Collecting Bathymetric Data using a 3D Forward Looking Sonar
What is 3D FLS?
Over the past 14 years, the use of 3 dimensional forward looking sonar (3D FLS) for real-time navigation has been adopted globally by a growing number of vessel operators. FarSounder’s 3D FLS technology insonifies the area (the sonar’s field of view) ahead of the vessel (up to 1000m) and allows navigators to determine not only the range and bearing to navigational hazards within the field of view, but also their depth in the water column. In addition to real-time information about navigational hazards, the depth of water ahead of the vessel is also obtained. Determining the depth of the seafloor ahead of the vessel as measured by a 3D FLS is useful for real-time navigation, but are there other applications for this data?
Vessels of Opportunity - Collecting Bathymetric Data Using a 3D FLS
A recent International Hydrographic Organization (IHO) report shows that about 50% of the US coastal water a depth of less than 200m has not been systematically surveyed, and about 39% has been designated as “re-survey required”. Global coverage, especially in polar regions, is even less robust. Meanwhile, mariners knowingly operating in poorly or uncharted areas, or in areas with seasonally dynamic bathymetry are relying on 3D FLS for real-time navigation in growing numbers. An interesting opportunity exists to use navigational 3D FLS to collect and save bathymetric data, especially in conditions where charts are outdated or do not exist.
How accurate is bathymetry data collected using a 3D FLS? What are the advantages and what are the limitations? And in what ways is it different from data collected using traditional down looking sonar technology? These are all questions that need to be fully understood if the full utility of bathymetric data collected using a 3D FLS can be realized. FarSounder has taken the first steps toward answering these questions by performing two short surveys in Narragansett Bay, RI, and comparing the results to survey data collected by the National Oceanic and Atmospheric Administration (NOAA).
A full write up of the procedures, processing, and results can be found in “Applications of Today’s 3D Forward Looking Sonar for Real-time Navigation and Bathymetric Survey”, a paper that we presented at the MTS/IEEE OCEANS17 conference in Anchorage, AK. A short summary of some of main results is presented below.
Benchmarking 3D FLS Bathymetry
Figure 1: FarSounder's chart overlay with Patience Island survey area footprint displayed.
An example of one of the survey areas used in the study is overlaid onto a nautical chart in Figure 1. Color is mapped to depth where blue and yellow correspond to deep and shallow water, respectively. This area was chosen for the survey because of the rather abrupt change in depth on one side of the chanel, with a more gradual change on the opposite side.
The survey shown in Figure 1 was conducted by temporarily installing a FarSounder 3D FLS onto the University of Rhode Island’s R/V Cap’n Bert via a pole mount. When bathymetric data is collected, it must be corrected for height of the tide during the time of the survey. This can be accomplished in a number of ways (eg. using data reported from nearby tide stations, a tide gauge or predictions based on historical values). For this study, predictions were provided by the C-Map Professional+ chart engine (see the paper for comparison of different tide correction methods).
Once corrected for tide, roll, pitch and sensor offset, bathymetry data from the two surveys considered was compared to NOAA reference data from three distinct surveys for the area and resulted in 4.9% and 2.3% mean absolute percent deviation in the two surveys studied. Figures 2a and 2b present a comparison between the measured and reference data, with the 3D FLS measured value presented on the y-axis and the corresponding NOAA reference on the x-axis.
Figure 2a: Comparison between NOAA and 3D FLS Bathymetry in Patience Island survey area.
Figure 2b: Comparison between NOAA and 3D FLS Bathymetry in Newport Bridge survey area.
More information about the processing and collection of this data can be found in the paper. Overall, the 3D FLS data collected over these regions agrees well with NOAA reference in the same area. Additional corrections that can be applied that may reduce error in the data, including sound speed profile correction, are discussed in the paper.
Conclusions
A number of questions remain to be explored when it comes to collecting and processing 3D FLS bathymetry data, however the results of the surveys presented in the paper show that is a promising new way to collect bathymetric data, especially on vessels with 3D FLS already installed for real-time navigation.
At FarSounder we are hard at work developing new 3D FLS technology, processing algorithms, and collecting data to further understand how forward looking navigation sonar can be used to supplement traditional down looking survey measurements. We are working on further comparisons and evaluating additional metrics such as Total, Vertical, and Horizontal Propagated Uncertainty (TPU, VPU, HPU), 100% coverage swath width, and compliance with IHO’s special order and order 1a/b survey requirements.
