Data Collection: Survey Methods and Equipment

A wide variety of data types are acquired during offshore survey operations, including biological and geomorphological data.

Seabed Imagery

Digital still cameras and video cameras are deployed in underwater housings and secured to a variety of job-specific frames that are lowered from the vessel to the seabed. Due to low levels of natural light at depth and the loss of colours as a result of light absorption, the cameras are equipped with lights necessary to obtain clear images of even the smallest marine species living on the seabed. When surveys are conducted in very deep waters (>200m), images may be taken using camera mounted on a Remotely Operated Vehicle (ROV) operated by the crew onboard the survey vessel.

A sledge-mounted camera system used on Cefas Endeavour (left) © Neil Golding/JNCC 2012. A dropframe-mounted camera system being deployed (right) © Neil Golding/JNCC 2012.

Different types of camera are used depending on the weather conditions and type of seabed being surveyed. When the seabed is relatively flat, the camera can be mounted on sledge and towed along the seabed behind the vessel. However, in areas with rocks, boulders or other hard substrata, a drop-frame camera is preferred. This can be lowered from the vessel and towed at a fixed height above the sea floor, or “bed-hopped” along a line. Cameras mounted on ROVs have the ability to be piloted to a feature of interest, and some are able to collect samples with a robotic arm. However, they are typically much more expensive to operate than the other systems discussed above.

An ROV on deck of the RV Cefas Endeavour (left) during CEND1406 © Cefas 2006. An ROV system being deployed from the RRS James Cook (right) © Fionnuala McBreen/JNCC 2011.

All these systems are connected to the vessel with an umbilical cable which provides real-time videos of the seabed. While the video footage is recorded the operator can also choose to capture still images if they see something of interest!

More information on underwater photography and video techniques used on survey can be found can be found in the MESH Recommended Operating Guidelines.

Anton Dohrn and Rockall MV Franklin survey 03/2009 from JNCC.

Video footage taken at Anton Dohrn Seamount cSAC at a depth of 1333m. Lophelia pertusa, gorgonians, feather stars and brisingids are all visible © JNCC 2009.

Seabed types

Example of an image taken with a drop-camera on CEND1108 at Solan Bank Reef (top left) © JNCC 2008. A still image taken with a drop camera onboard MV Franklin during the Anton Dohrne Seamount and Rockall survey (top right) © JNCC 2009. Image of a seapen Pennatula phosphorea captured using a camera sledge during 1515S (bottom) ©JNCC/MSS 2015.

To view some ROV footage taken at The Canyons MCZ during JC125 in August 2015, please see https://codemap2015.wordpress.com/.

Grab sampling

Grabs are use to collect samples of sediment and marine species living beneath the surface of the seabed. The samples are used for quantitative assessments of the seabed habitats and sediment grain size analysis. Most grabs have a levered scoop or jaws that stay open as the grab descends to the seafloor and closes as the grab touches the bottom.

Different types of grabs are used depending mainly on the properties of the seabed being sampled. A Van Veen grab or a Day grab may be used for sand and muddy seabeds, whilst the Hamon grab can be used to collect coarser sediments.

Hamon grab (left), this particular one, used on the Cefas Endeavour, is equipped with a camera to collect a picture of the undisturbed seafloor before the sample is collected © Neil Golding/JNCC 2012. Day grab (middle) and Van Veen grab (right) ©Cefas 2014.

Other types of seabed samplers such as box corers or Shipek grabs can be employed when a sample with an undisturbed sediment surface (on retrieval) is required (e.g. to undertake chemical analysis). More information on box coring is reported in the MESH Recommended Operating Guidelines for Box Coring.

Shipek grab © Cefas 2006.

In some cases grabs are equipped with cameras, to be able to observe the undisturbed surface of the seabed and seabed fauna and flora.

Once the grab sample reaches the ship deck, it is sub-sampled for Particle Size Analysis(PSA) and the remaining sediment is sieved to eliminate the fine sand, silt and mud, and the animals living in (and on) the sediment are preserved for laboratory analysis.

Wet sieving of a sample collected with a grab (left) © JNCC, 2012. Life on a cobble, collected in a grab sample during CEND0513 (middle) © Eleonora Manca/JNCC 2013. Some of the macrofauna collected is selected for the taxonomic collection (see here for more information). Sampling of sponge spicules (right) © Mike Nelson/JNCC 2013.

Trawls and dredges

Both trawls and dredges are use to sample macro organisms living on or near the seabed, and are towed behind the vessel. Trawls are designed to skim the surface of the seafloor, whilst dredges are heavier and can dig into the sediment. Some trawls are made of a metal frame and are used over rough or rocky areas (rock trawls).

MESH Recommended Operating Guideline for Trawls and Dredges contain more information on the devices and data analysis.

A trawl being recovered on board the RV Cefas Endeavour during CEND0313 (left) © JNCC 2013. Retrieving the sample from a rock dredge on board the RV Cefas Endeavour during CEND1108 (right) © JNCC 2008.

Acoustic Survey Techniques

Different types of acoustic surveying systems © MESH project.

Acoustic Ground Discrimination System (AGDS)

AGDS use a single-beam echosounder data to measure the acoustic properties of the seabed. The signal strength gives a measure of the ‘roughness’ or ‘hardness’ of the seafloor and can be interpreted with ground-truthing data (photos, video and grab samples) and related to the seabed type (rock, sand, gravel etc) or the habitat type. Whilst they provide a coarse resolution in comparison to multi-beam systems they do have the advantage of being relatively cheap and are easily deployed on a variety of vessels. More information can be found in the MESH AGDS Guidelines.

Multibeam echosounder (MBES)

Multibeam echosounders (MBES) provide detailed information on water depth and seabed morphology (form and structure) by focusing pulses of acoustic energy directly beneath the ship’s hull in a fan-shaped coverage of the seabed. Transducers then record the strength, direction and elapsed time of the acoustic reflection from the seabed and these digital signals are decoded to determine seabed depths (bathymetry) and features from a wide area under the survey vessel. The MBES, along with all other survey equipment onboard, is coupled with a GPS (Global Positioning System) to determine the exact position of the ship. For more details on swathe mapping please see http://www.seabedmapping.com/swath-mapping.html.

MBES data acquisition has been an integral part of many of JNCC’s offshore seabed surveys as is the most efficient method for quickly collecting bathymetry data from a wide area. For example, MBES data have been used to help to determine the boundaries of protected features on the seabed and define boundaries of Marine Protected Areas.

Technical information on MBES systems, calibration and data quality control can be found in the recommended operating guidelines for swath bathymetry surveys, in the MESH web pages.

Multibeam depth profile and 3D view of the largest pockmark within the Scanner Pockmark SAC/SCI boundary during CEND19x12 © JNCC/Cefas 2012. Drumlin features visible in a hillshade output from a multibeam echosounder in the northeast corner of North St Georges Channel rMCZ during CEND0513 © JNCC/Cefas 2013.

Side scan sonar

Like MBES, side scan sonar also uses acoustic signals to detect underwater objects and features. The angle at which the the acoustic beams are transmitted results in distinctive ‘shadows’ being cast behind objects and features, helping JNCC scientists build a picture of what the seabed looks like. The strength of the returned acoustic signal varies with the characteristics of the seabed being surveyed, which helps differentiate between hard and soft sediments. To collect these data, a torpedo-shaped 'towfish' containing the sensors is towed behind the survey vessel. Signals are transmitted via the tow cable to a ship-based processing unit where the acoustic data are interpreted. A good example of how JNCC have used side scan sonar is in trying to map features such as the biogenic Annex 1 reef, Sabellaria spinulosa.

These MESH Side scan sonar guidelines contain detailed information on calibration, deployment, operation and data quality checks that we use in our side scan sonar surveys.

A processed side-scan image from CEND2315 (left) ©JNCC/Cefas 2015. Recovering a side-scan sonar towfish during CEND0513 (right) © Eleonora Manca/JNCC 2013.

Sub Bottom Acoustic Profiler

These instruments are generally towed behind the vessel and are used for the collection of information on the shallow geology and sedimentary structure of the seabed. Like side scan sonar and MBES they emit an acoustic wave, however the frequency used is low and the wave is able to penetrate the seabed and reveal shallow reflecting layers within the seabed. To find out how one of our recent surveys to the Croker Carbonate Slabs collected data using a type of sub-bottom profiler known as a ‘boomer’, please see their 'To boom or not to boom' blog post.