Comparison of bathymetric systems

Both the interferometric multibeams and beamforming multibeams measure range and angle to a series of points on the seabed. A beamformer mathematically forms a set of “beams”, and detects the range to the seabed in each beam. An interferometer measures the angle of the incoming sound wave fronts in a time sequence of samples. Slant range is obtained from the time of the sample and speed of sound.

In summary, beamformers measure range for each of a set of angles, and interferometers measure angle for each of a set of ranges.

ADVANTAGES & DISADVANTAGES OF THE TECHNIQUES

Parameter / Function

Interferometer

Beamformer

Notes

Number of depth measurements

6000+

60-120

Depends on range

Range vs. water depth

10 - 20

3-5

Beamformer footprint becomes unacceptably large at far range.

Amplification / processing channels

4-5

60 +

In a harsh environment, simplicity is important

Outboard transducer electronics

Passive

Active

The outboard component of an interferometer is extremely robust, and cheaper to replace if damage does occur

Outboard transducer size and weight

350x160x60mm

5 kg (air)

120x190x450mm

16 kg (air)

Dimensions for a common portable beamformer. Many beamformers are much larger.

Horizontal resolution at range

Good

Poor

Beamformer footprint becomes unacceptably large at far range.

Angular coverage

260°(including 20° overlap)

90°- 180°

 

Co-incident sidescan

True

Partial

An interferometer collects amplitude in the same way as its bathymetry: as a time-series.

Profile data density

Increases with reducing grazing angle

Decreases with reducing grazing angle

Higher complete profile data confidence with an interferometer.

Ability to resolve several targets at the same range

No

Yes

 

Ability to resolve several targets at the same angle

Yes

No

 

Profile data density

Increases with reducing grazing angle

Decreases with reducing grazing angle

In the first 5 m of horizontal range, a beamformer collects slightly more depth samples. Beyond that, an interferometer collects many more.

 

ADVANTAGES OF INTERFEROMETERS

Wide Swath Width
An interferometer produces a swath width of 10 to 15 times water depth, depending on sonar conditions. This advantage is particularly clear in shallow water (less than 30 m).
USGS: “Operating Swath of the bathymetric system ranged from 15 to 20 times water depth in depths less than 15m.”

Coverage
Bathyswath measures thousands of depths along every profile, across a very wide swath. Its coverage of the seabed is thus unparalleled.

Simplicity
A beamformer requires digitized signals from each of dozens of transducer elements, which must then be highly signal-processed. This results in a requirement for many amplifiers, wires, connectors and processors. These components must either be present in the wet-end of the system, or highly complex cabling is required to pass through the hull of the platform. Such complexity must inevitably result in reliability problems.
In contrast, Bathyswath requires only three or four individual signal channels. The innovative use of electronic processing limits the requirement for many processors. Only the transducers need to be placed in the water, and these are entirely passive: they contain no electronic components at all other than the piezo-electric elements and are completely potted in plastic compound. They are thus extremely compact and robust.

Weight and Size
The simplicity of Bathyswath results in a lightweight, portable package with a small footprint and simple cable requirements.

Flexibility
Interferometric systems can similarly be configured to work from a range of platforms, and in a range of configurations. Bathyswath permits simple and rapid deployment

Cost
The cost of ownership is relatively low when compared to beam-forming multibeams

Seafloor imagery
Interferometers generate both high quality bathymetry and seafloor imagery (sidescan)

COMPARISON WITH MULTI-BEAM BEAMFORMERS
 

Effect of Beam Width at Far Range
Beamformers have a finite width of beam, detecting anything within this beam as signal. Their resolution is thus related to the width of beam. At far ranges, where the beams make a small angle with the seabed, horizontal resolution is poor. In contrast, Bathyswath produces a small pulse of sound and measures returns from this pulse across the seabed. The footprint of this pulse thus remains small, and the resolution of an interferometer remains good even at far range.

               Bathyswath interferometer                     Standard multibeam

Angular Restrictions
Beamformers form a limited number of beams, in a limited angular sector. They therefore cannot survey up to the sea surface unless special mounts are used. In contrast, Bathyswath can survey a full 260° sector with 20° overlap in its standard configuration.

Lower Resolution
With less than a hundred depth measurements in each sonar cycle, it is often necessary for a beamformer to interpolate between measurements in order to simulate full coverage.

Roll Sensitivity
Beamformer profiles, being a much smaller percentage of water depth than an interferometer, are subject to large movements relative to vessel track.  This can lead to unsurveyed areas at the swath edges if the survey vessel is subject to rolling motion from wave encounter.
 

 

 

 

Swath width 25m

Swath width 100m