|
|
|
|
|
|
|
© GeoAcoustics 2008
|
Application Note 9 Shallow Water Surveys using the GeoAcoustics GeoSwath
Introduction Good knowledge of shallow water bathymetry is vital to a wide range of marine activities. Surveys for navigation channels, ports and harbours, lake and dam management, environmental mapping, marine archaeology, marine construction, cable landfalls and hydrodynamic modelling all rely on accurate and detailed information on the water depth. Such surveys are mainly in waters less than 50m deep, with many in the zone from 20m water depth to the shoreline. High resolution, complete coverage, high accuracy, and detection of all significant features are particularly important in these circumstances. Recently published survey specifications emphasise these points, and pay particular attention to the shallow water regime. Examples of this include the International Hydrographic Organisation (IHO) Special Publication S-44 Edition 4 and the United States Army Corp of Engineers (USACE) Hydrographic Surveying Standards. A sonar system for shallow water surveys must also be able to operate from shallow draft vessels, be easy to deploy and use, and reduce the time to produce a deliverable chart. Wide swath sonars are particularly suitable for shallow water surveys, and the application of new interferometric multibeam technology to shallow waters has developed rapidly in recent years. In 1999 GeoAcoustics launched the GeoSwath, a fully integrated wide swath survey tool, which made this technology available in a system designed for the commercial user. This paper describes how the data from the GeoSwath meets the requirements of high quality bathymetric surveys, and examples from survey datasets are included which demonstrate the sonar’s advantages in shallow water.
Surveying with the GeoSwath Small boat mounted sonar surveys are the best solution for high resolution full coverage bathymetry around coastal regions, harbours, rivers and lakes. Here a small boat has the shallow draft and manoeuvrability needed to operate around shorelines and hazards, and sonar has the penetration to cope with turbulent conditions and the deeper parts of the survey. These surveys are typically done at speeds of between 5 and 10 knots. Many overlapping survey lines are run to give complete coverage of the survey area, with data often required up to the waters edge. The GeoSwath is a fully integrated sonar survey tool designed for high resolution surveys, especially from small survey craft. The GeoSwath uses phase measuring (interferometric) technology to provide the advantages of wide swath and high resolution in a compact and robust system which is suitable for deployment in shallow waters, an area where a beamforming multibeam or towed side-scan have particular problems.
A 250kHz GeoSwath sonar includes a pair of transducers (35cm by 15cm by 6cm) mounted on a V bracket, cables up to 40m long and a sonar control computer which contains all the sonar electronics. The V bracket also houses the heave/pitch/roll sensor, or motion reference unit (MRU). GeoAcoustics manufactures a transducer bow mount system designed to make the most of the transportability of the GeoSwath. This bow mount is quick to deploy and requires no vessel modification, and is suitable for short surveys (up to 1 week or so). It weighs about 75kg and is rapidly deployable on a locally sourced small vessel. Alternatively the transducers can be mounted on a side pole mount or permanently hull mounted on dedicated survey vessel.
Once mounted on the vessel, calibration is required to define the relative positions and orientations of the survey sensors. This can only be done with sufficient accuracy by running a survey pattern (often called a patch test), and using self consistency of survey results to determine the offsets. The patch test will consist of a sequence of survey lines that can be compared in pairs to check the latency, roll, pitch and yaw offsets between the transducer head, the motion reference unit and the heading reference. The GeoSwath provides semi-automated calibration software which allows full quantitative quality control of the calibration process. Calibration is a key part of obtaining accurate survey data, and the GeoSwath’s shallow water capabilities are a major advantage in getting a good calibration data set. Finally the depths on a patch of seafloor can be directly checked using multiple soundings, where the uniquely high data density available from the GeoSwath means that the standard deviation can be measured and used to quantify the survey accuracy.
Once the calibration survey has been completed the survey can begin. On-line software tools allow complete monitoring of the sonar and ancillary equipment, and there are a variety of ways to visualise the output data so that the surveyor can maintain good quality control. Usually the surveyor will keep on display a scrolling waterfall of bathymetry, a side-scan waterfall, a coverage plot and a real-time profile of the seafloor under the vessel, as well as a selection of other windows to monitor ancillary equipment. The processed data from each line can be added to the binned fairsheet and side-scan mosaic as the survey is carried out, giving confidence that coverage is complete and that overlapping data from different lines matches well. The on-line display of the side-scan with the bathymetry also allows the surveyor to immediately identify features that may be of interest for a more detailed look.
Data processing software for on-line processing and for post-processing back at base is a vital part of a commercial survey sonar. The GeoSwath software has been designed to enable the surveyor to make the most out of the masses of data collected. The processing of swath survey data consists of 2 main functions: rejection of outliers, then statistical combination of the remaining soundings into binned survey data. The GeoSwath software is designed to provide the tools a surveyor needs to perform efficient processing of the large amount of data collected while maintaining good quality control of the final output.
During processing the very high resolution ping-by-ping data is converted into a binned bathymetric chart - typically 1m binning is used in shallow water surveys. A side-scan mosaic can also be created, which allows feature interpretation and gives a confidence check of feature detection. A 3D flythrough of the survey area can be viewed, and the side-scan and bathymetry can be overlaid to further aid interpretation of the data. The side-scan and bathymetry records from individual lines can be re-examined to look at specific features at higher resolution, if required. Immediate feedback on survey results is important so that the surveyor can quality check the data as it comes in and ensure the required accuracy standards are maintained. Data quality control tools are an important part of the software provided with the GeoSwath, giving a bin-by-bin measure of the data density, distribution and confidence, both on-line and in postprocessing. Survey Specifications for Shallow Water Surveys
The IHO Standards for Hydrographic Surveys are increasingly being used as the basis for hydrographic work around the world. They have been issued as voluntary guidance to IHO member states, and are intended mainly for surveys for nautical charting for marine navigation. S-44 Edition 4 includes changes that reflect the recent technology advances in commercial swath sonars, especially the advantages of using wide swath surveys in shallow navigable depths. The IHO S-44 is being adapted by maritime authorities around the world in their issued standards for commissioned survey work, and the performance of a sonar intended for use in shallow water surveys needs to be measured against the applied standards. The IHO S-44 Special Order is the standard aimed at critical areas in shallow waters so is the minimum standard that a shallow water swath survey sonar must meet, although most authorities are setting more exacting requirements on commercial shallow water surveys than those in the basic S-44.
Land Information New Zealand (LINZ) has published “Provisional Swath Sonar and Survey Specifications” (LINZ 2003), a paper reviewing proposed changes to the LINZ Hydrographic Survey Specifications (HYSPEC). High depth accuracy, 100% coverage and target detection standards are key parts of this. Currently S-44 is included in HYSPEC without interpretation, but the LINZ mandate is much broader than S-44, and it is recognised that S-44 contains significant ambiguity when it comes to applying new sonar technology to achieve the required survey result. In China the GeoSwath wide swath sonars are being adopted as the system of choice in river development projects, with survey requirements often much stricter than IHO, especially in large dredge monitoring applications. As well as accurate bathymetry, a shallow water sonar must enable the surveyor to find, accurately position, and identify features of interest. Different applications would have different priorities for this: for navigational charting the finding of features that could present a hazard to shipping is vital, for hydrological modelling there may be submerged shallow channels that would affect water flow, and for maritime archaeology there could be small variations in bathymetry that indicate a possible wreck site for further investigation. When choosing a swath sonar for shallow water work the question needs to be asked: Can the sonar find a feature of 1m in extent, and can it accurately map all features at this scale to the required accuracy?
GeoSwath Specifications
The Performance of the GeoSwath in Shallow Waters The 250 kHz GeoSwath is capable of up to 100m water depth surveys, with a 300m maximum swath width. In shallow waters the maximum swath width will be about 12 times water depth, up to this limit. This swath width limit come from the sonar signal attenuation and the resulting sonar signal to sea noise ratio. Sonar signal attenuation results from the absorption by seawater, the spreading of the sound with range, and the scattering characteristics of the seafloor. The maximum range of the 250 kHz GeoSwath due to absorption and spreading loss is about 150m, so in a shallow water survey the limiting factor on the swath width will be the scattering and absorption characteristics of the seafloor. Beyond about 6 times water depth to each side of the transducers (12 times water depth total swath width) very little of the sonar signal will be scattered back to the source due to the shallow grazing angle of the incident sound. On softer sediments with significant absorption the sonar signal will be more attenuated, and this can further limit the swath width. In an area of unknown softer bottom types it is prudent to design the survey to allow for 8 times water depth swath width. This will give plenty of overlap for most of the survey, while maintaining coverage over regions of soft sediment. The GeoSwath takes a depth measurement by finding the angle of return of the sound scattered from the seafloor at a series of times after the sonar pulse is transmitted. This series of angle and range solutions are then built up into the seafloor profile under the vessel. The short time interval between angle measurements gives the system its high range resolution, while the accuracy of the angle measurement gives the high angle resolution. Range resolution (sample rate) is user controlled, and is usually set to a few cm. Angle resolution is determined by the sonar signal to noise ratio, and is typically about 0.05 degrees in shallow hard sand at the swath edge. Combining these gives a depth accuracy of about 10cm at the 95% confidence level across the whole swath in shallow waters, for a single sounding. In a delivered survey the accuracy of the binned depths is greater than that of the individual soundings, as there are many soundings in each bin which are statistically combined to produce the final chart. These system specifications show that the GeoSwath sonar is expected to well exceed IHO shallow water survey specification requirements across the whole swath. But what are the observed errors in real GeoSwath data? To get a complete answer to this we need to look at the absolute accuracy, the accuracy compared to other techniques, and the repeatability of the depths measured. The absolute accuracy of the GeoSwath has been found using a test tank of known dimensions. In a 4m deep 5m wide test tank the GeoSwath gave the true depth to within 2cm, across the whole tank (Hogarth 2003).
The true level of confidence of a chart depth from a GeoSwath survey is found by comparing many soundings within one bin on a flat area of seafloor. The standard deviation of a sounding can be found from the distribution of depths in one bin, and the standard error of the mean depth of the bin is given by dividing the standard deviation of individual depths by the square root of the number of depths used to find that mean. The 95% confidence level is given by twice this standard error. This type of calculation is best illustrated by an example. From a survey of Bristol Docks a standard deviation of 7cm was found from a 5m bin containing 4000 soundings from several survey lines over a flat area about 6m deep. In the main survey region most of the 1m bins had more than 20 soundings. This gave a 95% confidence level error of less than 5cm across the whole survey. Note that this survey accuracy is measured from the data collected on the day, and is independent of the sonar’s brochure specification. It also includes contributions from all sources of error, including the ancillary sensors and the calibration. These measurements show that the accuracy and repeatability of depths found by the GeoSwath are well within the IHO S-44 Special Order specifications. The Resolution and Swath Width Advantages In shallow waters you need high resolution in order to be able to use appropriate bin sizes. This resolution needs to be achieved over a wide swath in order to give useable survey productivity. The major problem with beamforming multibeams in shallow waters is that the footprint over most of the swath is too large for useful bin sizes, because most of the survey is at low grazing angles.
In a seafloor with fine scale features, obtaining accurate bathymetry at the scale of the bin size requires more that one beam per bin. In an analysis of beamforming multibeams by the Ocean Mapping Group, University of New Brunswick, it was found that “… topographic and textural features that are around or below the beam footprint size will result in misleading fine scale bathymetry. A solitary oblique narrow beam bottom detection should not be assumed to be representative of either the shoalest depth in the ensonified area or the average depth within the area. …only through averaging of several closely spaced beams can one come up with a reliable estimate of an average depth.” (Hughes-Clarke 1998). This averaging within a bin is not available from a beamforming multibeam in shallow waters using any reasonable bin size and swath width, while it is easily achieved by the GeoSwath interferometric system. The survey requirement that 1m features are to be mapped accurately means that the maximum bin size should be 1m, and there should be at least 2 soundings per bin to ensure accuracy of the mean depth recorded. Given this requirement, what swath width can be used by a typical beamformer? Below 20m water depth a 1.5 degree beamformer will not have 2 soundings per 1m bin, even directly under the vessel. In 5m water depth the 2 soundings per bin limit is reached at only 8m out from the transducers. The graph compares the GeoSwath data density (using typical sonar settings) to the performance of a 1.5 degree and a 0.5 degree beamforming mulitbeam in 5m of water. While the GeoSwath maintains more than 2 points per bin across the whole swath (and beyond 2.5m horizontal range has many more), the 1.5 degree beamformer falls below 2 points per bin at 9.5m range, and the 0.5 degree beamformer at 16m range.
Using the requirement of at least 2 soundings per 1m bin the swath width limits for the beamformers can be calculated at different depths, and this is shown in the attached figure. Even a 0.5 degree beamformer is limited to 4 times water depth total swath width in 10m of water. This limitation clearly does not apply to the sonar technology used in the GeoSwath, as its resolution increases towards the swath edge. Technology Comparison in a Shallow Water Deployment
A beamforming multibeam intended for shallow water applications may have a 120 degree swath and 1.5 degree beamwidth. As this is a coastal survey it would not be unusual to expect about 5 degrees of roll in a small survey vessel. A beamformer’s outer beams track along the seafloor as the vessel rolls, so the swath width for this system must be restricted to 110 degrees to ensure full bottom coverage. This gives a range in 5m depth of just 7m per side. Line spacing with 10% overlap would be 12.6m. But the depth may go to 3m. Gaps will appear in the survey over these critical shoals unless the line spacing is kept to about 8m and the vessel sticks to the survey lines to within 1m. This is unrealistic both in terms of survey time and vessel navigation, and demonstrates why a beamformer is considered inappropriate technology for this type of shallow water survey application.
When using the GeoSwath for this survey the line spacing might be chosen to be 8 times typical water depth with 20% overlap. In 5m depth this gives about 30m line spacing. Roll does not affect the GeoSwath swath width, and the wider swath means that this 8m overlap will allow reasonable leeway to the helmsman in following the survey line. Shoals to 3m and less will still be mapped across the whole swath width, as the GeoSwath does not have the limited field of view that restricts a beamformer. There will be many soundings per metre even at the swath edges, allowing full confidence checking and detection of features and shoalest points.
Calibration Matters A good calibration is a vital step in producing high quality survey data, and in shallow water surveys this is especially important. The small angle from horizontal at the swath edges means that roll calibration is vital for accurate depth results and good matching of adjacent survey lines. A calibration is best done with lots of data from two overlapping swaths, and in shallow waters this can only be achieved efficiently using a system that maintains high resolution and high data density out to the swath edge. If only shallow water is available for the patch test, as in a lake, river, or port, the limited swath width of a beamforming multibeam will limit the calibration accuracy. On top of this any seafloor texture on the scale of the bin size can lead to errors in the calibration if the footprint is a similar size to the binning used. For the GeoSwath survey there will be 10's of data points in each 1m bin, even at the swath edge, and in 5m water depth the swath edge will be typically at 25m-35m range. The multiple data points per bin across the whole of this wider swath gives more accurate calibration, and the extra data also allows the calibration accuracy to be measured quantitatively directly from the data. The GeoSwath provides a comprehensive set of tools for this type of analysis, so the surveyor can have more confidence in the quality of the calibration results, even in the shallowest surveys. The Wide View Angle
In contrast, a typical ‘shallow water’ beamforming multibeam might have a 120 degree field of view. In 5m of water this will only allow a 17m swath width at best (less if we take into account the large footprint at the swath edge and rejection of outer beams). In addition, any structures extending to near the sea surface will be outside this 120 degree fan, so will not be detected unless the vessel passes virtually over the top of them. The wide view angle of the GeoSwath means that the survey productivity stays high even in shallow regions. For example a 2km by 2km by 5m deep survey area can be covered in about 8.5 working hours. It also means that channel walls will have full coverage to the surface, and there is confidence that all structures will have been ensonifed no matter how near to the waterline they extend.
The GeoSwath Technology To use a sonar appropriately and maintain good quality control of the survey data the surveyor should have an appreciation of the sonar technology being used and its capabilities. The GeoSwath uses different techniques from beamforming multibeams in order to overcome the problems they experience when used in shallow waters. This section gives and introduction to the GeoSwath technology in order to make it clear why the GeoSwath does not have the beamformer’s limitations. The GeoSwath uses phase comparison angle measurement, commonly described as interferometric multibeam or bathymetric side-scan. The sonar is typically configured with a port-facing and a starboard facing transducer, with the face of each transducer aligned at about 30 degrees from the horizontal. Each transducer has one transmit stave and multiple receive staves. The transmitted pulse is similar to that used by side-scan sonar: very wide in the across track direction (greater than 150 degrees), narrow in the along track direction (about 1 degree), and about 30 microseconds long (~7.5cm, although this can be adjusted by the user). The beampatterns from the port and starboard transducers overlap under the vessel, giving extra coverage in this area. The receive staves have a similar beampattern to the transmit staves. They are connected to electronics that measure the amplitude and phase of the sonar signals scattered from the seafloor. The relative phase delay between the receive staves is decoded to give the angle of return of the sonar signal. Note that phase delay is a time delay measurement so it can be determined very accurately (to a fraction of a percent). These relative phases allow the angle of return of the sonar signal to be measured to a fraction of a degree, and measurements are taken at very short intervals (the interval is user selectable, down to every wavelength). It is this rapid and accurate phase measurement that gives the GeoSwath its very high resolution. Each sonar ping from the GeoSwath provides a range series of angles to the seafloor. A port and starboard ping together give the seafloor profile under the vessel, and a series of pings taken as the vessel moves along the survey track gives the swath of soundings. The amplitude of the sonar return is also recorded, giving the side-scan image. There are two main sources of error in the accurate location of a GeoSwath sounding, arising from the sea noise and the length of the sonar pulse. The pulse length limits the range accuracy, although the measurement interval can be set to be shorter than the pulse length to allow averaging. The sea noise (mainly from short range thermal sources) adds a random, uncorrelated phase component to the sonar signal on each stave of the transducer. The angle error induced by sea noise is usually what limits the sonar range and swath width, as the sound returned from the seafloor dies away at low grazing angles and the sonar pulse is attenuated by absorption in the sea. In shallow water the ranges are comparatively short so the signal strength is high, and the sonar pulse can be kept quite short. This allows the high angle accuracy of the phase measurement technique to be used to its best advantage, and the results of this can be seen in the earlier section on system accuracy. Two questions are often raised with the phase measurement technique: the issue of simultaneous returns from two features at different angles and the lower data density under the transducers. The keys to overcoming these are the short pulse length, rapid phase measurements and very low noise electronics used in the GeoSwath Sonar.
The lower data density directly under the transducers arises because the range change from one measurement to the next is large at this point. Rapid measurement of the phases means this is minimised. For the shallow water surveys being considered here, in 5m water depth a 30 µsecond sample rate corresponds to an across-track separation of the first two depth measurements of about 50cm. This sample rate is typical of what is used in a normal survey, and provides at least 2 soundings per metre: further out from the vessel the sounding density rapidly increases, to over 40 soundings per metre, as shown earlier in the comparative graphs of data density. Conclusions A full coverage survey of the sea floor in less than 50m water depth can now be performed with high productivity, high resolution and low cost. In the shallowest depths (20m to the water’s edge) the GeoSwath sonar has key advantages over other survey technologies in accuracy, productivity, data density, feature detection and identification, and ease of deployment. This application note shows how the GeoSwath gives:
Summary of the GeoSwath Advantages When using the GeoSwath for a shallow water survey you get:
References: Hogarth, P., 2003, “Achieving High Accuracy Using Wide Swath Bathymetry Systems” GeoAcoustics Limited, Great Yarmouth, UK. Hughes-Clarke, J.E.,1998, “The effect of fine scale seabed morphology and texture on the fidelity of SWATH bathymetric sounding data”, Proceedings of the Canadian Hydrographic Conference, Victoria B.C., Canada. IHO, 1998, “IHO Standards for Hydrographic Surveys, 4th edition”, International Hydrographic Organisation Special Publication 44, International Hydrographic Bureau, Monaco. Jakobsson, L., 2000, “The Swedish Implementation of S-44, International Standards for Hydrographic Surveys”, Swedish Maritime Administration, Sweden. LINZ, 2003, “Provisional Swath Sonar and
Survey Specifications”, Land Information New Zealand. USACE, 2002, “Engineering and Design -
Hydrographic Surveying”, United States Army Core of Engineers Publication EM
1110-2-1003: Also see GeoSwath Application Notes 1 to 8 from GeoAcoustics Limited for further details of individual survey deployments and results. More details on GeoSwath |
The GeoSwath is available in two
frequency versions, 125 kHz (for up to 200m water depth) and 250 kHz (for up to
100m water depth). The higher frequency version has smaller transducers and
higher resolution, so is more appropriate for smaller vessels in shallow
deployments. Most of the data shown here is from this version. 
A typical deployment on a vessel of
opportunity will take 2-3 hours from arrival at the jetty. This includes
physical mounting of the survey equipment on the vessel, installing the dry end
equipment in the cabin, laying of the cables between the components and
interfacing the various items of ancillary survey equipment to the GeoSwath.
Ancillary equipment will include a positioning system (GPS), a heading reference
(gyrocompass), a navigation computer and a sound velocity profiler. The GPS
aerials are attached high on the vessel superstructure, and all offsets between
the various sensors are measured and entered into the GeoSwath software. System
checks are run to ensure all the equipment is communicating and the vessel can
then set off to begin calibration.
In the cabin the GeoSwath processing
unit has the footprint of a desktop PC and the ancillary equipment (GPS and
navigation computer) take up little more space, so it can all be installed at a
single surveyor’s station. If the vessel does not have mains power then the
system will run off a small (3KVA) generator and un-interruptible power supply
(UPS). A helmsman’s display is usually also set up to aid survey line
navigation - an example of the layout in a small vessel cabin can be seen in the
image.
Swath bathymetry surveys done in shallow
waters will typically be aimed at mapping fine scale features, requiring higher
resolution and more demanding accuracy requirements than most deepwater
surveys. The shallow water survey will usually use bin sizes of 1m so the
survey sonar needs to be able to detect all features of this size and larger.
These features must be located sufficiently well by the sonar to allow accurate
charting and re-visit for further investigation. Confidence of detection of all
such features in the survey area is important and coverage must be complete in
the area specified. Most surveys will also require an estimate of the accuracy
of the depths found. There are several internationally recognised standards for
shallow water surveys which incorporate these requirements, the most widely
quoted being the International Hydrographic Organisation Special Publication
S-44 (IHO 1998).
An example of adapting the S-44
standards is from the Swedish Maritime Administration (SMA), in the “Swedish
Implementation of S-44”, often referred to as SS-44 (Jakobsson 2000), which was
put into practice by the SMA in May 2000. This emphasizes that the SS-44
standards are “Requirements on survey accuracies, seafloor coverage and the
possibility to identify minor features including accuracies in positioning of
navigation aids and important features”. An Exclusive Order is defined for
areas of particular sensitivity, with allowable depth errors of about half those
in S-44 Special Order. The SMA’s SS-44 states that 100% coverage is to be
achieved, and that at least Special Order should apply to all Fairway areas from
0-20m deep. All Nautical Objects over 1m in extent should be detected, and an
object’s depth should be positioned to better than 1m horizontal accuracy (this
is beyond IHO S-44 requirements). The depth accuracy is, as in the IHO
standards, required at the 95% confidence level.
Comparisons have been made between
surveys from a GeoSwath and two single beam echosounders over an area 130m by
100m by about 10m deep (Hogarth 2003). This found that the distribution of
differences between a GeoSwath and a single beam echosounder survey was similar
to the differences between two single beam echosounder surveys. The differences
in both cases are due to small scale variations of the depth within the sonar
footprint, and amount to no more than a few cm. The histogram of the grid
differences between the various surveys from this report is shown in the figure.
Consider a regional coastal survey where
the water depth is typically 5m but may shoal to 3m, with 1m binning required.
A 100% coverage survey is required, and the shoalest points must be found.
To overcome some of these limitations a
dual head configuration could be deployed. This is an expensive option as the
beamformer’s transducers are complex and bulky, and it provides little extra
useable data as the across-track footprint of a 1.5 degree beam in 5m water
depth is over 1m at the edges of a 138 degree swath (corresponding to about 5
times water depth).
The high resolution bathymetry from the
GeoSwath combined with the side-scan data output also gives two key advantages
in detecting features anywhere in the swath. Multiple depth hits in each bin
give confidence that the depth is accurate and that features of the scale of the
bin size are being accurately represented while the side-scan gives a visual
image of the sonar return at high resolution across the whole survey swath.
This side-scan image can be used as a direct check that no features were missed
by the bathymetry. Again the IHO S-44 Special Order standard requires “It must
be ensured that cubic features greater than 1m can be discerned by the sounding
equipment. The use of side scan sonar in conjunction with a multibeam
echosounder may be necessary in areas where thin and dangerous obstacles may be
encountered.” With the GeoSwath the side-scan is co-registered with the
bathymetry so they can be used in conjunction to give confidence of coverage and
to help identify features. This capability has also been used by surveyors to
help interpret the bottom sediment type in the survey area.
The GeoSwath’s swath width is not
limited by field of view issues as the pair of transducers gives coverage of
over 250 degrees. In flat shallow areas this allows very wide swath widths,
limited only by the sonar characteristics of the seafloor at low incident
angles. Typically the swath width is limited to about 12 times water depth,
when the scattered returns at very shallow grazing angles become too weak for
phases to be accurately measured. In very shallow situations this limit can be
exceeded (in 2m water depth swath widths of 40m or more have been seen, although
again this depends on the bottom type). In situations where the seafloor is not
flat, for example navigation channels, estuaries, harbour walls, beaches, and
river banks, the GeoSwath’s wide angle of view allows data to be collected right
up to the shoreline. This wide field of view also means that the whole of the
water column in the survey area has been ensonified, giving confidence of
detection of any shoal features or structures.
Simultaneous returns from two directions
arise in shallow waters at the edges of U-shaped channels and by harbour walls.
This is where the range to the vertical structure is the same as the range to
the nearby seafloor, so the two returns can interfere at the transducer and
cause errors in the phase decodes. The multiple return staves in the GeoSwath
transducer give several redundant measurements of the angle, so inconsistent
phases caused by interference can be detected. However, phase errors will only
occur if the two returns are of very similar intensity. In practice the
intensity of sound returned from a patch of seafloor or harbour wall varies
rapidly (as can be seen in side-scan plots). While interference will cause some
data points to be rejected by the phase processing, there are many points where
one of the returns is much more intense than the other and can be used. The
rapid sample rate of the GeoSwath means that even with several points rejected
the data density will be high, and this can be seen in images of survey data of
harbour walls.
