The BELLHOP Manual and Users Guide: PRELIMINARY DRAFT Michael B. Porter Heat, Light, and Sound Research, Inc. La Jolla, CA, USA January 31, 2011 Abstract BELLHOP is a beam tracing model for predicting acoustic pres- sure fi elds in ocean environments. The beam tracing structure leads to a particularly simple algorithm. Several types of beams are imple- mented including Gaussian and hat-shaped beams, with both geomet- ric and physics-based spreading laws. BELLHOP can produce a vari- ety of useful outputs including transmission loss, eigenrays, arrivals, and received time-series.It allows for range-dependence in the top and bottom boundaries (altimetry and bathymetry), as well as in the sound speed profi le. Additional input fi les allow the specifi cation of directional sources as well as geoacoustic properties for the bounding media. Top and bottom refl ection coeffi cients may also be provided. BELLHOP is implemented in Fortran, Matlab, and Python and used on multiple platforms (Mac, Windows, and Linux). This report describes the code and illustrates its use. 1 2 Contents 1Map of the BELLHOP program5 1.1Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 1.2Output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 2 Sound speed profi le and ray trace9 3Eigenray plots17 4Transmission Loss21 4.1Coherent, Semicoherent, and Incoherent TL . . . . . . . . . .25 5Directional Sources27 6Range-dependent Boundaries31 6.1Piecewise-Linear Boundaries: Dickins seamount . . . . . . . .31 6.2Plotting a single beam . . . . . . . . . . . . . . . . . . . . . .34 6.3Curvilinear Boundaries: Parabolic Bottom . . . . . . . . . . .35 7 Tabulated Refl ection Coeffi cients39 8 Range-dependent Sound Speed Profi les41 9Arrivals calculations and broadband results45 9.1Coherent and Incoherent TL. . . . . . . . . . . . . . . . . .45 9.2Plotting the impulse response . . . . . . . . . . . . . . . . . .51 9.3Generating a receiver timeseries . . . . . . . . . . . . . . . . .53 10 Acknowledgments57 3 4 1Map of the BELLHOP program 1.1Input The overall structure of BELLHOP is shown in Fig. (1). Various fi les must be provided to describe the environment and the geometry of sources and receivers. In the simplest case, which is also typical, there is only one such fi le. It is referred to as an environmental fi le and includes the sound speed profi le, as well as information about the ocean bottom. However, if there is a range-dependent bottom, then one must add a bathymetry fi le with range-depth pairs defi ning the water depth. Similarly, if there is a range- dependent ocean sound speed, the one must an an SSP fi le with the sound speed tabulated on a regular grid. Further, if one wants to specify an ar- bitrary bottom refl ection coeffi cient to characterize the bottom, then one must provide a bottom refl ection coeffi cient fi le with angle-refl ection coeffi - cient pairs defi ning the refl ectivity. Similar capabilities are implemented for the surface. Thus there is the option of providing a top refl ection coeffi cient and a top shape (called an altimetry fi le). Usually one assumes the acoustic source is omni-directional; however, if there is a source beampattern, then one must provide a source beam pattern fi le with angle-amplitude pairs defi ning it. BELLHOP reads these fi les depending on options selected within the main environmental fi le. Plot programs (plotssp, plotbty, plotbrc, etc.) are provided to display each of the input fi les. 1.2Output BELLHOP produces diff erent output fi les depending on the options selected within the main environmental fi le. Usually one starts with a ray tracing option, which produces a fi le con- taining a fan of rays emanating from the source. If the eigenray option is selected, then the fan is winnowed to include only the rays that bracket a specifi ed receiver location. The fi le format is identical to that used in the standard ray-tracing option. Ray fi les are usually used to get a sense of how energy is propagating in the channel. The program plotray is used to display these fi les. Usually one is interested in calculating the transmission loss for a tonal source (or for a single tone of interest in a broadband waveform).The transmission loss is essentially the sound intensity due to a source of unit strength. The transmission loss information is written to a shade fi le which 5 BELLHOP STRUCTURE Model Output rays (.ray) plotray eigenrays (.ray) pressure fi eld (.shd) plotshd plottlr plottld arrivals (.arr) plotarr source time series (.sts) source timeseries generator cans m-seq lfm Generate Timeseries receiver time series (.rts) plotts BELLHOP Model Input environment (.env) plotssp altimetry (.ati) plotati bathymetry (.bty) plotbty top refl ection coefficient (.trc) plottrc bottom refl ection coefficient (.brc) plotbrc source beam pattern (.sbp) 2D SSP (.ssp) plotssp2D Figure 1: BELLHOP structure. 6 can be displayed as a 2D surface using plotshd, or in range and d