New Developments in Multi-beam Backscatter Data Collection and Processing
Lockhart, Edward Saade and Jerry
Fugro Pelagos, Inc.
About the Technology
Multibeam backscatter sampling
Continuing advances in multibeam echosounder technology make possible additional enhancements to the already valuable swath ensonification. A broad base of users is increasingly utilizing the power of multibeam echosounders (MBES) to address their unique applications; these users include hydrographers, navigators, engineers, fisheries biologists, marine geologists, military planners and mine countermeasures specialists.
We have been working to produce improved data products and presentations for many of these user groups by taking advantage of the acoustic backscatter content from MBES data. The advantages of the backscatter information derive from:
1. precise co-registration of the backscatter with the multibeam bathymetry data set;
2. improved signal-to-noise compared to conventional imaging sonar; and
3. increased resolution (smaller pixel size) for the final products.
These data characteristics are discussed below and examples of various data formats illustrate the results, which are more beneficial to users because they are intuitive images of the undersea environment.
About the Technology
During 2001, Fugro Pelagos, Inc (FPI) conducted extensive MBES operations in the US waters of the Pacific Ocean, collecting over 15,000 sq km of high-resolution multibeam and backscatter data. Working closely with Reson, Inc and TritonElics, Inc, we have jointly added the capability to record the raw backscatter data from each beam for each ping of the Reson 8000 series systems. We refer to this as “Snippet” or our proprietary terminology is Footprint Time Series (FTS).
Registration of the backscatter data with the across-track bathymetry is based on the intersection of the slant range with the digital seafloor profile. Therefore, the backscatter value is placed at the correct depth on an irregular seafloor. More specifically, the final data product will yield the image pixels precisely located in 3-dimensions, a capability developed by FPI to support NOAA-sponsored fisheries habitat and tsunami regional surveys conducted this year.
The MBES collects a series of backscatter records across-track for each ping. These backscatter data are mosaicked on the terrain as noted above using pixel size no greater than 0.1% of the water depth per pixel (ie 5cm pixels in 50 meters water depth). The placing of imagery on terrain results in more accurate placement of the acoustic data. Importantly, the hull-mounted transducer offers significant improvements in the positioning of the sonar beams compared to towed sensors.
To date, FPI have collected and processed backscatter data in the full range of ocean depths, from just a few meters to over 5000, using sonar frequencies of 200, 100, 50, 24, and 12 kHz. While some loss of resolution with depth is to be expected, data quality in all instances has been impressive. The snippets appear to provide useful information at any depth.
Multibeam backscatter sampling
Side-scan sonar data are acquired by forming a large beam on either side of the towfish. Time series data are sampled across these beams by summing all of the returns from any giving time into one pixel. Some multibeam systems have emulated this data through digital beam forming, referring to the resultant image as “pseudo side-scan sonar”. An example of pseudo-side-scan imagery is presented below. The diagram graphically illustrates the single fan-shaped beam pattern with the broad time-arcs that sum into each time sample. The imagery example shows typical side-scan results over a rocky outcrop near Kodiak, Alaska.
Multibeam backscatter is acquired by sampling an individual time series for each beam in the system. Sampling only occurs in the region of the bottom detect rather than throughout the water column (time-arc). Multiple beams may have samples acquired at the same time. However, those samples will be only assigned as backscatter returns their respective individual beam footprint. As a result, the signal-to-noise ratio of the backscatter record is greatly improved. This is demonstrated in the diagram below and the resulting image improvement for the same portion of the seabed off Kodiak Island is illustrated.
In its raw form, the boundaries between the individual beams can be seen in a waterfall display (top image). Once processed, in either the multibeam firmware or post-processing software, the data become seamless and are indistinguishable from side-scan sonar data (bottom image).
The backscatter time series from an individual beam is being referred to as a Snippet by Reson or as a Footprint Time Series (FTS) by TritonElics. Shown here is a snippet from beam 5 (69 degrees from nadir) from a Reson SeaBat 8111. The 8111 is a 100-kHz sonar with 101 1.5-degree beams. The backscatter is sampled at 5kHz. The sample shown here, from an outboard beam, is 0.0292 seconds in length and represents roughly 22 meters of seafloor. The shape of the beam is clearly visible. The low point near the center of the beam shows data clipped in acquisition by an overly aggressive analog gain. Nearer to nadir the FTS is shorter and sharper. At nadir it has the appearance of a single beam echo sounder return.
This represents the backscatter in its most basic raw form. Sonar power, gain, pulse width, spreading loss, etc are logged along with the snippets so that they can be normalized and reduced to a calibrated standard.
All examples below are from the same portion of our Kodiak test range. The area is 4 km East to West and 4 km North to South. Depths range from 140 m in the north corner to 13 meters near the center.
The following example shows color stratification of multibeam bathymetry data combined with sun-illumination shading. The shading shows added relief detail and texture, both of which are valuable for most user groups. This is a fairly common style of data presentation along with monotone sun illuminated images.
Multibeam backscatter imagery is shown in the following data example. This data set is valuable for geological mapping, habitat assessments and for engineering such as submarine cable route selection. The lighter pattern in the southern portion of the image is a sedimentary deposit that would not be detected by multibeam echosounder data alone. The image below was produced at 25 cm resolution. Zooming in shows the image in greater detail. A full resolution version of this image is available here (warning - large image).
Multibeam bathymetry and backscatter data were collected simultaneously, so the co-registration between the two data sets is precise. This allows for excellent integration of both types of information.
The perspective view of the Kodiak test area in the example below illustrates draped imagery, which provides the user with a powerful visualization tool. The precise co-registration makes this image relatively noise-free and unambiguous. This degree of agreement between bathymetric surfaces and seabed backscatter imagery is not possible with typical towed sonar systems.
There is virtually no difference between a standard multibeam installation for hydrographic surveying and an installation for backscatter acquisition. That said, it should be noted that multibeam systems are optimized for bottom detection and not backscatter sampling. So, small amounts of noise that have negligible effect on bathymetry data quality may have a detrimental effect on the backscatter data. For example, a 200 kHz multibeam can be operated simultaneously with a 100 kHz single beam echosounder without any measurable interference in the bathymetric data. However, the interference pattern from the single beam will be clearly visible in the backscatter record.
High frequency (>100 kHz) systems can be installed on small vessels of opportunity either as a hull mount or as a pole mount. Pole mounts are particularly effective in fiberglass or wooden vessels. A hull mount may be more appropriate for steel or aluminum vessels, particularly were rough weather is expected. Acquisition hardware is generally rack mounted to conserve space. These systems are used to survey harbor and coastal areas. The backscatter in shallow water rivals conventional side scan for sediment demarcation and target detection. Particularly where the water depth results in a depth to swath ratio of 10-20%.
|Essential Features of a small boat
Both high and low frequency devices can be installed on large vessels. Hull mounting is generally advisable due to the difficulty in stabilizing a long pole, particularly at increased survey speeds. A hull mount also provides the opportunity to shroud the MBES, reducing noise due to turbulence. Larger vessels may also be outfitted with more than one MBES for operation over a large range of water depths. These installations can become relatively complex.
After some quick processing, either in real time or in the office, backscatter data looks a lot like side scan. Shadows tend to be shorter since the MBES is higher in the water column but positioning is improved remarkably. For many applications, backscatter data can be substituted directly for side scan. The added benefit is that there is a bathymetry layer precisely co-registered with the backscatter layer. As seen in the sample below, from a pier face in San Diego Bay, the bathy layer can be queried while viewing the backscatter layer.
For some applications FPII has delivered bathymetry and backscatter products to clients using a web server. This method allows users without application specific software to view and manipulate the data. The samples shown below are taken from the web server. The bathymetry data (on the left) and backscatter (on the right) were collected simultaneously during a fisheries habitat survey. The value added nature of the backscatter data can be seen during the progressive zooms.
Note the reflectivity variations in the backscatter indicating changing sediment types. The bathymetry alone does not give information on sediment type.
The image below demonstrates the improved resolution of the backscatter data with respect to the bathymetry. Many hundreds of backscatter samples are collected across each bathymetry footprint.
It is also possible to overlay bathymetry and backscatter and create an image using varying amounts of either image. A purely bathymetric model will display the general roughness of the seafloor and provide information on the region geomorphology. Blending some of the backscatter with the bathymetry add information on sediment types and illuminates smaller features. Removing all the bathymetry from the image will produce a standard backscatter mosaic.
Multibeam backscatter can also be used as an inspection tool. The precise positioning and co registration combine to provide excellent images of pipelines (Movie) and other detailed objects. Note the suspension in the pipeline in the final frame of the movie.
Data representations such as those above are useful to all user groups. For example, a biologist does not need to know how to interpret sonar images when the presentations are so well integrated. Engineers can position their project elements with confidence using this informative and spatially correct imagery. Much of the strength of the new processing described above is in results that do not require interpretation because they are so intuitive.
1. Quantitative rather than qualitative: Processing and interpretation can be automated
2. Precisely georeferenced imagery and bathymetry
3. Improved Signal to Noise ratio
4. Increased survey speed (up to 8 knots)
5. Sun-illuminated, color-enhanced processing highlights seafloor or harbor floor features
6. No risk to towed equipment
7. No additional acquisition coast over multibeam
1. Requires precise mobilization for multibeam
2. Demands new processing and interpretation skills
3. Depends on trained on-line operators for good results
4. Backscatter technology is new unknown to much of the survey community