A Consistent, User Friendly Interface for Running a Variety of Underwater Acoustic Propagation Codes

Transcription

1 Poceedings of ACOUSTICS 6 - Novembe 6, Chistchuch, New Zealand A Consistent, Use Fiendly Inteface fo Running a Vaiety of Undewate Acoustic Popagation Codes Alec J Duncan, Amos L Maggi Cente fo Maine Science and Technology, Cutin univesity of Technology ABSTRACT A numbe of undewate acoustic popagation modelling pogams have been placed in the public domain by thei authos. Diffeent pogams ae equied fo diffeent situations, but togethe they can pefom the majoity of common modelling tasks. All these pogams ae input file diven, but with diffeent file fomats, and each equies consideable time and effot to be expended in ode to wok out its specific equiements. The pogams also geneate output data in a numbe of diffeent fomats, making the setting up of softwae fo visualisation o futhe pocessing a time consuming pocess. Seveal yeas ago the Cente fo Maine Science and Technology publicly eleased a fee Gaphical Use Inteface (GUI) that povides a simple, consistent inteface fo unning the vaious popagation outines in the Acoustic Toolbox, a collection of undewate acoustic popagation modelling pogams witten by Mike Pote fom HLS Reseach. The GUI is witten in Matlab, and also povides a vaiety of output data visualisation tools. It povides eady access to pogams fo acoustic field calculation by nomal modes, wavenumbe integation, and ay and beam tacing, and can also compute eflection coefficients fo complicated layeed seabeds. These codes ae only suitable fo modelling ange-independent envionments, with the exception of the ay code which can handle ange-dependent bathymety. An expanded vesion of this inteface has ecently been eleased and can be downloaded fo fee fom CMST's web site. The pimay enhancement is to povide fully ange-dependent modelling though the inclusion of suppot fo the RAM family of paabolic equation codes witten by Mike Collins at the US Naval Reseach Laboatoy. This pape descibes the capabibilities of this new vesion of the GUI, and povides examples of its use. INTRODUCTION The complexity of undewate acoustic popagation makes numeical modelling an essential tool fo eseaches and an extemely desiable tool fo educatos, because in many cases modelling povides significant physical insight that isn t othewise available. A numbe of diffeent models have been developed ove the yeas, each with its own ange of applications, and many of them ae feely available ove the intenet. (See, fo example, Unfotunately these codes ae invaiably file diven, and only sketchily documented, making them a daunting pospect fo a new use, and definitely not something you can let the aveage student loose on. Seveal yeas ago the Cente fo Maine Science and Technology publicly eleased a fee Gaphical Use Inteface (GUI) that povides a simple, consistent inteface fo unning the vaious popagation outines in the Acoustic Toolbox, a collection of undewate acoustic popagation modelling pogams witten by Mike Pote fom HLS Reseach. The GUI is known as the Acoustic Toolbox Use Inteface (Ac- TUI V1.6). It is witten in Matlab, and also povides a vaiety of output data visualisation tools. The pogams suppoted by AcTUI V1.6 ae listed in Table 1. They include a high-fequency popagation code (BELLHOP) that can handle ange-dependent bathymety (but not sound speed), and seveal low-fequency, angeindependent codes (KRAKEN o KRAKENC togethe with FIELD, and SCOOTER togethe with FIELDS). Acoustic Toolbox Use inteface and Post pocesso (AcTUP V.l), addesses the fist of these poblems though suppot fo two customised vesions of the Range-dependent Acoustic Model (RAM) witten by Mike Collins fom the US Naval Reseach Laboatoy. RAM is an extemely efficient paabolic equation (PE) code that copes natually with angedependent envionments and ovecomes the pinciple limitation of the PE method: lack of accuacy fo enegy popagating at lage angles to the hoizontal (Collins 199, Collins 1993, Collins 1999). It is hoped to addess the lack of a fully ange-dependent high-fequency code in the nea futue though the povision of a fully ange-dependent vesion of BELLHOP. This pogam is cuently unde development at CMST. AcTUP V.l includes the following additional featues: Compatibility with the 6 elease of Mike Pote s Acoustic Toolbox (Pote, 5). A edesigned Run Definition class combining model envionment and popagation paametes into a single stuctue. This offes inceased obustness, facilitating full documentation and ecovey of all model paametes. Ability to set up batch uns of one o moe popagation codes using the same use fiendly inteface used fo single uns. A edesigned and moe connected use inteface to cope with the significantly expanded functionality. See Table 1 fo a full list of the codes suppoted by AcTUP V.l. The biggest limitation of AcTUI V1.6 is the lack of suppot fo a low-fequency, ange-dependent code, although a fully ange-dependent high-fequency code would also be useful. The latest public elease of the GUI, which is known as the Acoustics 6 471

2 - Novembe 6, Chistchuch, New Zealand Poceedings of ACOUSTICS 6 Table 1 Popagation codes suppoted by AcTUI V1.6 and AcTUP V.l. Name Desciption V1.6 V.l BELLHOP Beam tacing pogam Y Y BOUNCE Plane wave eflection Y Y coefficient fo layeed seabeds FIELD Used with KRAKEN o Y * Y KRAKENC to compute tansmission loss fo ange-independent envionments. FIELDS Used with SCOOTER to Y * Y compute tansmission loss fo ange-independent envionments. (Wavenumbe integation by fast-field method) KRAKEN Real nomal modes Y Y KRAKENC Complex nomal modes Y Y RAMGEO Low fequency tansmission N Y loss in ange- dependent envionments - fluid seabeds. RAMSGEO Low fequency tansmission N Y loss in ange- dependent envionments - elastic seabeds. SCOOTER Computes depth dependent Geens function (hoizontal wavenumbe spectum) fo angeindependent envionments. Y Y Y * indicates specified code is included but is un automatically when one of the associated codes is un and doesn t appea explicitly on the menus. THEORY This section is a vey bief intoduction to the theoetical basis of the vaious popagation models suppoted by AcTUP V.l. A detailed account can be found in Jensen et. al., (). All acoustic popagation codes ae ultimately based on the wave equation: (, 1 1 p ρ (, t ) p(, = f (, ρ (, c( ) t Hee ρ is density, p is pessue, t is time, is the position vecto, is the spatial gadient opeato, c is the sound speed in the medium, and f is a focing tem which epesents the acoustic souces. In most situations in ocean acoustics changes in density occu ove elatively lage spatial scales, apat fom at boundaies between media (eg. at the seabed) whee the density is effectively discontinuous. In this case, within a single medium, educes to the moe familia fom: () (, p p (, 1 = f (, () c t Changes in media can be handled by solving () independently in each medium, and using continuity conditions between media (usually continuity of pessue and nomal velocity) to evaluate unknown coefficients. Unfotunately () involves fou independent vaiables (thee spatial and time), and diect numeical solution using finite diffeence o finite element techniques is impactical except in situations whee the computation domain is a elatively small numbe of acoustic wavelengths in size. Most poblems of pactical inteest in undewate acoustics involve computational domains seveal odes of magnitude lage than this, so it is necessay to educe the dimensionality of the poblem, and to intoduce assumptions that allow the use of moe efficient numeical techniques. All of the codes discussed hee stat by assuming a hamonic iωt focing tem and pessue, i.e.: f (, = f ( ) e and iωt ( = p( ) e p, whee ω is the angula fequency. Substitution into () leads to the Helmholtz equation: ( ) + k( ) p( ) f ( ) p = (3) whee ω k() = is the acoustic wavenumbe. Equation c() (3) has the advantage that it can be solved one fequency at a time, making it moe tactable than (), paticulaly if the solution is only equied at a single fequency o ove a small ange of fequencies. Acoustic popagation poblems often involve point souces and have cylindical symmety, o at least have envionments that vay slowly enough in azimuth that the azimuth dependence can be ignoed when computing the tansmission in a paticula diection. In this case = (, z), whee is the hoizontal ange and z is depth, 1 = +,.and (3) now has only two independent vaiables. Howeve, fo many undewate acoustic z poblems the solution of even this two-dimensional equation equies too much numeical effot, and futhe assumptions ae necessay. Thee ae a numbe of options: Depth sepaated Helmholtz equation If we assume that the envionment is ange-independent then the sound speed and bounday conditions ae functions of z only. If we futhe assume a single point souce at z = zs, =, we can sepaate (3) into depth and ange equations: d p dz and p ( k, z) + ( k k ) p( k, z) = Aδ ( z z ) ( z ) = p ( k, z ) J ( k ), k dk whee k is a sepaation constant that tuns out to be the hoizontal component of the wave vecto, and p( k, z) is the solution of (4) fo a given k. A plot of p( k, z) against k fo a paticula depth is known as the hoizontal wavenumbe spectum, and if A = 1/ π it is called the depth dependent Geens function. A is a constant that depends on the amplitude of the souce, δ is the Diac delta function, and J is the Bessel function of the fist kind of zeo ode. s (4) (5) 47 Acoustics 6

3 Poceedings of ACOUSTICS 6 Thee ae two ways of solving these equations. The fist is numeical solution of (4) fo vaious values of k, followed by numeical integation of (5). This is the wavenumbe integation method used by SCOOTER/FIELDS. SCOOTER solves (4), and FIELDS caies out the integation in (5) using an appoximation, valid moe than a few wavelengths fom the souce, that allows (5) to be evaluated by a Fast Fouie Tansfom. This is known as the fast-field method. The esult is an accuate and easonably efficient method of solving ange-independent poblems that can deal with seabeds compising abitay fluid and elastic layes. The second method is to find the nomal modes of (4). These ae the values of k = k that esult in non-tivial solutions of the m homogeneous fom of (4), i.e. (4) with a ight hand side of zeo, and coespond to the poles of the hoizontal wavenumbe spectum. The coesponding solutions to (4), Ψ m ( z) = p( km, z), ae known as mode functions. Unde these conditions the integation in (5) educes to a summation ove the poducts of the mode functions evaluated at the souce and eceive depths: i = Ψ (6) ρ (, z) 4 ( z ) = p s m 1 m ( z ) Ψ ( z) H ( k ) s m whee H is the Hankel function of the fist kind of ode zeo. This esults in an extemely efficient method of solution of ange-independent poblems because the mode functions only have to be evaluated once, and then the pessue can be evaluated at any ange by using a simple summation. The main disadvantage of this method is that the nomal modes only account fo enegy that is tapped within the waveguide by total intenal eflection, so mode solutions tend to be inaccuate at shot ange whee enegy that is only patially eflected fom the seabed is significant. KRAKEN finds the nomal modes and mode functions by looking fo eal values of k, wheeas KRAKENC seaches m fo the equied values of k m in the complex plane. The seach along the eal axis is a much easie numeical poblem than the complex plane seach, but limits the envionments that KRAKEN can deal with to those without elastic seabed layes (an elastic halfspace is OK though). The complex plane seach caied out by KRAKENC allows it to deal with elastic layes and also to account fo some of the patially eflected enegy though what ae called leaky modes. Leaky modes coespond to values of k m that lie off the eal axis even in the absence of seabed attenuation. Howeve, KRAKENC doesn t always find all of the modes, which can lead to significant eos. The mode functions computed by eithe of these pogams can be used by FIELD, which evaluates the pessue using (6). It is also possible to extend the nomal mode method to ange-dependent poblems by evaluating the modes at diffeent anges and then using one of two methods fo detemining how the mode amplitudes at one ange depend on those at pevious anges. The adiabatic mode method assumes enegy emains in a given mode, wheeas the moe computationally intensive coupled mode method allows enegy to couple between modes. Range-dependent nomal modes ae not cuently suppoted by AcTUP. Ray and beam methods These methods ae based on assuming the following fom fo the pessue: iωτ ( ) () B() e p = (7) m whee B ( ) and ( ) - Novembe 6, Chistchuch, New Zealand τ ae slowly vaying functions of position along a paticula ay. This can be shown to be a high fequency assumption. Substitution into (3) and consideable mathematical manipulation (see Jensen et. al. fo details), leads to the following vecto equation, which can be numeically integated to obtain the ay paths: d 1 d 1 = c (8) ds c ds c Hee s is the distance along the ay. In the ay appoach the amplitude of the signal is detemined by consideing changes in the aea delineated by adjacent ays, howeve this leads to a pediction of infinite amplitude when ays coss (i.e. at caustics), and zeo amplitude in egions whee no ays penetate (shadow zones). The beam appoach, on the othe hand, consides each ay to be the cente of a beam with a (usually) Gaussian intensity pofile and computes the signal by summing the contibutions of all beams with significant amplitude at the eceive location. Beam codes educe poblems with caustics and shadow zones and can be used at lowe fequencies than ay codes. Howeve, they ae still inheently high fequency codes. BELLHOP is a beam code of this type. It can be un using eithe beams with a Gaussian intensity pofile, o beams with a tiangula pofile that simulate a standad ay code. BELLHOP equies a bottom eflection coefficient file, which AcTUP geneates automatically by unning anothe pogam fom the Acoustic Toolbox, BOUNCE, which calculates the plane wave eflection coefficient of a layeed seabed. BELLHOP can also ead in a bathymety file that specifies the wate depth as a function of ange, which allows it to handle some ange dependence. Paabolic equation The paabolic equation (PE) method assumes a solution in the fom of an outgoing cylindical wave: p( ) =ψ (, z) H ( k) whee (, z) (9) ψ is assumed to be a slowly vaying function of ange, and k is a efeence wavenumbe. Substituting into the homogeneous (zeo.h.s.) fom of (3) and making the futhe assumption that enegy is popagating at small angles to the hoizontal (the paaxial appoximation) leads to the standad paabolic equation: ik ψ ψ c + + k 1 = ψ z c whee c is the efeence sound speed coesponding to k. This equation is only fist ode in ange and, given the field at one ange, can be eadily integated to obtain the field at a subsequent ange. This leads to an efficient ange maching algoithm. Although is only accuate fo popagation at angles close to the hoizontal, ecent developments have led to PE algoithms that elax this estiction to the point whee it is no longe an impediment. The Range-dependent Acoustic Model (RAM) developed by Mike Collins (Collins 1993) implements an extemely efficient algoithm that allows a tade-off between the ange of Acoustics 6 473

4 - Novembe 6, Chistchuch, New Zealand Poceedings of ACOUSTICS 6 popagation angles that can be accuately modelled, and computation speed. This tade-off is implemented though the use specifying the numbe of tems to use in a Padé expansion that appeas in the equations. Moe tems give accuacy ove a wide ange of angles but esult in longe computation times. RAM comes in seveal vaiants: RAM - fluid seabeds, seabed layeing specified elative to the wate suface. (Stata ae hoizontal.) RAMGeo - fluid seabeds, seabed layeing specified elative to the seabed. (Stata follow bathymety.) RAMS - As fo RAM but can handle elastic seabeds. RAMSuf - as fo RAMGeo, but also inputs a file specifying the height of the top bounday of the wate column as a function of ange. RAMGeo is moe consistent than RAM with the way in which BELLHOP handles ange-dependent bathymety, and was theefoe selected fo use with AcTUP. The vesion supplied with AcTUP has been modified by CMST to poduce an additional output file that AcTUP can convet to the standad Shade file fomat used by the acoustic toolbox outines. To allow fo modelling situations whee shea wave popagation in the seabed is impotant, CMST developed a modified vesion of RAMS that uses the same seabed laye specification method as RAMGeo. This is known as RAMSGeo. This pogam equied significant modifications to be made to the oiginal code and, although it has been tested successfully on a numbe of standad poblems, should be used with a cetain degee of caution. ACOUSTIC TOOLBOX USER INTERFACE AND POST-PROCESSOR (ACTUP) Pinciple of opeation A simplified block diagam of the way AcTUP woks is shown in Figue 1. The envionment and code paamete edito allows the use to load, modify, and save the envionment specification (sound speed pofiles, seabed chaacteistics etc.) and othe paametes elating to the popagation calculation (souce and eceive depths, eceive anges etc.). This block also allows the use to specify the popagation code that will be un, and any paametes that ae specific to that popagation code. All of this infomation is saved in the un definition file. The code engine uses the infomation in the un definition to wite the input file that is equied by the popagation code that it has been told to un. In some cases the un definition will equie seveal codes to be un in sequence (eg. BOUNCE then BELLHOP, o KRAKEN followed by FIELD), in which case seveal input files will be equied. The next step is fo the code engine to un the popagation codes in the equied sequence. The codes ae compiled.exe files and ae un using Matlab s! command. The popagation code(s) will poduce one o moe output files. The post pocesso includes an extensive set of outines fo eading the output files and plotting the esults. Use inteface AcTUP is menu diven, and most paametes ae enteed via dialog boxes. The exceptions to this ae sound speed pofiles, which can eithe be enteed into a dialog box o ead fom a file, and bathymety, which is always ead fom a file. Figue 1. Simplified block diagam of the opeation of Ac- TUP Cae has been taken to use detailed pompts in an attempt to make the opeation of the pogam as intuitive as possible. The fist item on the main menu shown in Figue povides access to the envionment and code paamete edito. The edito has changed substantially since AcTUI V1.6, patly to suppot ange-dependent envionments and patly to logically sepaate editing of the envionment (sound speed pofiles, wate depth and seabed geoacoustic popeties) fom editing of code-independent popagation un paametes (souce depth, eceive depths, anges, etc.). Code-dependent paametes ae now also specified by way of this edito, athe than at un-time as was the case in the ealie vesion. All paametes ae now saved as pat of the un definition, allowing much impoved epoducibility and documentation of uns compaed to the ealie vesion, which didn t save the code-dependent paametes. Whee multiple paamete values ae allowed they can be specified using any valid Matlab vecto specification. Fo example, specifying the fequency as 1:.5:5 would un the popagation code(s) at.5 Hz intevals fom 1 Hz to 5 Hz. The same method can be used to specify eceive depths. Fully ange-dependent envionments ae handled though the intoduction of an envionment aay, which is simply a numbe of envionment specifications, each coesponding to a paticula ange. The envionment aay edito window is shown in Figue 3 and includes a list of cuently specified envionments and contols fo editing, inseting, deleting and moving individual envionments. Thee envionments have been specified in the example shown hee, coesponding to anges of m, 1m, and 5m. AcTUP can automatically linealy intepolate between these envionments and output intepolated wate column and seabed sound speed pofiles to the popagation code. This can significantly impove the accuacy of esults calculated with the RAM vaiants because they assume step changes in wate column and seabed popeties at the anges at which they ae specified. In the exam- 474 Acoustics 6

5 Poceedings of ACOUSTICS 6 - Novembe 6, Chistchuch, New Zealand ple shown hee the pofiles will be intepolated and output to the popagation code at a 1m ange inteval. Figue 4. Envionment edito menu Figue. Main menu folde. This povides a simple way of setting up batch uns of abitay combinations of codes and paametes and is useful, fo example, fo unning a seies of time consuming calculations ovenight. Note that it is not necessay to use this option if fequency is the only paamete to be vaied, as this is handled moe simply by specifying a vecto of fequencies in the un definition. AcTUP includes an extensive set of plotting tools, which ae accessed via the plotting tools menu shown in Figue 5. These tools povide a simple and vey flexible way of visualising the esults of popagation code calculations, including tansmission loss plots and plots of intemediate esults such as mode shapes, depth-dependent Geens functions, and seabed eflection coefficients. EXAMPLE AcTUP can be used fo a wide vaiety of acoustic popagation poblems, so the choice of an example to demonstate its capabilities is athe abitay. The example chosen fo this pape is based on the sound speed field and bathymety shown in Figue 6. The sound speed field coesponds to a notional wam coe eddy in the uppe 5m of the wate column. Figue 3. Envionment aay edito window Individual envionments ae specified as a seies of layes in the same way as in ealie vesions. An example of the envionment edito window is shown in Figue 4, and has a vey simila layout and set of contols to the envionment aay edito window. The popeties of each laye ae specified as a function of depth fom the top of that laye. The wate depth is taken fom a bathymety file if one is specified, othewise the maximum depth given in the fist laye is used. The tansmission loss plots shown in figues 7 and 8 wee calculated using RAMGeo fo a fequency of 6 Hz, a souce depth of 1m and a fluid seabed with a sound speed of 1749 m.s -1, a density of 1941 kg.m -3 and no absoption. Figue 7 was calculated fo the ange-independent sound speed by applying the m ange sound speed pofile at all anges. Figue 8 is the fully ange-dependent esult. Figue 9 is a ay tace computed using BELLHOP fo the ange-independent sound speed case (BELLHOP can t deal with angedependent sound speed). The main menu shown in Figue includes thee un options that utilise the code engine shown in the block diagam in Figue 1. The Run Cuent model fo ACTIVE Popagation Code option is the most commonly used and uns the cuently specified popagation code using the paametes in the cuent un definition. The Run Cuent Model fo ALL Popagation Codes option uns all popagation codes using the paametes in the cuent un definition and is mainly included fo testing the opeation of the inteface. The Run Model(s) fom File(s) option uns the models and paametes specified by all the un definitions in a use specified Acoustics 6 475

6 - Novembe 6, Chistchuch, New Zealand Poceedings of ACOUSTICS 6 Figue 7. Tansmission loss at 6 Hz calculated using RAMGeo fo ange-independent sound speed pofile. Figue 5. Post pocessing menu Figue 8. Tansmission loss at 6 Hz calculated using RAMGeo fo ange-dependent sound speed pofile shown in Figue 6. Figue 6. Sound speed and bathymety fo ange-dependent example. Shading epesents sound speed, line epesents bathymety. Figue 9. Ray tace computed using BELLHOP fo the ange-independent sound speed pofile. Compaing figues 7 and 8 shows that the eddy significantly inceases the tansmission loss in the uppe 5 m of the wate column fo anges between 14 km and 4 km, but educes the tansmission loss at slightly deepe depths. This is because the uppe tuning points of the ay paths ae depessed by the wame wates of the eddy. Rangeindependent codes, and codes that can only handle angedependent bathymety, would not show this effect. 476 Acoustics 6

7 Poceedings of ACOUSTICS 6 CONCLUSIONS The elease of AcTUP V.l adds fully ange-dependent low fequency undewate acoustic popagation modelling to the aay of modelling tools povided by its pedecesso, AcTUI V1.6. The impoved inteface geatly simplifies the task of unning acoustic popagation models, especially when uns at multiple fequencies ae equied o whee it is desiable to apply seveal diffeent popagation codes to the same poblem. As was the case with its pedecesso, the inteface will be extemely useful as an educational tool as it allows students and instuctos to quickly and easily un a vaiety of popagation codes on poblems designed to highlight diffeent aspects of the physics of undewate acoustic popagation. The inteface will also be useful fo eseaches in undewate acoustics, paticulaly because it povides a quick and easy way of obtaining a fist look at a popagation poblem. In many cases it will be the only tool equied, but it is not a panacea: the desie to make the inteface as staightfowad as possible fo the aveage use has esulted in some loss of flexibility, and thee will be situations whee specialist modelles will need to wite thei own inteface code in ode to cay out a specific task. Thee is a substantial wish list of potential impovements to AcTUP that will be implemented ove time. Chief of these ae: Suppot fo a fully ange-dependent high fequency code. It is hoped to have a fully ange-dependent vesion of BELLHOP developed and incopoated into the inteface ealy in 7. Suppot fo coupled and adiabatic ange-dependent nomal mode codes. FIELD has the capability to compute tansmission loss in ange-dependent envionments, by - Novembe 6, Chistchuch, New Zealand eithe of these methods, using modes computed by KRAKEN o KRAKENC at a numbe of diffeent anges. Howeve, this capability is not suppoted by AcTUP V.l. Pe-pocessing functions to ead envionment data fom standad data bases and fom standad field data file fomats. AcTUP V.l can be downloaded fom the Cente fo Maine Science and technology web site: ACKNOWLEDGEMENTS The development of AcTUP V.l would not have been possible without financial suppot fom the Defence Science and Technology Oganisation (DSTO). REFERENCES Collins, M. D. 199, A self-state fo the paabolic equation method, J. Acoust. Soc. Am., 9 (4), Octobe 199, pp Collins, M. D. 1993, A split-step Padé solution fo the paabolic equation method, J. Acoust. Soc. Am., 93 (4), Apil 1993, pp Collins, M. D. 1999, Uses Guide fo RAM vesions 1. and 1.p. ftp://ftp.ccs.nl.navy.mil/pub/am/ram/am.ps Jensen, F. B., Kupeman, W. A., Pote, M. B., and Schmidt, H., Computational Ocean Acoustics, Spinge- Velag, New Yok, ISBN Pote, M. B., 5, Acoustic Toolbox, ox/ Acoustics 6 477