Home | Contact ST  
Follow ST

Feature Article

BOAGS: An Integrated Seabed and Water Column Sampler
CSIRO Designs Customized Instrument To Optimize Ecosystem Evaluation on the Continental Margin


Matthew Sherlock
Electronics Engineer
Marine Instrumentation Group
Dr. Rudy Kloser
Team Leader
Deepwater Ecological Status and Prediction

Mark Underwood
Scientific Equipment and Technology Group Leader
Marine and Atmospheric Research Division of the Commonwealth Scientific and Industrial Research Organisation
Hobart, Tasmania

The Upper Slope around Australia. The green areas indicate mapped
areas, the red unmapped.
Australia, like many other nations, faces the challenge of managing an enormous marine exclusive economic zone (EEZ). This management is being guided by an ecosystem-based approach that incorporates the development of regional marine plans, including spatial zoning such as marine protected areas and fisheries spatial management. To ensure that this zoning is effective requires an understanding of ecosystem structure, function and dynamics over multiple spatial and temporal scales, aided by direct observations. Direct observations are expensive, so acoustic remote sensing combined with targeted optical and physical sampling is often used as a cost-effective alternative.

As an example, Australia's continental margin (defined here as the area where water depth ranges from approximately 150 meters to 1,500 meters), is a narrow strip characterized by high productivity and diversity. While supporting major ecological and economic resources (fishing, oil and gas), this area is poorly understood, yet heavily exploited in parts.

A simple first step to assisting management of this region is to map the spatial scales of the types of terrain and key components of the biotic assemblages, in order to define marine habitat patches and key ecological features (e.g., canyons, seamounts and deep reefs).

To achieve this objective, the Australian continental margin is being systematically mapped through research and transit voyages between ports. Mapping with multibeam acoustics gives information about the depth, roughness and hardness of the seafloor, and in combination with other variables, the potential habitat for different biota. From these maps, optical and physical samples can be targeted to place these features within an ecological context for management objectives.

The standard approach to acquiring the relevant data is to separately and sequentially deploy a range of instruments, including a Smith-McIntyre sediment grab sampler; an optical system; a conductivity, temperature and depth (CTD) sampler; and acoustic sensors. Deploying any of these instruments to 1,500 meters is time consuming. In the case of the Smith-McIntyre sediment grab, additional redeployment time is required if the sampler misfires. Misfires can be due to hard terrain or equipment failures.

BOAGS Solution
To more effectively collect the data needed to determine marine habitats and ecology, a tool needed to be developed that could cost-effectively provide the rapid ecological context of these maps with optical, acoustic and physical samples over a range of terrain types at depths of up to 3,000 meters.

Australia's Commonwealth Scientific and Industrial Research Organisation's (CSIRO) solution was to develop the Benthic Optical, Acoustic and Grab System (BOAGS), which combines several observational technologies to deliver a platform that maximizes scientific return from a single shipboard instrument deployment. BOAGS couples a sediment grab sampler with video and still cameras, an echosounder and a CTD to deliver a platform that greatly enhances sampling efficiency and the quality of data collected. In addition to increased efficiency, collecting the datasets concurrently offers additional advantages. For example, a grab sample with no optical sensing of the seafloor does not provide the necessary information on the macrofauna and its patch structure over both hard and soft terrain. The BOAGS system is able to provide this.

The underwater package being lowered onto its cradle, with electronics canisters, cameras and lights visible.

The system's design is based on a modular philosophy with the ability to, for example, swap out the benthic sampler and replace it with additional acoustic transducers, should the need arise. A winch loaded with a 6,000-meter optical fiber tow cable is used to deploy the system from the vessel to a rated working depth of 3,000 meters. The tow cable delivers power to the platform with real-time data and live video transmitted continuously through the optical fiber from the platform to the surface. The winch has both local and remote control stations. The latter allows the operator to actively control the depth of the BOAGS platform from a console within the ship in response to data and video from the system.

System Details
Benthic Grab. The BOAGS structural frame has been designed with the Smith-McIntyre sediment grab as the core of the platform, along with the infrastructure for the other observational sensors that constitute the system. This has required special adaptation of the grab design to allow integration with the larger BOAGS platform.

The jaws of the grab and the bottom-contact trip plates protrude beyond the base of the BOAGS platform so that they are first to impact the seabed during the sampling phase. The shape of the base minimizes the likelihood that other structural elements of the platform will impact the seabed before the grab, which is particularly important on a sloping or uneven seabed.

At the top of the platform, where the deployment cable connects to the structure, there is a sliding bar that is linked to the jaws of the grab. Once the grab has been triggered, the sliding bar is released and is free to move upward as cable tension returns. This causes the jaws to close, capturing the sample. The full weight of the BOAGS platform maintains closure of the jaws during recovery to the surface.

An additional feature of the BOAGS sediment grab is the inclusion of an arming motor. Locking pins fitted to the trip-plate mechanism prevent accidental activation of the grab until they are removed. This eliminates grab activations that sometimes occur in rough sea conditions where waves slap against the trip plates as the grab enters the water. The pins are also important for preventing accidental grab closure if the system makes unintended contact with the seafloor in moderate to rough sea conditions: Since the BOAGS system is designed to be an observational tool used in close proximity to the seabed (to capture video and still images prior to collection of the grab sample), typical height above the seafloor is between one and three meters—in moderate to rough sea conditions, ship heave will cause the BOAGS platform to move up and down close to the seafloor, with potential for accidental contact. Grab closure is prevented until the arming motor is activated by the operator, which then extracts the locking pins just prior to collection of the sample.

Optics. Further operational improvement over traditional Smith-McIntyre sampling is provided with the ability to view real-time video of the seafloor. This allows an operator to review and select the area for sampling, reducing the risk of misfires due to the presence of boulders or other impediments to Smith-McIntyre sampling.

The BOAGS platform is fitted with two video cameras to provide the operator with live video. The first camera, a Hitachi HV-D30P, is a high-sensitivity color unit. This camera has very good low-light capability and high resolution and is used as the principal survey camera for bottom characterization. Illumination is provided by two light-emitting diode (LED)-based Multi SeaLite devices manufactured by DeepSea Power & Light (San Diego, California), each providing around 2,500 lumens in water. The composite video signal from the Hitachi camera is transmitted continuously to the surface for display, and the video is used by the system operator as a visual cue for control of winch wire-out in order to maintain the BOAGS platform's position just above the seabed.

The second video camera works at close range, solely to observe the status of the grab. The camera is a relatively inexpensive color camera that is coupled with a small, custom-made five-watt LED light. The composite video signal from this camera is also transmitted continuously to the surface and is used during the grab-sampling phase to visually verify that the grab has closed after recovery from the seabed.

The system is also fitted with a paired digital still-camera system and strobe for high-resolution imagery of the seabed and the water column. The mechanical mounts for the two cameras are carefully designed to ensure the alignment and position of the cameras is accurately maintained. The complete paired digital camera assembly can be removed as a submodule from the platform without disturbing the camera housings. This aspect is critical to ensuring that the calibration of the system is not altered. In-water optical calibration is conducted in a large tank or pool using a reference frame that is photographed at different orientations. Custom software is then applied to the images to characterize the system and produce a calibration model. Using image-analysis software enables sizing of any biota captured in the frame of both cameras during a survey.

The cameras used in the system are Canon EOS 5D Mark II's, which are coupled with a Canon 580 EX II strobe. Full E-TTL exposure capability is maintained in the system. A custom camera synchronization circuit accepts signals from the operator console to allow photographs on demand. The system can also be configured to take photographs at regular time intervals or automatically in response to the detection of acoustic signatures by the Simrad (Horten, Norway) EK60 echosounder system. In this way, the digital stills subsystem provides the ability to capture very high-resolution images of the seafloor and water-column images of biota.

Acoustic Sensors. The Simrad echosounder integrated with the BOAGS system operates at a frequency of 120 kilohertz and is coupled with a Simrad split-beam 120-kilohertz deepwater transducer. The EK60 has been re-engineered in-house to permit it to fit in the pressure case.

The echosounder enhances the capability of the BOAGS system by allowing the collection of calibrated acoustic data from the surface to the seafloor for estimates of biota biomass. At 120 kilohertz, the useful range for the sounder is approximately 400 meters.

The BOAGS system has been designed to allow addition of extra acoustic frequencies for species identification. In the case of lower frequencies where the size of transducer becomes significantly larger, the system has been designed so that it can be reconfigured by replacing the sediment grab module with an echosounder module. This integration is relatively seamless.

Additional Sensors. A Sea-Bird Electronics Inc. (Bellevue, Washington) SBE 37-SI MicroCAT CTD instrument measures water conductivity, temperature and depth at a data rate of two hertz. Serial data from this instrument is transmitted to the surface continuously during a deployment for display and archival. An altimeter on the system provides information on the platform's distance from the seabed, which is particularly useful in situations where the bottom is sloping or the topography is unknown. The altimeter is a Simrad model 1007 with a typical range of 200 meters.

The BOAGS platform provides a highly flexible sampling tool that not only improves the efficiency and reliability of collecting sediment samples, but greatly enhances understanding of the seabed and water-column habitat by providing visual context at the sample site. In addition, the CTD and acoustics sensors on the platform add significant value by providing physical and biological measurements of the water column during each deployment.

The BOAGS system achieves its design goal as a rapid assessment tool for ocean observation.

The authors would like to acknowledge the contribution of the colleagues who assisted in the manufacture and testing of the system: Tim Ryan and Jeff Cordell, who assisted in the implementation and at-sea testing of the winch and BOAGS system; David Kube and the CSIRO Engineering Technical Services team, who worked to construct the system; Ian Helmond, for mechanical design of the BOAGS platform and for meeting the challenging task of integrating the Smith-McIntyre sediment grab to allow it to work with the BOAGS system; and Dr. Robin Wilson from the Museum of Victoria, for providing advice on grab designs and sample collection methods.

For further information or a full list of references, contact Matthew Sherlock at Matthew.Sherlock@csiro.au.

Matthew Sherlock has more than 20 years of experience as an electronics engineer with the Commonwealth Scientific and Industrial Research Organisation in Hobart, Tasmania. He has worked on the development of numerous subsea instrument platforms, including towed acoustic and video systems.

Dr. Rudy Kloser is a team leader of the Commonwealth Scientific and Industrial Research Organisation's deepwater ecological status and prediction group, with a Ph.D. in applied physics specializing in marine acoustics. The focus of the team is to provide the underpinning observation science for the ecosystem-based management of Australia's deepwater marine living resources.

Mark Underwood manages the Scientific Equipment and Technology group of the Commonwealth Scientific and Industrial Research Organisation's Marine and Atmospheric Research division. He is an electronics engineer with 20 years' experience with marine and Antarctic science and instrumentation.

-back to top-

-back to to Features Index-

Sea Technology is read worldwide in more than 110 countries by management, engineers, scientists and technical personnel working in industry, government and educational research institutions. Readers are involved with oceanographic research, fisheries management, offshore oil and gas exploration and production, undersea defense including antisubmarine warfare, ocean mining and commercial diving.