DTIC ADA533925: Effect of Internal Solitary Waves on Underwater Acoustic Propagation

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by  Defense Technical Information Center

P A P E R
Effect of Internal Solitary Waves on
Underwater Acoustic Propagation
AUTHORS
ABSTRACT
Peter C. Chu Internal solitary waves (ISWs) were observed on 07:52–09:36 GMT July 30,
Chung-Ping Hsieh
2005, in the Philippine Sea near Taiwan from high-resolution temperature sam-
Naval Ocean Analysis pling.TheeffectofISWsonacousticpropagationwasidentifiedusingtheNavy’s
and Prediction Laboratory,
ComprehensiveAcousticSimulationSystemthroughcomparisonbetweenrange-
Naval Postgraduate School independent and range-dependent sound speed profiles. The ISWs enhance the
soundpropagationslightly(0–3dB)innear-range,weakenorenhancethesound
propagation(−20to20dB)evidentlyinmidrange,andalwaysweakenthesound
Introduction propagation(upto−20dB)infarrange.TheISW’seffectontheacousticpropagation
N varieswithsoundfrequencyandsoundsourcedepth.Thisworkprovidesamethod-
onlinear propagating internal
ologytoanticipatepossibleerrorsintransmissionlossestimationinanoperational
solitarywaves(ISWs)areaubiquitous
framework,ifnofurtheradditionaldataareavailable.
feature of the coastal ocean. They
Key Words: Acoustic detection, CASS/GRAB, Coastal monitoring buoy, Internal
have been observed as sharp depres-
solitary wave, Ray path
sions of a near surface pycnocline
thatareoftenseenatthesurfaceasal-
ternating bands of slicks and rough
patches (i.e., Apel et al., 1985; Chu
FIGURE 1
and Hsieh, 2007b). The effect of
ISWs on sound propagation has been TopographyofthewesternPhilippineSeaandsurroundingareaswiththesymbol“+”indicating
investigated extensively in the conti- the location where the ISWs were observed. Note that the ISWs were observed in deep water
nental shelves such as the Yellow Sea with water deptharound 5km.
(Zhou and Zhang 1991) and north
of Lisbon (Rodriguez et al. 2000).
Zhou and Zhang (1991) claimed that
ISWs contribute to an important loss
mechanism for shallow water sound
propagation. However, Rodriguez
et al. (2000) argued that ISWs cause
successive signal attenuation and sig-
nal amplification (called acoustic fo-
cusing). It is noted that the signal
attenuation or amplification as ob-
served and modeled by these studies
dependonfrequencyandpropagation
angle of the nonlinear internal waves.
Usually, the effect of ISWs on
sound propagation is masked by
boundary interactions. For a deep sea
suchasthePhilippineSea,thesurface
and the bottom acoustic interactions
are weak. Thus,theeffect ofISWson
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Marine Technology Society Journal
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1. REPORT DATE 3. DATES COVERED
2010 2. REPORT TYPE 00-00-2010 to 00-00-2010
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Effect of Internal Solitary Waves on Underwater Acoustic Propagation
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Naval Postgraduate School,Naval Ocean Analysis and Prediction REPORT NUMBER
Laboratory,Monterey,CA,93943
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
Internal solitary waves (ISWs) were observed on 07:52?09:36 GMT July 30 2005, in the Philippine Sea
near Taiwan from high-resolution temperature sampling. The effect of ISWs on acoustic propagation was
identified using the Navy?s Comprehensive Acoustic Simulation System through comparison between
rangeindependent and range-dependent sound speed profiles. The ISWs enhance the sound propagation
slightly (0?3 dB) in near-range, weaken or enhance the sound propagation (−20 to 20 dB) evidently
in midrange, and always weaken the sound propagation (up to −20 dB) in far range. The ISW?s
effect on the acoustic propagation varies with sound frequency and sound source depth. This work
provides a methodology to anticipate possible errors in transmission loss estimation in an operational
framework, if no further additional data are available.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 7
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
fi
sound propagation can be effectively face, surface winds, air temperature, Sound Speed Pro les
identified. Recently, ISWs were ob- and air pressure were measured. Be- During07:52–09:36GMTJuly30,
served during 07:52–09:36 GMT lowtheoceansurface,thetemperature 2005, 10 temperature profiles were
July 30, 2005, in the Philippine Sea was observed at 1, 3, 5, 10, 15, and observed between 0 and 20 m from
(water depth deeper than 5 km) near 20 m as shown in Figure 2. During the CMB sensors (at 1, 3, 5, 10, 15,
Taiwan (Figure 1) from the free drift- the observational period (July 28– 20 m) with a sampling rate of
ing coastal monitoring buoy (CMB) August 7, 2005), the CMB traveled 1/(10 min). About 424 temperature
(Figure 2) deployed by the U.S. Na- 229.14 km along the track (Figure 3) profiles were obtained between 25 and
val Oceanographic Office (Chu and with an average speed of 0.267 m/s. 140 m with sampling rate of 1/(15 s)
Hsieh 2007a,b). Thesurfacewindswerelight,fluctuating from the fifteen thermistors attached
TheoriginaldesignofCMBwasto from 3.3 m/s (minimum) to 4.2 m/s to a wire rope from CMB (at 25, 30,
collectdataevery10minneartheair– (maximum) with a mean wind speed 35,40,45,50,55,60,65,70,75,80,
ocean interface. Above the ocean sur- of3.8m/s(Figure4). 100, 130, 140 m). One temperature
FIGURE 2
FIGURE 3
ThefreedriftingCMBusedintheWPSsurvey.
Fifteenthermistorsareattachedtoawirerope Track ofCMB (from July 28 toAugust 7,2005) deployedby the NavalOceanographic Office.
extendingfromthecodeofCMB(20mdeep)
to140mwithhigh-frequencysamplingrate
(every15s).
FIGURE 4
Wind speed during0700–1200GMT July 30,2005,observed from CMB.
September/October 2010 Volume44 Number 5 11
FIGURE 5 profile was used between 140 and
760mfromanexpendablebathyther-
MeanSSP for range-independent simulation.
mograph and one temperature profile
below 760 m from the Navy’s Gen-
eralized Digital Environment Model
(GDEM)(averageofJulyandAugust)
nearesttotheCMB.
The low-rate sampling data above
25 m (10 profiles) could not resolve
propagation of ISWs. Below 140 m,
only one profile was available. Thus,
a temporally averaged profile was cal-
culated from 10 profiles to represent
the upper layer (above 25 m depth)
thermal structure. The temporally in-
variant data in the upper layer (above
FIGURE 6
25 m) and the lower layer (below
Soundspeedbetween25and140musedformodelsimulationfromthetemperaturefieldobserved 140 m) was used for the whole obser-
from0752to0936GMTJuly30,2005:(a)time-varying424SSPsand(b)depth–timecrosssection vational period (07:52–09:36 GMT
ofthesoundspeed.Notethatduringtherange-dependentacousticsimulationusingCASS,therange-
July30,2005).Thus,424temperature
dependentportionoftheSSPswasatdepthsbetween25and140mfrom0752to0936GMTJuly30, profilesfromthesurfacetothebottom
2005.OtherwisetheSSPisthesameasthemeanprofileshowninFigure5.
wereobtainedwithtemporalvariation
onlybetween25and140m.Chuand
Hsieh(2007b)identifiedthepropaga-
tionofISWswithaperiodof7min.
Salinity was not measured in this
survey. To calculate sound speed, the
GDEM (average of July and August)
salinity profile (nearest to CMB) was
used. The sound speeds calculated
from the GDEM salinity profile and
time average of 424 temperature pro-
files formed the mean sound speed
profile (SSP) (Figure 5). The sound
speedscalculatedfromtheGDEMsa-
linityprofileand424temperaturepro-
files generated the range-dependent
SSPs (Figure 6). Comparison of the
acoustic propagation between range-
dependent and range-independent
determined the effect of ISWs on
acoustic propagation.
Modeling
Sonar Model
Arange-independentgenericsonar
model has evolved into the Navy’s
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Marine Technology Society Journal
range-dependent Comprehensive Oceanographic Office’s database (Na- the nominal acoustic direction. When
Acoustic Simulation System with the val Oceanographic Office System In- the propagation angles are larger,
Gaussian Ray Bundle eigenray model tegration Division, 1999a). Bottom these velocities can be much smaller,
(CASS/GRAB) for acoustic and sonar reflection effects are modeled using producing different length scales in
analysis. It predicts range-dependent the Rayleigh scattering model. Two whichtimevariabilitymaybepredom-
acoustic propagation in the frequency frequencies(3.75 and 7.5kHz)with a inant.Here,arangeof10kmisselected
band between 600 Hz and 100 kHz bandwidthof500Hzandthreesource/ to investigate the ISW effects on the
(WeinbergandKeenan1996;Keenan transmitter depths (SD) (5, 100, and soundpropagation.
and Weinberg 2001). Test rays are 200m)areused.Eigenraysaregenerated
sorted into families of comparable in the mono-static active mode maxi-
numbers of turning points and bound- mum surface/bottom reflections less ISW Effects
ary interactions. Ray properties are than 30°. The vertical angle changes TheCASS/GRABmodelisprimed
power averaged for each ray family to from −89° to 89° with the increment with range-independent SSP (Fig-
produce a representative eigenray for of0.1°. ure5)andrange-dependentSSPs(Fig-
thatfamily.Targetecholevelandrever- The calculated ISW propagation ure 6). Three source depths (5, 100,
berationlevelarecomputedseparately. velocities are around 1.5 m/s. From and 200 m) were chosen to place the
CASS/GRAB predicts the sonar 07:52 to 09:36 GMT July 30, 2005 source above, within, and below the
performancereasonablywell,givenen- (104min),theISWstraveled9.36km. thermocline. In addition, wind speed
vironmentalinputdatasuchasbottom It is noted that the estimate of spatial of 3.8 m/s, 0° source angle, and 10°
type,SSP,windspeed,andtiltangleof scale(i.e.,9.36km)isbasedontheas- Gaussian ray bundle were used as in-
thesoundsource(WagstaffandKeenan sumption of small propagation angles putsforalloftheCASS/GRABmodel
2003). It hassuccessfully modeled tor- between the ISW propagation and runs. We chose transmission loss (TL)
pedo acoustic performance in shallow
water exercises off the coast of South-
FIGURE 7
ern California and Cape Cod and is
currently being developed to simulate Transmissionloss(left)andraytrace(right)from−10°and+10°ofrange-independentcasesof
mine warfare systems performance in f=3.75kHzatdifferentsourcedepthsof(a)5,(b)100,and(c)200m.
the fleet (Keenan and Weinberg 1995;
ChuandVares2004).
Model Integration
The Wilson (1960) equation for
temperature–salinity–sound speed
conversion is used. GRAB defaults to
the Leroy equation (Leroy, 1969) for
sound speed conversions, where nu-
merically stable polynomials are fitted
to Wilson’s data. The environmental
parameters for the CASS/GRAB in-
put file (Naval Oceanographic Office
System Integration Division, 1999a,b)
consist primarily of data taken at the
CMBbuoy.Thesurfacewindcollected
by the CMB during the ISW event is
used as an input into CASS/GRAB.
Thewaterdepthis5,400m.Thebot-
tomsedimenttypeissiltyclaywithout
apparentlayerstructurefromtheNaval
September/October 2010 Volume44 Number 5 13
(an output from CASS/GRAB) as the to −87 dB for Area 3 (Figure 7). For weakensaround−20dBinArea3(Fig-
primary parameter to investigate the higher frequency (f = 7.5 kHz), the ure 9). Such a feature in Area 2 is very
ISW effects on sound propagation. TLiscomparabletothatwithlowerfre- evident for the shallow sound source
Here, TL always takes on negative quency (f = 3.75 kHz) in Area 1 and (SD = 5 m) and less evident for the
values. Area 2 but enhances in Area 3 with deeper sound source (SD = 100,
varyingfrom−78to−90dB(Figure8). 200 m). However, a strong surface
Range-Independent weakening zone with Δ(TL) ∼−20 dB
Range–depthcrosssection(range= TL Difference occurs for deeper sound source. The
10 km; depth = 1000 m) is divided The TL difference [Δ(TL)] be- thickness of this surface weakening
intothreeparts,withArea2beingoc- tween range-dependent (TL ) zone is around 15 m for SD = 100 m
dependent
cupied by the Gaussian ray bundle and range-independent (TL ) and50mforSD=200m.Theevident
dependent
(midrange). Left and right of Area 2 represents the ISW effect on sound weakeninginArea3isforbothfrequen-
are defined by Area 1 (near range) propagation. Since TL is negative, ciesandthreeSDvalues.
and Area 3 (far range) (Figures 7a and the sound waves attenuate more be- SinceISWs(usuallyforatwo-layer
8a). For a range-independent SSP, cause of ISWs when Δ(TL) < 0 and fluid)canpropagateatdifferentangles
the TL has distinct characteristics attenuate less when Δ(TL) > 0. The (i.e., the crests of the ISWs might not
among the three areas: (1) weak TL sound signal enhances little in Area 1 be normal to the range–depth plane),
for Area 1, (2) intermediate TL for [i.e., Δ(TL) ∼0–3 dB], weakens (up to the maximum value of |Δ(TL)| is a
Area 2, and (3) strong TL for Area 3. −20 dB for SD = 5 m and −10 dB for useful estimationofeffectof ISWs on
For f = 3.75 kHz, the TL varies from SD = 100, 200 m) and enhances (up acoustic transmission. For a shallow
−40 to −65 dB for Area 1, from −65 to 20 dB for SD = 5 m and 5–8 dB source depth (SD = 5 m), the maxi-
to −73 dB for Area 2, and from −73 for SD = 100, 200 m) in Area 2, and mum value of |Δ(TL)| is 20 dB. For
source depth at 100 and 200 m, the
maximum TL reduction because of
FIGURE 8
ISWpropagationis−20dB.Thesees-
Transmissionloss(left)andraytrace(right)from−10°and+10°ofrange-independentcasesof timatescouldbeusedasareferenceto
f =7.5 kHzatdifferent source depths of (a)5,(b) 100, and(c) 200m. assesstherelevanceofusingtherange-
dependent estimate if Δ(TL) for each
individual case falls outside this range,
that is, the focusing errors might be
largerthanthetimevariability.
It is noted that the color contour
plots(Figures8and9)havesignificant
steppystructuresalongrays.Suchfea-
tures occurred in a recent statistical
study on the estimation of TL and its
statistical properties on the basis of
observations of ocean acoustic data
acquired during the Asian Seas In-
ternational Acoustics Experiment
(ASIAEX) 2001 in the East China
Sea (Dahl et al. 2004). During
ASIAEX, the environmental param-
eters are first estimated. Then on the
basis of the likelihood that each of
these environmental models fits the
ocean acoustic data, each model is
mapped into TL. Such “steppy” may
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Marine Technology Society Journal
FIGURE 9 (2)Weshouldbecautioustointer-
pret these results because of the mean
EffectofISWsonacoustictransmissionΔ(TL)(dB)withISWpropagationspeedof1.5m/sfor
SSPprofilecalculatedfrom10routine
(a)sourcedepthof5mandf=3.75kHz,(b)sourcedepthof5mandf=7.5kHz,(c)source
depthof100mandf=3.75kHz,(d)sourcedepthof100mandf=7.5kHz,(e)sourcedepthof CMB measurements for the top layer
200mandf=3.75kHz,and(f)source depthof200mandf=7.5kHz.Notethatthesound (0–25 m). A large vertical gradient
wavesattenuatemorewithISWsthanwithoutISWsForΔ(TL)<0,andotherwiseforΔ(TL)>0. may be created at a depth of 25 m.
Such a vertical gradient also occur in
therange-independentcase.Theeffect
ofsuchaverticalgradientmaybeoffset
because of the modeled differences in
acoustic propagation between the
range-independent and the range-
dependentSSPs. In a futurestudy,an
oceannumericalmodelisneededtoas-
similate this observational data to get
continuous temperature profiles from
thesurfacetothebottomoftheocean.
(3) Although the modeling results
reconcile previous measurements of
transmission loss fluctuations near
ISWs,itispointedoutthatthecompli-
cated ISW effects on sound propaga-
tion in the Philippine Sea identified
here using the CASS/GRAB model
must be verified by acoustic obser-
vations. Experiments similar to the
Asian Seas International Acoustics Ex-
periment (ASIAEX) are needed for
thisregion.
be caused by changing in amplitudes pactsoundpropagationwithabasicpat- Acknowledgments
and separations of acoustic signals in ternoflittleenhancement(0–3dB)in The Naval Oceanographic Office
suchawaythatthecumulativetimein- near-range, alternate attenuation (up
and the Naval Postgraduate School
tegral of the pulse shape matches the to −20 dB for SD = 5 m and −10 dB
supported this study. The authors
more “steppy” cumulative integral of for SD = 100, 200 m) and amplifica-
deeply thank Ms. Ruth E. Keenan
thetrueeigenrayormodearrivalpulse tion (up to 20 dB for SD = 5 m and from the Scientific Application In-
shape. 5–8dBforSD=100,200m)inmid-
ternational Corporation, Mashpee,
range,andattenuation(upto−20dB)
Massachusetts, and Mr. Melvin D.
infarrange.Anevidentsurfaceweak-
Wagstaff at the Naval Oceanographic
Summary ening zone exists in midrange with Office for their guidance on the
(1)Thisstudyshowstheimpactof strongest Δ(TL) ∼−20 dB for deep
CASS/GRABmodelingandsimulation.
nonlinear internal waves in sonar per- sound source (SD = 100, 200 m)
formance estimation using the CASS/ but does not exist for shallow sound
GRABmodelwithrange-independent source (SD = 5 m). The thickness of Lead Author:
and range-dependent SSPs calculated thissurfaceweakeningzoneisaround PeterC.Chu
from the high-frequency temperature 15 m for SD = 100 m and 50 m for Naval Ocean Analysis and Prediction
observational data. ISWs strongly im- SD = 200 m. Laboratory
September/October 2010 Volume44 Number 5 15
NavalPostgraduateSchool,Monterey, Naval Oceanographic OfficeSystems Inte-
CA,93493 gration Division.1999a. Software design
Tel:[phone] document for the GaussianRay Bundle
Fax:[phone] (GRAB) eigenray propagation model. Missis-
sippi:OAML-SDD-74,StennisSpaceCenter.
E-mail:[email protected]
pp. 445.
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