Application Overview

The 3DSS sonar employs patented Computed Angle-of-Arrival Transient Imaging (CAATI) in place of interferometry in order to separate seafloor returns from sea surface, water column and multipath interference. And as a result, the 3DSS is ideally suited for shallow water mapping applications in water depths of less than 1m to greater than 40m. The inherently wide swath coverage and high sounding density along with bathymetry accuracies that exceed IHO Special Order specifications provide surveyors and scientists with the best instrument for all shallow water mapping requirements.

Advantage of CAATI in Bathymetric Performance

3DSS-DX-450 data example showing bathymetry results for a water-column target cluster above the seafloor (28m average depth). The data is displayed twice, first with processing using the 3DSS patented Computed Angle-of-Arrival Transient Imaging (CAATI) methodology (foreground target cluster) and then again (same data) using least squares interferometry (background target cluster).

The target cluster is separated from the seafloor using CAATI to resolve multiple simultaneous angles-of-arrival (e.g. from the seafloor and water-column targets). In comparison, least squares interferometry incorrectly merges the water-column target cluster with the seafloor and introduces a bathymetric artifact.

The variability of the bathymetry is also reduced using CAATI as a result of the inherent separation of seafloor backscatter arrivals and concurrent multipath interference.

Bathymetric Uncertainty and Swath Widths

3DSS-DX-450 data example showing uncertainty results for 2.7m average water depth and 0.3m survey grid

In the example, the achieved swath width for a 0.3m grid while maintaining the Special Order IHO TVU (Total Vertical Uncertainty) specification is 21m to port and 26m to starboard for a total span of 13.4 times the mean depth at nadir.

Also in the example, the IHO Special Order specification is exceeded to achieve better than 10cm depth uncertainty 95% of the time for the 0.3m grid out to a swath width of 17m on port and 19m on starboard (9.7 times water depth). The 10cm uncertainty degrades to approximately 20cm in a region spanning approximately 2.5m near nadir and coverage in this region is also reduced.

Increasing the bin size or the ping density within the grid cells reduces the TVU. In the example data set, the ping rate of 12.5Hz (i.e. 50m range setting) and vessel velocity of 3.7Knots provide for 2.4pings per 0.3m bin. Ping density and bin size should be considered when comparing uncertainty results across surveys or systems.

Only roll corrections derived from the onboard MRU have been applied. Improved motion correction (e.g. heave, pitch and yaw) will further improved uncertainty results by reducing the striping evidenced in the Raw Depth Bins display.

A second 3DSS-DX-450 data example showing uncertainty results for 22m average water depth and 1m survey grid

In this example, the achieved swath width for a 1m grid while maintaining the Special Order IHO TVU (Total Vertical Uncertainty) specification is 100m to port and 85m to starboard for a total span of 8.6 times the mean depth at nadir.

Coverage and uncertainty are degraded in a region near nadir spanning approximately one half of the sonar altitude.

In the example data set, the ping rate of 4.3Hz (i.e. 150m range setting) and vessel velocity of 2.0Knots provide for 4.2pings per 1 bin. Ping density and bin size should be considered when comparing uncertainty results across surveys or systems.

Only roll corrections derived from the onboard MRU have been applied. Improved motion correction (e.g. heave, pitch and yaw) will further improved uncertainty results by reducing the striping evidenced in the Raw Depth Bins display.

Bottom type and geometry (e.g. cross-track slope) also influence achievable swath widths for a specified TVU. A sonar altitude of greater than 10m will generally achieve swath widths of less than 12 times altitude.

Bathymetric Coverage and Acoustic Shadows

3DSS-SX-450 (single side 3DSS) data example for 150m sonar range setting and an average water depth of 30m. Hypack software is used for the display.

In the example, acoustic shadows due to seafloor features and geometry are preserved in the bathymetry and mirror the shadows of colocated side scan imagery. Exact bathymetric data coverage is therefore provided and data is not interpolated or extrapolated into regions with little or no acoustic coverage.

In the shown figure, note that the bathymetric waterfall display scrolls in the opposite direction to the side scan display and so must be flipped visually in the vertical dimension for comparison. The single ping bathymetry profile display corresponds to the image line at the top of the side scan display, and with the bathymetric line entry at bottom of the bathy waterfall display.

Sounding Density and Angular Resolution

Application example showing the achievable cross-track sounding density (independent depth measurements) and angular resolution (i.e. equivalent cross-track beamwidth) as a function of cross-track distance for an individual 3DSS sonar ping at an operating altitude of 10m above a flat seafloor.

In the example, sounding density is smallest near nadir. At a cross-track distance of approximately 2.5m (1/4 of the sonar altitude) the sounding density is approximately 15 soundings/meter and the associated angular resolution is approximately 0.4 degrees. At larger cross-track distances the sounding density quickly improves to more than 35 soundings/meter and an angular resolution of less than 0.1 degrees.

In comparison with multibeam echo sounders, 3DSS soundings are each derived from an individual range sample rather than multiple range samples within a beam. For a specified cross-track grid resolution, averaging over the high 3DSS sounding density provides depth uncertainty benefits analagous to the benefits of the multiple range samples but coarse sounding density of beamformed echo sounders. The difference is that the swath coverage for the 3DSS sonar is not limited by the echo sounder beamwidth and is only governed by the uncertainty specification within each grid cell. Multibeam results are also limited by the same uncertainty specification, however a 1 degree multibeam echo sounder operating at 10m altitude, for example, can only achieve 0.5m grid cell resolution for a total swath widths of 2.8 times the sonar altitude, while the 3DSS sonar achieves up to 14 times altitude.