MAPPING FISHERIES HABITATS BY ENHANCED MULTIBEAM ACOUSTIC DATA IN ALASKA

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1 1.0 ABSTRACT MAPPING FISHERIES HABITATS BY ENHANCED MULTIBEAM ACOUSTIC DATA IN ALASKA Jerry Wilson Fugro Pelagos, Inc. Bill Gilmour Fugro Pelagos, Inc. Dr. Gary Greene Moss Landing Marine Laboratories New acoustic data processing methods developed by Fugro Pelagos, Inc. (FPI) have greatly improved the ability to display textural characteristics of the seafloor. This paper illustrates how digital multibeam swath bathymetry and acoustic backscatter images can be used to produce higher resolution for applications for developing marine benthic habitat maps. These techniques may resolve unique bottom features such as structural geology, faulting, columnar basalts, sedimentary bedrock, glacial erosion and other features that have been found to be important types of bottom fish habitats. Data examples of all of these features are shown with correlative groundtruth data showing various target species. Recent multibeam bathymetric mapping of the eastern Fairweather Ground of Alaska using snippet processing of Reson backscatter data has provided the Alaska Department of Fish and Game (ADFG) with high-resolution marine benthic habitat mapping that may refine the way rockfishes are managed in southeast Alaska. FPI s unique usage of multibeam echo sounding backscatter processing software; Reson power, gain, transducer beam pattern corrections; and leading edge data collection tools, such as FPI Precise Timing and the Applanix TrueHeave, result in especially useful acoustic representations of the benthic seafloor environment. A flow diagram of data collection and processing that results in superior positioning of the acoustic data is presented. This systematic approach ensures that the geographic location of the backscatter image is precisely co-located with the sounding data from the multibeam echo sounder (MBES), resulting in a true 3-Dimensional display of the seafloor and the substrate. Comparative examples of these data products and their application are provided. 2.0 INTRODUCTION The recent key recommendation of the United States Commission on Ocean Policy to address marine resources from an ecosystems management perspective underscores the desirability of quantifying fundamental ecosystem resources such as habitat. Fisheries managers and scientists of ADFG have been using enhanced seafloor acoustic mapping technology to do exactly this to assess benthic habitats (see References at end). This work was funded by the National Oceanic and Atmospheric Administration (NOAA). For example, Essential Fish Habitat (EFH) is a term introduced in the Magnuson-Stevens Fishery Conservation and Management Act (FMCA) in the 1990s to promote understanding of the impacts of activities affecting the spawning and nurturing areas of offshore, anadromous and estuarine managed species. With resources being over-fished, it was apparent that the National Marine Fisheries Service (NMFS) could not rebuild the fisheries only by controlling harvest, but needed to influence activities that impacted recruitment. The technology described in this paper contributes to the assessment of what habitat is essential and to the overall quantification of the resources that support fisheries. These examples are from benthic habitat surveys conducted over two seasons over the eastern Fairweather Ground, Figure 1.

2 Fairweather Grounds Figure 1. Location of Fairweather Ground survey area This survey area is about 10km by 27km, ranges in depth from 50m to 160m, and is shown in Figure 2. It is characterized by both hard and soft bottom, and both habitat types have a considerable diversity in detailed features. This variety of habitat characteristics can result in a wide range of fish species distributions. Figure 2. Eastern Fairweather Ground survey area showing bathymetry from multibeam echo sounding There are several benefits to conducting benthic habitat surveys using multibeam echo sounder technology. Principle values include complete ensonification of the seafloor and acoustic backscatter information that is co-registered with the depth values. This leads to data products that give especially

3 useful depiction of seafloor habitats. Further, the seafloor features are more accurately positioned than is possible using a towed sensor. From an operational point of view, the surveys can be conducted at higher speeds than with a side scan sonar towed at depth. This results in overall efficiencies, which are especially beneficial in operational areas like the Gulf of Alaska where weather windows are brief. 3.0 DATA ACQUISTION AND PROCESSING 3.1 Enhanced Multibeam Echo Sounding Recognition of the superior data and coverage provided by MBES has been widespread. In addition to the significant value of ensonification of a swath, as compared to a profile of bathymetric data, the acoustic data have been frequently presented as pseudo side scan sonar. Improved technology and increased bandwidth from the sonar head since the pioneering days of Simrad in the 1980s have provided the opportunity to develop enhanced acoustic imagery by manipulation of the backscatter information within each MBES beam. This work has resulted in improved MBES data products using new acquisition and processing techniques. Working closely with Reson, Inc. and Triton Elics, Inc., starting in 2001, FPI jointly added the capability to record the raw backscatter data from each beam for each ping of the Reson 8000 series systems. This is colloquially referred to as snippet and our in-house terminology is Footprint Time Series (FTS). The FTS data produced by a Reson system is raw. Each beam has a time series for each ping. The data are not altered by the system in any way. Simrad MBES also log backscatter data in a format that contains a distinct time series for each beam. Initial products were stitched together from each beam within the Reson system, providing a highresolution image that was exported and processed as a side scan sonar record. Continued developments enabled processing of the full captured FTS resulting in further improvement in image quality and more automatic processing techniques. Recent developments have resulted in additional enhancements to the MBES data by improved timing and Applanix TrueHeave implementation. 3.2 Acquisition Improved Timing Accuracy in Multibeam Acquisition Recent developments have resulted in additional enhancements to the MBES data by improved timing and Applanix TrueHeave implementation. Timing in multibeam acquisition and processing refers to the ability to accurately synchronize multibeam sonar data with navigation, attitude and heading data. Small errors in timing can occupy a large portion of the overall error budget. These errors can be made immeasurably small using a new timing scheme developed by FPI. Traditionally, time stamps within XTF files were assigned by the acquisition hardware. A millisecond counter within the acquisition package kept track of all incoming data streams and was used for synchronization. This allowed varying latencies to develop and remain unaccounted for. These latencies result in positional and attitude errors, giving a rippled appearance to sun-illuminated DTMs. An improved scheme has been developed and implemented by FPI for use in all of its hydrographic surveys.

4 The new approach requires the following systems: Reson SeaBat Applanix POS/MV TritonElics ISIS (XTF) CARIS HIPS Modifications were made to the POS/MV firmware, as well as the ISIS and HIPS software and XTF file structure, to support the new timing. Under the new scheme, the POS/MV is taken as the physical and temporal center of the universe, producing zero latency between the position, heading and attitude records. Millisecond counters within the acquisition package are ignored. There is a small but constant latency between the ping time and the position, heading and attitude. This latency is easily identifiable and accounted for. In the resulting data, timing errors are essentially eliminated. Results from the timing scheme have been very positive. Tests demonstrate that IHO special order accuracies can be maintained in rough sea condit ions and swells. Details of the new scheme, including system topology and interface control specifications, have been provided to all of the MBES community. The modified firmware and software is open and available to all users of the specified equipment. 3.3 TrueHeave Application The implementation of the enhanced timing paved the way to integrate Applanix TrueHeave. This added technology for benthic habitat mapping improves sounding accuracy and permits zero latency data acquisition. All data are precisely stamped on the UTC epoch. The result is illustrated in Figure 3. Figure 3. Illustration of TrueHeave improvement in data accuracy; vertical amplitude of graph about one metre. TrueHeave also provides efficiency gains in the field. It reduces the time needed in survey turns. Further, it allows for a larger weather window of data acquisition. 3.4 Data Processing Registration of the backscatter data with the across-track bathymetry is based on the intersection of the slant range with the digital seaflo or 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. 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

5 (i.e. 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. In deeper water, the resolution of some side-scan sonar systems will be better than current multibeam systems due to the frequency required and also footprint size. Positioning of the FTS data will be more accurate than the side-scan mosaic. In shallow water, the systems look similar. Positioning on the FTS mosaic will be more accurate than the side-scan mosaic. The FTS mosaic will also be free of surface return and water column noise. Reson 81xx Multibeam Sonar GPS Position POS MV Motion Reference Bathymetry and Backscatter WinFrog Navigation and Line Planning WinFrog Raw File Navigation Chart Attitude and Heading Triton Elics ISIS Sonar and Multibeam Acquisition Position XTF File XTF File CARIS Process Bathymetry Bathy and Flags Fugro Pelagos Snippet Processing Terrain Corrected Imagery Bathymetric Surface Models Backscatter Mosiac Figure 4. Flow diagram of data collection and processing 4.0 RESULTS These techniques have been applied to the surveying of eastern Fairweather Ground in Alaska for the purpose of benthic habitat mapping for management of fisheries resources. This survey was for ADFG, and was funded by NOAA. The data results were interpreted and classified by the Center for Habitat

6 Studies and Moss Landing Marine Laboratories, California. Please see References for information about the seafloor classification scheme. The survey was performed using a Reson 8111 MBES. The acoustic backscatter data were processed to 0.5m and 5m bins according to the way they were to be utilized. 4.1 Benthic Habitat Data Examples The bathymetric data from the survey is a very valuable set of information even without the backscatter. As shown in Figure 5, the seafloor morphology can be seen in great detail. This digital ele vation model provides the framework for the high-resolution acoustic imagery. Figure 5. Detail of the bathymetry results in the eastern Fairweather Ground fishery area, as illustrated by colorbanded depth differences and shaded relief. The processed acoustic backscatter image for the same area as Figure 5 is shown in Figure 6. The mosaicked survey swaths build up the image of the acoustic contrasts of the seafloor habitats. Some pronounced differences in sedimentary habitat are seen as well as in the reefs. While the emphasis of this survey was on rockfish, it clearly has value for demersal fish, infauna and epifauna habitats as well.

7 Figure 6. Processed acoustic backscatter image of the eastern Fairweather Ground fishery area With the high resolution of the acoustic image, details of the rocky reefs and sedimentary seafloor are clearly seen in Figures 7 and 8. These two images illustrate a pronounced difference in the habitat roughness, or rugosity, which is a key parameter for distribution of different fish species.

8 Figure 7. Contrast in seafloor rugosity is depicted in this detailed portion of the overall backscatter data set. Figure 8. Contrast in seafloor rugosity is also depicted in this detailed portion of the overall backscatter data set.

9 Moss Landing Marine Laboratories interpreted the combined bathymetric elevation model and acoustic backscatter imagery for differences in benthic habitats. The overall classified seafloor habitats are shown for the survey area (to 2003 data coverage) in Figure 9. Figure 9. Color-coded benthic habitat map produced by Moss Landing Marine Laboratories, using their seafloor habitat classifications described in the References. Details of the southern portion of the Figure 9 habitat map are shown in Figure 10, where the benefit of draping the interpretation over the bathymetry surface is clear.

10 Figure 10. Detail of benthic habitat classification draped over the bathymetry surface. Results from the latest survey in 2004, which focused on extending the data set over the volcanic cone, have resulted in spectacular imagery of the seabed. Work is ongoing in the interpretation and classification of this data. 4.2 Groundtruth Observations Figure 10 includes an arcuate structure that was inspected using a manned submersible (Figure 11) and found to have columnar basalt at its apex. Fisheries biologists have seen an association of some target rockfish species with this type of underwater terrain. Figure 11. Delta manned submersible used for groundtruth observations of fishery habitats

11 The ADFG fisheries scientists and managers and the Moss Landing Marine Laboratories geologists planned groundtruth observation operations using these high-resolution seafloor maps. The detailed distribution of the various habitats meant that the limited submersible time could be effectively focused in a range of habitats without undue duplication of effort. The scientists made direct observations and recorded them using underwater photography, as illustrated in Figure 12. Figure 12. Underwater photographs of habitat and fish species from a submersible dive on the arcuate feature, which proved to have volcanic substrate 5.0 CONCLUSIONS The net benefits of multibeam echo sounding with backscatter when these technology enhancements are implemented include: Improved signal-to-noise ratio compared to side scan sonar No water column noise Precise co-registration of imagery with bathymetry Allows mosaicking of data at increased resolution Increased accuracies and efficiencies

12 Using the enhanced acoustic seafloor imagery from high-resolution multibeam echo sounding technology can produce useful information for fisheries managers and scientists. Effective habitat assessment starts with effective baseline mapping. Multibeam backscatter technology and mapping techniques are effective first steps toward ecosystem management of marine resources. REFERENCES Greene, H.G., O Connell, T., Erdey, M., Bissarro, J., Brylinsky, C. and Lockhart, D., Marine benthic habitat characterization of a commercial fisheries area, Fairweather Ground, Alaska: a possible technique beneficial to SOPAC. SOPAC Misc. Rpt. 549, p. 23. Greene, H.G., Yoklavich, M.M., Starr, R., O Connell, V.M., Wakefield, W.W., Sullivan, D.L. MacRea, J.E. and Cailliet, G.M., (1999). A classification scheme for deep- water seafloor habitats: Oceanographica ACTA, v. 22, n. 6, p Greene, H.G., Bizzarro, J.J., Tilden, J.E., Lopez, H., and Erdey, M.D., The benefits and pitfalls of GIS in marine benthic habitat mapping. Amer. Assoc. for the Advancement of Science Annual Meeting, Seattle, (in press). Greeene, H.G., Bizzarro, J.J., O Connell, and Brylinsky, C.K., (in review). Mapping Potential Marine Benthic Habitats in Geographic Information Systems : a classification scheme. In Greene, H.G. and Todd, B.J., Mapping Marine Benthic Habitats, GeoHab Volume 1, Geological Association of Canada, Special Volume (in prep.). Greene, H.G., O Connell, V.M., Brylinsky, C.K., and Bizzarro, J.J., Habitat Mapping of Eastern Fairweather Ground, SE Alaska: An Important Commercial Groundfish Area, 13 th Western Groundfish Conference, Victoria, B.C., Cananda. (in press).