seanat manual v3 - cbd...seanat+manual,+4january2014+ + 4+ 4 sound+propagation+modelling+...

Centre for Marine Science and Technology .

Subsea Environmental Acoustic Noise Assessment Tool (SEANAT) 4 January 2014

1 Introduction

The Subsea Environmental Acoustic Noise Assessment Tool (SEANAT) provides a suite of tools for the modelling of underwater sound fields associated with anthropogenic underwater noise sources within the German Economic Exclusion Zone (EEZ). The software allows users to configure model scenarios, run underwater sound propagation models in realistic acoustic environments, compute received levels, and visualize the resulting sound fields.

The sections below describe setup and use of the SEANAT software. System requirements and site access are described in Section 2. A brief overview of the system design is presented in Section 3. Finally, Section 4 describes use of the SEANAT site, in the context of a sample scenario.

2 System Requirements and Setup

The SEANAT software resides on a secure web server, and is accessed using a web browser. As such, the only requirements for use of the software are an Internet connection, a JavaScript-‐enabled web browser, and a SEANAT user account.

SEANAT supports most browsers, including current versions of Internet Explorer, Firefox, Safari, Chrome, and Opera. JavaScript must be enabled in the browser’s security settings in order for the SEANAT site to function correctly. JavaScript is enabled by default on most modern browsers, but may be restricted or blocked depending on a user’s specific security settings. If JavaScript is not available, an error message will appear at the top of the Main page upon login. If this message appears, please refer to your browser’s documentation to enable JavaScript or add the SEANAT site to the list of trusted sites.

The SEANAT site is available at the following URL:

http://cmstacoustics.com/SEANAT

Login (Figure 1) requires a user name and password, which may be obtained from the SEANAT administrator.

SEANAT manual, 4 January 2014 2

Figure 1: SEANAT login screen

3 Overview

The SEANAT system includes the following main components:

• Web-‐based graphical interface • User-‐specific configuration and results files • Databases of source spectra, environmental data, and model parameters • A high-‐performance computing environment on which sound propagation models

are run

The user only interacts directly with the first of these components, i.e. the graphical interface. However, the remaining components will be referred to in the sections that follow.

Each user name on the SEANAT system is associated with a secure user workspace on the server, within which the user’s configuration and results files are stored. Estimates of underwater noise are generated within the context of a scenario, which consists of a configuration (one or more acoustic sources and their associated modelling grids, an acoustic environment, and a set of model parameters) and the raw and processed model results generated using that configuration. Multiple related scenarios may be grouped into a named project, and a user may have multiple projects. Figure 2 should make this structure clearer.


Figure 2: Hierarchal organization of user workspaces, projects, and scenarios within the SEANAT software

The SEANAT site is organized into the following five pages, corresponding to the main steps involved in generating estimates of underwater noise:

• Main Page: Selection or creation of a project and scenario • Configuration Page: Definition of scenario settings, visualization and validation of

the setup • Model Runs Page: Initiation and management of sound propagation modelling tasks • Model Results: Calculation of post-‐processed received levels, generation of final

products (e.g., images)

These steps and pages are discussed further in the following section. Note that each page includes page-‐specific help, accessed by clicking the “Show page help…” link at the top of each page. There is also a main Help page, accessed via the main site menu bar.

Sound propagation modelling is carried out using two models. RAMGeo, a modified version of the widely used Range-‐dependent Acoustic Model (RAM) (Collins et al., 1996), is used for lower frequencies, up to 2 kHz. For frequencies higher than 2 kHz, sound propagation is modelled using the Bellhop model (Porter and Liu, 1994), using the Bounce model to estimate the bottom reflection coefficient.

SEANAT

User 1 User 2 User 3

Project 1 Project 2

Scenario

1

Scenario

2


4 Sound Propagation Modelling

The following sub-‐sections describe the process of creating and running a typical model scenario, using a simple demo scenario involving pile driving at two locations in the North Sea.

4.1 Select or Create a Project and Scenario

Projects and scenarios are created and/or selected from the Main Page (Figure 3); selecting a scenario is the first post-‐login step for all work done via the SEANAT site. As described above, a user may have one or more projects, and each project may contain one or more scenarios. The user first selects an existing project from the pull-‐down list at the top of the Main Page, or creates a new project by typing the name of the new project into the “Create new project” text entry box and clicking “Create” (Figure 3). Any scenarios associated with the project (in the case of an existing project) are listed in the “Select existing scenario” pull-‐down menu; the scenario name may then be selected or created as for the project name. A message appears at the bottom of the main page indicating that the scenario has successfully been selected (Figure 3), and the remaining pages (described below) are populated from the scenario configuration file on the server.

Existing projects and/or scenarios that are no longer in use may be deleted by clicking the “Delete current project” or “Delete current scenario” buttons (Figure 3). The system presents a confirmation prompt before deleting the project or scenario, but deletion cannot be undone after deletion has been confirmed via this prompt.

Two additional items may be seen in the upper-‐right corner of the Main page (Figure 3), and on all other pages of the site: the “Show page help” link displays brief page-‐specific help, and the “Log Out” button logs the user out of the site.


Figure 3: Main page, after the user has created the scenario “North Sea pile driving” within the project “Demo project”

4.2 Configure the Scenario

Once a scenario has been selected from the Main page (Section 4.1), the Configuration page is used to define the scenario or edit the scenario configuration. As outlined in the sub-‐sections below, the Configuration page consists of tables used to define the acoustic sources to be included in the scenario and the parameters common to all sources in the scenario.

An interactive map (Figure 4) under the Acoustic Sources heading of the Configuration page provides a visual summary of the scenario configuration. Source locations and the associated acoustic field grids are plotted over a default base map, as are bathymetry contours and an outline of the German Exclusive Economic Zone. Moving the mouse pointer over the source locations or bathymetry contours displays the feature labels. The map includes standard pan and zoom controls, an indicator of the current cursor position, and a layer selector allowing the user to control which layers are visible on the map (Figure 4). Note that the layer selector is hidden by default, and is accessed by clicking on the small blue box in the upper right corner of the map. The map is updated whenever the source or configuration tables are updated.


Figure 4: Sample configuration map, showing two source locations off the coast of Germany. Source locations and associated field grids are shown in red, bathymetry contours are shown in blue, and the German EEZ is outlined in black. The map includes pan and zoom controls (upper left corner), a pull-‐out layer selector (upper right corner, normally collapsed), and the current cursor position (lower right corner). Moving the mouse pointer over the source locations or bathymetry contours displays the feature labels.

4.2.1 Environment and Model Parameters

The two tables under the "Environment and Model Parameters" heading (Figure 5) describe parameters common to all acoustic sources included in the scenario. The first of these two tables, titled “Acoustic Environment”, allows the user to select databases of bathymetry data, water column profiles, and sub-‐bottom profiles. The same set of databases is used for all acoustic sources included in the scenario, but site-‐specific profiles are extracted from the data sets using the source locations and acoustic field grids. The following is a brief description of the databases available within each category:


• Bathymetry: Bathymetry data are taken from the General Bathymetric Chart of the Oceans (GEBCO) (BODC, 2009), which has a resolution of 30 arc-‐seconds. For speed of loading, the SEANAT database contains two sub-‐sets of the GEBCO data set, one for the North Sea and one for the Baltic Sea. Bathymetry data are displayed on the interactive map as contours (Figure 4). Note that a global bathymetric chart such as GEBCO should be used with some caution near-‐shore and/or in shallow waters, where the resolution may not be adequate to capture finer features, and where assumptions made with regards to the vertical datum (GEBCO assumes a vertical datum of mean sea level) may not apply to all data sets included in the database.

• Water column profiles: Profiles of temperature and salinity are taken from the World Ocean Atlas data set (NODC, 2005), and are used to compute profiles of density and sound speed as per the formulae outlined in Fofonoff et al. (1983) and Millero and Li (1994), respectively. The WOA climatological data are available on a 1° grid and have a vertical resolution of 10 m. Four WOA data sets are available, one per season. In order to ensure full water column coverage over the region modelled, the SEANAT engine seeks WOA profiles within 1° of a given source location, and then retains the deepest profile if more than one is found. Note that only one WOA profile is available for the Baltic Sea, for a location of 54.5°N, 14.5°E; this profile, which has a maximum depth of 30 m, has been extended to 60 m by assuming a constant sound speed between 30 and 60 m. This profile is labelled “Baltic” in the list of available water column profiles, and should be used for model runs in this region. The water column profile(s) extracted for the source location(s) defined under the “Acoustic Sources” heading may be plotted using the “Plot Water Column Profiles” button (Figure 5, Figure 6). A custom water column profile may also be defined by clicking on the “Custom profile…” button (Figure 5), which brings up a dialogue box (Figure 7) allowing the user to define depth-‐dependent profiles of water column sound speed and density. The density profile is optional; a constant density of 1025 kg/m3 is assumed if no profile is specified. The profile name (Figure 7) is the name that will be shown in the water column profile selection box (Figure 5). Custom profiles are stored at the project level (Section 3), and are available to all scenarios within the project. Because the profile name uniquely identifies a profile, it cannot be the same as any of the default application data sources. The profile name should also be unique within the project unless the user wishes to overwrite a previously defined custom data source; a warning is provided in this case, as one or more scenarios may be affected.

• Sub-‐bottom profile: Two sub-‐bottom profiles are currently available. Both assume a smooth bottom, and the following values at the top of the sediment layer: compressional sound speed of 1475 m/s, density of 1100 kg/m3, and sound speed attenuation of 0.3 dB/λ. The compressional sound speed and density increase with depth below the sea floor in both profiles, but the gradients are steeper for the progressive profile (Figure 8). The profiles of geoacoustic properties may be plotted using the “Plot Sub-‐Bottom Profile” button (Figure 8). Similarly to the water column profiles, a custom sub-‐bottom profile may be defined by providing a profile name and depth-‐dependent values for the sound speed, density, and attenuation.


The second table, titled “Propagation Model Parameters” (Figure 5), defines the frequency range and model parameters to be used for the sound propagation models. The frequency range is entered in terms of the minimum (as low as 50 Hz) and maximum (up to 200 kHz) frequencies for which the models are to be run. The frequency range should be selected to provide good coverage over the range of frequencies where significant levels of noise are generated by the source and where target species are reasonably sensitive, bearing in mind that computation time increases with the number of frequencies to be modelled. The same frequency range is used for all acoustic sources. Pre-‐defined sets of model parameters are selected from the list beside the "Parameter Set" label. Currently only one set of parameters, suited for most modelling situations, is available. Parameter values included in this configuration are included in Appendix A for reference purposes.

Clicking the "Update configuration" button at the bottom of the “Environment and Model Parameters” section (Figure 5) saves the environment and parameter settings to file and updates the bathymetry contours on the map (Figure 4). Any informational or error messages are displayed at the bottom of the section. To ensure that model results in a scenario are consistent with the current configuration, updating the configuration file will delete any incompatible model results (after confirmation by the user that this is the desired result). Any modelling jobs currently being run will also be cancelled. If the user wishes to run a modified scenario while retaining previous results it is therefore essential to create a new scenario.

Figure 5: Sample Environment and Model Parameters tables for a location in the North Sea, summer water column profile, and linear sub-‐bottom profile. The frequency range is based on the source spectrum.


Figure 6: Sample plot of water column sound speed profiles, for the source locations shown in Figure 4. Data are taken from the WOA-‐summer data set.

Figure 7: Custom water column profile dialogue box


Figure 8: Sample plots of geoacoustic properties, for the linear (top panels) and progressive (bottom panels) profiles


4.2.2 Acoustic Sources

The tabbed tables under the "Acoustic Sources" heading (Figure 9) describe the sources of underwater noise to be considered as part of the current scenario. A scenario may have one or more acoustic sources. Sources are selected, added, or removed using the tabs at the top of the table area. Specifically, clicking the “Add a source…” button adds a source tab, and clicking the small “x” on a tab deletes the source (after confirming that this action is in fact desired). Each source table contains the following two sections:

• Source Characteristics: This section describes the location and source spectrum for a single acoustic source, and includes the following parameters:

o Source name: This is a unique name to be used to refer to the source. The name will be used as the tab title once the source configuration has been submitted. This can be any name that is significant to the user, so long as it is unique within the scenario (i.e., no other tab has the same title) and contains only legal characters (letters, numbers, spaces, and underscores). Two sources have been defined in the example shown in Figure 9, named “Pile driver 1” and “Pile driver 2”.

o Source spectrum: Pre-‐defined sources are selected from a pull-‐down list. Both sources shown in Figure 9 make use of the “Pile driving – 100 kJ” source spectrum, corresponding to an impact pile driver with an impact energy of 100 kJ. Source details may be viewed by clicking the “Details…” link beside the source selection menu. An optional offset (in dB) may be applied to the spectral levels, e.g. to adjust for hammer energy. The source spectrum (adjusted by the specified offset) may be displayed using the "Plot Source Spectrum" button below the table; a sample plot is shown in Figure 10. Custom source spectra may also be defined by clicking on the “Custom spectrum…” button, bringing up the New Source Spectrum dialogue box (Figure 11). Once submitted, the name of the custom spectrum appears in the pull-‐down list alongside the default spectra provided by the software. As with the custom environmental profiles discussed in Section 4.2.1, user-‐defined spectra are stored at the project level, and are shared by all scenarios within the project.

o Source location: The source location is defined by specifying the location units (either decimal degrees or UTM coordinates) and the x and y coordinates of the source. Depending on the units selected, coordinates can be typed into the table as a latitude and longitude or as a northing and easting; for example, the location of Pile driver 1 in Figure 9 is specified in decimal degrees. If coordinates are to be entered as a northing and easting, a numeric UTM zone must also be provided using the optional field that appears when the units are set to “UTM”. (The North Sea component of the German EEZ is in UTM zones 31 and 32 whereas the Baltic Sea component is in zones 32 and 33.) Alternatively, clicking the "Capture location from map" button allows the user to select a source location in decimal degrees by clicking on the live map. Clicking on the map in this mode updates the text input boxes, and places an orange dot on the current position. Clicking on the


map again will further update the source location, until either the "Stop capture" button is selected or the source information is committed to file. Once the source location is committed to file, the source is displayed as a red dot on the map (e.g., Figure 4). Note that source locations must be within the German EEZ.

o Depth below sea surface: The depth below the sea surface is specified in metres. If the depth is greater than the bottom depth, a sub-‐bottom source is assumed. This is compatible with RAMGeo, the model used for lower frequencies, but not with the Bellhop model used for frequencies greater than 2kHz.

• Acoustic Field Grid: This section describes the radials along which the sound propagation model(s) will be run for the specified source. Two fields are included:

o Azimuths: Azimuths are specified as degrees clockwise from north, and are entered as a list with items separated by spaces, commas, or semi-‐colons. The azimuths list may also be defined using a beginning, end, and step value, using the following notation: (start azimuth):(azimuth separation):(end azimuth). For example, the input "0:20:180" is equivalent to "0,20,40,60,80,100,120,140,160,180".

o Maximum ranges: Maximum ranges (in kilometres) may be input either as a list (with items separated by spaces, commas, or semi-‐colons) with the same number of items as the list of azimuths, or as a single value to be applied to all radials.

Once a source configuration containing a source location and a set of radials and azimuths is committed to file, the source is displayed on the map as a red dot with red lines representing the model radials (Figure 4).

The source configuration is committed to file by clicking on the "Update Source Information" button (Figure 9). Note that information entered into the form is not persistent until the information is stored to file, and so will be lost if the user navigates away from the form without submitting.

If the source information is successfully committed to file, the source locations and acoustic field grids are displayed or updated on the map (Figure 4). If the configuration update is unsuccessful for any reason, previous settings will be retained, and an error message will be shown below the Acoustic Sources table.

As with updates to the environment and model parameters, most non-‐trivial changes to the source configuration will result in the deletion of previously generated results (after confirmation by the user that this is the desired result). For example, changing a source spectrum will delete processed model results, and changing a source location will result in deletion of previously generated transmission loss files and cancellation of any active modelling jobs. If the user wishes to run a modified scenario while retaining previous results it is therefore essential to create a new scenario.


Figure 9: Acoustic Sources section of the Configuration Page. Two sources have been configured in this example, named “Pile driver 1” and “Pile driver 2”.

Figure 10: Sample source spectrum plot, for a 100kJ pile driver


Figure 11: Custom source spectrum dialogue box. Note that the units shown for the “Level”

column depend on the source type (continuous or impulsive).

4.3 Run the Models

Once a scenario has been configured (Section 4.2), it is possible to run the sound propagation models in order to generate estimates of transmission loss. At this stage the scenario is broken down into smaller units that can be handled by the SEANAT sound propagation models. Specifically, a single instance of a modelling run, or job, is based on a single source location and depth, radial, frequency, and set of environmental and model parameters.

Modelling jobs are handled by SEANAT’s grid computing environment, wherein several modelling jobs can be run simultaneously by a set of dedicated computational nodes. An individual user submits his or her jobs to a queue, and the runs are processed as soon as a computational node is available. Depending on the current system load, the jobs may be processed immediately, or may have to wait until other users’ jobs have completed. The SEANAT front-‐end is used to submit jobs to the run engine and to manage existing jobs.

If a scenario has been selected and configured, the available sources, frequencies, and azimuths are displayed in the list boxes at the top of the Model Runs page (Figure 12). The list of azimuths displayed depends on the acoustic source(s) currently selected; selecting one or more sources from the "Source" menu updates the "Azimuths" list with the corresponding list of azimuths, as previously defined on the Configuration page.


Modelling jobs are submitted to the run engine by selecting one or more acoustic sources, frequencies, and azimuths from the menus at the top of the Model Runs page and clicking on the "Submit to Scheduler" button (Figure 12). Model-‐specific input files are then created and submitted to the run engine queue, a process that may take several seconds, during which time the submission and management controls are locked (controls are greyed out and labelled “Busy…”). Once the jobs have been submitted to the computational grid, they are listed in the job status table (Figure 12); the table includes scroll controls for viewing long lists of jobs. Jobs will be shown as "queued and active", then "running", and finally "finished". The table refreshes every 10 seconds until all runs are complete. Note that it is not necessary to wait until a batch of modelling runs has finished running before submitting another batch.

The "Manage selected jobs" menu below the job status table (Figure 12) allows the user to perform the following administrative tasks:

• Pause jobs: Jobs are held back until the user selects "resume" or (in the case of jobs that were already running when they were paused) all other jobs have completed.

• Resume jobs: Paused jobs are allowed to return to the queue as normal. • Cancel jobs: Jobs are stopped and removed from the queue, and any model results

associated with the jobs are deleted.

Jobs to manage are selected via the check boxes in the "Job ID" column of the job status table. Jobs may be selected individually, or the box beside the column heading may be checked/unchecked to select/unselect all jobs in the list.

Once one or more jobs have finished running, clicking "Fetch Results" moves the model results from the grid computing environment back to the main user workspace, making them available for display and post-‐processing. Only finished jobs are retrieved; running jobs are not interrupted. The fetching process may take up to a few minutes, depending on the number of model runs involved.


Figure 12: SEANAT Model Runs page. In this example 12 jobs have been submitted to the computational grid.

4.4 Post-‐Process and Plot the Model Results

Once model results have been fetched from the Model Runs page, post-‐processing is carried out from the Model Results page (Figure 13). This page is split into sections for the handling of transmission losses and received levels, respectively. Available model results, in terms of sources, frequencies, and azimuths, are listed at the top of each section. As on the Model Runs page, the menus of frequencies and azimuths are updated when a source is selected.

Frequency-‐dependent transmission losses may be plotted as a function of range and depth by selecting the desired sources, frequencies, and azimuths from the lists under the "Transmission Losses" heading and clicking "Plot Transmission Loss" (Figure 13). Optionally, the colour limits used for plotting may be specified via the input boxes below the selection boxes. The message "Processing. Please wait..." appears while the plots are being generated (which may take some time if a large number of plots were requested),


and the resulting plots are then listed in the image gallery below the menus; sample output is shown in Figure 14. Plots within the gallery may be browsed using the controls above the plot panel, or using the menu to the left of the gallery (Figure 14).

Transmission loss data may also be downloaded via the “Download Data Files” button (Figure 13). Raw data files for the selected sources, frequencies, and azimuths are compressed (zip format) and downloaded to the user's computer. The compressed archive contains raw data in plain-‐text files (one for each individual model run), organized into directories for each source and azimuth selected. The individual file names indicate the model used and the modelling frequency. Within each plain-‐text file, transmission loss data are presented with one row per receiver range and one column per receiver depth. The first column after the receiver range column contains the range-‐dependent bottom depth.

Once the user is satisfied that the transmission loss estimates are sensible, they may be combined with the source spectra selected on the Configuration page to generate frequency-‐dependent received level estimates for each acoustic source. This is done by selecting the desired sources, frequencies, and azimuths and clicking “Compute Received Levels” (Figure 13). Available received levels are then listed in the menus under the "Received Levels" heading. If all available frequencies are selected for received level computations, the overall (sum of received energy over frequency) received level is also computed; this resulting file is listed as “sum” in the list of frequencies under the “Received Levels” heading (e.g., Figure 13).

Once received levels have been generated, they may be plotted using the controls under the "Received Levels" heading (Figure 13), and the corresponding data may also be downloaded. Three plot/download types are available:

• Received levels as a function of range and depth: As with the range-‐depth plot option under the “Transmission Loss” heading (Figure 14), one plot is generated for each source, frequency, and azimuth selected. The colour legend used for plotting of received levels may be modified by specifying minimum and maximum received levels for the colour scale. Additionally, checking the “Apply threshold levels” box (Figure 13) generates a colour scale with breaks at 140 dB re. 1 µPa2s (avoidance threshold) and 160 dB re. 1 µPa2s (temporary threshold shift level). Clicking "Download Data" initiates download of plain-‐text files, as for the transmission loss data.

• Received levels as a function of geographic location: The map view (e.g., Figure 15) accepts one frequency and one or more sources and azimuths, as well as the desired depth below the sea surface and whether or not received levels from different sources should be summed; the latter controls are made visible when the plot type is set to “Received level vs. geographic location” (Figure 13). If a numeric plot depth is provided, the map represents a horizontal “slice” through the sound field at the depth indicated. If the depth is greater than the bottom depth, the sub-‐bottom sound field will be shown in the case of RAMGeo (Bellhop does not model sub-‐bottom sound propagation). Alternatively, ticking the “maximum over depth” check box generates a map where the level at each x, y point represents the maximum level


over all water depths modelled for that point (sub-‐bottom grid points are excluded). If more than one source is selected from the source menu and the “Combine received levels from all sources” box is checked, the received levels from the various input sources are interpolated to a common spatial grid and then summed incoherently in order to generate a map of the total received level from the selected acoustic sources. Optionally, axis limits and colour scale options may be specified; note, however, that axis limits may be modified internally before display if the limits specified will prevent the map from rendering correctly. Finally, selecting "Download Data" for this output type yields a plain-‐text file of received level as a function of geographic location, as well as a shapefile of received level contours. The shapefile download includes all three files (extensions .shp, .shx, and .dbf) needed for display by standard GIS software.

• Received spectrum at a specified location: This option accepts one or more sources and azimuths (all frequencies are used by default), as well as a target depth (again, either a numeric depth or maximum-‐over-‐depth) and location. The target location may be specified as a range and azimuth from one of the selected sources, as a latitude and longitude, or as UTM coordinates. Sample output is shown in Figure 16. Optionally, an audiogram may be defined and/or selected to plot alongside the received spectrum or spectra. A plain-‐text file containing the data points plotted may also be downloaded.

Figure 13: Model Results page, showing available raw and processed results after all runs shown in Figure 12 have been run, fetched, and converted to received levels. Optional settings for the plot of received levels as a function of geographic location are shown.


Figure 14: Sample plot of transmission loss as a function of range and depth. The source location and bottom depth are shown as a black asterisk and black line, respectively. The menu to the left of the plot is updated as plots are created (here, 10 plots have been generated); the menu and Previous/Next controls are used to navigate the Plots gallery.

Figure 15: Sample map of SEL for Pile driver 1, for values maximized over depth. The shoreline is shown in gray.


Figure 16: Sample source spectrum plot, for a location 2 km east of Pile driver 1

5 References

Collins, M.D., R.J. Cederberg, D.B. King, and S.A. Chin-‐Bing, 1996. Comparison of algorithms for solving parabolic wave equations. Journal of the Acoustical Society of America 100(1): 178-‐182.

Collins, M.D., 1997. User’s Guide for RAM Versions 1.0 and 1.0p. Naval Research Laboratory, 14pp.

British Oceanographic Data Centre (BODC), 2009. The GEBCO_08 Grid, version 20091120, General Bathymetric Chart of the Oceans (GEBCO), http://www.gebco.net.

Fofonoff, N.P, and R.C. Millard Jr., 1983. Algorithms for computation of fundamental properties of seawater, UNESCO Technical Papers in Marine Science, Vol. 44, 53 pp.

Millero, F.J., and X. Li, 1994. Comments on “On equations for the speed of sound in seawater” [J. Acoust. Soc. Am. 93, 255–275 (1993)]. Journal of the Acoustical Society of America 95(5): 2757-‐2759.

National Oceanographic Data Center (NODC), 2005. World Ocean Atlas 2005, http://www.nodc.noaa.gov/OC5/WOA05/pr_woa05.html.


Porter, M.B., 2006. The BELLHOP Manual and Users Guide. HLS Research, La Jolla, CA, USA., 57pp.

Porter, M.B., and Y-‐C Liu, 1994. Finite-‐Element Ray Tracing. Theoretical and Computational Acoustics 2: 947-‐956.


Appendix A: Default Model Parameters

Sound propagation modelling parameters included in the “Default Parameters” parameter set (Section 4.2.1) are outlined in Table 1. The parameter values shown in the table are suitable for most environments, and are included here only for reference.

As noted in Section 4.2.1, the acoustic models are run at third-‐octave band centre frequencies from 50 Hz to 200 kHz. RAMGeo (a parabolic equation model) is used for frequencies up to and including 2 kHz, and Bellhop (a ray tracing model) is used for higher frequencies (Table 1). The Bounce model is used in conjunction with Bellhop in order to compute the bottom reflection coefficient. Note that while a frequency cut-‐off of 2 kHz is suitable for most environments in the German EEZ, Bellhop may tend to over-‐estimate transmission losses in very shallow water for frequencies near the cut-‐off.

Detailed information on the RAMGeo and Bellhop parameters listed in Table 1 is available in the RAM and Bellhop manuals (Collins, 1997; Porter, 2006). SEANAT uses the following approach to select frequency-‐appropriate parameters for RAMGeo:

• The resolution of the computational grid is defined in terms of the wavelength, providing a balance of accuracy and computational efficiency. In this approach, the depth grid spacing is specified as a multiple of the wavelength, and the range step is specified as a multiple of the depth increment. Maximum values are used for each in order to avoid overly coarse grids at low frequencies. The output values are then sub-‐sampled on a computational grid with depth and range step increments as defined in Table 1. Because the range step is defined in terms of the wavelength-‐dependent depth increment rather than as a fraction of the maximum range specified in the Acoustic Field Grid table on the Configuration Page (Figure 9), the farthest range modelled may be some fraction of a range step less than the maximum range.

• An attenuating layer is added to the bottom of the sediment half-‐space to simulate loss of sound energy into deeper sediment layers. The thickness of this layer, as a number of wavelengths, and the attenuation assumed within the layer are defined in Table 1.


Table 1: Default parameters used for sound propagation modelling

Parameters common to all models Model frequencies (Hz) 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630,

800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000, 10 000, 12 500, 16 000, 20 000, 25 000, 32 000, 40 000, 51 000, 64 000, 81 000, 100 000, 130 000, 160 000, 200 000

Output depth grid spacing (m) 1 RAMGeo parameters Frequency range where the model is applied (Hz)

50 – 2000

Computational depth grid spacing (number of wavelengths)

0.1

Maximum computational depth grid spacing (m)

1

Computational range step (multiple of the depth grid spacing)

2

Maximum computational range step (m)

2

Output range step (m) 50 Reference sound speed (m/s) 1500 Number of terms in rational approximation

5

Number of stability constraints 1 Maximum range of stability constraints

0

Sediment layer thickness (number of wavelengths)

10

Bottom attenuation layer thickness (number of wavelengths)

10

Maximum attenuation (dB/wavelength)

10

Bellhop parameters Frequency range where the model is applied (Hz)

2001 – 200 000

Run type Semicoherent TL calculation Beam type Gaussian beams Number of beams 5000 Range of beam angles (degrees) -‐80 – 80 Beam angular step Automatically determined Number of range steps 1000

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