CHAPTER 6 RESULTS FIGURE 8.- DATA WORK FLOW FOR BACKSCATTER PROCESSING IN HYPACK

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1 CHAPTER 6 RESULTS 6.1. Backscatter Workflow Comparison Currently, INOCAR owns and operates RESON and Kongsberg multibeam systems for nearshore surveys. The RESON system is integrated with HYPACK Hysweep software, while the Kongsberg systems are integrated using the Kongsberg Seafloor Information System (SIS) onboard the survey vessels. However, the software used for processing multibeam data are CARIS HIPS&SIPS and FLEDERMAUS. In this context a brief description of the process used in this study and the Geocoder implementation of each software is provided below using the conceptual design of Geocoder described by Fonseca and Calder (2005 and 2007) HYPACK Workflow FIGURE 8.- DATA WORK FLOW FOR BACKSCATTER PROCESSING IN HYPACK The GEOCODER implementation in HYPACK has been conceived as a continuation of the bathymetric workflow. For that reason all the survey lines were reformatted to HYPACK files (. HSX) using the tool called Side Scan Data Reformatter. Then using HYSWEEP Editor the bathymetric workflow is followed to generate a (.GSF) file for each survey line. The GSF 27

2 files are the primary source for the GEOCODER workflow. Currently the HYPACK implementation supports data from RESON and Kongsberg systems in their three record modes: Side scan, beam average intensities, and beam time series data. Once the GSF files are created it is necessary to set the configuration under three software options: Calibration Parameter, Program Option and Mosaic Option. Survey information is entered into the Calibration Parameter Option. The Program Option setting defines the Angular Range Analysis and Statistic Analysis. Under the Mosaic Option information to define mosaic limits, pixel size, blend style, all the corrections that will be applied to backscatter,r and the Angular Variation Gain mode are set. TABLE 10.- MULTIBEAM SYSTEM OFFSET. Transducer Configuration Inertial Measurement Configuration X Offset 9.04 m Roll Offset -1.6 Y Offset 0.00 m Pitch Offset 2.4 Z Offset 1.31 m Yaw Offset -1.1 While Calibration Parameters were not read automatically from the survey files, it was necessary to set them manually with the values shown in table 10. Correction options applied were Transmission Power and Reception Gain, Area Correction, Spherical Spreading, Lambert and Splines Slant Range. In the Assemble Method both sides of the swath were selected, and Adaptive algorithm with filter size of 300 samples for AVG option was chosen. Although all those corrections were selected in the software there is no way to document and verify that they have already been applied. The resulting mosaic shows many artifacts both along and across track, and there is also a line out of contrast with its neighbor lines; preliminary analysis indicates that there is 5dB offset between each this line and the others. The next step in the Data Workflow for backscatter is the creation of the beam pattern correction file. The correct extraction of the beam pattern correction is one of the keys to a good result for the seafloor classification, and it is a necessary step to conduct the Angle Range Analysis. This extraction should be conducted over a patch of seafloor where the nature of the 28

3 seafloor is known (figure 10, left). The intention is to establish the difference between the theoretical beam pattern over a type of sediment against the real beam patten developed by the multibeam system, and then apply the corresponding correction throughout the corrected backscatter time series. Contrast Artifacts FIGURE 9.- BACKSCATTER MOSAIC GENERATED USING THE HYPACK IMPLEMENTATION OF GEOCODER. The patch of the seafloor used to extract the beam pattern correction is located between the ping number 800 and 1150 from line number 9 of the survey collected on June 13, As established by the analysis of the bottom samples, the sediment type present in this area is fine sand. HYPACK is able to extract the beam pattern correction files only when the histogram extracted from the data has been established. The software implementation used is designed to 29

4 determine automatically the decibel range of the entire data set. This decibel range is associated with a histogram that is used during the mosaic construction, Angular Range Analysis, statistics analysis, and also when GEOCODER assigns a spectrum of color-codes across the range. In this project, the first approach to beam pattern extraction failed. HYPACK was unable to create the files from Kongsberg.all files converted to HYPACK.gsf files. This problem was overcome by recalculating the db Shift tool under histogram option from the main menu. FIGURE 10.- HYPACK EXTRACTION OF THE BEAM PATTERN CORRECTION The beam pattern graphic (figure 10, right) shows three different curves: the blue one corresponds to the model, the yellow one correspond to the multibeam system pattern, and the red one correspond to the difference between the previous ones. This difference value correction is applied to the entire data set during the Angular Range Analysis (ARA) and the results are illustrated in figure 11 (lower right). For context, the results were exported to a (.*DXF) file shown in the figure

5 The final product shows some inconsistent lines when compared with neighbor lines, but most of these are lines run as cross check lines, to fill gaps in coverage, or lines where the backscatter intensity fell outside the range of the project s histogram. Although the graphic product shows different type of sediment, it is somewhat difficult to select segmented sites with the same angular response. FIGURE 11.- HYPACK ANGULAR RANGE ANALYSIS RESULTS CARIS Workflow The GEOCODER implementation in CARIS has also been done in such a way that it is a continuation of the bathymetric workflow. Once the Bathymetric Surface is created, the data are passed to a quality control function before starting with Imagery Processing in order to eliminate undesirable data such as depth outliers. Currently the CARIS implementation supports data 31

6 sources from RESON and Kongsberg systems in their three record modes: Side scan, beam average intensities, and beam time series data. The intensity data from each survey line is converted into a Geo-referenced Backscatter Raster (GeoBaR) in order to be assessed in geo-referenced space before final mosaic creation. Before creating the GeoBaR for each survey line, Beam Pattern Correction Files are needed. These are created from a survey line gathered over a flat seafloor with known sediment type. Creation of the beam pattern correction files require the preliminary creation of its GeoBaR without beam pattern correction. Once the beam pattern correction file is obtained, it is applied to the entire data set and the initial GeoBaR is rebuilt. FIGURE 12.- BACKSCATTER MOSAIC GENERATED USING THE CARIS IMPLEMENTATION OF GEOCODER. The patch of the seafloor used to extract the beam pattern correction is the same area used in the HYPACK analysis and is located between ping numbers 900 and 1350 from the line number 9 collected on June 13, As established by the analysis of the bottom samples, the sediment type present in this area is fine sand, and the adjustment was done using the Set Sediment Class tool. The survey parameters are entered when the vessel file is created for the bathymetric workflow; in that all the corrections are applied during the GeoBaR creation, the following correction and setting were selected: Processing Engine, Geocoder; Source Data Type, Time 32

7 series; Slant Range Correction, Beam Pattern Correction; Angular Variation Gains, Adaptive; AVG size filter, 300 samples; Despeckle, Moderate Method. Once the final GeoBaR was created there were no large contrast differences between lines, thus permitting the creation of the mosaic. This mosaic is an extra layer of the Bathymetric Surface created during the Bathymetric workflow. Compared to the backscatter mosaic obtained with HYPACK implementation, this mosaic shows less across-track and along-track artifacts, and also shows three areas with distinctly different intensities. FIGURE 13.- CARIS EXTRACTION OF THE BEAM PATTERN CORRECTION Although this implementation includes the Angular Analysis Tool and also Sediment Classification Analysis, the final results are stored into the GeoBaR files. If a graphic product is desired, it is necessary to use a different software focused on graphic solutions. The information obtained from the Angular Range Analysis can be exported to ASCII files. 33

8 FLEDERMAUS Workflow In contrast to the HYPACK and CARIS implementations, the FLEDERMAUS implementation is independent of the bathymetric workflow and because of this parallel implementation it has become a more flexible tool. Currently the FLEDERMAUS implementation of GEOCODER supports data sources from RESON and Kongsberg systems in their three record modes: Side scan, beam average intensities, and beam time series data. FIGURE 14.- FLEDERMAUS WORKFLOW OF GEOCODER IMPLEMENTATION A notable factor in this implementation is that all the survey parameters are read directly from the survey line files, while the Processing Parameters are divided in two categories. Under the Backscatter category, the following options were selected: Tx/Rx Power Gain Correction, Beam Pattern Correction, and Keep data for ARA analysis; Backscatter Range calibrated and Beam Angle Cut off between 0 and 90 degrees. Under AVG category the adaptive algorithm with a window size of 300 samples was selected. Mosaic Parameters used for the project were: fifty percent of line blending, blend mosaic style and db Mean filter type. Once these setting were applied, FLEDERMAUS created the mosaic. 34

9 FIGURE 15.- FLEDERMAUS MOSAIC OF CORRECTED BACKSCATTER As shown in figure 15, the backscatter mosaic is free of across- track and along-track artifacts. The Beam pattern correction file may be created before or after running the Angular Range Analysis. In this project the beam pattern correction was created after the mosaicing and before the ARA process; the correction file is created by selecting a piece of survey line over a known seafloor sediment type using the segmentation tool. In the final graphic product shown in the figure 16, sites with the same acoustic angular response are easily identified. As the Sediment Classification graphic produced by FLEDERMAUS maintained consistency between cross survey lines, this product will be used to correlate analysis results with ground truth samples taken in the survey area. 35

10 FIGURE 16.- FLEDERMAUS ANGULAR RANGE ANALYSIS RESUTS 6.2. Seafloor Samples Constrain 33 ground truth samples were examined using both the photographs taken during the field activities as well as the physical samples stored at CCOM. The results simulate the field classification conducted by hydrographers worldwide and are shown at the table 11. During the summer 2011 and as part of CCOM project, nine more samples were obtained around Gerrish Island. Samples GI_4.8 and GI_4.9 were located within the interested area (table 10). The procedure employed for this ground truth collection was exactly the same as that used in the previous missions. 36

11 TABLA 11.- GROUND TRUTHING SAMPLES, FIELD CLASSIFICATION Station Latitude Longitude (N) (W) Field Classification GI_ vegetation I_ vegetation GI_ vegetation _ gravel GI_ vegetation GI_ vegetation and rock GI_ gravel and vegetation GI_ vegetation GI_ rocks and sand GI_ gravel GI_ gravel and rocks GI_ gravel GI_ vegetation GI_ sand and rock GI_ rocks and gravel GI_ sand and mud GI_ vegetation GI_ sand, gravel and rocks GI_ sand GI_ sand and mud GI_ gravel GI_ vegetation GI_ sand GI_ sand GI_ sand GI_ sand GI_ sand and mud GI_ sand GI_ sand GI_ sand GI_ sand GI_ rock, gravel, vegetation The video records taken at each station were used to extract a characteristic image that will allow us to differentiate between seafloor environments (figure 16). Both the field classification results and sample location images were geo-referenced and will be further used in this research to compare with the graphic angular range analysis results. 37

12 TABLE 10.- SAMPLES (GRAB/VIDE) POSITION, SUMMER 2011 BETWEEN WEST SISTER AND EAST SISTER ISLAND. Station Lat (N) Long (W) East (x) North (y) Field Classification GI_ rocks and vegetation GI_ rocks and vegetation The images obtained in the project area show many sites covered by vegetation and an extended sandy area. From an anchoring perspective, both for safety of navigation and for environment protection, anchoring practicable is not in the vegetated areas, and the anchoring area delimitation must be confined to the sandy area. To further clarify the results, a bathymetric grid has been combined with the Angular Range Analysis results; this creates a natural way to delineate areas by topography; results are shown in the figure 17. It is simple to follow the perimeter line that differentiates between the dark blue and green areas; and in this way create bounds of an area accepted for anchoring. FIGURE 16.- IMAGES EXTRACTED FROM THE VIDEO RECORDS AROUND EACH GROUND TRUTH SAMPLE STATION. 38

13 FIGURE 17.- COMBINATION OF BATHYMETRY AND ANGULAR RANGE ANALYSIS RESULTS TABLE 11.- LABORATORY ANALYSIS RESULTS FROM THE SELECTED BOTTOM SAMPLES GRAVEL (%) SAND MUD (%) STATION VERYCOARSE COARSE MEDIUM FINE VERYFINE (%) (%) (%) (%) (%) GI_ GI_ GI_ GI_ GI_

14 During the comparison between observations in situ with the GEOCODER results, dark blue areas were associated with seafloor covered by vegetation. The green color corresponds to sandy sites, but the yellow and red spots within this area are related to silt and clay sediment respectively according to the acoustic model used by GEOCODER. FIGURE 18.- LABORATORY ANALYSIS COMPARED WITH ARA ANALYSIS RESULTS Samples over yellow and red spots were verified using the laboratory analysis described in the methodology, and the results are shown in table 11. Comparing the laboratory results and the angular range analysis results show that the entire green/yellow/red area is dominated by fine sand (figure 18), the gravel and mud content are almost the same across the area. Coloration changes between yellow and red show consistency with the sampled increment of very fine sand content. 40