BATHYMETRIC SURVEY UTILISING SINGLE AXIS
SCANNING SONAR AND MS 1000 PC-BASED SONAR PROCESSOR
Bathymetric surveys are typically conducted using a fathometer mounted on a vessel, and running a set of pre-plotted track lines within a selected area. This approach is acceptable for many hydrographic survey applications. It fails, however, to address real-time dredging operations, or surveys close to docks or marine installations when very detailed depth information is required adjacent to these structures.
In a typical hydrographic survey, the spacing of survey grid lines is generally determined by the following:
• the desired density of acoustic coverage
• the ability of the survey vessel to maintain track
• the beam width of the fathometer
• the seabed gradient
• structures (man-made or natural)
Rarely is a line spacing of less than 5 metres used.
With this conventional survey method it is impossible to obtain depth information under any floating equipment, and there are physical limitations in how close the survey boat can approach another vessel or marine structure.
The following information examines this specific bathymetric survey problem, and provides a solution to obtaining highly accurate, and repeatable bathymetric measurements from a fixed position.
The accuracy of any bathymetric survey is critically dependent on a combination of acoustic and positioning factors. There are points effecting “accuracy”, some of which include:
In-water Speed of Sound:
A sonar simply measures its echoed returns in time. The time from the sound transmission to receiving the echoed return is halved and that value is multiplied by the in-water speed of sound to produce a one-way measurement. Any variance in the actual speed of sound to the propagation value used by the algorithm in the sonar to calculate range will result in some measurement error. Knowing the in-water sound speed is important, regardless of which type of sonar system is used.
Transducer Beam Width/frequency:
Transducers are specified as to their frequency and beam patterns. The general rule-of-thumb is that the higher the frequency the better the resolution; the narrower the beam pattern the smaller the acoustic footprint. The typically frequency range of survey fathometers is from 35 kHz to 210 kHz.
Beam angles of a single beam survey fathometer average between 7 to 10 degrees, although some high frequency fathometers are equipped with even narrower beams (3º-5º).
The beam angle refers to dimensions between the outer limits of the main lobe of the received signal (which is determined by the size and shape of the transducer). It is possible, however, to detect signal through side lobes; these lobes are an inherent (but unwanted) component of a transducer. Although side lobe sensitivity is significantly less than the main lobe, they are still able to detect strong return side echoes and result in an incorrect range measurement in the plotted or digitized return. This is a critical issue when working adjacent to structures. When working in close proximity to a dock or bridge pier a conventionally mounted fathometer transducer will likely record the acoustic side echoes from the structure’s vertical sides – and not actual distance to the seabed below.
Another component of the problem is the relationship between a very narrow transducer beam pattern and the realities of utilising a conventional survey fathometer from a moving vessel. The fathometer beams are designed to be wide (7-10 degrees) because the gear must operate from a platform which is subject to pitch and roll. A very narrow acoustic beam works against the user if the vessel is rocking back and forth. Although a wider beam will provide increased acoustic coverage, very fine detailed seabed relief data is lost because the sonar’s acoustic footprint covers too large an area. This is much like trying to use a wide roller to paint a detailed picture.
A major factor to the accuracy of any survey is understanding the relationship between the positioning (navigation) and where the position of the acoustic data is plotted.
The accuracy of positioning systems used in offshore survey operations typically ranges from 0.5 to 3 metres horizontal (depending on various factors that include the vessel’s proximity to land and the design and accuracy of the equipment employed). This error may be acceptable for offshore operations; however, surveys close to fixed structures or real-time dredging operations often demand a much higher level of position resolution. In a conventional survey, navigation control and side echo interference problems are magnified when utilising a bathymetric survey vessel near marine structures or dredge barges. Navigation signals are often blocked and/or suffer from multi-path reflections; the acoustic returns are subject to both side lobe and multi-path
interference, and the ability to steer the survey vessel on a given course is typically restricted. Consequently, accurate bathymetry coverage using a survey vessel within 5-10 metres of any marine structure or vessel has a history of being poor.
The result of combining inherent limitations in positioning and tracking a vessel, and the acoustic geometry – for the application being addressed herein – is unreliable bathymetric data.
Utilising a combination of onshore and offshore survey techniques, and employing the right equipment will provide the solution to the problem of detailing sediment deposition, or scour and erosion around structures, or material removal during dredge operations. With this combination of techniques and equipment, highly accurate, and repeatable bathymetric measurements can now be obtained.
Overview of the Solution Requirements:
The offshore survey navigation problem (with a typical accuracy of 0.5 – 3 metres) necessitates using an alternative positioning method to measure the location of the sonar head, and its reference azimuth.
The solution to the positioning component of the problem is relatively simple because the survey is within sight of the shore (or structure), which permits the use of highly accurate land-based survey methods and equipment.
With respect to the sonar and positioning control there are four main rules to follow:
1. Keep the sonar head stable during the data collection process so that its location and azimuth can be accurately positioned.
2. Position the 0º reference of the sonar head so that it is pointed vertical.
3. Utilise a higher frequency sonar with a narrower beam pattern.
4. Position the sonar in an optimum geometric arrangement – mid-water or near the seabed to minimise acoustic side bounce effects.
The hardware required for this survey solution is readily available.
Conventional survey equipment (measuring tape, transit, EDM) is all that is needed to fulfil the navigation component of this survey solution. When RTK GPS is available, it will permit decimetre level geographic positioning. Remember that conducting survey operations around bridges will often require that you work under the deck structure that will likely block or at least limit satellite coverage.
A narrow beam, high frequency scanning sonar is used instead of a conventional survey fathometer. Kongsberg Mesotech manufactures 1071-series scanning sonars designed specifically for high-resolution profiling and imaging applications. Although there are several
available frequencies the equipment recommended for this application operates at 675 kHz, and has a conical beam pattern of 1.7 degrees (single axis sonar).
A mechanical deployment arrangement is required to rigidly offset the sonar from the vertical face of the structure, or below the hull of the dredge barge.
Using the MS 1000 allows the image and profile data to be recorded to the PC’s hard-drive or printed in real-time or from the recorded files using a variety of Windows-supported printers. The MS 1000 also features the capability of time-tag recording external sensor data (input into the the PC running the MS 1000 program via serial or USB ports on the PC).
The survey procedure will vary slightly depending on whether the application is for dredging operations, or a survey close to a fixed structure.
A typical field operation’s procedure is as follows:
• Determine what co-ordinate system will be used for the survey.
• Establish an optimum profile pattern and determine the ideal position for the sonar head (horizontal and vertical offset from the structure or barge) required to obtain the desired acoustic coverage. This may require several positions.
• Pre-plot profile lines and their respective identification numbers.
• Prepare a mechanical mounting frame for the sonar head.
It is important that the mechanical deployment system allow the sonar head to remain rigid and stationary during data acquisition. The arrangement must allow X, Y, Z position, and the zero reference azimuth of the head to be accurately determined.
Mechanical deployment systems used for this type of operation tend to follow three basic design arrangements:
1. Pole/frame deployment
2. Cage/tripod deployment
3. Depressor/”Bat Wing” deployment
Note: In situations where the vertical angle of the sonar head is subject to some movement, a Motion Reference Unit (MRU) should be used to record the pitch, roll, and heave of the sonar head.
• Deploy the sonar using the selected mechanical arrangement.
The sonar head typically is positioned 2 to 15 metres above the seabed and the same range away from the face of the structure (position may vary depending on mechanical arrangement, sonar coverage, scanning range, and water depth).
• Ensure the orientation of the sonar head is recorded along with all positioning information.
At least one of the personnel conducting the survey must understand basic co-ordinate systems and land surveying requirements. A junior level engineer or survey technician is recommended for this position.
• Complete the sonar scan and verify that the data was recorded.
Each scan will take approximately 20 seconds (or less) to complete. Sonar range and scanning speed will change the time required for each scan. However, at 20 metres range and at the highest resolution the time required will be less than 1 minute.
An azimuth change of 5 degrees is typically used to generate a star-pattern of profiles from a single position (this azimuth change must be mechanically completed.
• Complete the star-pattern of profiles and recover the sonar head.
• Use BathyXYZ (a stand-alone program available though Kongsberg Mesotech) to process the recorded MS 1000 profile points to create files in an X,Y,Z format. This format is used in most off the shelf 3D data rendering programs.
• Record the water (tide) level during profile data acquisition.
Scanning Sonar Bathymetry Method Advantages and Disadvantages:
The advantages of this bathymetry method include:
• The survey can be repeated at any time without specialised hydrographic equipment or personnel.
• The measurement resolution exceeds conventional bathymetric survey methods (leading edge detection of 2.5 mm at ranges to 25 metres).
• The survey can be completed without interruption to vessel traffic (there must obviously be vertical distance between the keel of the vessel and the seabed if a vessel is at dockside).
• The amount of data coverage exceeds conventional bathymetric survey coverage.
• The overlap of profile coverage permits better data correlation and confidence.
• This survey method is typically less expensive than conventional bathymetric surveying.
• The method greatly increases positional accuracy, and minimises acoustic side lobe
• The sonar position can be set to avoid surface aeration or fresh/salt water interface problems.
The disadvantages of this bathymetry method include:
• Data accuracy is critically dependent on keeping the sonar head stable during each profile; if it is not stable, an MRU must be used to record the sonar head movement.
• The X, Y, Z position and zero reference azimuth of the sonar head must be accurately calculated.
• The signal range is limited to the “acoustic line of sight.”
• The method described is not currently well known as a solution to this type of bathymetric survey.