Bathyswath productivity

 

Survey Productivity

The time in which a given seabed area can be surveyed depends on the distance between the survey lines and the forward speed of the survey vessel.

 

Line Spacing

The spacing between survey lines is determined by a combination of range limit and accuracy required. There must also be some overlap allowed to account for variations in the survey line followed. Otherwise, any small helmsman’s errors will cause gaps in coverage of the seabed.

 

Pulse Repetition Frequency

The time taken for a ping cycle is that for a round trip from the transducers, to the farthest range, and back again. The speed of sound in water is about 1500 meters per second. For example, a 150 meter ping takes 0.2 seconds. This gives a pulse (or ping) repetition frequency (PRF) of 1 / 0.2 = 5 per second (or Hertz).
The software allows the nominal sonar range to be set in meters. The corresponding PRF is calculated in software and used in data acquisition.

 

Platform Speed and Along-Track Coverage

Bathyswath is capable of providing total ensonification of the bottom at practical and efficient survey speeds
The distance between pings along the track of the vessel is determined by the pulse repetition frequency (PRF) and platform speed. In order to minimize cross talk between the two sides, the system can be used with alternating sonar transmissions, port and starboard. Thus, in the alternating mode, this distance is doubled:

d = 2 . V . PRF

where:

  • d is along-track distance between pings, in meters
  • V is platform speed in meters per second (Halving the speed in knots gives a good estimate)
  • PRF is the pulse repetition frequency, in seconds.

Bathyswath also provides the option of firing both transducers simultaneously. This doubles the coverage rate, so that the along-track ping spacing reduces to (V.PRF). However, this mode should be used with caution in surveys with a requirement for high bathymetry accuracy, because some cross-talk between the channels is likely. That is, the signals from one side can affect the other side.

Coverage is also determined by the width of the sonar beam. A narrower beam gives better resolution, but carries a greater risk of missing targets between beams. At 50 metres, the 234kHz and 468kHz beams cover 0.43m, and the 117kHz beam covers 0.74m.

Increasing the speed over the ground will reduce survey time, but will also reduce the along-track coverage. Five or six knots is generally a good compromise. At 5 knots, with a 100m range, giving 6.7 pings per second, each side is covered every 75cm along-track. At 10 knots, this spacing doubles. In the 5 knot example, ground is covered at 500 square metres per second, or 1.8 square kilometres per hour.

When using the system in simultaneous pinging mode, on a typically flat seabed, the directionality of the two transducers is sufficient to prevent the signals from one side appearing on the other. However, if one side contains a very strong reflector (e.g. a harbour wall), or is very weak (e.g. contains acoustic shadows), then there can be “cross-talk” between the sides. The operator needs to be aware of the risks and priorities. Typically, it may be safest to use alternating mode where bathymetric accuracy is paramount, and simultaneous mode when using the system to detect small objects on the seabed. When high coverage is required in a limited area, and channel cross-talk is a problem, it may be beneficial to ping on one side only, thus doubling the along-track coverage on that side.

Ranges

 

117 kHz

234 kHz

468 kHz

Minimum depth (1)

0.3 m

0.2 m

0.1 m

Operational slant range (2)

300 m

150 m

75 m

Maximum slant range (2)

350 m

200 m

80 m

Swath width vs. water depth ratio

Typically between 10:1 to 15:1

(1)There is no real physical limit to the depth at which measurements can be made by an interferometric system. Close to the transducer, the sonar system is operating in the near-field domain, where sound rays are not yet fully formed. This means that the accuracy of the angle measurement is not as good as a fraction of the sonar range as it is in the far field zone, but the very small ranges (within the 15-times depth limit) means that the depth measurement accuracy remains easily within that specified. A practical limit is close to that of the size of the transducer itself.

(2)The operational slant range is the one you should get in most cases. The maximum slant range is the one you should get in the best environmental conditions.

 

 

 

Swath width and coverage angle

Environmental conditions

Poor

Standard

Good

Swath width vs. water depth ratio (1)

10:1

12:1

15:1

Max. swath coverage angle - β angle (1)

157°

161°

165°

(1)Theoretical value  => Not limited by sonar range ( θ angle)

 

The following table shows the optimum water depth for which the maximum swath coverage angle and width are reached without being limited by the operational sonar range.

 

Environmental conditions

 

Poor

Standard

Good

Swath width vs. water depth ratio

10:1

12:1

15:1

 

Operational slant range

Water depth (Optimum)

Swath width (max.)

Water depth (Optimum)

Swath width (max.)

Water depth (Optimum)

Swath width (max.)

468 kHz

75

14.7 m

147.1 m

12.3 m

148.0 m

9.9 m

148.7 m

234 kHz

150

29.4 m

294.2 m

24.7 m

295.9 m

19.8 m

297.4 m

117 kHz

300

58.8 m

588.3 m

49.3 m

591.8 m

39.6 m

594.7 m

 

Shallow waters

  • Bathyswath can be used in very shallow water but I like to keep about 0.5m under water under the transducer heads.
  • In shallow water the use of a phase differencing (interferometric or PDBS) sonar really pays off against using a multibeam (MBES). With a MBES you usually get 3.5x water depth (wd) across all wd; a PDBS can give 10 to 15 x wd per side in very shallow water, so 20 to 30 x wd. We tend to be conservative as there are other things come into play such as vegetation and relief that can reduce this and use 5 to 7x wd per side so, in general, you should see 15m+ wide swaths in 1m water, 30m+ in 2m water, and around 60m swath in 4m.

Overlapping the swaths

In general it is a good idea to run survey lines with overlap. At a minimum say 10% to take account of line keeping errors. The type of survey being undertaken can determine the overlap, with the highest level requiring 200% coverage (100%overlap per side) but for reconnaissance surveys just full bottom coverage is required, which is in effect no overlap.

  • Some people have concerns about the nadir region with PDBS. This area has less data density and which in flat bottoms can appear as a gap. In our experience when there is relief and features/objects in nadir these do appear in the data so in effect no meaningful data is lost, but to counter this it could be worth considering running a survey pattern to overlap this area. This can be done with 100% overlap per side, which could be required anyway (see 3) or is a good idea to ensure full ensonification of the seabed. This survey pattern though effectively doubles the survey time from if minimal overlap required. A good alternative is to run alternating lines between 100% or better overlap and 10% overlap. In this way the nadir region is addressed with only a third reduction in survey time. You can also use an optional third, forward-facing transducer.
  • there is a very quick drop off in ranges in extremely shallow water so across the region you may be using diffferent survey line intervals. So in 1m water you could be using, for example, 15m line spacing, whereas in 4m this could be 60m

 

Grazing angle and spreading loss

Line spacing is the distance between adjacent survey lines. The spacing is determined by the sonar horizontal range expected at that depth, and the amount of overlap required. The horizontal range expected depends on the water depth under the sonar-head, as well as the seabed type and the sea state.
The term "Horizontal range" is used to describe the sonar coverage from one transducer. For a twin transducer configuration, the total swath width, from port edge to starboard edge, is therefore twice this range.
 
The horizontal range is limited by two factors: grazing-angle and spreading loss. The grazing angle limit is related to the angle that the sound ‘beam’ makes with the seabed.
 
Directly under the transducers, sound is reflected directly, and there is little loss when sound is scattered by the seabed.
Moving away from the transducers, much of the sound is reflected away from the transducers, but enough sound is scattered back for the seabed to be properly detected.
 
At the grazing-angle limit, the sound makes a very small angle with the seabed. Most of the sound is reflected away, and the signal scattered back from the seabed is too small to be detected. Actually, the configuration of the Bathyswath transducers is similar to sidescan sonars: the swath width ends at the point where the backscattered signal is not sufficiently above noise to enable detection of the angle of the backscattered wave.
 
Backscattered acoustic signals from a seabed generally follow “Lambert’s Rule”; that is, the backscattered signal falls with the square of the cosine of the angle of incidence of the acoustic “ray”.
 
 

Plotting this rule, we can see that the Lambert’s Law function is low at depth to swath width ratios above about 15:1.
 
In poor seabed conditions or turbid waters, this ratio can be reduced to about 10:1 or even less. Bottom types such as soft mud or peat can indeed reduce the expected range by as much as 30%. Sand, rock and shingle all give good sonar backscattering.
 
The spreading loss limit is simply caused by the sound spreading outwards, and being absorbed by seawater. The rate of absorption is related to the frequency of the sonar signal. The spreading loss limit is thus determined by the distance from the transducers to the farthest point on the seabed (the slant range).

 

 

 

 

 

Sidescan productivity

To get the best out of the sidescan capabilities of the PDBS then survey speeds should be around 3 knots. Surveying at around 5 to 6 knots though works well. I have surveyed at 8 to 10 knots but this does begin to compromise along track coverage.

In our experience the 234kHz system has slightlybetter performance at nadir than 468kHz system and provides better ranges at depth. But if your survey area is less than 10m there is not much to separate them.

The nadir region can be also be effectively covered by using a 3rd forward looking transducer. A forward look transducer can enhance safety in shallow water giving the helmsman an idea of rapid shoaling or objects ahead of the vessel. Transducer size and weight may be a consideration if using from small vessels/platforms. The 468kHz transducers are roughly half the size of the 234kHz ones.