Using Forward Looking Sonar in Multibeam Surveys

  • Posted on: 24 February 2011
  • By: Matthew Zimmerman

A number of FarSounder's installations are on survey vessels. The most obvious use of our 3D forward looking sonar products is to provide realtime obstacle avoidance information to the survey vessel. By definition, if an area is being surveyed, then the bathymetry is unknown. These customers can use our equipment to help protect the very expensive survey equipment. Certainly, if the survey is part of a post dredging verification, the risk of damaging equipment is low. However, there are many instances where danger to ship, crew, and equipment are real:

  • Post Disaster (think missing oil rigs post Katrina or dangerous harbor conditions post earthquake in Haiti)
  • Arctic and Antarctic areas (think growing eco/adventure tourism in Antarctica and opening of the Northwest Passage to commercial vessels)
  • Charting New Shipping Lanes (think grounding of the Shen Neng 1 on the Great Barrier Reef in April 2010)

Using our current products for their originally envisioned purpose can be helpful today for some survey customers. However, the FarSounder development team has not stopped here. We're currently developing new capabilities for use with our existing hardware.

Upcoming Survey Applications

Over the past 3 years, while we were developing our FarSounder-1000 product, we were also looking at different uses for our sonar technology. One exciting area we identified is Forward Looking Multi Beam (FLMB) surveying. Our vision is that a customer may purchase a FarSounder unit for real-time navigation use and at the same time be able to use the same hardware as a survey tool. Just a few of the various particular applications we envision are:

  • Work boat is operating in a poorly charted area and requires real-time obstacle avoidance. At the same time, they would like to start building their own charts of the area where they are working. Using the ship's echo sounder, this can be slow as the swath coverage is narrow. Adding a dedicated survey multibeam is cost prohibitive...
  • Coast guard or other government agency responds to a post disaster area. Prior to disaster, chart coverage was good. Post disaster, it is unknown if new debris is blocking channels. Real-time obstacle avoidance is necessary to safely bring in supplies, get the port back operational, etc. Basic chart updating is needed but availability of dedicated survey vessels is not sufficient to cover the large areas of interest...
  • Research vessels are towing a sensor on a towfish in relatively shallow water. Forward looking obstacle avoidance sonar on the mother ship allows for advance warning for towfish operators of sub-sea obstacles. Towfish may contain a variety of sensors, but due to towfish depth, multibeam survey swath width is not very wide. Survey quality from FarSounder sonar may be sufficient for scientific needs without the expense of adding multibeam sonar to mother ship...
  • Large commercial vessels operating in riverine environments often know there is a channel that is "deep enough" somewhere. However, with changing seasons, the channel may shift requiring frequent survey updates. For the most part, vessel operators do not need high resolution surveys, they just need to know if the channel center has moved 5 meters or 50 meters. Due to the large areas of interest, availability of dedicated survey vessels is not sufficient. However, local pilot boats could be equipped with real-time navigation sonar which could also provide chart updating capabilities every time they transit to meet the large commercial vessels...

Forward Looking Multi Beam (FLMB) Concepts

When people think "multibeam" they typically think, 2D downward looking, cross track sonar. However, technically a multibeam sonar is any sonar that generates multiple beams with a single ping. FarSounder sonars do exactly this. So technically, ours is also a multibeam sonar. The big differences between our sonars and tranditional survey multibeams are: ours face forward, ours produce beams in azimith and elevation, our beams are wider but have a huge amout of footprint overlap. These differences are very important for realtime navigation. They can also provide some benefit for survey applications as well:

  • Dual use equipment:  realtime obstacle avoidance and bathymetric surveying
  • Large overlap footprints = high speed surveying: vessel speeds 2-3 times traditional multibeam surveys

We have begun working on a proof of concept integration of FarSounder sonar data into Hypack, one of the commercial survey software tools popular here in the US. Already, we have done some simple comparisons between data from our surveys with data from historical sources and the results seem reasonable. We are now in the process identifying improvements in our data quality, calibration, external sensor registration (heading, position, etc), and roll and pitch correction.

FLMB Initial Results

For the first proof of concept, we performed a small survey near the Newport Bridge in Rhode Island's Narragansett Bay.

survey bathymetry survey bathymetry

The image on the left shows the survey track. It was not very "pretty" as we were only moving at about 4 knots with strong cross winds and currents. We weren't using any fancy survey track management tools, just a simple chart plotter with vessel track history shown. The image on the right shows the survey results after 5 meter binning in Hypack. The deep water channel is clearly seen in the survey and the depths correspond pretty well to the chart. 

survey bathymetry
survey bathymetry

For out first survey attempt, we were pretty happy with these results. Soon after producing these results, we performed a similar test in shallow water outside the locks at Port Canaveral, FL. The top picture at right shows all the raw soundings from our FLMB survey. The bottom picture at right shows that same data after 5 meter binning in Hypack.

The water depth in this survey was much shallower than in our first proof of concept. However, our survey showed some deep scour directly in front of the gate and along the center of the channel. This is not surprising since the locks drain by simply opening the gates and letting the water rush out between the two doors. This causes some strong, localized water flows which erode the channel floor.

Our preliminary survey results show that the concept of Forward Looking Multi Beam surveying appears viable. Obviously for hydrographic purposes, survey quality is an important feature to evaluate. We'll discuss survey data quality in a future blog post.