally, an example showing the peak wave power in Galway Bay, obtained using a Spectral Wave Model in summer storm conditions, made using DHI MIKE Coupled Models. If on average wave power is above 30kW/m, energy generation is viable.
A benthic lander is a large three-legged frame with oceanographic instruments and sensors attached to it. These measure a range of parameters in-situ at the seabed; such as in this case, the current speed, temperature, salinity and turbulence. They are designed to operate in some cases 1000s of meters below the sea surface. Weights or ballasts are used to make the otherwise positively buoyant lander land down on the seafloor.
Here it was deployed from the Irish research vessel the Celtic Voyager in Galway Bay, West of Ireland, during a cruise by the National University of Ireland, Galway. The lander remains monitoring the conditions at the seabed for one month, in this case at depth of ~25m. Whilst out at sea during this period, it observes the impact of storm waves on the sediment transport. By making measurements at various heights above the seabed, it can obtain a profile of the benthic boundary layer and allow us to study how this changes during a storm.
By adding different sensors, you can also measure the chemical and biophysical properties of the water at the sediment-water interface. In-situ measurements allow us to study in the natural laboratory of the sea, without the need to remove anything. The measurements obtained by benthic landers are often used to verify as well as compliment laboratory results made under controlled situations.
It also has an acoustic positioning transponder which responds to the ship’s positioning call, to locate it for collection after its deployment. The weights or ballasts are released, with the buoyancy from the yellow floats allowing the lander to float back up to the surface.
