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Towcam Survey System

A description of the system used to collect video and still imagery of the pockmarks.

        

Fig. 4

Towcam is a towed body system that collects continuous composite colour video imagery of the seabed over a large area (i.e. transects many kilometers in length). It is towed at a constant altitude (generally about 2-4 m above the seabed) at a speed of about 2-4 knots (i.e. about 4-8 kilometers an hour).   The field of view is about 1-2 m wide. (Click here to enlarge image of the Towcam survey system conceptual diagram.)

Fig. 5The aluminum body is of open construction and depends upon a combination of dead weight and adjustable wings to generate a depressing force.  It is 1.9 m long x 1.7m wide x 1.1 m high and weighs 327 kg on deck.  (Click here to enlarge image of the Towcam vehicle.) It is fitted with the following:

  • Sony XC-999 colour video camera
  • Two Nuytco Research Ltd. Model HMI 600W metal halide (HMI) gas discharge lights
  • Insite Tritech Scorpio digital still camera (6.1 mega pixel)
  • Quantum Q Flash T1 strobe (150 watt/sec)
  • Harbor Branch Oceanographic Institution Model LSR-6000-635-10-6 laser scale (10 mW, red-orange)
  • Mesotech 807 acoustic altimeter (30 m range)
  • Pitch, roll, pressure and water temperature sensors

Electrical power is supplied to the body from the surface via the tow cable so there is no limit on tow duration although digital camera storage capacity and strobe battery life limit still photos to approximately 400 per tow.  An acoustic transponder attached to the body establishes location relative to the ship using a Trackpoint II ultra-short baseline acoustic tracking system.

Fig. 6

The body is towed on a ¾ inch double armoured stainless steel conductor cable manufactured by South Bay Cable Corp.  When deployed on large survey vessels, a 50 HP Swann winch with 700 m of cable is used. (Click here for enlarged image of Towcam being launched from CCGS Hudson.) While the maximum operating depth of this configuration is currently 200 m, depths in excess of 300 m could be achieved by increasing the dead weight of the body.  On smaller survey vessels, a lightweight aluminum Harrison and Robbins 20 HP winch is used with about 400 m of cable.  This limits the working depth to about 150 m.  Constant altitude above the seabed is maintained by adjusting the amount of cable payed out.  In manual flight mode, a scientist in the ship’s laboratory controls the winch while observing body performance on computer and video monitors.  In automatic flight mode, a closed loop control system compares the altimeter reading to the desired operating depth and sends “haul-in” and “pay-out’ signals to the winch to maintain a constant altitude automatically.

Fig. 7Real-time video and low-resolution still imagery are displayed in the laboratory.  The former is also displayed on the bridge to assist the quartermaster in ship handling.  Video imagery and navigation data are recorded on both a digital VCR and a DVD recorder for later analysis. Time-referenced data on body behaviour are recorded on a PC.  During a tow, observations of interesting features, referenced to video time code and UTC, are recorded on paper to assist subsequent analysis of the tapes.  Digital still photos can be taken manually or automatically at preset intervals (as short as 3-4 seconds) to remove any bias.  They are previewed in the laboratory in real time and recorded internally in the underwater camera. After each tow, the photos are downloaded and matched to the navigation and body attitude data in order to define their precise geographic positions and the area imaged. An explanation of the method used to calculate the area encompassed by the underwater still and video images is described in the appendix “U/W Photographs and Video Bitmaps Image Area Calculation”.

The resolution of the video imagery depends upon the altitude of the body over the seabed and the towing speed, which are both influenced by current and sea state.  Under reasonable conditions, it is possible to discern physical features of the seabed as well as surface-dwelling organisms such as sand dollars, brittle stars, sea cucumbers and various species of fish.  The laser beams (spacing 10 cm) allow the scaling of seabed features and organisms.  Image size depends on the altitude of the body but is on the order of 1-2 m2. Post-processing of the still images greatly improves the resolution of physical and biological features such that it is possible to resolve features on the order of a few millimeters, and allows the examination of organism-microhabitat associations in an undisturbed state.

Towcam has proven to be an excellent tool for conducting general reconnaissance surveys on the scale of tens of kilometers.  The only limit on transect length is endurance of the operators and we have run transects in excess of 30 km.  Towcam is very robust and has the potential to carry other sensors such as sidescan sonar.  It can be used over any kind of seabed (e.g. mud, sand, gravel, cobble, boulder, bedrock, etc.) as long as the relief is relatively low and the water is not turbid.  Towcam has the potential to become an excellent stock assessment tool for commercial fisheries such as scallops and perhaps even some species of ground fish (including juveniles), for ground-truthing acoustic geophysical survey data and conducting benthic habitat studies.

A short segment of Towcam video from a transect across pockmark WAI in Passamaquoddy Bay can be viewed by clicking on the following link [ Sample Towcam Video ]. A higher quality video file can be viewed by clicking on the following link [ ftp://starfish.mar.dfo-mpo.gc.ca/pub/ocean/Pockmarks/141300.avi ]. This latter file is very large (67 MB) so several minutes will be required to download it. The heavy sediment load in the waters of Passamaquoddy Bay degrades the quality of the video. The red lines that converge toward the sea floor in the middle of the image are produced by the laser scale that projects a pair of dots 10 cm apart onto the bottom. The occasional red flash that occurs is produced by the strobe light associated with the still digital camera. The text at the top of the image describes the time (UTC) and position (latitude and longitude) of the towing vehicle. While the Towcam control system attempts to maintain the vehicle a constant height above the sea floor, there is some lag in its response so that, as the vehicle descends into the pockmark near the beginning of the video then exits from it near the end, its altitude varies somewhat. Because there is a large amount of suspended particulate matter in Passamaquoddy Bay the vehicle had to be towed at about 1.0 to 1.5m above the bottom to compensate for the resultant poor visibility.

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