Long-range backscatter from the Mid-Atlantic
Nicholas C. Makris and Jonathan
Ridge
M. Berkson
Naval ResearchLaboratory, Washington,DC 20375
(Received2 June 1993; acceptedfor publication10 October 1993)
Acousticbackscatterreturnsmeasuredwith a vertical sourceand horizontalreceivingarray are
beamformedand chartedto respectivescatteringsiteson the westernflank of the Mid-Atlantic
Ridge (MAR). Well-definedpatternsof high-levelbackscatterresemblingbottom morphology
occurwithin the direct path and at convergencezoneranges.Measuredbackscatteris compared
with modeled two-way transmissionloss using the wide-angleparabolic equation (PE) and
high-resolutionsupportingbathymetry. The level of backscatteris found to be strongly
dependentupon two-way transmission
loss (TL). Specifically,TL selectsprominent
bathymetric features from which high backscatteris returned, with backscattermaxima
correspondingto TL minima. Scatteringstrengthestimatedfor the wide area surveyedshows
low variance and relatively minimal spatial variation. Apparently, distinct scattering
contributionsare sowell mixedin long-rangebackscatter,that dominantvariationsare generally
due to propagation.Theseresultsindicatethat it is possibleto forecastthe rangeand azimuth
of prominent MAR backscatterreturns simply by propagationmodeling if high-resolution
bathymetryis available.They alsohaveled to the developmentof a new techniquefor ambiguity
removal in reverberation data measured with a horizontal line array. The method takes
advantage of environmental asymmetry provided by TL in highly range-dependent
environmentsand doesnot require multiple measurements
as do other existingmethods.
PACS numbers: 43.30.Bp, 43.30.Gv, 43.30.Vh
INTRODUCTION
mountsor continentalmargins,penetrateor curtail sound
channelswhich are otherwisegenerallyfree from bottom
In July and August of 1991 acoustic reverberation
interaction. In thesesituations,grossbathymetric surveys
data were acquired above the westernflank of the Midare sufficientfor a first-ordermodelingof TL.
Atlantic Ridge (MAR). A singleresearchvesseltowed a
The ARC standsapart becauseit was carried out in
vertical source and horizontal receivingarray to obtain
the previouslyuninvestigated
MAR whereruggedbathymquasimonostatic
measurementsof ocean-bottomreverberetry projectssmall-scalefeaturesinto the deepsoundchanation at low frequency.The three-weekexperimentcovered
nel. As a result, bottom limitation of deep-sound-channel
a large area and employeda broad range of waveformsas
well asimpulsivecharges.Data wererecordedlongenough propagationis extensiveand highly variablefor sourcesat
to addressa variety of direct-pathand long-rangereverber- depths used in typical sonar systems.Reverberationin
such complexenvironmentshas not been thoroughlyination issues.Sinceseastateswere generallybelowBeaufort
vestigated
in the past, and is interestingbecauseof the
Force 1, interferencefrom surfacereverberationwas negnumerous
returns
found over range and azimuth which
ligibleand data qualitywashigh. Referredto asthe Acousgenerally
serve
as
false targets in active sonar measuretic Reconnaissance
Cruise (ARC), the experimentis part
ments.
Only
with
the
recentavailabilityof high-resolution
of an on goingSpecialResearchProgram (SRP) on botbathymetry,
collected
in supportof SRP acousticdata, has
tom reverberation sponsoredby the Office of Naval
it
become
possible
to
study
the relationshipbetweenreverResearch.
1'2In Augustof 1992,a supporting
database
of
beration
and
small-scale
geomorphology
in the MAR.
high-resolution
bathymetry
3 wasobtained
for the region
Such
analysis
is
not
only
useful
for
sonar
design
in highly
previouslysurveyedby the ARC.
reverberant
environments,
but
may
also
provide
a
practical
Our goal is to understandthe relationshipbetween
method
for
remote
survey
of
complex
bathymetry.
In this
backscatter returning from multiple convergencezone
paper,
we
show
that
long-range
backscatter
is
highly
cor(CZ) ranges and complex geomorphologyin the deep
related
with
small-scale
geomorphology
and
that
this
corocean.Previousinvestigatorshave found a correspondence
relation
can
be
largely
explained
by
TL
modeling
with
betweenlong-rangebackscattermeasuredvia towed array
andgross
bathymetry.
*-8In ananalysis
of Tyrrhenian
sea high-resolutionbathymetry.
To describeour method of charting backscatterwe
data, Nghiem-Phu and Tappert have shown that rangefind it convenientto introduce the following definitions.
dependentmodeling of two-way transmissionloss (TL)
We definean "observation"as a singleactiveinsonification
with the parabolicequation (PE) can determinethe general characteristics of backscatter without detailed inforand subsequent
towed-arraymeasurementof reverberation
mationaboutindividual
scattering
sites.
9However,
thisas wherethe array positionand orientationcan be considered
well as other past studieshave focusedupon environments constant.We definea "reverberationmap" as the magnitude of acousticreturnsfrom a singleobservationcharted
where isolatedlarge-scalescatteringfeatures,such as sea1865
J. Acoust. Soc. Am. 95 (4), April 1994
1865
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to the horizontallocationof respectivescatteringsites.To
generatereverberationmapsfrom ARC data, we first estimate the rangeand azimuth of scatteringsitesby two-way
travel time analysisand beamforming.Sincebeamformed
line-arraydata havean inherentfight-left ambiguityabout
the array's axis it is especiallychallengingto identify true
from virtual scatteringsitesin the MAR whereambiguous
returns are broadly distributedin range and azimuth.
The ARC tow-shiptrackswere designedto providea
generalsurveyof backscatterin the vicinityof the western
MAR. They are not optimal for the existingmethodsof
ambiguityremovalwhich rely upon multiple adjacentob-
servations
at different
arrayheading.
1ø'11
Therefore,
weintroducea methodbasedupon two-way TL modelingwith
thewide-angle
PE,12andusethisin oursubsequent
data
analysis.This method has a strong physical foundation
which takesadvantageof naturallyoccurringasymmetries
in the MAR environment by incorporating the highresolutionbathymetry database.Unlike other ambiguity
resolutionmethods,it doesnot require multiple data observations.Further, it can be applied objectivelyto the
entire data set. This is in contrastto ambiguityresolution
by comparisonwith bathymetrywhich is extremelysubjec-
B. Long-range propagation corridors
Somegeneralcharacteristicsof the acousticfield incident upon the oceanbottom can be readily deducedfrom
source, sound speed, and bathymetric information. The
source/receiverdeploymentgeometryis indicatedin Fig.
2, along with miscellaneous
operationalcharacteristics
of
thearrays.
Duetonegligible
horizontal
variation,
13a single
representativesound-speed
profile,shownin Fig. 3, can be
used to describethe water column. While the density,
soundspeed,and attenuationof the bottom vary in both
the horizontal and vertical, information about these parameters is limited. However, we do know that sedimentof
varyingthicknessand relativelyhomogeneous
acousticim-
pedance
covers
a majority
of thesurvey
area.
14Therefore,
we assumethe bottom is comprisedof sedimentwith uni-
formdensity
1.5g/cm3,sound
speed
1700m/s,andattenuation0.5 dB/•t for all range-dependent
acousticmodeling
donein this paper.
An exampleof deepsoundchannelpropagationcharacteristicto the ARC is shownin Fig. 4 for a range-depth
cross section of the ocean. An idealized fiat bottom at 5-kin
depthis usedto demonstratethe CZ structureof the source
beam when undisturbedby bottom interaction,which is a
tive, and biased toward discrete or well-defined returns.
rare situationfor our survey.Propagationmodelingis done
Once ambiguityis resolved,backscatterlevelsare unwith the wide-anglePE for the environmentalparameters
ambiguouslyprojectedonto bathymetry for comparison. discussed
above.The sourcearray is accuratelymodeledby
We findthatprominent
returnsoccurat n+« CZ ranges ten omnidirectionalmonopolesat depthscorrespondingto
and are well correlatedwith ridgeswhich have extended thoseof the actual projectors.The model sourceelements
scarpsfacingthe observation,wheren =0, 1 for the present are horizontallysteeredand operateat 270 Hz. The ½omsurvey. However, resolution is too poor to determine binedTL is normalizedso that a singlesourcelevel may be
whether this correlationis solelydue to a propagationefappliedto obtain sound-pressure
level at any point in the
field.
fect which insonifiesthe backfacingscarpsof prominent
ridges, or whether it is also due to the orientation and
The sourcebeamcontainsthe highestamplitudesound
geomorphologyof scattererswithin the ridges.Finally, we
wavesby orders of magnitude.The beam has a complex
structuredue to interferencecausedby the source'sproxcombinemodeledTL with unambiguousbackscatterlevel
imity to the pressurereleasesurface;the numeroussideto estimate scatteringstrength over the wide-area surlobes of the untapered source array; and the refractive
veyed.Our techniqueintroducestwo advancesover more
propertiesof the water columnwhich lead to deep-soundtraditional scatteringstrengthestimation:(a) TL is comchannel propagation.It has a first deep vertex approxiputed using a full-field propagationmodel rather than a
1
mately 33 km from the source, which definesthe • CZ
raytrace;(b) the area term of the sonarequationis incorporated by a spatial convolutionrather than a constant range,the regionwithin which is referredto as the directscale factor.
path area. Here beamvertexoccursbetweenroughly3900and 4500-m depth. For practical considerations
then, the
beam will propagateundisturbedin the water columnfor
bathymetryexceeding4500 m, will be partially blockedor
reflected for bathymetry between 4500 and 3900 m, and
I. A HIGHLY BOTTOM-LIMITED
ENVIRONMENT
will be totally blocked or reflectedfor bathymetry shal-
A. Observation geometry
We analyze only a small fraction of the total ARC
reverberationdata set for the presentstudy, focusingon
data measuredcentral to the bathymetry database.We
classifyobservationsby track, or run number (R), and
emissionsequence,or epoch number (E). The observations for the presentstudy are then labeled R7E10 and
R7E15 in Fig. 1 wheretrack informationis superposed
on
an image generatedfrom the high-resolutionbathymetry,
sampledat 200-m intervals. For example, track 7 runs
from north to south.
1866 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
lower than 3900 m.
To consolidatethis information,we employthe following terminology. "Conjugate depth" refers to the depth
below the minimum sound speedwhere the soundspeed
returnsto the value at the sourcecenter.Conjugatedepth
occursat roughly 3900 m for the ARC. (By Snell'slaw,
horizontal rays emanating from the sourcewill vertex at
conjugatedepth.) The term "excessdepth" refersto the
differencebetweenbathymetricdepthand conjugatedepth.
And, "positive excess depth" occurs for bathymetric
depthsexceedingthe conjugatedepth.
To give an overview of the anticipatedblockagesin
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1866
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Depth
(m)
1700
,j
200
,
,
-I'
,i,,,
,
,
,,
3300
.'
4900
,.
.....ß ' ß ß ß "Q
.... ß ' ß ,i' ß !
.........
1 O0
I
200
ß '""
' ß
km
FIG. 1. Hydrosweep
bathymetry
fromSRPgeophysical
survey
of 1992gridded
at200-mintervals?
Thedotted
whitelineindicates
thetracks,
orstraight
line runs,of the researchvessel.The coloreddots indicatethe positionof monostaticbackscatter
observations
relevantto this investigation.
The
observations
arelabeledR7E10 andR7E 15for the northernandsouthernobservations,
respectively.
The irregularborderwithinthe figureindicatesthe
limit of hydrosweep
coverage.
multiple CZ propagation,we have plottedthe bathymetry
wherepositiveexcessdepthis available,in Fig. 5, and note
that this compriseslessthan 50% of the area surveyed.
I
1
Two concentric circles centered about observationR7E15,
bathymetry.Theseindicatethe • and 1• CZ radii as if there
was no bottom interaction. In reality, however, bottom
1
interaction is extensive.The • CZ circle'sintersectionwith
positive excessdepth is limited to relatively narrow azi-
closest to the track crossing,are superposedon the
muths to the southwest and southeast of the observation.
Research
Only alongtheserestrictedazimuthsdo we anticipatemultiple CZ propagationto be possible.At all other azimuths,
bottom interactionwithin the direct-path area will cause
Vessel
the source beam to be attenuated
,
and redistribution
in ver-
tical angle, resultingin much weaker long-rangeinsonification. Therefore, only a sparseazimuthal distributionof
scattering
sitesat 1«CZ rangewill beinsonified
by water320
Morizontal
33
m
Receiving
Array
m
,
,
Tilt
Vertical
< 2ø
Source
Array
FIG. 2. A sketchof the researchvesseltowing the vertical sourceand
horizontalreceivingarrays.Array depthsand orientationchangewith
ship'sspeed.Conditionsshownare for a typicaltow speedof 3.5 kn. The
elementspacingis 3.66 m for the vertical array and 2.50 m for the horizontal array.
1867 J. Acoust.Soc. Am.,Vol. 95, No. 4, April1994
bornepaths.This is significantbecausethe strongestbackscatter is expectedto return from siteswhere waterborne
paths are available.
To demonstratethe environmentalsymmetrybreaking
potentialavailablein range-dependent
propagationmodeling, we have strategicallyselectedradialsfor PE runs. The
radials extend from observationpositionR7E15 and are
comprisedof ambiguouspairs (a,a') and (/•,/•') aboutthe
array axis. Radials a and /•, respectively,intersect the
southwestern
and southeastern
azimuthswherepositiveex1
cessdepth is availableat • CZ. This is displayedin Fig. 5.
Bathymetryfrom the high-resolutiondatabaseis extracted
alongeachradial to supplyrangedependence
in eachrun.
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1867
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iDepth
<3900
20•
x
ß
!
-2000
lorn
• -3ooo
,,,
100
-5000
[5
200
4900
km
FIG. 5. Positiveexcessdepthwith respectto the conjugatedepthof the
source-arraycenter.The white dotted linesindicatethe approximatelo-
cation
ofthesource-beam
vertex
at« and1«CZranges
fromcentral
-6000
'
1490
1500
'
1510
•
'
1520
Sound
'
1530
Speed,
c,
observationR7E 15 which is indicatedby a dark dot. The solidwhite lines
indicateradial pairs (a,a') and (B,/T) symmetricabout the array axis.
'
1540
1550
1560
m/s
FIG. 3. A sound-speed
profiletypicalof thosemeasuredvia XBT during
the ARe. The castwastaken approximately18-km southof the central
observation
R7E15 on run 7. Archivaldatausedbelowthe 2000-mdepth.
To be consistentwith ARC steeringand sourcefrequency
for the data analyzed in this paper, the modeledsource
elementsare steeredto a depressionangle 6øfrom horizontal and operateat 270 Hz. Range-depthcrosssectionsof
the propagationalong each radial are shown in Fig. 6.
First, considerpropagationalong radial a where a water-
ing the backscatterprocessshould be made. For each of
the (ct,ct') and (B,B') runspresentedin Fig. 6, backfacing
scarpsare generally insonifiedby the sourcebeam while
forward-facingscarpsare in the acousticshadow.Stronger
backscatteris thereforeexpectedfrom backfacingscarps.]
Theseexamplesillustratethe substantialdifferencesin
TL that can exist at ambiguousscatteringsitesdue solely
to asymmetryin bathymetryalong ambiguousradials. In
later sectionswe extend theseresultsto objectivelyeliminate right-left ambiguityfrom backscatterdata over wide
areasby comparingTL at ambiguousscatteringsites.
bornepathis available
to a bottomsiteat roughly1«CZ.
Here TL is substantially
lowerthanat the ambiguous
1«
CZ sitealongradial ct'. This is becausealongct', the source
beam is terminatedwithin the direct-patharea. Therefore,
backscatterof much higheramplitudeis expectedfrom the
1«CZ bottomsitealonga. Propagation
runsforB vsB', in
Fig. 6, show a similar scenario.That is, a waterbornein-
sonification
of thebottomat 1«CZ occurs
alongB butnot
alongB'. [One additionalobservationusefulto understand-
TL
dB re
I
m)
<60
II. REVERBERATION
MAPS
A. Data acquisition and reduction
Although a wide variety of waveformswere usedduring the ARC, from narrow-band continuouswave (cw)
tonals to broadband hyperbolic frequency-modulated
(hfm) emissions,
we limit our presentanalysisto backscatter from cw signalsof •-= 2-s pulseduration. This is principally because the correspondingrange resolution Ar
=c•-/2 = 1.5 km, for a nominalmeansoundspeedof c= 1.5
km/s, is on the order of the cross-rangeresolutionfor the
rangesrelevantto the presentstudy. (Cross-rangeresolution is simply the azimuthal resolutionof the horizontal
receivingarray multiplied by the range of the scattering
site.)
>120
50
100
km
FIG. 4. Transmission
loss(TL) for a range-independent
crosssectionof
the ocean using the sound-speedprofile of Fig. 3 and the ten-element
verticalsourcearray shownin Fig. 2. Convergence
zonestructuretypical
of the regionis evidentas are sidelobes
in the beampatterndue to the
untaperedarray, whichis steeredto 0ødepression
anglefor thisexample.
Modelingwasdonewiththewide-angle
PE.12
1868 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
Azimuthal dependenciesof the backscatterdata are
determinedby beamformingtime seriesdata receivedon
the array elements. Data were beamformed, filtered, and
demodulatedto a 128-Hz sample rate from the original
rate of 2048 Hz. The data were then refiltered
to within
a
3-Hz band about the 270-Hz centerfrequencyto eliminate
out-of-bandnoise,leadingto the beam-timeformat necessary for our mapping procedure.
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1868
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(a)
TL
(dB
2
re
1 ra)
•
<80
50
(b)
100
km
0•'-- 112ø SE•
>140
50
(c)
1 O0
km
•'= 241ø SW•
TL
(dB
re
1 m)
<8O
>140
50
100
km
FIG.6.Transmission
loss
(TL)forrange-dependent
cross
sections
oftheocean
along
theradials
depicted
inFig.5.Ineach
case,
propagation
to1«CZ
is partiallyor fully blockedin the direc•path for onememberof eachsymmetricpair, and is clearfor the other. (The sourceis modeledwith •he
wide-angle
PE asan untapered
ten-element
verticalarray,shownin Fig. 2, steeredto a 6ødepression
angle.)In general,backfacing
scarpsareinsonified
by the sourcebeamwhile forward-facing
scrapsare in the acousticshadow.S•rongerbackscatter
is thereforeexpectedfrom backfacingscarps.
1869 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1869
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kill
Reverb
(dB re i •Pa)
105
200-
100-
45
100
200
km
FIG.7. A reverberation
mapforobservation
R7E15.
Highest
level
returns
occur
within
thedirect
pathandat1«CZrange.
Allreturns
aremirrored
aboutthe array axis, roughly 183øazimuth,due to right-leftambiguity.Poor resolutiondue to endfireoccurswithin roughly :• 30øof the array axis.
scatter,bathymetry,and TL. Referring to Fig. 6, backfacing scarpsare insonifiedby waterbornepathsat both sites
a and/•, while forward-facingscarpsare in the acoustic
shadow.The scarp at site a, where a broad backscatter
return is charted,corresponds
to the face of a prominent
abyssalhill, while the scarpsat/•, where narrower returns
are charted,belongto smallerridgesdistributedon a shallower ledge.
III. TRANSMISSION
LOSS MAPS
For comparisonwith reverberation,we are interested
in the two-way TL, which is proportionalto reverberation
level R in the sonar equation
R=--TLf--TL•+S+
l0 logA+ W,
(3)
where S is definedas scatteringstrength, W is the source
strength,A=rArB(O) is the scatteringarea, r is the range
to the centerof A, TLf is the forwardtransmission
loss
from the vertical sourcearray to bottom sites,and TLo is
the back transmission
loss from bottom
sites to the hori-
zontal receivingarray.
To mapTLf andTL, overthewideareassurveyed
in
0.04
eachbackscatterobservation,we extract bathymetryalong
R = 110-km radialsat evenlyspacedazimuthal increments
A0= 1ø about each observation.Along each radial, we
compute the acousticfield in a two-dimensionalrangedepth crosssectionof the oceanusing the wide-anglePE
with the same frequencyand environmentalparameters
used in Sec. I, Fig. 6. The TL at 1 rn above the bottom
(roughly 2./6) is extractedat evenlyspacedrangesalong
0.03
0.02
o
40
i
i
60
70
Reverb
80
dB
90
re
100
110
120
1 micPa
each radial. For each observation, the radials are combined
to producea TL map over the entiresurveyarea.To fill the
areas between radials, the TL within +/--A0/2
FIG. 8. A histogram of wide-area backscatterlevels for observation
R7E15 with mean level 73 dB re: 1/tPa, median72 dB re: 1/tPa, and
standarddeviation8.5 dB. Correctingfor array gainand finitebandwidth,
the noisefloor corresponds
to typical deep-oceanambientnoiselevels.
1871 d. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
of each
radial is giventhe valueat the radial without interpolation.
For TLf we usethe sameverticalsourcearray,steering,
and normalization as in Fig. 6. For TLo we use a point
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1871
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TL•
(dB re
<75
200.
i
TL b
m)
<65,
zoo:
•135•__>121
100
200
100
200
km
FIG. 9. Forwardand backtransmission
lossmapsTLf and TLb for
observationR7EI5. Modeling is done for radials at 1ø increments.A
generalcorrespondence
can be found betweenTL minima in thesefigures
and backscattermaxima in Fig. 7.
sourceat the depth and central range of the horizontal
receivingarray and invoke reciprocity, with the understandingthat variationsin TL b alongthe receivingarray's
aperture
arenegligible.
MapsofTLf andTLbareshown
in
•40
160
180
200
TLf+TLb,
dB
220
re
240
260
1 m
FIG. 11. A histogram
of wideareaTLf+TL• for observation
R7E15
with mean level 191 dB re: I m, median 191 dB re: 1 m, and standard
deviation 14 dB. Two-way TL valuescorrespondingto waterbornesites
comprisethe lower tail of this distribution.
Fig. 9. While thereis a generalcorrespondence
betweenthe
two maps,sharperfeatures
arefoundin TLf dueto the
narrow beamwidthof the vertical array. This is especially
so in the direct-pathregionwhere the incidentfield of the
point sourceis more uniformly distributed on the ocean
bottom.
The TLf+TLo mapfor observation
R7E15is shown
in Fig. 10. Direct-path and long-rangesitesinsonifiedby
waterbornepathshave the lowesttwo-way TL. Theseoc1
1
cur within • and at 1• CZ ranges,respectively,the latter
being contingent upon the availability of a long-range
propagationcorridor. Two-way TL to sitesat intermediate
ranges is also low when the sourcebeam is diverted by
bottom interactionon the forward trip. However, bottom
interactionon the return trip, which must occur by reciprocity, often causesa substantialincreasein overall TL
for suchsites.If part of the sourcebeam is rescatteredinto
CZ propagationfrom theseintermediatesites,its trajectory
will not intersectthe receivingarray. Suchredistributionof
TLf+TL b
(dB
re
1 m)
<160
200-
acousticenergyis only of significancein bistatic observation geometries.Due to uncertaintyin geoacoustic
parameters of the seafloor, the level of modeled TL at interme-
diate sites insonifiedafter previous source-beambottom
interactionis highly approximate.Sincethe angleof bottom reflectionis primarily morphologyrelated, the location of these intermediate sites is more dependable.For
sitesinsonifiedby waterbornepaths,which are more central to the analysisof this paper, knowledgeof the geoacousticparametersof the bottom is unnecessaryfor dependable TL modeling. While the corresponding
reverberationmap in Fig. 7 has ambiguitieswhich smear
and mirror the data about the array's axis, many features
are closelymatchedby the two-way TL of Fig. 10.
The associatedhistogramin Fig. 11 showsa broad
distributionof valueswith standarddeviationsignificantly
largerthan that of the backscatterlevels.Sitesinsonifiedby
waterbornepaths correspondto the lower tail of the distribution. Correspondinghigh-level backscatteroccupies
the uppertail of the distributionshownin Fig. 8. Like the
histogramof backscatterlevel, the two-way TL distribution is alsolog normal, as is confirmedby the 0.98 normalized correlation
with a Gaussian
100-
I
200
km
>220
coefficient obtained after cross correlation
distribution
of the same mean and vari-
ance. Again, this is suggestiveof an intermittent process,
which for the TL case, can be readily attributed to the
infrequentintersectionof the sourcebeam with the ocean
bottom. The measurementof a log-normaldistributionfor
backscatterlevel, Fig. 8, is consistentwith the notion that
two-way TL drives the level of backscatter.
To facilitate our comparisonwith measuredbackscatter, we incorporatethe area term of the sonarequationby
a runningspatialintegrationof the antilogof negativetwoway TL as windowedby the spadally varying resolution
rArfl(O) set by the receivingarray'sbeamwidthand wave-
formbandwidth.
[Wesubstitute
theantilogof TLf+TL 0
for Ts(r,t) in Eq. (4) of Ref. 10.] This incorporatesthe
FIG. 10. Two-waytransmission
lossmapof TLf+TLb for observation
area term of the sonarequationby effectivelysmearingthe
R7EI5. A general
correspondence
canbefoundbetween
TL/+ TL• minima in this figureand backscattermaxima in Fig. 7.
1872
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
TLf+TL0 mapto the resolution
of the backscatter
data
N.C. Makris and J. M. Berkson:Long-rangebackscatter 1872
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TLz+TL b
km'
-101ogA
(dB
re
1 m)
<9O
200"
turesfound in the reverberationmap. For example,at long
rangethe lineatedbackscattermaximaat sitea is precisely
selectedby an extendedTL minima, confirmingour previous deduction.
The
discrete
returns
associated
with
the
waterbornepath to site/%are alsopreciselyselectedby TL.
Also,thereis generally
higherTLf+ TLb-- 10log.4 to the
west and east of the observationwhere correspondingly
low backscatteris measured,partly due to the north-south
orientation
lOO-
While
[__3>150
100
200
km
FIG. 12. A mapof TLf+TLb--10 logA for observation
R7EI5. The
areaterm is incorporated
by convolving
the antilogof --(TLf+TLb)
with the spatiallyvaryingresolutionwindowsetby the receivingarray's
beamwidth and waveform bandwidth. Right-left ambiguity is not included.A generalcorrespondence
canbe foundbetweenminimain this
figure and backscattermaxima in Fig. 7.
of the endfire beams.
a broad azimuthal
distribution
of sites in the
direct path have distinctTL minima, theseare more difficult to comparewith backscatterdue to the excessive
overlap of ambiguousreturns. For comparativepurposes,we
have artificially applied right-left ambiguity to
TLf+TLb-10 log.4. [We substitute
the antilogof the
two-way TL for œ(r,t) and sets(r,t) = 1 in Eq. (5) of Ref.
10.] The result is shownin Fig. 13. A great similarity can
now be seenbetweenbackscattermaxima and ambiguous
TLf+ TL•-- 10log.4 minimain thedirectpath,aswellas
at longer ranges where there is generally a pronounced
TLf+TLo--10 log.4 minimafor everyprominent
backscatter maxima.
To quantifythis comparison,we computethe normalwithout includingright-left ambiguity,as is shownin Fig.
ized cross correlation
12.The newmapis referredto asa TLf+ TLb-- 10log.4
modeled
TLf+TL•--10 log.4,shownin Figs.7 and 13,
map.
respectively.We convertto linear units which are proportional to root-mean-squarepressureand cross correlate
It is now evident that TL selectsthe prominent fea-
between
measured
backscatter
and
TLf+TL b
klT['
-101ogA
(dB re 1 m)
<90
200-
100-
>150
100
200
km
FIG. 13. Sameas Fig. 13 exceptright-left ambiguityis now included.A generalcorrespondence
can be foundbetweenminimain this figureand
backscattermaximain Fig. 7. SeeSec.III for quantitativecrosscorrelationwith backscatter.
1873
d. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
N.C. Makris and d. M. Berkson:Long-rangebackscatter 1873
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'dB
kill'
re
(dB
re
105
1 pPa•
1 m)'
>75
-
2oo•
(:2"
,,
:.ooz
•". •,,.
, •,
"'" i!•,,,•.• o.,-,•"'
,
..
-
•.,;•i
'.•
?..%'
.I 4" :'•
,
:..
......
45
.
,,•.
.....
-
•<135.
km -
(dB re
>90
2OO
1 m)
>90
-.
1oo
<150
.........
I ....
100
•"•''i
•
<150
.
.............
200
I .........
km
100
I ....
200
km
FIG. 14.Reverberation
andTLf mapsforobservation
R7E10.Weapproximate
TLf+TL 0by2TLf forR7E10,andagainincorporate
theareatermby
a running
integration.
Mapsof2TLf-- 10logA withandwithoutambiguity
appear
below.SeeSec.III forquantitative
cross
correlation
withbackscatter.
10R/20with 10--(TLf+TL6--101øgA)/20.
The rationalefor this
conversionis to first investigatecorrelation within the
physicalunits of the problem. The resultingcorrelation
coefficientis 0.59. However,there is alsoa strongphysical
motivationfor correlatingthesequantitiesdirectlyin decibels, as they are presentedin the figures.Besidesbeing
linearly related in the sonarequation,our analysisshows
that backscatterlevel R and two-way transmissionloss
This is most likely due to the uniform spatialintegration
usedin the modeling.Sincethe cross-rangeextent of endfire sitesgenerallyexceedsthe scaleof geomorphological
variation,discussed
in the next section,significantscattering strengthvariationis likely within the spatialresolution
of endfire sites. This could easily account for the noted
correlation function is a secondmoment characterization,
with mapped reverberation,also shown in Fig. 14 for
R7EI0. In particular, the highestlevelbackscatterfeatures
difference.
We generate
a TLœmap for observation
R7E10,as
TLf+TL o have normaldistributions.
Sincethe cross- shownin Fig. 14. Again, there is a strongcorrespondence
its interpretationis unambiguousfor variableswhich have
normal distributions.That is, given a statisticallysignificant numberof samples,a high correlationunequivocally
showsdependence
while a low correlationshowsindependence for Gaussian variables. Therefore, the normalized
correlation coefficient of 0.63 obtained after cross correlat-
at 1«CZ areprecisely
selected
byTL minimasouthwest
of
the observationposition.This siteis in the generalvicinity
of the elongatedfeature at site a in observationR7EI5.
However, the magnitudeand shapeof thesereturnsvaries
significantly from R7EI5. Since both observationswere
thatsmall
ingR with(TLf+ TL0-- 10logA) isnotonlyquantitative, within15km (¬CZ) of eachother,weconclude
but unambiguousin demonstratingthat spatialvariations
in chartedbackscatterare significantlydependentupon
two-way transmissionloss. Refinementsin charting the
backscatteras well as modelingthe geoacouticparameters
of the bottomfor TL computations
will only improvethe
correlation.
At endfire, however, relative backscatter levels are
lowerthanarepredicted
bymodeled
TLf+ TL0-- 10logA.
1874 d. Acoust.Soc. Am., Vol. 95, No. 4, April1994
variationsin measurementpositioncan lead to large variationsin the level of backscatterat a givensite.To facilitate qualitativecomparisonwith backscatter,we approxi-
mate TLœ+TL0 by 2TLf for R7EI0, and again
incorporatethe area term by a running integration.The
result is also shownin Fig. 14 for unambiguousand am-
biguous
cases
respectively,
where2TLœ--10logA ishigher
thanTLœ+TL0--10logA withinthe directpathdueto
N.C. Makrisand d. M. Berkson:Long-rangebackscatter 1874
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(dB
kmß
re
1 •iPa
Reverb
(dB
1
re
1 m)
>75
200•
<13
km ß
200
101ogA
101ogA
(dB
re1m)t
-
100
ß ß ß ß ß ß ß ß ß I ß ß i i ß ß ß ß ß I i i ß ß
100
200
km
100
200
km
FIG. 14.Reverberation
andTLf mapsforobservation
R7E10.Weapproximate
TLf+ TL0by2TLf forR7E10,andagainincorporate
theareatermby
a running
integration.
Mapsof2TLf-- 10log,4withandwithoutambiguity
appear
below.SeeSee.III forquantitative
cross
correlation
withbackscatter.
10•/2ø with 10-(TLf +TLb-101øgd)/20.
The rationalefor this
conversionis to first investigatecorrelation within the
physicalunits of the problem.The resultingcorrelation
coefficientis 0.59. However,there is alsoa strongphysical
motivationfor correlatingthesequantitiesdirectlyin decibels, as they are presentedin the figures.Besidesbeing
linearly relatedin the sonarequation,our analysisshows
that backscatterlevel R and two-way transmissionloss
This is most likely due to the uniform spatial integration
usedin the modeling.Sincethe cross-rangeextentof endfire sitesgenerallyexceedsthe scaleof geomorphological
variation,discussed
in the next section,significantscattering strengthvariationis likely within the spatialresolution
of endfire sites. This could easily account for the noted
difference.
We generate
a TLf mapfor observation
R7E10,as
TLf+TLb have normaldistributions.
Sincethe cross- shownin Fig. 14. Again, there is a strongcorrespondence
correlation function is a secondmoment characterization,
its interpretationis unambiguousfor variableswhich have
with mapped reverberation,also shown in Fig. 14 for
R7E10. In particular,the highestlevelbackscatterfeatures
normal distributions.That is, givena statisticallysignificant numberof samples,a high correlationunequivocally
showsdependence
while a low correlationshowsindependence for Gaussian variables. Therefore, the normalized
at 1«CZ areprecisely
selected
byTL minimasouthwest
of
correlation coefficient of 0.63 obtained after cross correlat-
the observationposition.This site is in the generalvicinity
of the elongatedfeature at site a in observationR7E15.
However,the magnitudeand shapeof thesereturnsvaries
significantlyfrom R7E15. Since both observationswere
thatsmall
ingR with(TLf+ TLb--10log`4) isnotonlyquantitative, within15km (¬CZ) of eachother,weconclude
but unambiguous
in demonstratingthat spatialvariations variationsin measurementpositioncan lead to large variin charted backscatterare significantlydependentupon ationsin the level of backscatterat a givensite.To facilitwo-way transmissionloss. Refinementsin charting the
tate qualitativecomparisonwith backscatter,we approxibackscatter
as well as modelingthe geoacoutic
parameters mate TLf+TLo by 2TLf for R7E10, and again
of the bottomfor TL computations
will only improvethe
incorporatethe area term by a running integration.The
correlation.
result is also shownin Fig. 14 for unambiguousand amAt endfire, however, relative backscatter levels are
lowerthanarepredicted
bymodeled
TLf+ TL•-- 10log`4.
1874 J. Acoust.Soc. Am., Vol. 95, No. 4, April1994
biguous
cases
respectively,
where2TLf-- 10log.4ishigher
thanTLf+TLo--10 log.4 withinthe directpathdueto
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1874
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0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
2000
2500
3000
3500
Depth,
4000
4500
5000
5500
m
FIG. 16. A histogramof bathymetryintersectingthe circularregionsurveyedby backscatterdata in Fig. 7, with mean depth 3785 m, median
depth 3833 m, and standarddeviation467 m.
100
200
km
FIG. 15. Lambertianshadedbathymetricrelieffor a light sourcecentered
abovethe regionat infinity. Site a corresponds
to a typical abyssalhill,
and • to a shallowledgeon the Mid-Atlantic Ridge.
the aboveapproximation.A normalizedcorrelationcoefficient of 0.54 is measured between backscatter
the combined
transmission
level R and
loss-area terms of the sonar
equation -- (TLf+ TLb--10log•4)
"m Awayfromfracturezones,ridgestendto
for observation faults...•s.
R7E10. Converting to linear units as before, the correlation coefficient
those obtained
is 0.47. These results are consistent with
for observation
R7E15.
In summary, we find that the level of backscatteris
stronglydependentupon TL. Two-way TL selectsthe sites
from which high-levelbackscatteris returned, with backscattermaxima correspondingto TL minima. Small vari-
ationsin measurement
position,
lessthan¬CZ, canleadto
large variationsin the level of backscatterreturnedfrom a
given site due to bathymetry related variations in TL.
Larger variationsin measurementpositioncan alsolead to
spatialvariationsin backscatter.However, thesemay simply be due to shadowzonesassociatedwith CZ propagation, and are not necessarilybathymetry related.
IV. WESTERN
MID-ATLANTIC
GEOMORPHOLOGY
are believedto be geologicallydistinctfrom thosefound on
southern edge. Northern ridge boundariesare known as
"insidecorners"and have irregular morphology.Southern
ridge boundariesare known as "outsidecorners"and have
more regularly lineatedtendenciestypical of abyssalhills.
"Compared to outsidecorners,insidecornershave excess
elevation,high-residualgravity anomaly, steepand often
irregularly oriented slopescreated by multiple high angle
RIDGE
A. Bathymetry and geomorphology
Two well-definedcorridorsof excessdepth run across
the surveyarea, as is evident in Figs. 1 and 5. These are
often referredto as "segmentvalleys"or "fracture zones"
sincethey are causedby separationof the local plate segment. They are best seenin Fig. 15 where depth perspective is simulatedby artificiallyshadingthe bathymetryas a
Lambertian surfacewith a light sourceat infinity directly
abovecenter.Darker shadingindicatessteeperslope.The
lower segmentvalley runs east-westalongthe very bottom
of the image. The central segmentvalley follows a more
circuitoussoutheastto northwestroute. Ridge boundaries
found on the northern edgeof the central segmentvalley
1875 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
have axes parallel to the MAR, extendingfrom North to
South. They also tend to be distributedregularly with a
spatialperiod of about 5 kin. (The faint diagonalstriations
throughoutthe shadedbathymetryat about 20øinclination
from horizontal are an artifact of the data acquisition.
Their pattern is related to the swathscoveredby the researchvessel.) A histogramof the depth distributionfor
the entire survey area is presentedin Fig. 16. The mean
depth is within a standard deviation of the conjugate
depth, insuringthe extensivebottom interactiondiscussed
in previoussections.
B. Directional derivatives of bathymetry
We are also interestedin the relationshipbetweenreverberationand seafloorslope. For example, does highlevel backscatterreturn predominantlyfrom steepslopes?
If so, is it biasedtoward slopesfacing the observationposition?To study this we have computeda directionalderivative (DD) of the bathymetryby taking the dot product
of the gradient vector with a unit vector pointing in thedirectionof observationR7E15 from the given field point.
This is useful since it gives an overview of the vertical
slopesencounteredon a path radially outward from the
source,as soundtravels.A spatialmap of DD can be used
to detect the horizontal orientation of ridgeswith vertical
slopesfacingthe observationposition.The result is shown
in Fig. 17. Sincethe gradientis computedby finite differences,the DD resolutionis no finer than the 200-m grid
interval of the bathymetry.
Here we identify ridgesas lineationsof high DD magnitude. For a given ridge, surfacesfacing the observation
N.C. Makrisand J. M. Berkson:Long-rangebackscatter 1875
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DD
(tan•)
>.5
0.025
0.02
o .o15
<-.5
-0.6
-0.4
-0.2
0
Slope,
100
200
km
FIG. 17. A directionalderivative(DD) of the bathymetryequal to the
dot productof the gradientvectorat eachpoint in the field with the unit
vector pointingfrom the locationof observationR7EI5. This givesthe
slope orientationalong the path sound wavestravel from the source.
Positivevaluesindicateslopesfacingthe observation(back facing), and
negativevaluesindicateslopesfacingaway (forward facing). Slopesare
givenby tan q•,whereq•is the slopeanglefrom horizontalalongthe DD.
pointhavepositiveDD, surfaces
facingawayhavenegative
DD, and surfacesnormal have zero DD. Since the ridge
axesgenerallyrun north-south,we often find north-south
lineationsof high DD magnitudeeastand west of the observationpoint corresponding
to abyssalhills. To the north
and south,ridge axesare generallyparallel to the observation direction where we find DD maxima diffuselydistributed in speckledpatternscorresponding
to ridge edgesand
crests.Thesepatternsare better definedat segmentvalley
walls along the fracture zone central to the observation,
and are most coherentfor the irregularly shapedinside
cornerswhich give much higher DD valuesthan outside
corners, as is anticipated. Abyssal and segmentvalley
floorstend toward zero slope.
A histogramof the DD distributionfor the surveyarea
is presentedin Fig. 18 where a sharplypeakedand symmetric distribution with 8ø variance about zero slope is
found. Slopesfacing the observationpositionare as likely
to be found along a propagationradial as slopesfacing
away. However, tails of the distributionbeyond + 25ø are
not accurate. This is due to limitations
0.4
0.6
FIG. 18. A histogramof DD intersectingthe circularregionsurveyedby
backscatterdata in Fig. 7, with mean slope0.2tY,median0.23øand standard deviation8.tY from horizontal. Slopesnear zero predominate,and
neitherslopesfacingor opposingthe observationare favored.
20 dBoccurfor spatialfrequencies
greater
than1 km-•.
These are an artifact of registrationerror along neighboring measurementswathsand do not reliably correspondto
actual bathymetric variations.
V. COMPARISONS
WITH
BATHYMETRY
,
To correlatebackscatterwith geomorphology,a color
reverberationmap is projectedonto the black and white
shadedbathymetryrelief. The resultingimageis presented
in Fig. 20 for observationR7E15. To insure that bathymetric variationsare not obscuredin the relief, only high
level backscatteris presented.
While cross-rangesmearingand fight-left ambiguity
make the image somewhatconfusing,closeinspectionre-
dB
•
I m2/km
-2
'70
2.0
knorth
of the measurement
systemand coarseness
of the samplinginterval.
To obtain a quantitative estimateof the scalesover
which bathymetry varies, we have computed the wavenumber spectrum,shown in Fig. 19. The dominant spectral componentis in the low wave-numberregime corresponding to length scales greater than about 5 km.
Variationswithin the array'srangeresolution( 1.5 km) are
lesssignificanton average.However,from the DD map in
Fig. 17, steepslopesdo exist within thesesmaller scales.
While they are less frequently encountered,steep slopes
associated
with lineatedridgesare more likely to terminate
shallow-anglepropagation,typical at long ranges, than
level areas which tend to occur in valleys below the conjugatedepthcontour.High wavenumberspikesof roughly
1876
0.2
tan
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
1.0
<1o
2.0
k_.t
km-1
FIG. 19. A two-dimensional wave number spectrum of hydrosweep
bathymetry
overa 102.4X102.4km2 spatialwindowwithinthe region
shownin Fig. 1. The dominant spectralcomponentsoccur for length
scalesgreaterthan about 5 km. High wave-numberspikesof roughly20
dB occurring
for spatialfrequencies
greater
than1 km-• do notcorrespondto actualbathymetricvariations.Theseare an artifact of registration error along neighboringmeasurementswathsof the bathymetry.
N.C. Makris and J. M. Berkson: Long-range backscatter
1876
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Reverb
(dB re I •lPa)
>95
20*
loo
<75
.....
!
.........
100
!
200
....
km
FIG. 20. Color backscatterlevels from observationR7E15 projectedonto correspondingLambertian shadedbathymetry, see Fig. 16. Right-left
ambiguityhasnot beeneliminated.Higher levelbackscatterreturnsoften resemblethe bathymetricfeaturesto which they are mapped.Only prominent
backscatteris shownto avoid obscuringbathymetricvariations.
words, little information about high-level backscatteris
lost by the choiceof a 10-dB threshold.On the other hand,
a general correspondence
betweenprominent bathymetry
and high-level backscatteris now evident. In particular,
the backscatterlineation at site a discussedin previous
sectionscorrespondsto an abyssalhill to the southwestof
the observation. The well-defined pattern of high-level
backscatterfound in the direct path traces the ridge contourssurroundingthe segmentvalley which runs centralto
TLf+ TLb--10log•/ at ambiguous
scattering
sites.If the the observationposition.
The relationshipbetween prominent backscatterand
difference
in TLf+ TLb--10log•/is greaterthana chosen
the ridge structureis further elucidatedin Fig. 22 by overthreshold, we associatethe true scatteringsite with the
placeof lowerTLf+TLo--10 log•/ and map the corre- lay on a DD map. Here prominent returns can be more
spondingbackscatterlevelsto this site. If the differenceis
easilyassociatedwith specificridges.For example,this is
less than this threshold we do not make a distinction, and
the case within the direct path, as well as at long-range
sitesa and/•. However, it is also evident that the resolution
project backscatterlevel onto neither site. We choosea
thresholdof 10 dB which constitutesa significantenough window is frequentlywider than scarpswithin the ridges.
difference
in TLf+ TLo--10log•/to ruleoutfalseidenti- That is, returns are typically mappedto areaslarger than
fication in most cases.
the width of associatedscarps.As a result, quantitative
The resultingunambiguousbackscatterlevelsare procomparisonsbetweenslopeorientationand backscatterare
not possiblewith the givenmeasurements,
and are left for
jectedonto shadedbathymetryand presentedin Fig. 21 for
observationR7E15. Again, only high-level backscatter future analysiswith higher resolutiondata. (Preliminary
( > 70 dB re: 1/zPa) is presentedto insurethat bathymetric results from high resolutionmeasurementsmade during
the main acousticsexperimentshow a strongrelationship
variationsare not obscured.We first note that folding the
backfacing
scarps
andbackscatter.
2ø)
figure about the array's axis yields the same prominent between
However, an associationbetween backfacing scarps
returnsdepictedin the ambiguousFigs. 7 and 20. In other
veals that many high-level backscatter features closely
match correspondingbathymetry. However, we do not
base our distinction between true and virtual scattering
sitesupon visualcorrelation.While this methodmay often
be accurate,as well as intuitive, the probability of false
correlationis also high givencross-rangeresolutionwhich
is often only slightly less than the separation between
ridges.
Instead, we resolvefight-left ambiguityby comparing
1877
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
N.C. Makris and J. M. Berkson:Long-rangebackscatter 1877
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Reverb
(dB •e • I•Pa)
>85
200-
100--
<70
I
I
I
!
I
I
I
I
I
I
I
I
I
100
I
!
I
I
I
i
I
200
i
I
i
i
k•
FIG. 21. Same as Fig. 20 except ambiguity has been eliminated with a TL-based environmentalsymmetry breaking technique.Only prominent
backscatteris shownto avoid obscuringbathymetricvariations.Highest level backscatteris found where waterbornepaths are available.Well-defined
patternsin backscatteroften correspondto bottom morphology,followingridgesprotrudingabovethe conjugatedepth contour.
DD
(tan(•)
>0.36
<-0.36
and prominentbackscattercan still be made in the present
study with the aid of the TL modelingpresentedin Sees.I
and III. Specifically,the resemblancebetween high-level
backscatterand bathymetry can be largely attributed to
spatial variations in TL. We focus on the numeroussites
which are insonifiedby waterborne paths and therefore
return the most prominent backscatter.This is because
there is little questionregardingthe wide-anglePEs accuracy in modelingTL along waterbornepaths for a rangeindependentwater columnin the deepocean.For example,
comparethe TL maps in Figs. 10 and 12 with the bathymetric maps shown in Figs. 1, 5, 15, and 17. Within the
direct-path, segment valley ridges facing observation
R7E15 are broadly outlined by TL minima along the conjugate depth contour. And, lineationsof minimum TL oc-
curat 1«CZ ranges
onbackfacing
scarps
for ridgeswhich
l•O
-200i
- kmß
FIG. 22. White contoursof unambiguous
backscatterexceeding80 dB re:
1/•Pa are projectedonto the color map of DD shownin Fig. 17. Welldefinedpatternsin backscatteroften follow ridgeswith extendedbackfacingscarps.Backscatterresolutionfor the givensignalis generallytoo
poor resolvethe scarps.
1878
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
protrude above the conjugatedepth contour over a wide
azimuth, as is the caseat site a. In general,ridgeswhich
crossthe propagationpath are more likely to show up as
TL lineations since they span broader azimuthal sectors
than thoserunning parallel to the propagationpath. As a
consequenceof the predominantly north-south running
ridge axesof the westernMAR, ridgeseastand west of the
observationare more likely to yield backscatterlineations.
N.C. Makris and J. M. Berkson:Long-rangebackscatter 1878
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tribution of scatteringstrength is consistentwith the low
s
resolution of the measurement. That is, contributions from
(dB)
>
-10
200--
elementalscatterersare well mixed in the long-range,lowresolution
100--
-
-
-
--
mm <-70
--
--
-
1•o
ß200i ....
km
backscatter
returns.
The scatteringstrengthvaluespresentedare usefulas a
lower bound for the region.This is becausethe backscatter
data presentedcannot resolvethe dominant contributors,
as is suggestedin the previoussection.Sincethesecontributors are under-resolved,their scatteringstrengthis likely
underestimated.(As a point of interest, we note that the
lower bound measured, 47 +4 dB is roughly consistent
with Lambert'slaw S--20 log(sin %b)
-/• and the Mackenzie coefficient/•= --27 dB (Ref. 21) for the predominantly
shallowgrazingangles,%b
< 10, of this survey.However,we
believethat this result is merely coincidental.Recent work
with high-resolutionFM signalsin the samearea indicate
that actual scattering strengths are significantly
higher.
•6,n)
FIG. 23. Scatteringstrengthovera wide-areafor observationR7E15. The
estimateis basedupon the unambiguousbackscatterlevels,see Fig. 21
andcaption,andtheunambiguous
TLf+ TLb-- 10logA levelspresented
VII. CONCLUSIONS
in Fig. 12 for a 233 dB re: 1 #Pa and 1-m sourcestrength.
Two primary conclusionsare drawn from this initial
analysisof long-rangebackscatterin the western MAR:
Vl. SCATTERING
STRENGTH
(a) there is a high correlation between prominent backscatter
and detailedbottom morphology;(b) the rangeand
We estimatescatteringstrengthS over a wide area by
azimuth
of prominent backscattercan be determinedby
adding unambiguousbackscatter level R to modeled
propagation modeling if high-resolution bathymetry is
TLf+ TL0-- 10log,4 andsubtracting
the sourcestrength
available.In general,measuredbackscattermaxima correW= 233 dB re: 1/•Pa and 1 m. The result appearsin Fig.
spond
to modeled transmissionloss (TL) minima when
23 for observationR7E15 where we find uniformly low
charted
to respectivelocationson the seafloor. These spascattering strength values. (Here we have used the full
tial extremaare most pronouncedin the direct path and at
rangeof unambiguousbackscatterlevelsas opposedto just
1
n +• CZ rangeswhen propagationis not blockedat shorter
the prominentvaluesoverlain on relief bathymetry in Fig.
21.) A histogram is shown in Fig. 24 where scattering rangefor the givenazimuth, where n--0, 1 for the present
study. At theseranges,backscattermaxima typically folstrengthis distributednarrowly aboutthe median,roughly
low
extendedridgesalong the local conjugatedepth confollowing the anticipatedlog-normal distributionfound in
tour.
This is partially explainedby propagationmodeling
backscatter level and transmission loss. A 0.96 normalized
which
showsthat forward propagationpaths tend to incorrelation
coefficient is obtained after cross correlation
sonify
the
backfacingscarpsalong ridges which protrude
with a Gaussian distribution of the same mean and variabove
the
conjugate
depth contour. This leadsto the corance. The low variance and generallyuniform spatial disrespondingTL minima which apparentlyplay a fundamental role in driving the level of backscatter.Whether the
prominenceof these returns is also related to scattering
propertiesof the associatedridgesand their orientationis
still uncertain. This uncertainty arisesbecausethe resolution of the measurementspresentedin this study is often
too poor to adequatelyresolvethe scarps,let alone more
elementary contributors to the scattering processwhich
may residewithin the scarps.
Although we do not yet fully understandthe scattering
process,we have learneda great deal about reverberation.
Our analysishas provideda simplemethod of forecasting
the range and azimuth of prominent backscatterin the
MAR. Either by a crude tracing of CZ circleson an excess
depth map, or a more accuratemodelingof transmission
100
-80
-60
-40
-20
0
losswith a full-wave propagationmodel suchas the wideS, dB
anglePE. Both of the abovemethodswere usedin designFIG. 24. A histogramof the scatteringstrengthvaluesat sitespresented
ing the main acousticsexperimentof the SRP. The prelimin Fig. 23, with mean --47 dB, median --47 dB and standarddeviation7
inary resultsof this more recentexperimentshowthat both
dB. Since the signal used cannot resolvedistinct bathymetricfeatures
methodsare extremely dependablein forecastingpromiwhich may dominatethe backscatterprocess,thesestatisticsare primarily
nent backscatterin the MAR. 2ø'•3
useful as a lower bound for the surveyarea.
600
300
1879
J. Acoust. Soc. Am., Vol. 95, No. 4, April 1994
N.C. Makris and J. M. Berkson: Long-range backscatter
1879
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We havealsoestimatedscatteringstrengthover a wide
area and have obtained a lower bound of --47 +4
dB for
the survey.Our estimatesprovide a lower bound because
well-defined morphological features, which apparently
play a major role in the backscatterprocess,are underresolvedby the cw measurementsusedin this study. More
recent scattering strength estimates made with much
higherresolutionFM data haveyieldedsignificantlyhigher
Bruce Palmer, John Perkins, and Peter Vogt. We would
alsolike to thank the membersof the SRP communityfor
many useful discussionsand insights. In particular we
thank the ARSRP
chief scientist John Orcutt of SIO for
his encouragement,and Brian Tucholke of WHOI for
making the SRP hydrosweepbathymetry available.This
work wasmadepossibleby partial supportfrom our sponsorsat ONR, Mohsen Badicy and Marshall Orr.
valuesfor the sameregion.
22Nevertheless,
the estimates
presentedhere showa low varianceand generallyuniform
spatial distribution.This is consistentwith the high cross
correlationmeasuredbetweenlong-rangebackscatterand
TL in this study.Evidently, distinctscatteringcontributors
are so well mixed in the large areas integratedby longrange returnsthat the most significantvariationsin backscatterare generallydue to propagation.This further supports the idea of backscatterforecastingby propagation
modeling. It also suggeststhat the possibilityof remote
bathymetric
surveyby backscatter
measurement
5'6'24
is
practical.However,a more negativeimplicationis that any
of a number of scatteringmodels will be able to match
long-rangeMAR backscatterdata, regardlessof whether
they are correct,so long as a sufficientpropagationmodel
is used.
Investigationsinto the fundamentalscatteringprocess
arebettersuitedto the site-spe•cific
Main AcousticsCruise
of July 1993.This bistaticexperimentwasdesignedspecifically around three sites detailed in the hydrosweep
bathymetry.High-resolutionmonostaticand bistaticreverberation measurementswere made at various rangesand
orientations from known scarps. Short-range measurements should enable resolution of distinct small-scale con-
tributorsto the scatteringprocess.While, long-rangemeasurements
will
show
how
these
accrue
in
distant
reverberation.Vertical arrays were also deployedbefore
selectbackfacingscarpsto explorethe verticalstructureof
the scattered field near the scatterers. These data are to be
comparedwith hydrosweepas well as finer scalebathymetry and geologicaldata recently collectedto further elucidate the scatteringprocess.In addition,tow-shiptracks
in the Main AcousticsExperimentwere designedwith ambiguity removalas an essentialtheme. Star patternsrather
than polygonswere usedextensivelyto provideredundant
measurements
at differingarray headingsat a point of convergence.This wasnecessary
becausethe presentstudyhas
shownthat small differencesin measurementposition,as
small as CZ/4, can lead to drastic differencesin backscat-
ter due to bathymetryrelatedvariationsin TL. Preliminary
results from the '93 cruise indicate that the environmental
symmetrybreakingtechniquefor ambiguityremovalpresentedhere is consistentwith ambiguityremovalby com-
parison
ofdatafrommultiple
adjacent
arrayheadings.
2øA
detailed
reportontheseresults
isin preparation.
23
ACKNOWLEDGMENTS
This work hasbenefitedgreatlyfrom discussions
with
and assistance
from our NRL colleaguesMichael Collins,
Dalcio Dacol, Gary Gibian, Timothy Krout, William Kuperman, Richard Menis, Gary Murphy, Peter Ogden,
1880 J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
1A.B. Baggeroer
andJ. Orcutt,"Overview
oftheONR Reconnaissance
Cruisefor the AcousticReverberationSpecialResearchProgram,"in
OceanReverberation,editedby D. Ellis, J. Preston,and H. Urban (Kluwer, Dordrecht, 1993).
2j. M. Berkson,
N. C. Makris,R. Menis,T. L. Krout,andG. L. Gibian,
"Long-range measurementsof sea-floor reverberationin the MidAtlantic Ridge area," in OceanReverberation,edited by D. Ellis, J.
Preston, and H. Urban (Kluwer, Dordrecht, 1993).
3Bathymetry
datawereacquired
by the Hydrosweep
15-kHzmultinarrow-beamechosounding
systemfrom the R/V Ewing. Brian E. Tucholkefrom the Woods Hole OceanographicInstitutionwas the Principal Investigator,Martin C. Kleinrock and Jian Lin also from the
WoodsHole Oceanographic
InstitutionwereCo-principalinvestigators.
4R. C. Spindel
andJ. R. Heirtzler,"Long-Range
EchoRanging,"
J.
GeophysRev. 77, 7073-7078 (1972).
•D. E. Shifter,E. R. Franchi,
J. M. Griffin,andB. B. Adams,"HydrographicReconnaissance
of LargeUnderseaTopographyusingScattered
Energy," in Rec. 1980 IEEE Electron. AerospaceSyst. Conventions,
270-274 (1980).
6F.T. Ersldne,
G. M. Bernstein,
S.M. Brylow,
W.T. Newbold,
R. C.
Gauss,E. R. Franchi, and B. B. Adams, "BathymetricHazard Survey
Test (BHST Rep. No. 3), ScientificResults and FY 1982-1984 Processing,"NRL Rep. 9048 (1987).
7j. M. Berkson
andT. Akal,"Simultaneous
Reception
of Long-Range
Long-FrequencyBackscatteredSound by a Vertical and Horizontal
Array," IEEE J. Ocean. Eng. OE-12, 362-367 (1987).
sj. R. Preston,
T. Akal,andJ.M. Berkson,
"Analysis
ofbackscattering
data in the Tyrrhenian Sea," J. Acoust. Soc.Am. 87, 119-134 (1990).
9L. Nghiem-Phu
andF. Tappert,
"Parabolic
equation
modeling
of bottom reverberationusing seafloorroughnessspectra,"J. Acoust. Soc.
Am. Suppl. 1 88 (1990).
løN. C. Makris,"Imagingocean-basin
reverberation
via inversion,"
J.
Acoust. Soc. Am. 94, 983-993 (1993).
ll R. A. Wagstaff,
M. R. Bradley,
andM. A. Herbert,"TheReverberation
Array HeadingSurface,"in OceanReverberation,editedby D. Ellis, J.
Preston, and H. Urban (Kluwer, Dordrecht, 1993).
12M.C. Collins,"A self-starter
for theparabolic
equation
method,"
J.
Acoust. Soc. Am. 92, 2069-2074 (1992).
13N. C. Makris, J. M. Berkson,T. L. Krout, and R. Menis,
Environmental/Navigation,Sections7.1-7.4, in "AcousticReverberation SpecialResearchProgramAcousticReconnaissance
Experiment,
Initial Report," ScrippsInstitutionof Oceanography(1991).
i4B. E. Tucholke,
M. C. Kleinrock,
andW. K. Stewart,
"Geological
and
GeophysicalCharacteristicsof the Acoustic ReverberationCorridor,
and their Relevance to the Conduct of G&G Fine-ScaleSurveys,"
AcousticReverberationSpecialResearchProgram ResearchSymposium, ScrippsInstitution of Oceanography(1993).
l•B.D. Steinberg,
Principles
of,4perture
and,4rray
System
Design
(Wiley,
New York, 1976).
16Apaperonmapping,
inverting,
andprojecting
high-resolution
reverberationdata onto bathymetryis in preparationfor submissionto J.
Acoust.
Soc. Am.
l?N.C. Makris,J. M. Berkson,
R. Menis,andG. L. Gibian,"Mapping
Direct Path ARC Backscatteronto High Resolution Bathymetry,"
Acoustic ReverberationSpecialResearchProgram ResearchSymposium, ScrippsInstitution of Oceanography(1993).
lSB. E. Tucholke
andJ. Lin,"Detachment
Faulting
andIts Relation
to
Geologic Structure at First- and Second-Order Offsets of SlowSpreadingRidges,"EOS ?3, 286 (1992).
19B.E. Tucholke,J. Lin, and M. C. Kleinrock,"CrustalStructureof
SpreadingSegmentson the WesternFlank of the Mid-Atlantic Ridgeat
25ø 25' to 27ø 10' N," EOS ?3, 537-538 (1993).
2øN.C. Makris,J. M. Berkson,
R. Menis,andL. Alvelino,Preliminary
N.G. Makrisand J. M. Berkson:Long-rangebackscatter 1880
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 18.38.0.166 On: Mon, 12 Jan 2015 18:31:36
Results:Correlation betweenReverberationand Geomorphology,in
"AcousticReverberationSpecialResearchProgramMain AcousticExperiment,Initial Report," ScrippsInstitutionof Oceanography(1993).
p.M. OgdenandF. T. Erskine,"Low-frequency
Surface
andBottom
ScatteringStrengthsMeasuredUsing SUS Charges,"in OceanReverberation, edited by D. Ellis, J. Preston, and H. Urban (Kluwer,
Dordrecht, 1993).
22N.C. Makris,J. M. Berkson,
andR. Menis,"HighResolution
Back-
1881
J. Acoust.Soc. Am., Vol. 95, No. 4, April 1994
scatterfrom the Mid-Atlantic Ridge," J. Acoust. Soc. Am. 94, 1802
(1993).
23Apaperontheresults
of theMainAcoustics
Exp.of 1993isin preparation for submission to J. Acoust.
Soc. Am.
24i.Dyer,A. B. Baggeroer,
J. D. Zittel,andR. J. Williams,"Acoustic
Backscattering
from the Basinand Margins of the Arctic Ocean," J.
Geophys.Res. 87, 9477-9488 (1982).
N.C. Makris and J. M. Berkson:Long-rangebackscatter 1881
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 18.38.0.166 On: Mon, 12 Jan 2015 18:31:36