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Review of Ocean-Acoustic Models

Paul C. EtterNorthrop Grumman Corporation

Electronic SystemsBaltimore, Maryland 21203

Abstract - This paper describes acoustic models that can

generate analytical metrics in support of naval operations,

particularly in coastal oceans. Coastal environments are

generally characterized by high spatial and temporal

variabilities. When coupled with acoustic spectral

dependencies of the surface and bottom boundaries, these

natural variabilities make coastal regions very complex

acoustic environments. Thus, accurate modeling of the acoustic

environment is essential for prediction of sonar performance in

coastal oceans. The current inventory of ocean-acoustic models

comprises 126 propagation models, 19 noise models, 26

reverberation models and 34 sonar-performance models.

Approximately 18 percent of this inventory is tailored for

shallow-water applications. When coupled with coastal

atmosphere-ocean models, these ocean-acoustic models can

generate sophisticated prognostic and diagnostic metrics in

support of naval operations in coastal oceans.

I. INTRODUCTION

Ocean acoustics entails the development and

employment of acoustical methods to image underwater

features, to communicate information via the oceanic

waveguide, or to measure oceanic properties. In the present

context, ocean acoustics encompasses both the science and

the technology necessary to deploy functioning acousticalsystems in support of naval operations.Broadly defined, modeling is a method for organizing

knowledge accumulated through observation or deduced

from underlying principles. Modeling applications fall into

two basic categories: prognostic and diagnostic. Prognostic

applications include prediction and forecasting functions

where future oceanic conditions or acoustic sensorperformance must be anticipated. Diagnostic applications

include system-design and analysis functions typically

encountered in engineering tradeoff studies.

II. CHALLENGES IN COASTAL ENVIRONMENTS

Coastal environments are generally characterized by

high spatial and temporal variabilities. When coupled with

attendant acoustic spectral dependencies of the surface andbottom boundaries, these natural variabilities make coastal

regions very complex acoustic environments. Specifically,

changes in the temperature and salinity of coastal waters

affect the refraction of sound in the water column. These

refractive properties have a profound impact on thetransmission of acoustic energy in a shallow-water

waveguide with an irregular bottom and a statistically

varying sea surface. Thus, accurate modeling and prediction

of the acoustic environment is essential to an understanding

sonar performance in coastal oceans.

Physical processes controlling the hydrography of shelf

waters often exhibit strong seasonal variations. Annual

cycles of alongshore winds induce alternating periods of

upwelling and downwelling. The presence of coastal jets

and the frictional decay of deep-water eddies due to

topographic interactions further complicate the dynamics of

coastal regions. Episodic passages of meteorological fronts

from continental interiors affect the thermal structure of the

adjacent shelf waters through intense air-sea interactions.

River outflows create strong salinity gradients along the

adjacent coast. Variable bottom topographies and sediment

compositions with their attendant spectral dependenciescomplicate acoustic bottom boundary conditions. At higherlatitudes, ice formation complicates acoustic surface

boundary conditions near the coast. Waves generated by

local winds under fetch-limited conditions, together with

swells originating from distant sources, conspire to

complicate acoustic surface boundary conditions and also

create noisy surf conditions. Marine life, which is oftenabundant in nutrient-rich coastal regions, can generate or

scatter sound. Anthropogenic sources of noise are common

in coastal seas including fixed sources such as drilling rigs

and mobile sources such as merchant shipping and fishing

vessels. Surface weather, including wind and rain, further

contribute to the underwater noise field. Even noise fromlow-flying coastal aircraft can couple into the water column

and add to the background noise field.

Over the past decade, naval mission requirements have

shifted from open-ocean operations to shallow-water (or

littoral) scenarios. For convenience, shallow water will be

defined by water depths less than 200 meters. This has notbeen an easy transition for sonar technologists since sonar

systems that were originally designed for operation in deep

water seldom work optimally in coastal regions. This has

also held true for modeling and simulation (M&S)

technologies, which have undergone a redefinition andrefocusing to support a new generation of multistatic naval

systems that are intended to operate efficiently in littoralregions while still retaining a deep-water capability.

Shallow-water geometries increase the importance of

boundary interactions, which diminish acoustic energy

through scattering and also complicate localization of dieselsubmarines and coastal mines due to multipath propagation.

Moreover, the higher levels of interfering noises

encountered in coastal regions combined with higher levels

of boundary reverberation mask signals of interest. In

advance of naval deployments, synoptic meteorological and

oceanographic (METOC) measurements are often requiredin remote or hostile (i.e., harsh or heavily defended) coastal

environments to forecast acoustic sensor performance.

Coupled atmosphere-ocean-acoustic models could reduce

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the need for hazardous in-situ data collection by numerically

computing initial states for the embedded acoustic models.

Recently, acoustical oceanographers have employedocean-acoustic models as adjunct tools that can be used to

conduct rapid environmental assessments (REA) in remote

locations. Due to an increased awareness of the potential

technological impacts on marine life, naval commanders and

acoustical oceanographers must also be aware of new

environmental regulations governing the acoustic emissionsof their sonar systems.

In shallow water, interactions of the acoustic fields with

the sea bed require an understanding of the sedimentary

structure of the bottom to a level of detail that is usually not

required in deep-water environments. In the forward-

propagation case, this means that a significant amount ofinformation is necessary to properly characterize the bottom

boundary to ensure the generation of high-fidelity model

outputs. This generally requires a good understanding of the

physics of bottom-interacting acoustics in diverse ocean

environments.

III. MODELINGCAPABILITIES

Ocean-acoustic models translate our physical

understanding of sound in the sea into mathematical

formulas solvable by computers. Consistent with previous

work [1], the principal categories of ocean-acoustic modelscomprise environmental, propagation, noise, reverberation

and sonar performance. The hierarchical relationship among

these model categories is illustrated in Fig. 1.

It is important to note the fundamental importance of

propagation models in the subsequent modeling of noise and

reverberation. The inclusion of system-specific parameters

(e.g., sonar and target) is implicit in this schematic. Moredetailed flow charts available in Ref. [1] make such

information explicit in the computational chain.

Environmental models include physics-based or

empirical algorithms that are used to quantify the boundary

conditions (surface and bottom) and volumetric effects of

the ocean environment. Such models include, for example,

sound speed, absorption coefficients, surface and bottom

reflection losses and surface, bottom and volume

backscattering strengths.

Figure 1. Generalized relationships among environmental models, basicacoustic models and sonar performance models [1].

Basic acoustic models comprise propagation (or

transmission loss), noise and reverberation models. Sonar

performance models are composed of environmentalmodels, basic acoustic models and appropriate signal

processing models.

The ocean environment is characterized in terms of

boundary conditions (surface and bottom) and volumetric

effects. Shallow-water environments are typically

characterized by a sloping bottom (wedge geometry);moreover, continental weather, river discharge or ice

formation heavily influence shallow-water conditions.

Deep-water environments tend to be characterized by a flat

bottom and relatively stable water-column properties.

Therefore, models of acoustic propagation, noise and

reverberation intended for deep-water applications seldomperform well in shallow-water. This performance dichotomy

stems from the need for range-dependent models in shallow

water whereas range-independent models may be adequate

for many deep-water applications.

As sound propagates through the ocean, the effects of

spreading and attenuation diminish its intensity. Spreading

loss includes spherical and cylindrical spreading losses in

addition to focusing effects. Attenuation loss includes losses

due to absorption, leakage out of ducts, scattering and

diffraction. Propagation losses increase with increasing

frequency due largely to the effects of absorption. Soundpropagation is profoundly affected by the conditions of the

surface and bottom boundaries of the ocean as well as by the

vertical and horizontal distribution of sound speed within

the ocean volume. Sound-speed gradients introduce

refractive effects that may focus or defocus the propagating

acoustic energy.Noise is the prevailing, unwanted background of sound

at a particular location in the ocean at a particular time. The

local noise field is thus characterized by temporal, spatial

and spectral variabilities. The noise generated by natural or

anthropogenic point sources is diminished through the

effects of propagation to the sonar receiver.Reverberation is sound that is scattered by the ocean

boundaries or by the volumetric inhomogeneities.

Reverberation-producing scatterers in the ocean can be

grouped into three classes: sea surface, sea floor and ocean

volume. Reverberation is produced by the sonar itself;therefore, the spectral characteristics are essentially the

same as the transmitted sonar signal. The intensity of

reverberation varies with the range of the scatterers (due to

propagation loss) and also with the intensity of the

transmitted signal.

The performance of a passive sonar (i.e., one thatdetects sound emitted from a target of interest) could be

modeled using the appropriate environmental descriptors

together with suitable propagation-loss and noise models.

Sonar-system characteristics would be included in

appropriate signal-processing models. The performance of

an active sonar (i.e., one that transmits an interrogationsignal and then detects the echo returned from a target of

interest) could be modeled using the appropriate

environmental descriptors together with suitable

propagation-loss, noise and reverberation models. Sonar-

system characteristics are included in the appropriate signal-processing models.

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A. Propagation Models

Propagation models are integral to the higher-level

modeling of noise, reverberation and, ultimately, sonarperformance (refer to Fig. 1). The categorization of

propagation models into five distinct techniques follows that

of Ref. [1]: Ray Theory, Normal Mode, Multipath

Expansion, Fast Field (or Wavenumber Integration) and

Parabolic Equation. A further division can be made

according to range-independent (1D, or depth-dependenceonly) or range-dependent environmental specifications,

where environmental range-dependence can be 2D (depth

and range) or 3D (depth, range and azimuth). Since all five

techniques are derived from the wave equation by restricting

solutions to the frequency domain, the resulting models are

appropriate for traditional sonar applications. (Solutionsobtained in the time domain would be appropriate, for

example, for modeling shock propagation in the ocean.)

Each of the five techniques has a unique domain of

applicability that can be defined in terms of acoustic

frequency and environmental complexity. These domains

are determined by the assumptions that were invoked in

deriving each solution. Hybrid formulations obtained by

combining two or more different techniques are often

developed to improve domain robustness. Table I (at the

back) provides a comprehensive summary of stand-alone

propagation models. Use of 1D (range-independent) modelsmay be an appropriate approximation for stable shallow-

water environments with locally flat bottoms; otherwise,

2D/3D models are recommended.

To provide compact summaries, propagation models are

arranged in categories reflecting the basic modeling

technique employed (i.e. the five canonical approaches) aswell as the ability of the model to handle environmental

range dependence. Such factors define what is termed

domains of applicability. Hybrid models occasionally

compromise strict categorization, and some arbitrariness has

been allowed in this classification process. The

environmental range dependence considers variations insound speed or bathymetry. Other parameters may be

considered to be range-dependent by some of the models,

although they are not explicitly treated in this summary.

The specific utility of these categories is further

explained below. In applying ocean-acoustic propagationmodels, the analyst is normally faced with a decision matrix

involving water depth (deep versus shallow), frequency

(high versus low) and range-dependence (range-independent

versus range-dependent ocean environments). The following

assumptions and conditions were imposed in construction of

Fig. 2, which was originally adapted from F.B. Jensen (seeRef. [1]):

Shallow water includes those water depths for whichthe sound can be expected to interact significantly with

the sea floor. Typically, a maximum depth of 200 m is

used to delimit shallow water regions.

The threshold frequency of 500 Hz is somewhatarbitrary, but it does reflect the fact that above 500 Hz,

many wave-theoretical models become computationally

intensive. Also, below 500 Hz, the physics of some ray-

theoretical models may become questionable due to

restrictive assumptions. A solid circle indicates that the modeling approach is

both applicable (physically) and practical

(computationally). Distinctions based on speed of

execution may change as progress is made in

computational capabilities. A partial circle indicatesthat the modeling approach has some limitations in

accuracy or in speed of execution. An open circle

indicates that the modeling approach is neither

applicable nor practical.

Fig. 2 has been modified in two important respectsrelative to previous versions [1]. Specifically, a range-

dependent capability has been explicitly added to the

multipath-expansion and the fast-field (or wavenumber

integration) approaches. This represents a significant change

from the corresponding chart presented in the 2003 baseline

[1]. This change is warranted by the substantial progressmade by modelers over the past several years.

Table I identifies thirteen new stand-alone propagation

models; the new models are bordered by a dashed line. New

models are those that have been added to the inventory since

2003. Twelve of the thirteen new propagation models are

range-dependent, and there is at least one new model in each

of the five categories. (Note that RAMGEO, a new addition

to the existing RAM/RAMS family, handles sediment layers

that are range dependent and parallel to the bathymetry.)

There are three community standard propagation models:

two range-dependent normal-mode models (ASTRAL andNAUTILUS) and one range-dependent parabolic equation

model (NSPE). Community standard models are those

computer codes that are configuration managed by the US

Navys Oceanographic and Atmospheric Master Library

(OAML).

Taken together, Fig. 2 and Table I provide a usefulmechanism for selecting a subset of candidate models once

some preliminary information is available concerning the

intended applications. Note that range-dependent models

can also be used for range-independent environments by

inserting a single environmental description to represent the

entire horizontal range.

Figure 2. Updated domains of applicability of ocean-acousticpropagation models (adapted from Ref. [1]).

B. Noise ModelsNoise models can be segregated into two categories:

ambient-noise models and beam-noise statistics models.

Ambient-noise models are applicable over a broad range of

frequencies and consider noise originating from surface

weather, biologics, shipping and other commercial activities.

Beam-noise statistics models predict the properties of low-frequency shipping noise using either analytic (deductive) or

simulation (inductive) methods.

RI RD RI RD RI RD RI RD

Ray theory

Normal mode

Multipath expansion

Fast field

Parabolic equation

L ow fre que ncy (< 50 0 Hz) RI: Ra nge-indepen den t en viro nment

High frequ enc y (> 500 Hz) RD: Ra nge -de pen den t e nvi ronment

Modeling approach is both applicable (physically) and practical (computationally)

Limitations in accuracy or in speed of execution

Neither applicable or practical

Model type

Applications

Shallow water Deep water

Low frequency High frequency Low frequency High frequency

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Table II (at the back) identifies two new stand-alone

noise models; the new models are bordered by a dashed line.

Noise models do not necessarily include sonar self-noisecomponents. New models are those that have been added to

the inventory since 2003. There is one community standard

ambient noise model (which is also a new model): DANM

(Dynamic Ambient Noise Model).

C. Reverberation Models

Reverberation models can be categorized according to

cell-scattering or point-scattering techniques. Cell-scattering

formulations divide the ocean into cells, where each cell

contains a large number of uniformly distributed scatterers.

Point-scattering formulations assume a random distributionof (point) scatterers. Table III (at the back) identifies nine

new stand-alone reverberation models; the new models are

bordered by a dashed line. New models are those that have

been added to the inventory since 2003. At present, there are

no (stand-alone) community standard reverberation models.

D. Sonar Performance Models

Sonar performance models combine environmental

models, propagation models, noise models, reverberation

models and appropriate signal-processing models to solve

the sonar equation.Available sonar performance models (subcategorized as

active sonar models, model operating systems and tactical

decision aids) are summarized in Table IV (at the back); the

eight new models are bordered by a dashed line. New

models are those that have been added to the inventory since

2003. Most of the active sonar models listed in Table IV areintended principally for use in ASW scenarios, although

four of these models (CASTAR, MINERAY, SEARAY and

SWAT) are designed for use in mine-hunting scenarios.

Model-operating systems provide a framework for the direct

linkage of data-management software with computer-

implemented codes of acoustic models, thus facilitating theconstruction of versatile simulation capabilities. Model-

operating systems are further distinguished from stand-alone

active sonar performance models by virtue of their ability to

conduct sensitivity analyses by computing components of

the active-sonar equation using alternative solutiontechniques. Since sonar model operating systems normally

utilize existing ocean-acoustic models and standard

oceanographic databases, these systems are unique only in

the sense of the number and types of models and databases

included, and the particular architectures, GUIs and other

features employed. Tactical decision aids represent a formof engagement-level simulation that blends environmental

information with tactical rules. These decision aids guide

system operators and scene commanders in planning

missions and allocating resources by exploiting knowledge

of the operating environment.

Table IV identifies five new stand-alone active sonarmodels, one new model operating system, and one new

tactical decision aid. ASPM and CASS are designated as

community standard models.

IV. COUPLED COASTAL-OCEAN MODELS

A review of requirements for coupling ocean-acoustic

and atmosphere-ocean models in coastal environments

suggests a broad architectural plan for their commonintegration. Issues critical to the further integration of

atmosphere-ocean-acoustic modeling technologies have also

been identified including high-level architectures (HLA),

data standards and V V & A (verification, validation andaccreditation) standards. Collectively, these initiatives seek

to promote modeling reuse and interoperability amongdiverse user communities.

In Fig. 1, the canonical (pyramid) hierarchy indicates

that environmental data (derived either from in-situ

measurements or from historical data) are used to initialize

propagation models. Once initialized, the propagation

models are used in tandem with noise or reverberationmodels, or both. Alternatively, a coupling scheme could be

used to initialize the propagation models with high-fidelity

forecast data (generated by coupled coastal-ocean models)

in place of in-situ data. By high fidelity is meant

spatial/temporal resolutions and coverages sufficient tosupport the initialization of 2D/3D range-dependent modelsin shallow-water environments. Such a capability is usually

beyond that afforded by traditional in-situ data collection

performed by isolated operating units.

Coupling ocean-acoustic models with coastal

atmosphere-ocean models could substantially enhance theprovision of timely and comprehensive environmental

descriptors to these acoustic models. When coupled with

coastal atmosphere-ocean models, these ocean-acoustic

models can generate sophisticated prognostic and diagnostic

metrics in support of naval operations in coastal oceans.

V.SUMMARY

This paper describes acoustic models that can generate

analytical metrics in support of naval operations in coastal

oceans. Coastal environments are generally characterized by

high spatial and temporal variabilities. Thus, accurate

modeling of the acoustic environment is essential for

prediction of sonar performance in coastal oceans.

The current inventory of ocean-acoustic modelscomprises 126 propagation models, 19 noise models, 26

reverberation models and 34 sonar-performance models.

Approximately 18 percent of this inventory is tailored for

shallow-water applications. These models are summarized

in charts that discriminate attributes useful in coastalapplications. Selection criteria based on these attributes are

presented to guide scientists and engineers in matching

appropriate acoustic-modeling capabilities with acoustic-

sensing requirements in the coastal ocean. When coupled

with coastal atmosphere-ocean models, these ocean-acoustic

models can generate sophisticated prognostic and diagnostic

metrics in support of naval operations in coastal oceans.

REFERENCES[1] P.C. Etter, Underwater Acoustic Modeling and Simulation, 3rd ed.

London: Spon Press, 2003.

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ANDES

AMBENT BBN Shipping Noise

ARAMIS BTL

CANARY USI Array Noise

CNOISE Sonobuoy Noise

DANES

DANM

DINAMO BEAMPL

DUNES DSBN

FANM NABTAM

Normal Mode Ambient Noise

RANDI - I / II / III

Ambient Noise Beam-Noise Statistics

Analytic

Simulation

TABLE I. SUMMARY OF OCEAN-ACOUSTIC PROPAGATION MODELS.THETHIRTEEN NEW STAND-ALONE MODELS ARE BORDERED BY A DASHED

LINE.

TABLE II. SUMMARY OF OCEAN-ACOUSTIC NOISE MODELS.THE TWO

NEW STAND-ALONE MODELS ARE BORDERED BY A DASHED LINE.

Technique

CAPARAY PLRAY ACCURAY GRAB LYBIN MPP RAYWAVE

FACT RANGER BELLHOP GRASS LYCH Pedersen RP-70

FLIRT Coherent DELTA HARORAY MEDUSA PlaneRay SHALFACT

GAMARAY FACTEX HARPO MIMIC PWRC TRIMAIN

ICERAY FeyRay HARVEST MPC RAYSON XRAY

AP-2/5 MODELAB ORCA ADIAB CPMS NAUTILUS WEDGE

BDRM NEMESIS POPP ASERT FELMODE PROLOS WKBZ

COMODE NLNM PROTEUS ASTRAL Kanabis PROSIM WRAP

DODGE NORMOD3 SHEAR2 CENTRO KRAKEN SHAZAM 3D Ocean

FNMSS NORM2L Stickler CMM3D MOATL SNAP / C-SNAP

COUPLE MOCTESUMA SWAMP

FAME NEPBR Integrated Mode

MULE RAYMODE

FFP OASES SAFARI CORE RD-OASES SAFRAN

Kutschale FFP Pulse FFP SCOOTER OASES-3D RDOASP

MSPFFP RPRESS SPARC RDFFP RDOAST

AMPE / CMPE HAPE OWWE Spectral PE

CCUB / SPLN / CNP1 HYPER PAREQ TDPE

Corrected PE IFD Wide Angle PDPE Two-Way PE

DREP IMP3D PECan ULETA

FDHB3D LOGPE PE-FFRAME UNIMOD

FEPE MaCh1 PESOGEN 3DPE (NRL-1)

FEPE-CM MOREPE PE-SSF (UMPE / MMPE) 3DPE (NRL-2)

FEPES NSPE RAM / RAMS / RAMGEO 3DTDPA

FOR3D OS2IFD SNUPE 3DWAPE

Use Single Environmental Specification

Fast Field or

Wavenumber

Integration

Parabolic

Equation

Multipath

Expansion

Range Independent Range Dependent

Ray Theory

Normal Mode

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Monostatic Bistatic Monostatic BistaticC-SNAP-REV ARTEMIS REVGEN Under-Ice Reverberation

DOP BAM RITSHPA Simulation

EIGEN / REVERB BiKR

MAM BiRASP

PAREQ-REV BISAPP

PEREV BISSM

PERM-2D BISTAR

REVMOD OGOPOGO

REVPA RASP

REVSIM RUMBLE

R-SNAP S-SCARAB

TENAR

Cell Scattering Point Scattering

Active RAYMODE MOCASSIN

ALMOST MOC3D

ASPM MODRAY

CASTAR MSASM

CONGRATS NISSM - II

ESPRESSO SEARAY

GASS SONAR

HODGSON SST

INSIGHT SUPREMO

INSTANT SWAMI / DMOS

LIRA SWAT

LORA UAIM

MINERAY

CAAM PRISM ASPECT

CASS SPPS IMAT

GSM - Bistatic NECTA

HydroCAM

Active Sonar Models

Model Operating Systems Tactical Decision Aids

TABLE III. SUMMARY OF OCEAN-ACOUSTIC REVERBERATION MODELS.

THE NINE NEW STAND-ALONE MODELS ARE BORDERED BY A DASHED

LINE.

TABLE IV. SUMMARY OF SONAR PERFORMANCE MODELS.THE EIGHT

NEW STAND-ALONE MODELS ARE BORDERED BY A DASHED LINE.