APPROACHES TO ACOUSTIC BACKSCATTERING MEASUREMENTS FROM
THE DEEP SEAFLOOR
C. de Mou{ier
MarinePhysicalLaboratory
ScrippsI nstitutionof Oceanography
Universityof California,SanDiego
La Jolla.California
I. ABSTRACT
Becausethe averageoceandepth is four kilometers,seafloorinvestigations are mostly remote sensingoperations. The primary means to
determinethe morphology,the structure,and the texture of the sealloor
are acoustic. This paper considersthe current seafloorremote sensing
approachesinvolving acoustic backscattering.The physicalconstraints
imposedby the oceanas a propagationmedium, by the seallooras a backscattering boundary, and by the measuring instruments are brielly
reviewed. The sonar systemscurrently used by the oceanographic
community for deep seaflooracousticbackscattering
measurements
deal with
theseconstraintsdifrerentlydependingon their specificapplicationand on
whetherthey are towedbehinda ship or mountedon her hull.
Towed sidescansystemssuch as Gloria II (U.K.), the SeaMapping
(SeaMARC) I and II, the DeepTow system
and RemoteCharacterization
of the Marine PhysicalLaboratory (MPL), and hull-mounted systems
such as Swathmapall give a qualitativemeasureof backscattering
by converting echo amplitudesto gray levelsto producea sidescanimageof the
seafloor. A new approachis presentedwhich usesa SeaBeair multibeam
to producesimilaracousticimages.
echo-sounder
of backscattering
have beenattemptedin
Quantitativemeasurements
recent experimentsusing the Deep Tow system and Sea Beam. Such
providesome insightinto the geologicalpropertiesresponmeasurements
sible for the acousticbackscatter,
with useful applicationsfor geologistsas
well asdesignersand operatorsof bottom-interacting
sonars.
2, INTRODUCTION
With an averagedepth of four kilometers,the oceanfloor is only
accessibleby specialpurposesubmersiblesfrom which observationsare
limited by the apertureof a view port. For this reason,most sealloor
investigationsare remote sensingoperationswhich use underwatersound
as their primary tool and core or grabsampleas well as bottom photography as ground truth. Sealloor acoustic measurementsare commonly
divided in two broadcategories:low frequency(< 100 Hz) seismicmeasurementsin reflectionor refractionwork, and high frequency(> 3 kHz)
rellectivity or backscattering
measurements.This paper focuseson the
high frequencyseaflooracousticbackscatter
which is on one hand a noise
backgroundagainst which active sonars must operate, and on the other
hand a source of information for geologists becauseits variations are
causedby changesin bottom type or bottom microroughness.
The systemscurrently used by the oceanographiccommunity to
measureacousticbackscatterfrom the deep sealloor are most often sides-
can sonars or echo-sounders,and occasionally specially designed
multifrequencyarrays. Sidescansonarc give a qualitative measureof
by convertingecho amplitudesto gray levelsin the process
backscattering
of forming an acousticimageof the seafloor,and their resolutiondepends
on a combination of parameterssuch as frequency, pulse length,
whose
beamwidth... [l]. The sameparametersapply to echo-sounders
primary output is numericalbathymetry. However, broad-beam() 30')
echo-soundershave been used to obtain quantitativenormal-incidence
measurementsin an effon to classify bottom types [2] or bottom
microroughness[3]. Basedon the author's work, this papershowsthat
multi-narrowbeamecho-sounderssuch as Sea Beam [4], [5] provide a
measmeansto obtain both quantitativeseallooracousticbackscattering
urementsat discreteanglesof incidenceand qualitativemeasuresin an
acousticimagingmodewhich is new to suchsystems.
In the following, the physicalconstraintswhich conditionthe perfortakenin the
manceof sonarsystemsare reviewedbriefly. The approaches
design of severaloperationalsystemsare compared,and recent experimentsinvolving SeaBeamand multifrequencyarraysare presented.
3. SONARDESIGN CONSTRAINTS
This sectionbriefly reviewsthe constraintsimposedby the oceanas
boundaryand
a propagatingmedium, by the seallooras a backscattering
of the sonar on the designof systemsto
by the physicalcharacteristics
More exhaustivetreatmentson the
measureseallooracousticbackscatter.
subjectmay be found in references[6] and [7].
As sound waves propaggte through the water column, they are
attenuated due to spherical spreading and absorption. The former
increasesas the square of range and the latter increasesroughly as the
is usuallyimbeddedin a
squareof frequency. This frequencydependence
logarithmicabsorptioncoemcienta in decibelsper meter (e.g.,a -10-3
dB/m at l0 kllz) which setslimits on the operatingfrequencyof a sonar
for a given range. The deep-waterambient-noiselevel is also frequency
5 to 6 dB per octavein the interval l-100 kHz.
dependentand decreases
At long ranges, attenuation dominates and adverselycounterbalances
improvementsin ambient-noisecharacteristics
obtainedby using higher
frequencies. Expressedin decibels, the attenuation terms add into a
transmissionloss:
TL - 2Ologrcr* ar
(l)
where the slant range r is in meters. The propagationmedium also
imposeshorizontal range limitations on sound wavesas they are refracted
137
due to variationsin the soundvelocitywith temperatureand pressure.In
practice,the rangelimitation due to ray-bendingdependson water depth
and bottom relief. At oceanicdepths ()4 km) the maximum horizontal
rangeattainablewith a sonarat the seasurfaceis between30 and 40 km
dependence
of backscattering
on bottom type is taken into accountin the
scatteringcoefficient.Ss which increasesby nearly 25 dB from clay,
through silt and sandto rock; and bottom roughnesscan causevariations
of severaldB in Ss for the samebottom type [9].
t81.
As mentionedabove,the effectsof bottom roughnessdependon the
Bottom relief entersas a geometricalparameterin the rangelimitaoperatingcharacteristics
of the sonar. To samplethe small scaleroughtion, but for a given relief, bottom type and bottom miiroroughnessconnessrather than bottom slopesrequiresa systemwith narrow horizontal
dition the backscatteringprocess. Whether a body backscatterssound
beamwidthand largebandwidth. Theseparametersare constrainedby the
effectivelydependson how its density and compressibilitydiffer from
physicalcharacteristics
of the transducerused.
thoseof the surroundingocean,and how its roughnessscalecomparesto
The
bandwidth
capability
of the transduceris usuallylimited to l0the acousticwavelength.There is no simpletheory to predictthe level of
150/o
of the centerfrequencyll2l. Sincethe rangeresolutionof a system
sealloor-backscattered
sound waves, and one relies on reported measurewith bandwidthw is AR - cl2w, the incentiveis to use high frequencies
ments (mostly done in shallowwater,e.g., references[9], and tl0]) and
for greater bandwidth and higher range resolution, but attenualion
geoacousticmodels[ll] to derive empiricalexpressionsfor bottom backimposeshorizontalrangelimitationsand it becomesnecessaryto operate
scatteringstrength as a function of grazingangle and/or frequencyfor
close to the bottom. The most common and simplestway of acheiving
varioustypesof substrates.
rangeresolutionin sonarsis to use short pulsedcontinuouswave signals
In practice,the bottom backscattering
strength BS, in decibels,is
(CW) but the shortestacheivablepluse length r is limited to r - l/w.
expressedas the sum of a backscattering
strengthper unit area(lm 2) S,
An alternativeapproachis to use long frequency-modulated
(FM) pulses
and an efrectivescatteringareaA :
with pz > I and Drocess
the returnsthroughmatchedfilters. The advantagesof this method are the possibilityto use lower frequenciesand srilt
BS-Sa+A
(2)
maintainadequaterangeresolution,and a theoreticalsignalto noisepowtr
with S, - ,Se* l0/ogqssi#a and A * l}tosrc{(lcr
gain over short CW pulseswith equalsourcelevel of l0/ry1s(wr) lU.
' 2 c o s ')
Howeverimplementationof the correlationprocessoraddscomplexityand
where as indiceted in Fig. l, r is the slant range, 0 the horizontal
costto the overallsonarsystemdesign.
beamwidthof the transducer,a the grazingangle. t is the transmitted
The beamwidthrequirementsare constrainedby the size of thc
pulse length, c-1500m/s the sound speed,and,Se is a scattering
transducerarray and by its operatingfrequency. Defining the beamwidth
coemcientindependentof grazing angle. For most sidescansonar oi
as the width of the mainlobeof the radiationpatrem3 dB down from its
echo-sounder
applications,a squarelaw dependence
on sirb (1K- 2) is a
on-axisresponse,the beamwidth0 in radiansis roughly the reciprocalof
reanonable
matchto existingshallow-water
measurements
and by extrapothe number of wavelengthacrossthe effectiveapertureof the anay i.e.
Iation to the few measurements
existing for the deep sealloor t6]. The
0-X/ L wrthl,-clf . L is the length of the aperturein meters,)t is the
acousticwavelengthin metersand / is the acousticfrequencyin Hertz.
Consequently,for a given frequency,the longer the array the narrower
the beamwidth. Howeverthe optimum sizeof a transduceris a compromise betweenits directionalityand its acoustic-power
output capability.
The maximum strain bearableby the transducerand cavitationlimit
the maximum powerto which the transducercan be excited. Thesecon.
strainsset a lower limit on the size of the efrectiveaperturenecessary
to
keepthe radiatedpowerper unit areabelowthe cavitationthreshold. The
power P per unit area necessaryto producecavitationincreasesas the
square of the ambient pressure Po: P-po2l?pc watts/cm2 where
pc-1.5 ldg cm-2s-tis the acousticimpedance
of water[l3l. A substantial gain in cavitationthresholdis thereforeachievedby operatingan array
a few decametersbelow the surfacewith the addedadvantageof reduced
refractioneffectsas one goes below the thermocline. This techniqueis
usedin most shallow-towed
sidescanarrays.
In the caseof sidescansonar arraysmounted on either side of a
towed vehicle for port-starboardcoverage,an additional constraint is
imposedon the choiceof operatingfrequencyby the existenceof mutual
interference (cross-talk)betweenthe arrays due to radiation of sound
from the back of eachanay. As a result, a mirror imageof returns on
one side is mapped on the other side (for examples,see [14]). In
generd, this problemis alleviatedby usingslightly differentoperatingfrequencieson eachside.
ACOUIIIC lrrcE
t
rlrE .
Finally, an estimateof the echo-to-noiseratio EN measurablewith a
given sonar systemis obtainedthrough the sonar equationwhich comFig. I SIDESCAN GEOMETRY. A transducer array at depth D and
binesthe variousaforementioneddesignparameters.In decibelunits. this
elevation H above the seafloor has a beam pattern with angular dimen_
equationhasthe form:
(horizontal)
sions0
andO (vertical). lt ensonifiesa strip of sealloorout
to horizontal range R at right angle to the direction of travel (anow). A
EN - SL - 2TL - (NL + t0tosrcw)+ aS + 2ltogrcb(e)
(3)
portion of the strip (broken line) is not ensonifiedas it is shadowedby
where ,Sl is the sourcelevel,2TL accountsfor the round-trip transmisthe hill' Backscattered
acousticintensitiesreceivedat the transducerare
sion loss (Eq. l) betweenthe sonarand the bottom, (rYl*10/og1ew)is
gray
oonvertedto
levels and mappedat increasingtimes of arrival (or
the noiselevel in the bandwidthp, ^B^S
is the backscattering
strength(Eq.
slant range r). In this case,the first retum is receivedH/750 s after
(@)accountsfor the transducer'sverticaldirectivity.
2) and 20log1qD
transmission.olr the acousticimage,specularretums appeardarkerthan
For most sidescansystemsthe amplitudecorrespondingto the echo
the normal backgroundand shadowzones are white. A R is the crosslevel is convertedto a gray level and output on a linescanrecorderto
track resolution,anda is the grazingangle.
create an acoustic image of the sealloor surveyed. In the process,it is
r38
Teble I OPERATIONAL CHARACTERISTICS
System
Glorh
P 6.5,S 6.7
Frcquency (ktlz)
Pulse lpngth
FM swp
l5
So MAIC u
2G,10
,25-10ru
4{)"48
Ser MARC I
P2?,S30
l, 2, 4. 8. 16
2 klIF
100lft
ll0
cw
cw
l5-32 ro
7ru
2,str
5.1.2or4
5 kHF
200 flz
2 1, 3 0
2,&
17,50
CrN-truk r@luri@ Ar(
20m
5m
5m
deDth
30{0 m
hull mount
5G100n
upto6km
l5-30
up to 36
I)
sltirude H
Horizontal rmge (km) I
Speed (knots)
AmvVehicle
t-22
225llz
T 2 7, 5 4
R16(20,27)
upto75ltr
1(1100
m
3/8 wrer depth
.5
up to l0
l-3
upto 15
t-2
38
l 5
t
t <
<
lengrh(m)
*idth (m)
5.33
lcnrth (m)
7.75
5.5
3
width (m)
.t
l 3
t2
It
T2t,R.2.E
T . t 6 ,R . 4
customaryto multiply the amplitudesby a time-varied-gain
to compensate
for transmissionlossesand keepthe returns within the dynamicrangeof
the recordinginstrument.
tt<
08
.7
The SeaMARC II systemis also a shallow-towvehicle with portstarboardcoverageand a tow-sp€edcapabilitycomparableto that ofcloria
II. It operatesat higher frequenciesand usesshort transmitpuls€sresulting in better cross-trackresolution but reduced horizontal ranges (Table
l). In order to maintaineven spacingbetweendatapointsin the sidescan
image, the bottom returns are sampledmore rapidly at close rangesthan
at far ranges. As with Gloria II, the recordsare automaticallycorrected
for slant range by assuminga llat plane at the mean depth below the vehicle. Due to its finer resolution, the qualitative bottom backscattering
measurementsobtained from SeaMARC II imagesare us€ful for bottom
slopeand texture determinationon a scaleof acousticwavelengths(-12
cm), and for largescaleregionaltrend determination.SeaMARC II usesa
pair of transducerarrays (e.g. Fig. l) on each side of the vehicle. By
measuringthe phasedifferencebetweenthe outputsof the two arrays,it
is possibleto determinethe angleof anival of a given retum and therefore compute the correspondingdepth and cross-trackdistance. ln s
post-processingoperation, approximatelyone hundred such pairs of
depths and cross-track distances are computed for every transmission
cycle, and are used to producea contour map of the swathof seafloorsurveyed. This bathymetryis an importantelementin the interpretationof
backscattering
measurements
madewith the system,becauseit offers the
potentialto remove bottom slopeeffectsfrom the data while in principle
retainingthe efrectsof bottom compositionand microroughness.
The SeaMARC I systemis a deep-towedversionof SeaMARC Il
with a lesserswathwidth (5 km maximum). Its use of higherfrequencies
and short pulse lengths yields sub-metercross-trackresolution; and its
inherent slow speed over the bottom yields an along-trackresolution
between.5 and 5 m, dependingon the pulse repetition rate (Table l).
Although this systemusesthe sametransducerarray pair configuration as
Sea MARC II, the phasemeasurementtechniqueis not implemented.
Without numericalbathymetry,it is thereforenot possibleto correct the
backscattering
measurements
for bottom slope. Nontheless,the fine resolution of the sonar allows detectionof changesin bottom texture over
areasof constantslopebetweenl0 and 100m in extent dependingon the
swathwidth chosen.
4, CURRENTSYSTEMS
The design constraintsoutlined above are dealt with differently
dependingon the intendedapplicationof the sonarsystem. In this section, severalsystemscurrently usedby the oceanographic
communityare
comparedon the basisof their designapproach,and of the characteristics
of the acousticmeasurements
obtained.
Six systemscoveringa wide spectrumof rangesin deep-ocean
work
havebeenchosenfor this comparison.They are the British systemGloria
I ll5], 1161,Swathmap[8], [17], [18], the Sea Mappinsand Remote
systems(SeaMARC I and lI) ll9l, t201,[21], SeaBeam
Characterization
and the Deep Tow instrumentpackage1221,1231.The operatingcharacteristicsof thesesystemsare listed in Table I wheresymbolsappearingin
Section3 and Fig. I havebeenrepeatedfor easeof correspondence.
Gloria II, SeaMARC I and lI and Deep Tow are original sidescan
sonarsystemswhereasSeaBeamis a multibeamecho-sounder
and Swathmap is a sidescanapplicstionof the SQS-26ASW sonarusedaboardU. S.
Navy frigates. The table is incompletefor Swathmapbecausesome of the
technicaldetailsof the systemare classified.In the Swathmapdesign,a
beamis steeredto one side of the ship's track only with a maximum horizontal rangeof about 36 km and a cross-trackresolutionof severalhundred meters. The high ship speedresultsin a poor alongtrack resolution
as transmit cyclesare spaced400.to 5fi) m apart at 20 knors (-l0m/s).
This systemis primarily a reconnaissance
tool designedto map at a rapid
rate the large scalesealloorrelief such as seamountsor fracture zones,
and the intermediaterelief (50-100m)typical of abyssalhills. Although
the acousticbackscatter
measuredwith Swathmapis mdulated by bottom
texture, it is dominatedby slope effects,and the recordsare qualitative
representationsof bottom slopesand regional trends.
Similar measurements,with greateralong and across-trackrbsolution, are obtained with Gloria II which recordsacousticbackscattering
from both sides of a shallow-towedvehicle. Different frequenciesare
used in the port and starboard arrays to avoid cross-talk between them.
To enhanceits signal to noise characteristics,this system uses a long
transmit pulse and processesthe echoesby match-filteringtechniques.
The records are automatically corrected from slant range to horizontal
range by projecting slant range onto a horizontal plane at the mean depth
below the vehicle [Searle,personalcommunicationl. Measurementsare
qualitativeand recordedacousticreturnsare usuallydominatedby bottom
slopeswith marginalindicationof textural changes[25]. The strengthof
this systemsis its ability to map with sufhcient detail large swathsof
sealloor (30 km or more) at an averagetow speedof 8 knots.
75,@
2 5 ,5 ,t , 2 5
<
up to 20
l0 kHz
1 5m
hull mount
l2-1250
m
up to ll
DcepTor
cw
Bcamsidth(rleg) 0 ,aD
Tow
Sq BsD
1 2l 5 t
cw
FM s*eep
100Hz
19
s t2
Pll
2.4w
t
Puls rcp (w)
B8ndwidth
Sr{huD
Through a combination of frequency, pulse Iength and tow speed,
the sidescansonars of the Deep Tow instrument packageachieve the
highestresolutionof all the systemslisted in Table l. The lateralcoverage to port and starboardis inherently limited to a srvrth about I km wide.
This system is primarily intended for fine scale studies of the seafloor
down to depths in excessof 7 km. The backscattermeasurementsare
qualitative gray scaledisplaysuncorected for slant range. Changesin boh
tom texture are readilyobservableand micro-relief (( ln ) such as small
fissuresis resolvableon such displays. However,the limited coverageof
the sonars make the system marginally useful for regional trend assess-
139
ment as it requireslong and costly surveys. The strengthof the Deep
Tow instrument packageis its versatilityas environmental(e.g. temperature, conductivity) and other geophysical(e.g. magnetics,subbottom
profiles)data can be collectedsimultaneouslywith the sidescandata; and
bottom photographscan be taken on the samelowering [22]. the ptrase
measurementtechniquementionedaboveis feasiblebut not implemented
on the DeepTow system.
5. NEWAPPROACHES
The systemsdescribedin section 4 all operateas sidescansonars.
of
measurements
As such they providequalitativeacousticbackscattering
the deep seafloor. This section looks at new measurementapproaches
such as SeaBeamor experimentalmulusing multibeamecho-sounders
suchas Deep
tifrequencyacousticarraysmountedon instrumentpackages
Tow [24].
Sidescansonarstypically hansmit and receive with the same fanshapedbeamwhich is narrowalongtrack and broadacrosstrack (Fig. l)'
The SeaBeam systemtransmitswith a similar Seometry'but it receives
with six(eennarrow beamsspaced2' 213 apafiathwartshipswithin f 20"
incidence. Each receive beam is 2" 213 wide. With this geometry'the
returns (Fig. 2) at discreteangles
systemis able to procbssbackscattered
of incidenceand calculatea set of depths and cross-trackdistancesfor
eachtransmissioncycle. This numericalbathymetryis then output in near
real-timeas a contouredchartof the swathof sealloorsurveyed.
the acousticsiglals to determinedepthsbut has
SeaBeamprocesses
no internal provision for recording the actual waveform. To preserve
these signqls,requiresa parallelacousticdata acquisitionsystemwhich
recordsdigitally the echo envelopeson magnetictape for later processing
and analysis.Sucha systemdevelopedat MPL wasusedby the author to
recordSeaBeamacousticdata [261. A typicalset of returnscqrresponding
to one transmissioncycleis illustratedin Fig. 2. Thesedatamakeit possi'
at discreteangles
measurements
ble to obtain quantitativebackscattering
causefluctua'
of incidence. Becausevariationsin bottom characteristics
and becausenumericalbathymetryallows
tions in the acousticbackscatter,
of
correctionsfor slope efrectsto be made, the geologicalcharacteristics
or bottom type) can be inferredin part
the sealloor(e.g. microroughness
from thesemeasurements
1211,1281,4291.
In a new applicationof Sea Beam system, the acousticreturns it
receivesare used in a sidescanJikemode by combiningechoeson either
side of vertical incidence. Working with digitally recordedSea Beam
acousticdata (Fig. 2) a peak detection processis used to obtain echo
o
amplitudesat incrementalslant rangesto port and starboard'This process
startsat the first arrival of eachtransmissioncycle' An exampleof the
resultingacousticimageis shown in Fig. 3 along with the corresponding
Sea Beam bathymetry,and a Sea MARC I image of the same region
includedfor comparison.In this example,the SeaBeamacousticimageis
not correctedfor slant rangeor for angleof incidence' However,as Sea
Beamcomputesa cross-trackbathymetricprofile every transmissioncycle,
slant rangecorrectionscan be performedwithout the ambiguitiesinherent
to the conventionalhorizontalplane method mentionedpreviously. The
correctionfor angleof incidenceentailsapplyinga time'varied-gainto the
signalsdisplayedin order to compensatefor the drop in backscatteras a
function of angle. Such a correctionis performedin most sidescansystems and yieldsan acousticimagehavingnearlyuniform resolutionout to
the edges.
This new acousticimaging applicationof Sea Beam gives textural
informationabout the seafloorand bringsout sealloorfeaturesnot discernible in the contouredbathymetry. Unlike conventionalsidescanimages
(Fig. l), acousticimagesobtainedwith SeaBeamdo not containshadows
becauseanglesof incidenceare limited to the range +20', (note that
most sidescansonarsdo not recorddatain this angularsector). The qualgiven by theseimagesis thereforerepresen'
itative measureof backscatter
and bottom slopes. The efrect of bottative of sealloormicroroughness
tom slopescan also be removed from the backscatterdata given that
quantitativeslopeinformationis availablein SeaBeam'sbathymetry.
The advantageof Sea Beam's discretenarrow beamscan also be
preservedby outputtingthe echo characteron eachbeam in a gray level
displaysuch as that of Fig. 4. Although this displayis more difficult to
interpret than conventionalsidescanimages,it gives bottom structural
detailswhich are smoothedout in the processingusedto createthe acoustic imageof Fig. 3b.
As mentionedin Section3, the effectsof the seaflooras an acoustic
boundaryare difficult to predict, and one relies on measbackscattering
modelsusableby designersof
ured data to derive empiricalbackscattering
sonar systems[30]. However,there is a definite need for deep sealloor
as the existingdata baseis very limited. Systemssuch as
measurements
as a function of angle
SeaBeamgive accessto quantitativemeasurements
of incidencewithin the limits of the availablebeamwidth. Recentexperiments to obtain quantitativemeasurementsas a function of frequency
havealsobeencarriedout usingmultifrequencyarrays(4.5, 9' 15' 2t' 60'
110, 160 kHz) mountedon the stem of the Deep Tow instrumentpackage,and projectingfan beamspointingaft 1241.
give non-uniqueanswers
In generalsinglefrequencymeasurements
about the nature of the sealloorbecausethere is an infinite combination
process'and
of roughnessand bottom type influencingthe backscattering
need to be validatedby direct bottom samplingor by bot'
measurements
tom photographsor television. On the other hand, multiple frequency
systemsallow simultaneoussamplingof severalroughnessscalesthereby
giving some indication of the respectivecontribution of roughnessand
process.Suchmeasurements
may even'
bottom type in the backscattering
tually help geologiststo acousticallydifferentiatebetweenbottom types'or
determine the size of bottom microrelief (e.g. furroun, manganese
have beenobtained
nodules,etc.). Similar multifrequencymeasurements
in an effort to determine
with conventionalhull-mountedecho-sounders
the sizeof polymetallicnodules[31].
6. CONCLUSIONS
In the remote sensingof the deep sealloor,acousticbackcattering
can be measuredwith sidescansonarsor echo-sounders.Sidescansonars
which allow
usually provide qualitativemeasurementsof backscattering
of
geologiststo make regionalstructuralanalysisand textural assessments
the seafloor,with a resolutiondependingon the operatingcharacteristics
of the sonar. A new applicationof the SeaBeamsystemin an acoustic
imagingmode has been shown to yield similar qualitativemeasurements
whoseusefulnessis enhancedby the precisionnumericalbathymetrynor'
mally availablewith this system. The SeaBeamalso offers the potential
as a function of angle
measurements
to obtain quantitativebackscattering
Fig. 2 ENVELOPESOF ECHOES RECEMD BY SEA BEAM'S 16
BEAMS. Beamsare equally spaced2" 213 apafi within +20" of the
ship's vertical axis. In this display, retums are not compensatedfor
ship's roll. Time is in secondsafter transmission.Amplitudeshave been
correctedfor transmissionlossby a time-varied-gainand are displayedin
volts.
140
km
0 0.5 I.0
sec
B
0'6 0.4 0.2 0 0.2 0.4 0.6
r
t
t
t
t
t
l
C k m
1'0 05 0 0.5 r'0
r
t
$
f
Fig. 3 ACOUSTICIMAGES AND BATHYMETRY. (a) Slant_range
correcredSeaMARC
I imageof the crestof the EastpacificRise at 10. N [32]. Distances
are true horizonrar
distances.(b) UncorrectedSea Beamimage of the samearea; the dashedline in (a)
representsthe corresponding
SeaBeamtrack. The cross-trackdimensionis in secondsfrom
first arrival (differentialslant range). (c) correspondingswathof sea Beambathymetryat
10 m contourinterval. Tick markspointdownhill.
141
t
r
l
E S T E P SO F zr lr n
A N G L EO F I N C I D E N CI N
3
4
5
2
2
l
0
l
3
5
4
PORT
STARBOARD
Fig. 4 ECHQ CHARACTBR OF INDMDUAL ROLL-COMPENSATED BEAMS. Same
SoaBeamar,ousticdegaas that of Fig. 3. At each angle of incidence,vertical grid lines rnark
each transmission cycle, and erhoes are displayed in bins.35s
th€ time of .6rst srivalfo
wide. The gap betweena grid line and the correspondingretum is the difierential dant rangp.
142
of incidence. Becauseof the complicatednature of the seafloorbackscatteringprocess,thesemeasurements
do not give a uniqueanswerabout
the natureof the seafloor,but they constitutea necessary
steptowardsthe
understandingof the processes
at work, and towardsthe constructionof
empiricalformulasfor sonardesignpurposes.
Further insight into the pr@essesinfluencingthe seaflooracoustic
backscatter
are obtainablewith murtifrequencyarraysas they ailow simurtaneoussamplingof severalbottom roughnessscales. Howeveracoustic
measurements
alone may never be sufrcient to give the exactnature of
the sealloor,and ground truth from some other sensingtechnique(e.g.
bottom samplingor photography)may still be necessary.
7. ACKNOWLEDGMENTS
work. Supportfrom the Office of Naval Research(Contract# N0001479-C-047Dfor the Sea Beamacousticbackscattering
experimentis also
gratefullyacknowledged.
lll
References
A. R. Stubbs,Sidescansonar,IEE proc, yol. l3l, part F No. 3,
1984,pp 243-256
l2l
L. Breslau,Classificationof sea-floorsedimentswith a shipborne
acousticalsystefi, WoodsHole Ocean. Inst. ContributionNo. 167g,
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reDrinted from
Current Practices and New Technology in Ocean Engineering - OED-Vol. 11
E d i t o r s : T . M c G u i n n e s sa n d H . H . S h i h
(Book No. 100206)
p u b l i s h e db y
T H E A M E R I C A N S O C I E T YO F M E C H A N I C A L E N G I N E E R S
3 4 5 E a s t4 7 t h S t r e e t , N e w Y o r k , N . Y . 1 0 0 1 7
Printed in U.S.A.