|Diagram showing the relative positions of the Motion Reference Unit (MRU), the interferometric sonar transducers, and the DGPS navigation antenna.|
Interpretation of swath bathymetric data is directly tied to proper system measurements and calibration prior to survey operations. Measurements include: offsets from DGPS antenna(s) to the Motion Reference Unit (MRU) and interferometric sonar transducers; the distance of the transducers and MRU below the water line; the distance in three dimensions between the center of the transducer face and the center of the MRU, as well as the angle the transducer face makes with respect to the reference plane of the MRU and vessel (Submetrix Training Manual,200). Calibration processes consist of: roll, position/navigation latency, heading, pitch, and depth offsets. Various ‘patch tests’ are run prior to survey operations to assess position, time, and angle offsets. Environmental factors such as tidal elevation and the speed of sound within the water column must also be considered. If the calibrations and measurements are not accurate, artifacts may appear within the dataset, hindering interpretation.
Below are a few examples of artifacts within a swath bathymetric dataset due to a roll misalignment (i.e. residual angular offsets that occur after mounting the MRU and sonar transducers on the boat), and absence of speed of sound measurements. For the interested reader, a thorough discussion of problems that may arise when collecting and processing swath bathymetric data are presented by the University of New Brunswick’s Ocean Mapping Group.
|The figures display bathymetric data acquired during a USGS nearshore geophysical cruise in 2002. The depth range is from roughly 7 to 20 meters, as displayed with the color bar. The bathymetric grid on the left contains roll misalignment artifacts, as depicted by the 'striping' within the grid. The figure on the right shows the same grid with the artifacts removed.|
A simplistic view of ‘roll misalignment’ and the resulting artifacts within a swath bathymetric dataset are presented above. The image on the left, shows ‘striping’ resulting from an incorrect angle measurement of the transducer from the horizontal. The image on the right has been corrected by compensating for the misalignment; ‘striping’ is no longer evident within the bathymetric image. The cause of the striping can be explained by referring to the graphic below. Assume the ship on the left is moving into the page, so the swath is emitted at an angle ‘up to starboard’. As the ship turns and moves out of the page, the emitted swath will be ‘up to port’. This ‘see-sawing’ results in artificial highs and lows between adjacent tracklines; displayed as ‘striping’ within the image. The figure on the right is in proper orientation.
Applying incorrect tidal data will yield a similar artifact within the dataset (i.e. artificial ‘highs’ and/or ‘lows’). Thus, it is critical to have accurate tidal measurements, either through direct measurement using DGPS and Real-time Kinematic techniques, tide stations, or use of a tidal model.
Measuring the speed of sound within the water column is critical to any bathymetric survey. This enables us to account for varying water column properties and their effect on the path of the outgoing acoustic ray (e.g. thermocline).
|The solid line shows refraction of the outgoing acoustic ray path as it travels through water masses of varying physical properties. Incorrect speed of sound measurements will cause improper placement of the bathymetric soundings, usually seen as a "smile " or "frown " in an across track plot of data soundings from a flat sea floor. For a detailed discussion of refraction artifacts see the University of New Brunswick 's Ocean Mapping Group (http://www.omg.unb.ca/AAAS/UNB_sea floor_Mapping.html)|
Within the diagram above, as an outgoing acoustic wave, represented by the solid line, travels through water masses of varying physical properties (e.g. temperature, pressure, salinity), the ray bends, or refracts. The change in the physical properties of the water mass cause a change in the speed of the acoustic ray as it moves from one water mass to another. By measuring the variations of speed of sound within the water column, the path of the acoustic ray can be accurately mapped, and the resulting depth sounding placed in its proper position on the sea floor.
Less than 100% Coverage
The WHSC conducts a variety of geophysical studies in many differing locations from nearshore, coastal to deep water environments. The survey plan designed for any study, regardless of environment, is dictated by the research goals, available funds and resources, and survey size. For example, a full (100 %) sea-floor coverage swath bathymetric survey may be required to address research objectives, or widely-spaced bathymetric tracks yielding less than 100 % sea floor coverage may suffice.
|Figure displaying the swath coverage during survey operations. In this example, swath bathymetric and sidescan sonar systems (green object towed at depth) are displayed. The survey is designed to acquire overlapping sidescan sonar data. The swath width of the bathymetric system is less than that of the sonar. In this example, water depth is ~ 15 m; sidescan sonar swath width is 400 m, bathymetric swath width is 100m. Ship on left is moving into the page; ship on right is moving out of the page.|
The image above shows a survey configuration designed to produce a full coverage (100 %) image of the sea floor using sidescan-sonar and a regional view of sea floor topography (< 100 % coverage). Generally, in shallow water (< 30 m), a sidescan sonar will achieve a larger swath width than a swath bathymetric system, enabling a greater area to be surveyed in a given amount of time. For example, in water depths of approximately 15 meters, the sidescan-sonar will achieve a swath width on the order of 400 m, while the swath bathymetric system may achieve 5 7x water depth, or 75 105 m. If the survey were configured to run with overlapping bathymetric tracks (or 100 % bathymetric coverage), survey time would increase 4 fold.
|Figure displaying the actual line coverage of the interferometric system during a USGS nearshore geophysical survey. Water depth was ~ 15 m; trackline spacing was ~ 350m, swath bathymetric coverage was ~ 100m.|
In order to generate a continuous bathymetric image from widely spaced bathymetric tracklines as displayed above, interpolation routines are used to fill data gaps. The example below displays an interpolated bathymetric image and a sidescan-sonar image of the same region.
Viewing geophysical data in three dimensions (or perspective views) by draping the sidescan-sonar imagery over the bathymetric grid greatly facilitates our interpretation of regional backscatter patterns and surficial geology. This data integration (especially in the field) also helps to identify smaller areas of interest that may be subsequently surveyed at 100% bathymetric coverage
Submetrix Training Manual, 2000, SEA Advanced products Ltd., Bath, England.