Surveyingtechnical

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Multibeam echosounders: trends in the hydrographic industryby John Fraser and Rich Lear, RESON

Decreasing quantity of experienced multibeam users juxtaposed with the increasing demand for qualitative and quantitative multi-layered (i.e. bathymetry and backscatter information of seabed topography and seabed material) multibeam sonar data results in growing demand for a comprehensive and efficient system for acoustic sonar mapping. This article explores, through discussions with multibeam sonar industry experts, the increasing demands for acquisition of "more/faster/better" sonar data through status quo or limited hands-on operator experience. Future generation multibeam sonars must be geared for high efficiency; a mantra for robust common-goal solutions.

Huge leaps in technology have occurred since the first primitive forays into depth sounding but technology also brings with it challenges. The dramatic increase in data quantities necessitate large investment in software and hardware and of course highly skilled personnel not only to operate but to support it.

The question has to be posed: Is today’s technology genuinely increasing a surveyor's data gathering capability in less time, using less equipment?

The investment in capabilities that translate directly into efficiency gains which of course in turn bring cost savings to operators. In this article, we will look at some of these features and what benefits they bring.

History

Through hydrographic survey history from the lead line; single beam; sweep, to ultimately multibeam. The hardware systems have always required skilled operators, that have been guided by education and experience. Since the theory has remained, how has hardware actually evolved?

With the introduction of the now familiar PC interface, and reduction of hand plotted charts, concurrently came multibeam systems available for all users – not just deep water military. Certain sonar manufacturers in the late 1980s pioneered the shallow water multibeam industry with the release of products such as the RESON SeaBat 9001. This system truly revolutionised this market and made multibeam accessible to a multitude of operators who, until then, had used single beams and sidescan almost exclusively in

shallow water. The success of such systems may be judged by the fact that they were in continuous production for over 17 years and many are still in use worldwide today.

Hydrographers then demanded a greater range of tools with which to collect high resolution bathymetry. Greater range; greater coverage; higher resolution and faster ping rate were all perquisites for previous generation multibeam systems. Still today some attributes can not be met

by all systems in the market place, however yet again next generation systems are raising the standards to be judged from. Significant development has taken place specifically on features and capabilities to maximise the considerable investment required to own and operate one of these cutting-edge systems.

Standards and shallow water

The hydrographic community wish to work to recognisable standards.

Fig. 1: In shallow water, object detection is normally paramount because of issues such as ship safety in harbours. In this case a combination of ultra-high resolution and coverage are critical.

Fig. 2: The plot shows theoretical equi-angle (blue) and equi-distant (green) spacing on a flat sea floor.

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Fig. 4: Roll stabilisation allows the swath width on the sea floor to be maximised thus increasing line spacing and reducing vessel time. The usable swath is that portion between the red lines.

The publication, “Standards for Hydrographic Surveys” (S-44), is one of the series of standards developed by the International Hydrographic Organisation (IHO) to help improve the safety of navigation. From the 1st Edition of S-44 entitled “Accuracy Standards Recommended for Hydrographic Surveys” published in January 1968 to the current 5th Edition “Standards for Hydrographic Surveys” changes have occurred in operational capability. As a result, the latest standard is not a specification, as that responsibility is given to individual hydrographic offices/organisations to prepare “specifications” based on the standards.

It is therefore important for those local persons defining standards, to ensure that the latest equipment capability be considered for existing or indeed new hydrographic purposes.

Specifications can be more system specific and as such will be quite

dynamic as systems change. In shallow water, object detection is normally paramount because of issues such as ship safety in harbours. In this case a combination of ultra-high resolution and coverage are critical. A system providing the combination of a swath width in excess of 4 x depth (a swath path of some 100 m in 25 m water depth) and the ability to robustly detect objects down to approximately 0,5 m³ in 25 m and down to less than the size of a soccer ball in 5 m water depth may be judged as favourable and offer greatest user benefit.

Survey efficiency

Multibeam features have been a priority amongst certain manufacturers, and they have put considerable effort into developing “real-world” features to aid the surveyor. For a long time manufacturers have been extolling the virtues of their systems based on detailed technical specifications

which to the typical surveyor mean very little and provide little practical benefit.

However, will other requirements change the equation? As a case study let us assume a survey was conducted in the following area:

l 50 m depth, 1000 x 1000 m area, 10% overlap client specification, 5 kts survey speed,

l 3 hits in a "1 m box" client specification

l System ping rate within 10% of theoretical maximum.

In reality for example using RESON systems

l RESON SeaBat 7125 – 2,7 hits per box – need to reduce speed to 4 kts

l RESON SeaBat 7101 – 1,9 hits per box– need to reduce speed to 2 kts

l RESON SeaBat 7125 – 1hr 30 min (narrower swath – less time)

l RESON SeaBat 7101 – 1hr 40 min (wider swath – more time)

So how are systems more efficient?

"Hands off" online

In the past, some sonar systems have required a great deal of operator intervention, in some cases needing a dedicated operator to monitor the system throughout all phases of operation. This obviously does not make sense and so modern systems developed in the last few years often now employ an “autopilot” type capability.

This feature means the operator can spend the majority of their time performing critical tasks such as data quality monitoring and less time adjusting the sonar system. The autopilot must be very reliable and robust even under difficult conditions. Autopilot should assist the operator by automatically setting certain sonar parameters from a pre-defined look-up table. Quite often multiple tables may be stored and loaded as required. In use, the autopilot system analyses bottom detect data from a number of pings. If a sufficient number of those pings are judged to have good bottom detect quality, then the average bottom depth and maximum usable swath angle are therefore determined.

Once the depth and maximum swath angle are determined, an appropriate range setting is determined and commanded to the sonar. The process

Fig. 3: An extract from data collected in both equi-angle and equi-distant mode near Santa Barbara harbour in January 2007.

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Fig. 6: The sonar head may be physically tilted (typically up to 30°) to one side to illuminate the desired area, a quay wall in this case, and the user interface is adjusted to match.

then repeats for the next number of pings and so on. If too few of these pings are judged to be good (for example, if the water depth exceeds the current range), then autopilot will command the next greater range setting to the sonar before analysing the next set of pings. In this way autopilot mode can be reset to the minimum range and it will incrementally try greater ranges until the bottom is found.

In addition to controlling the range, autopilot mode also automatically adjusts sonar parameters such as power, gain, and pulse length. A user editable table is made up of rows, each corresponding to a minimum and maximum range and including a set of parameters. Depending on which row the current range setting falls within, the parameters for that row will be commanded to the sonar. In this way, critical sonar settings as well as range are all automatically controlled when in autopilot mode. Typical variables include: range against power, pulse length, gain, spreading and absorption variables.

Beamforming

A majority of current multibeam systems allow equi-distant (ED) beamformed footprints which maximise productivity, although the number of beams vary according to the specific system geometry.

Traditional (and some contemporary) multibeam systems operate only in equi-angle (EA) mode. As its name suggests, in this mode the distance between the centreline of each beam is an equal angle. When viewed from above it becomes obvious that the

density of the beam centres on the sea floor decreases towards the edge of the swath.

The plot in Fig. 2 shows theoretical equi-angle (blue) and equi-distant (green) spacing on a flat sea floor.

It may be seen that in equi-angle mode (blue), sounding spacing is most dense at nadir and decreases towards the edge of the swath whereas in equi-distant mode (green) nadir density is maintained across the entire swath.

This has a critical impact on survey operations especially where object detection is a mandatory requirement.

The plot in Fig. 3 is an extract from data collected in both equi-angle and equi-distant mode near Santa Barbara harbour in January 2007.

In this example, equi-angle nadir spacing (red data) is approximately 6 cm (in 9 m water depth) However, at the edges of the swath, equi-angle spacing has increased to approximately 28 cm while the equi-distant (blue dataset) remains at 6 cm across the entire swath (see Fig. 3).

In a survey operation where a requirement exists for a specific number of soundings on an object, it may be seen that equi-distant spacing allows the entire swath to be used whereas equi-angle spacing quickly exceeds the specification.

Comparison of equi-angle and equi-distant footprints

The availability of equi-distant beamforming has increased

Fig. 5a and b: The operator may reduce the swath coverage sector from maximum to a predefined minimum angle using set increments. Often two modes – “high density” and “reduced beams” – may exist.

A B

Operating on 100 m range scale Ping rae 5Hz

Operating on 50 m range scale Ping rate 10 Hz

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productivity dramatically especially in survey operations where a number of soundings per grid cell is specified. The increased density in the outer beams maintains a high number of soundings across the swath allowing wider line spacing, less overlap, fewer survey lines, higher productivity, higher efficiency, and of course cost savings.

As an example, let us assume a RESON SeaBat 7125 multibeam system in 20 m water depth with a requirement to meet three soundings across and three soundings along-track to detect a 1 m "cube". The along-track soundings are determined purely by water depth and vessel speed so remain constant for both modes.

256 Equi-angle mode: In EA mode, 256 beams are formed at a spacing of 0,5° This translates to a spacing on the sea floor between soundings of approximately 18 cm which means that at nadir (directly under the transducer) the 1 m cube will be defined by some six soundings across-track and would meet the detection criteria. If one now performs the same analysis across the swath, it becomes apparent that it is only possible to place three hits on the cube until one reaches a swath angle of approximately 45°. This means that out of the available 128° swath from the 7125 only 90° is usable. 128° swath provides a swept

path of 4x water depth (80 m in 20 m depth) but of this 80 m width only 50% is usable.

512 Equi-distant mode: In ED mode, 512 beams are formed at a variable spacing to maintain the nadir footprint across the entire swath. This means that the 18 cm footprint and the ability to place six soundings on the 1 m cube is maintained across the entire swath meaning that a usable swept path of 80 m is generated. This translates to a huge efficiency gain, allowing line spacing to be reduced by 50% and completing the survey is around half the time it would have taken with EA footprints.

Roll stabilisation

Roll stabilisation allows the swath width on the sea floor to be maximised thus increasing line spacing and reducing vessel time. Real-time input from a motion sensor is used to steer each receive beam dynamically for every sample thus maintaining the swath vertical irrespective of vessel motion. Under zero roll conditions, the swath is vertical and the centre of the swath is directly below the vessel. When the vessel rolls, the swath is rotated and the projected swath on the sea floor is laterally displaced.

This has the effect of reducing the usable swath width due to the distortions on the edge. In the example in Fig. 4 the usable swath is that portion between the red lines.

Fig. 7: Installing a MBES on a ROV, ensuring alignment with Inertial heading sensor.

This means that the line spacing has to be reduced in order to maintain 100% coverage and avoid data gaps. The amount of overlap will depend on the amount of roll experienced on a particular day and so it is difficult to predict or plan line spacing accordingly.

With roll stabilisation active the swath stays vertical irrespective of vessel motion thus maximising swath. Roll stabilisation requires the input from a motion sensor and the operator is able to configure sensor type and communications protocol. A motion monitoring display is provided in order to quality assure the data being collected. Needless to say, implementing roll stabilisation requires extremely tight control over the system timing, not only of the multibeam but also the motion sensor.

Variable swath

A variable swath capability allows the swath width to be varied according to the environment, or operational requirements, for example pipeline inspection.

The operator may reduce the swath coverage sector from maximum to a predefined minimum angle using set increments. Often two modes – “high density” and “reduced beams” – may exist.

In high density mode, all beams are formed and are compressed into the reduced sector providing increased sounding density. In reduced beams mode only the natural number of beams are formed to populate the selected sector. It is important to note that such a feature does not affect the beam width which is a physical characteristic of the receiver array and operating frequency.

Reducing the sector while maintaining the same number of beams has the effect of decreasing the spacing between beams thus increasing sounding density. Variable swath should be available in all beam modes, equi-angle or equi-distant and may also be used in conjunction with roll stabilisation.

The benefits of this feature are twofold:

l Increased sounding density in a small sector may be used when tracking pipes or other

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Figs. 8a, b, c: Sounding chart; sounding chart with surface bathymetric shading; and aerial photograph with bathymetry.

linear features where the user is not interested in data outside a defined sector. The increased sounding density helps to define small targets.

l A reduced sector allows an increase in ping rate for any given depth. This has immediate effects when one thinks of requirements for a specific number of soundings across and along-track. It means that in any given water depth and vessel speed, the ability to meet these criteria is increased significantly by reducing the swath angle.

Data quality

An automated indicator of data quality for each sounding will assist the operator/data processor in manually determining the amount of resurvey or processing required. Online there are generally two different methods of flagging data, one is a rather coarse flag which defines whether a particular sounding has passed the two main criteria in the bottom detect assessment process – brightness and colinearity. This method is very robust and when processing data statistically one sees that if the sonar is set correctly, only around 0,5% of the soundings do not pass both criteria.

Total propagated error (TPE)

The calculation of total propagated error (TPE) is used to aid the surveyor in determination of sounding quality by statistical methods and is typically calculated in the data acquisition software. One of the inputs into the TPE calculation is the sounding measurement error as computed in the sonar system. Other error sources which are considered are position, draft, squat, load, tide, geodetic calculations, lever arm calculations, timing offsets, speed, heading, attitude, sound velocity and others.

The uncertainty value of each sounding is reported as part of the output record and allows the surveyor to continuously monitor the overall system performance during all phases of operations. For a direct display, the main user display may be coloured to display the uncertainty value. The data is also generally available in quality data acquisition software for use during processing.

Changing orientation

It is becoming more and more common to fuse different datasets such as bathymetry with laser or photography to further enhance the final data products. The ability to robustly define vertical structures is a key feature in all good premium multibeam systems and one that is used frequently.

In this type of application the sonar head may be physically tilted (typically up to 30°) to one side to illuminate the desired area – for example quay wall in Fig. 6 – and the user interface is adjusted to match. A sonar feature may then be employed to enable the use of modes such as equi-angle or equi-distant and indeed roll stabilisation.

Advanced multibeam systems may also employ a “pipeline mode” which maximises the concentration of beams in the centre section and also provides undisturbed sea bed towards the outer edges of the sector, again maximising efficiency in these specific applications.

Improved receivers

The old adage of poor quality in, poor quality out remains true in this as in most fields in our world. The purpose of a receiver sub-system to detect the minutest signal in the water and pick this out from a background cacophony of vessel noise (own vessel and other traffic); wave noise and other

sources. Improving receiver design is a major thrust and one which will help to improve the all-important data quality resulting in the end in more accurate soundings.

Portability and upgradability

Moving away from hardware and processing wizardry, traditionally, there have been two different configurations of shallow water MBES systems; one in which as much electronics as possible are located in the sonar head which is located underwater and the second where the electronics in the transducers are minimised and some sort of junction box used to house electronics. Both these approaches have advantages and disadvantages.

The highly integrated "surface heavy" approach reduces shipping weight and cost, installation and integration time, operator workload, vessel size and power requirements and provides a one-man operation suitable for rapid deployment on small vessels as well as a tidy installation each time.

The rapid improvements in computer hardware and processing power makes it obvious to place those items where they are easily upgradable – that is in the topside processor. Having the heart of a system in the wet end where severe constraints are placed on the designers simply does not make sense as upgrades will be expensive, time consuming and will more than likely require major engineering effort. Being able to simply replace a plug-in card in a PC provides longevity and a “future-proof” system. Less serviceable parts in the water arguably make a system more reliable with less potential down time due to failures.

Installations

As a general statement, most portable installations be the temporary or semi

a b c

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permanent are now installed under a best practice criteria. The practice of using fabric tapes and broomsticks to measure offsets is thankfully redundant. With the high resolution MBES system combined with RTK positioning, there is no doubt professional procedures for installation all involve land survey techniques using a total station or similar measuring device for both angles and distances. This is a test for a true certified hydrographic surveyor.

Deliverable product

With hardware capable of producing larger datasets, imaging complex structures, end clients also need to consider what is a suitable and appropriate data deliverable. Will a hardcopy paper chart suffice, or will an interactive dataset to be viewed in stereo using a freely distributed data viewing software package downloaded off the internet be preferable?

How will the data be used? Such a question falls beyond the scope of this paper, however when users consider hardware procurement, they must bear in mind how they are going to use the acquired data.

Conclusion

Today, industry users and clients of multibeam echosounder manufacturers are presented with an array of systems each promising unique capabilities. It is important for users not to lose focus on what features can actually be used by them for the benefit of operations. It is also critical that the system fundamentals such as bottom detection algorithms, beam forming ability etc. are not over looked. Clients should also consider reliability and service from the respective manufacturers.

However, users who procure systems must also consider not only survey standard compliance, but also maritime safety hardware compliance standards

such as CE certification and also IEC60945 (Maritime Navigation and Radiocommunication Equipment and Systems).

Irrespective of hardware capability, the user (hydrographic surveyor) must be educated, trained and experienced in conducting operations. End data is still only as good as the quality of the installation, integration and acquisition. Irrespective of data processing software technology, it is still a critical quality to have a qualified online surveyor, however to achieve the data in the first place requires a quality piece of sonar hardware and sonar processing ability.

Acknowledgements

Christian Blinkenberg, and Mairi Forrest, RESON Offshore Ltd

This paper was presented at Hydro 09 and is published here with permission.

Contact John Fraser, RESON, john.fraser@reson.com