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Mind the Gap! Forward-Looking Sonar Fills in Missing Data From Nadir 'Holiday'
Combining Forward-Look and Side Scan Imagery Provides 100 Percent Coverage, Saving Time in a Post-Tsunami Survey of Hawaii

By Dr. Thomas B. Reed IV
Oceanic Imaging Consultants Inc.
Honolulu, Hawaii

Significant progress has been made in the field of seafloor mapping since the era of lead lining and calling out 'by the mark, twain!' Echosounders, multibeam and side scan sonars have made the job of mapping the seabed easier. As the name implies, side scan sonar systems produce narrow beams of acoustic imagery out to the side of the sensor. We tend to think of each 'ping' of the sonar as illuminating, in a camera-like fashion, a swath of the seabed beneath the sonar. The reality is that coverage is a complex function of the beam pattern, the seabed geometry and the material properties. The results are often less than ideal.

It's no secret that the coverage and quality of data at nadir is generally poor. The side scan sonar's transducer mount angles, combined with each transducer's unique beam pattern, will dictate whether or not the seafloor at nadir is actually ensonified. If transducers are pointed downward, rather than sideways, it is possible to obtain acoustic returns from directly below the sensor. However, due to the vertical angle of incidence, acoustic shadows cannot be generated for near-nadir data, making object detection nearly impossible. Knowing this, surveyors will often choose to mount transducers to project more horizontally. Doing so will effectively expand the maximum range of usable data but will more often than not generate a gap in data coverage at nadir—a 'holiday.'

A typical result of merging forward-look data from BlueView's P450-45 2D imaging sonar with data from Falmouth Scientific's HMS-1400 side scan sonar. Shown here is a submerged dock and an overturned sailboat.

To compensate for this gap at nadir, the surveyor is required to run extra track lines in order to achieve full coverage of the survey area. A typical survey line plan is called the '15/45' pattern, in which spacing of adjacent survey lines alternates between 15 and 45 meters.

With the sonar at a range setting of 30 meters, lines could ideally be run at 45-meter line spacing, ensuring good overlap and rapid coverage. But because of the nadir gap, a different strategy is adopted. The first pair of lines are run with only a 15-meter line spacing, with the good data from the second pass filling the nadir gap of the first line, and the good data from the first line filling the nadir gap of the second line. The third line is then run at the usual 45-meter spacing, allowing good overlap, while extending the survey more rapidly. This type of survey plan allows the nadir gap of each line to be filled by data from the narrowly-spaced adjacent pass but increases the time required to cover the area by nearly 50 percent, costing more in acquisition and post-processing.

Bridging the Nadir Gap
While not optimal in terms of survey time, this method of alternating closely spaced lines with widely spaced lines does offer the surveyor 100 percent coverage but at the cost of 50 percent more survey time. One alternative to this approach would be to use the data from a forward-looking sonar as a 'gap-filler.' Forward-looking sonars can provide imagery ahead of the vehicle and are often used for obstacle avoidance and interactive (man-in-the-loop feedback) navigation.

To test this gap-filling technique, Oceanic Imaging Consultants Inc. (OIC) developed ray-tracing simulators for both a side scan and a forward-looking sonar. With a terrain model and navigation path fed into the simulator, it was possible to test various merging strategies. To continue this article please click here.

Dr. Thomas B. Reed IV received his undergraduate degrees in economic geology from Harvard University and the Massachusetts Institute of Technology in 1982 and his Ph.D. in marine geology and geophysics from the University of Hawaii in 1987. He founded Oceanic Imaging Consultants Inc. in 1993 to answer the growing need for seafloor mapping software, services and systems.

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