11 FIGS
GEOPHYSICS,VOL 50. NO 6 (JUNE 1985)P 989_1001,
Inference of manganese nodule coverage from Sea Beam acoustic
backscattering data
de Moustier*
Christian
tivity maps which servedas the baseof a month-long, fine-scale
study of the same site, with the Deep Tow instrument package
of the Marine Physical Laboratory of the Scripps Institution of
Oceanography (Spiessand Tyce, 1973; Spiess and Lonsdale,
1982).The purpose of the Deep Tow survey (ECHO leg 1) was
to assess,using a new multifrequency array, the near-bottom
backscatteringproperties of a manganesenodule field. In addition, an environmental impact study of nodule mining was
carried out through an extensivebox coring program (Spiesset
al, 1984).The choice of the site was motivated by the existence
of a good DOMES and OMA data basein the generalarea.
This paper discussesthe results of the Sea Beam acoustic
study, and usesthe Deep Tow data as ground truth.
ABSTRACT
Normal incidence reflectivity from a manganese
nodule field was measured with a 12 kHz Sea Beam
multibeam echo-sounding system, aboard the R/V
Thomas Washington and used to infer nodule coverage.
A reflectivity map of the area was produced using the
intensity of the specular return from each ping. The
patchiness of the nodule coverage is evidenced by definite highs and lows in the reflectivity pattern. Ground
truth was provided by near-bottom acoustic measurements and photographs taken with the Deep Tow instrument package of the Marine Physical Laboratory.
Agreement between the simple nodule coverage predictions from Sea Beam acoustic data and the bottom photographs taken throughout the area is 98 percent. Although the Sea Beam system is limited in its dynamic
range, this paper shows that it can be used very effectively to determine both topography and nodule
coveragein potential mining areas.
BACKGROUND
Manganese nodules remote sensing
Most techniquesused to prospect for manganesenodules on
the deep ocean floor rely on acoustic remote sensing, nearbottom photography and/or television, bottom sampling, or a
combination of thesemethods.
Two generalapproacheshave been taken for acousticremote
sensing of nodules. The first is a single-frequencyapproach,
using high-frequency( > 100 kHz) side-scansonars on a deeply
towed instrument (Spiess,1980),or using a shipboard subbottom profller to correlate the thickness of the transparent layer
with nodule abundance (Piper et al., 1979; Mizuno and Moritani, 1976).The second is a multifrequency approach fSumitomo Metal Mining Co. (MFES), Magnuson et a1.,1981,1982;
Spiess et al., 1984] which uses the frequency dependenceof
sea-floor acoustic reflectivity to infer nodule sizes and abundance.Both approachesrely on the observation of a difference
in acoustic backscatter between a sedimentary bottom laden
with nodules and a bare one.
Near-bottom photography techniques range from the conventional deeply towed camera systemsto the unmanned, deep
diving, free vehicle Epaulard (Duranton et al., 1980; Galerne,
1983).Ground truth for remote sensingof manganesenodules
is obtained through direct sampling of a nodule field. To this
end, dredging is used where large amounts of nodules with little
or no sediments are sousht. On the other hand. box cores
INTRODUCTION
Economic considerations make the exploitation of ferromanganesenodules in the deep ocean strongly dependentupon
extensiveassessmentof the density of nodule coverage at potential mining sites. Depending upon the method used to
survey a site, the cost in ship time and manpower can become
prohibitive. The approach described here derives its costeffectivenessfrom the use ofa Sea Beam echo-soundingsystem
operating at optimum ship speed (10-12 knots). By taking
advantage of the narrow-beam and multibeam capabilities of
the Sea Beam, I was able to combine detailed topographic
mapping with bottom backscattering characteristics to infer
nodule coverage.The data presentedhere were gatheredduring
two cruises in the northeastern tropical Pacific in May and
June of 1983.The first cruise (PASCUA leg 5) was a rapid Sea
Beam survey of an approxirnately 2O x 15 mile area southeast
of Deep Ocean Mining Environmental Study (DOMES) site C
(Figure 1), on an Ocean Mining Associates(OMA) trial mining
site. This survey provided the topographic and acoustic reflec*
Presented in part at the 106th meeting of the Acoustical Society of America in Sai Diego, November, 1983. Manuscript received by the Editor
September4, 1984;revisedmanuscript receivedDecember 12, 1984.
+Marine Physical Laboratory, Scripps Institution of Oceanography. University of California, San Diego, La Jolla, CA 92093.
This paper was prepared by an agency of the U.S. government.
989
de Moustier
990
provide an undisturbed sample of the bottom whenever an
assessmentof the geologic,biological, or acoustical properties
of both the nodules and their underlying sedimentsis needed.
Normal incidence acoustic reflectivity
There is extensiveliterature dealing with acoustic properties
of the ocean floor at various frequencies and angles of incidence, and with their geologic implications. Tyce (1976) and
Parrot et al. (1980)gave good reviews ofthe literature available.
Due to its multibeam, narrow-beam characteristics,the Sea
Beam system allows the measurement of the acoustic backscatter of the sea floor from each individual beam. An angular
relationship of acoustic backscattering can then be derived.
However, in this paper I take a first look at the data by
selecting the beam nearest normal incidence and leave the
others for later analysis.
Sincethe depth of this manganesenodule survey area is large
(4 500 m on the average),the radius of curvature of the acoustic
wavefronts reaching the bottom is much larger than SeaBeam's
acoustic wavelength (12 cm), and the wavefronts can be considered nearly planar. Therefore, I use this approximation in
the following development.
In general,the intensity of a plane acoustic wave is related to
the acoustic pressureP by
P2
I : -,
pc
(l)
l6o'w
t5o'
t4oo
I3o'
Q)
where o is the exponential attenuation coefhcient and R is the
Rayleigh reflection coeffrcient. Equation (2) assumes that the
bottom reflectsthe sound rather than scattersit. This assumption seems reasonable for a manganese nodule field whose
aggregate response was shown theoretically in Magnuson
(1983)to be reflectiverather than scattered.
For a plane wave normally incident on the boundary between two media of respectiveimpedance prc, and prcr, the
Rayleigh reflection coeffrcientis the ratio ofthe reflectedto the
incident pressurewaves:
^
Pzcz P(t
P 2 c 2+ P ( L
(3)
For the purpose, of this paper, medium I will be water and
medium 2 will be sedimentor nodules.The fraction on the right
side of equation (2) indicatesthat seawater absorbs some of the
energy, causing an exponential loss with distance from the
source, and that intensity decreases proportionally to the
square of distance due to geometric spreading.This is summarized in the sonar equation for an echo sounder obtained by
taking 10 logro ofboth sidesofequation (2):
EL:
SL - BL _ TL,
(4)
where
EL : l0 logto I : echo level,
wherep is the densityof the mediumand c the propagation
velocityin this medium.The productpc is calledthe characteristic acousticimpedanceof the medium.The bar indicatesthat
is averaged
oversometime(usuallytheintegration
the pressure
time of the instrument)(Urick, 1983).If 10denotesthe intensity
of soundat unit distancefrom the source,the intensityof the
normal incidencereturn I from the bottom at depthr is given
by
30'N
r : t "^ n ,(2r)''
11,
r2o"
I Io"
20.N
i 00N
o'
l0's
20's
30's
Frc. 1. Location of survey area. Deep Ocean Mining lnvironmental Studies (DOMES) sites A, B, C are shown by dots.
Inset: track chart of the PASCUA expedition leg 5'
SL:
1 0 l o g r o l o : s o u r c el e v e l ,
BL :
-20 logro R : bottom loss,
and
TL:20
logro(2r) -+ 2oar log,oe : transmissionloss.
Equation (4) was used to determine the reflection coefficientsof
various marine sediment types as a method of classification
(Breslau, 1967). Likewise, Hamilton (1970a, b) did extensive
work toward predicting in-situ acoustic and elastic properties
of marine sediments.Tyce (1976) focusedon the determination
of the attenuation coeffrcientin ocean sedimentswith a 4 kHz
near-bottom proliler. His data indicate a large variability in
seafloor reflectivity over short horizontal distances due to
changesin local bottom roughness and composition. Breaker
and Winokur (1967) observed an increasein the magnitude of
the echo level fluctuations with distance from the bottom'
Considering that most of the acousticreflection comesfrom the
first Fresnel zone which increasesin sizewith distancefrom the
bottom, they attributed these fluctuations to changes in the
reflective characteristics of the bottom over very short distances.Again, thesechangesare most likely due to variations in
bottom substrate and/or bottom roughness. The effect of
roughness is to change the phase relationships between the
different reflectors within a Fresnel zone. How rough a surface
appears depends on the ratio of the root-mean-square (rms)
bottom roughness o to the acoustic wavelength l" (Clay and
Medwin, 1971, chap. 10). Using the vertical component of
wavenumber K, one can distinguish three cases:
(1) ro ( 1, the surface appears to be smooth and the
amplitude of the backscatteredreturn is determined by
(coherentreturn);
the reflection coeff,rcient
Sea Beam Acoustlc Backscatterlng Dala
(2) xo > l, the surface appears to be rough and the
returnsare mostly incoherent;
(3) rco - 1, intermediaterange bridging the coherent
and incoherentregimes.
Clay and Leong (1973) pointed out that when ro = 1 and
when the spatial dimensions of bottom roughness are much
smaller than the diameter of the first Fresnelzone, the reflective
components add as the sum ofsquares and the returned mean
square signal is proportional to the number of reflectors in the
ensonified area. This means that bottom loss depends also on
the ensonified area and therefore on the beam width of the
measuring system. The diameter of the first Fresnel zone is
given by d : (2),h)rt2where l. is the acoustic wavelength and ft
is the distance from the sonar to the reflection plane (Clay and
Medwin, 1977, p. 50). For hull-mounted sonars such as Sea
Beam, this distance is approximately the water depth. In this
survey area Sea Beam's 12 cm wavelength yields a diameter of
about 33 m for the first Fresnel zone at 4 500 m depth. As
shown later, the rms roughnessin this manganesenodule area
is about 2 cm, so that ro - 1, and the spatial dimensionsof
roughness(10 cm or less)are rnuch smaller than the dimensions
of the first Fresnelzone
Other investigatorshave tried to correlate acousticfrequency
dependence of bottom reflectivity with bottom type and/or
bottom roughness(Mackenzie, 1960;Zhitkovskii et al., 1966).
This turns out to be a difficult problem when the acoustic
wavelength and/or the bottom are such that subbottom reflections can no longer be ignored. Note that when the bottom is
assumed horizontally stratified with plane interfaces between
layers, subbottom reflections from thin layers (Clay and
Medwin, 1977, chap. 2) yield a closed-form solution. However,
in order to perform a first-order analysis,simplifying assumptions consideringthe oceanbottom as locally homogeneousin
depth are made. An ideal ocean bottom can then be defined
where, for normal incidence, the dimensions of the bottom
irregularities compared to the acoustic wavelength and the
impedance mismatch at the water-sediment interface are the
main parameters in the determination of bottom reflectivity.
Higher reflectivity corresponds to a high impedance contrast
between sea water and bottom (generallyhard substrate)and a
locally smooth relief (dimensions smaller than the acoustic
wavelength).As the ratio ofthe size ofthe bottom irregularities
to the acoustic wavelength increases,the bottom irregularities
act as scatterers and the specular component of an echo is
lower Reflectivity is again lower when the impedancecontrast
is low (soft bottom with impedancesimilar to that of seawater).
A manganese nodule field consists of a soft bottom (sediments) on which a variable number of nodules lie. Nodule
acoustlc remote sensing is then possible, provided there is
enough impedancecontrast betweennodules and sediments.
Although no values are presently available for the acoustic
impedance of manganesenodules or surface sediments in the
survey area, the valuesgiven in Hamilton (1970b)for sediments
of the Pacific and in Magnuson et al. (1981)for nodules can be
used to compute an expected acoustic reflectivity difference
betweennodules and their underlying sediments.The sediments
are mostly pelagic silty clay (Spiess et al., 1984) for which
Hamilton gave an in-situ reflective coefficient of 0.1316 at
normal incidence and the corresponding bottom loss of 17.6
dB. For Pacific nodules, Magnuson gave a range of values for
both nodule density and compressionalwave speed.I use p2 :
991
I 95 g/cmr, cz:2 400m/s for a nodule,and from Urick (1983)
p r : 1 0 4 ' 1 5 g f c m 3 ,c r : I 5 2 0 m / s f o r s e a w a t e ra t 4 5 0 0 m
depth. Substituting these values in equation (3) yields a reflection coeflicient R :0.4923, which when expressed as the
bottom loss term in equation (4) becomesBL : 6.2 dB. When
the bottom depth is relatively constant,the transmissionloss of
equation (4) can be assumedconstant. Therefore bottom loss is
the only variable in equation (4), and the difference in reflectivity between a nodule and the underlying sediment is simply
the differencein bottom loss: 11.4dB.
Hamilton (1970b) also indicated there is little or no dependence of bottom loss with acoustic frequency for sediments
where there is no lower layer to reflect sound which interferes
with sea-floor reflections.Such is not the casefor a nodule field.
Studies by Magnuson et al. (1981, 1982)and the Sumitomo
Metal Mining Co., where manganesenodules were modeled as
sphereson an infinite flat plane, show that the bottom reflectivity of a nodule field can be expectedto reach a maximum for
a frequencycorrespondingto
ka:
(5)
l,
where k is the acoustic wavenumber and a is the mean nodule
radius. By comparing the results from measurementsat sea
with the foregoing models, some clues arise as to the validity of
the assumptions made. However, before I can discuss these
results,I need to characterizethe data baseand the measuring
systems,SeaBeam and Deep Tow, usedin this survey.
SEA BEAM ACOUSTICS
The Sea Beam system usesa multibeam, narrow-beam echo
sounder operating at a frequencyof 12.158kHz with a pulse
length of 7 ms and an echo processor to generate,in near real
time, contour maps of the ocean floor while the ship is under-
\
root
2<
intersection of the two beam patterns
which is delimited by 16
squares2J degreeson a side.
de Moustier
992
way. Figure 2 illustratesthe SeaBeamtransmit/receivegeometry. The transmitted beam pattern spans54 degreesathwartin thefore-aftdirection.It is pitch-stabilized
shipsby 2f degrees
to ensureverlical projection.As a result of the SeaBeambeam
forming,the receivingbeampatterncan be approximatedby 16
adjacentrectangles2 213 degreesby 20 degrees.The acoustic
energyreceivedat the ship comesfrom the intersectionof the
transmit and receivebeam patterns,that is 16 " squares"2 213
on a side.1
With contour mappingas a goal, the SeaBeam doesnot
preserveall the acousticinformation it receives.As a result
some valuable information not necessaryfor depth determination, suchas the amplitudeand the shapeof the echosignal,
is lost.
In order to preservethe echoamplitudeinformation,a parallel data acquisitionsystemwas built around an LSI lll23
minicomputer(de Moustier,1985).This allowsdigital recording on magnetictape the 16 Sea Beam detected,nonroll(TVG) and
compensated
beams,along with time-varied-gain
ship roll information.The TVG is designedto compensate
for
transmission
lossfattenuationand spreadingtermsin equation
(2)l and hasalreadybeenappliedby the SeaBeamsystemwhen
thedataarerecorded.Figure3 showsa typical16beamreturn,
not roll-compensated,
where the envelopesof the detected
acousticsignalsare plotted in analog-to-digital(A/D) units
versustime aftertransmission.
Theseunitscorresponddirectly
to soundpressure
levelswith TVG applied.In this instance,
the
seafloor is essentially
flat,resultingin the parabolaoutlinedby
the 16 returns.The amplitudeis highestin the near-specular
direction(first arrival in time)and decreases
rapidly from the
rFor a detaileddescription
of the system,seeRenardand Allenou
(197r.
centeroutward. The early, synchronousreturns on the side
beamsrepresentsidelobe energyfrom the specularreturn and
arerejectedin both depthandreturnamplitudeprocessing.
The SeaBeam systemhas not beencalibrated; thereforein
thefollowing all the measurements
arepresentedasrelative.
DATA
The datapresented
herewerecollectedduringa 40-hourSea
Beamsurveyand a subsequent
DeepTow near-bottominvestigation.Although the SeaBeamsystemmaps a swath of sea
floor three-quarters
of the oceandepth wide,a high degreeof
overlap betweenswaths(80 percentin places)was used to
provideas much redundantacousticdata as possible.Figure4
showsthe topographyof the surveyareacontouredfrom the
SeaBeamdata.The contoursareat 20 m intervals,implyinga
relativelyflat portion of seafloor with lessthan 200m of total
relief in 40 km. Aside from two north-southtroughs which
bottomout at 4 600m on the edgesof themap,themeandepth
is 4 500m.
The generalpatternagreeswell with the known north-south
perpendicular
orientationof sea-floorfeatures,
to the spreading
directionin this part of the PacificOcean.
At this depth the SeaBeamsystemtransmitsevery8 s. For
eachtransmission,
a setof 16returns(Figure3) is digitizedand
recordedon magnetictape.For a first look at thesedata,the
postprocessing
consistedin selectingthe highestamplitudein
eachset.The highestamplitudegenerallyrepresents
the nearspecularreturn,whichin the caseof a relativelyflat bottom,as
in thisarea.comesin first.
This set of peak amplitudesis then low-passfiltered by
applyinga running meanalong the track, averagingover the
40 0 0
3000
5
a
F
z,
1 { i
n'
9-
2000
3
4
I 000
a
4v
4 Y
5.8
6.0
6.3
6.5
seconds
6.8
7,0
FIc. 3. Acoustic signal envelopesof the 16 performed beams at the output of the Sea Beam echo-processorreceivers.The x-axis
r€presents time after transmission iir seconds. The y-axis represents the amplitude of the signals eipresse{ in AID units (G4095)
linear scale. Such data are recorded digitally on magnetic tape every transmission cycle, along with time-varied-gain and ship's roll.
No roll correction has been applied to the data at this stage.
Sea Beam Acoustic BackscatterlngDala
t25"25',w
t25'30'w
993
t25'20'w
t25"$',4
'..-115"
0'lt
14"
//
55',fi
N
J
-b(
140
50'h
"4 2
n
140
45',fi
ta
l4o
tt0'il
(\ui
'
) l'
14o
J0'lt
Frc. 4. Contoured map of the survey area from Sea Beam data. The contours are in uncorrectedmeters.The contour interval
is 20 m.
994
de Moustler
Ec. 5. S-hip'strack of_theSeaBeam surveyin the manganesenodule area (PASCUA leg 5). The peak amplitude from each ping
(Figure 3) has beenselected,low-passfiltered,and plotted along the track in A/D units linear scale(0-4095).Five intervals were
chosenon this scaleand pattern codedas shownin the key.
995
Sea Beam Acoustlc BackscatterlngData
sizeof the vertical beam'sfootprint: roughly five transmission
cyclesat 11 knots. ftre resulting data are plotted and pattern
codedalong the ship'strack on Figure 5. The A/D units linear
scaleis the sameasindic'atedin Figure 3.
The Deep Tow survey was concentratedin the southern
portion of the area wheie the Sea Beam survey density is
greatest.The SeaBeamacousticdata servedas a referencefor
the Deep Tow work during which camer4runs, near-bottom
multifrequency(4.5, 9, 15, 30, 110, 160 kHz) backscattering
measurements,and sidelooking sonar data (110 kHz) were
collected.As part of a mining environmentalimpact study, 16
box cores were also taken throughout the Deep Tow survey
area.The various elementsof the DeepTow work are summaet al.,1984).
rizedin Figure6 (Spiess
DISCUSSION
SeaBeam reflectivitydata
Inspectionof the peak-amplitudedata revealsa substantial
ping-to-ping variability as well as definite mean amplitude
highsand lows alongthe ship'strack. This point is illustratedin
Figure 7 where amplitude versusdistancealong the track is
plotted in conjunctionwith the centerbeamwidth profile. The
44 0 4 5 ' N
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IO-2I JUNE 1983
ECHO OI
TRACK
FISH NAVIGATION
I
635 m/in
1
I
r.onsponders
a
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8ox cores
,s\is<N
Runs
Eockscoil€r
3600+
Runs
IComero
'l2oo
-
t
+
.
'
-
48pO
t
84OO'
,
*
t
+
I2QOO
_
_
_-
lsqoo
a
-
t4o34:N
t 2 5 0 3 0W
Frc. 6. Deep Tow track chart showing camera runs, near-bottom multifrequencybackscatteringmeasurementruns, box core
locations,and navigationtransponderpositions.Coordinatesare in meterseastand north of 14"34'N,125"30'W.
996
de Moustler
in relativedecibels,with referenceto the
amplitudeis expressed
mean amplitude of the data displayed.The data shown in this
figure have been averagedover five transmissionrycles for
clarity; actual ping-to-ping variability is much greater.Some
small correlation with topography can be seen.However,rememberthat the vertical exaggerationon the depth profile is
On the
18.The slopesare thereforerarely in excessof 2 degrees.
left side of Figure 7,large fluctuationsin amplitudearc apparent, and the averagevalue is around - 6 dB. Comparedwith
the right side where the averageamplitude is +4 dB, the
bottom reflectivity has changedsignificantlyfrom one side to
the other.
Following this result,I divided the peak amplitudedata set
into five intervalson ihe A/D linear scale(0_4095)and pattern
codedeachone asshownin Figure 5.The datamatch,to within
130 A/D units dn the average,over all except three track
intersectionswhere there are slight offsets' Becausenormal
satellitenavigation is simply inadequatefor track positioning
at the level of SeaBeam'sresolution,one reliesupon matching
SeaBeam topography at crossingsand overlapsfor accurate
track positioning.However,accurateadjustmentsare diflicult
to achievewhen the seafloor surveyedexhibitslittle relief.asin
thisarea.
A histogram of the data distribution is shown in Figure 8.
The histogramsuggeststhat the data sufferamplitudeclipping
at 3 000 A/D units,sincethereare rio valuesbeyondthis point.
This unfortunatecharacteristicis an artifact of the data acquisition systemwhich wassaturatingat 7.3 Y . This problemhas
sincebeencorrected.In addition, it was found that the output
of the SeaBeamecho processorreceiversis limited to 8.5 V
As a result,
by themanufacturer.
insteadof the l0 V announced
rangeis apparentlyinadequateto ac'
the receivers'dynamic
commodateboth the low backscatterlevelsreceivedon the side
beamsfrom a smooth bottom and the high amplitrtdeassoci-
ated with some strong specularreturns. For this simplified
application, however,the nodule distribution results are not
significantlyaffected,sinceI arbitrarily choseto start the last
this amplitude clipA/D interval at 2 500units' Nevertheless,
ping doeslimit resolutionfor high-amplitudereturns and preventsdiscussionof the abilitiesof sucha sonar to characterize
similar nodule fields or other seafloortypes.From discussions
with the Sea Beam manufacturer,it seemsthat a first-order
improvementto this dynamicrangelimitation can be achieved
by a simplemodificationof the detectionamplifiers.Obtaining
the full range needed,however,might require modification of
the hydrophonepreamplifierswhich have beenfound to saturate occasionallyand would also require use of logarithmic
detectionamplifiers.
Drawn from the data in Figure 5, a contour map of reflectivity in the southernportion of the surveyareais presentedin
Figure 9. It usesthe samepattern codeas previouslydescribed.
stand out on this map: a regionof
Two main characteristics
strongreflectivityin th€ center,borderedto the eastand to the
w€stby stripsof much lower reflectivity.Someof the low values
correspondto the north-southtroughsapparentin the bathymetry (Figure 4). However,the low reflectivity extends3 to 4
km into the flat centerportion' Changesin relativereflectivity
of 12 dB or more (Figure 7) can thereforebe expectedas one
crossesthe leveledground area in an east-westdirection. Although this number is biased in trying to account for the
clipping of the high-amplitudereturn (I added2 dB to the 4 dB
averageamplitude shown on the right side of Figure 7), it is
reassuringto note that it is closeto the expectedvalue computed in the "Background" section(11.4dB). Sincewe were
nodulesite,I would like to associatethe
surveyinga manganese
in nodule coverage,with highwith
differences
changes
above
amplitude valuescorrespondingto a densecoverageand low
valuesindicatingabsenceof nodules.To do so, however,re-
4400
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td
-
U
\ l l - , i - \ ? . - \
4600
J
F
P e a kA m P l r t u d e
td
- - - Center BeamDePth
(V E x l8)
R el a t i v e M e a n = 0 d B
F
,l
lrJ
E
o
18 0 0
-10
l0
II
D I S T A N C En ,m r
3) is selected,low-pass4!!"t:9, and displayed1_19-91jvp
Frc. 7. The peak amplitudefrom eachtransmissioncyc-le.(Figure
11nfituae
in declbelsirormaliz'edb;-th; ;;;r of the portion bt oitfstrow'n. The mean is at 0 dB. The correspondingcenter beam depth
pi"nflt-pi"tt"O *ittr a'vertical exaggerationof 18 to give an indication of slope along the ship's track. Depth is glven ln
irncorrectedmetersand distancein nauticalmiles.
Sea Beam AcousticBackscatteringData
quires ground truth availableonly from near-bottom instruments with bottom photographic capabilities.Therefore,this
SeaBeam reflectivity work was done in ar areaintendedfor
DeepTow coverage.
Deep Tow groundtruth
It was a rewarding experienceto watch Deep Tow camera
runs unveilsnapshotsof tbe seafloor which matchedthe simple
predictedbottom pattern. Each camerarun (Figure 6) consistently output pictureswhich agreedwith the simplemanganese
nodulecoverageinferences
madefrom the SeaBeamreflectivity
picturesare shown on Figure 10.
data. Three representative
Each picture typifies an amplitudeinterval. The densenodule
to the dot patternedareason
coverageof photo A corresponds
Figure 9 (2 500 A/D units and greater).The intermediate
nodule coverageseenin photo B is found in areaswith the
brick pattern (2 0W-2 500A/D units) and the hatchedpattern
(2 000-1 500 A/D units). The two remaining patterns (1 500
A/D units or lower) turned out to be bare mud as shown on
photo C. Inspectionof over 3 000 Deep Tow bottom photographsand classification
accordingto the three types.shown
(dense,intermediate,and bare)yieldeda 98 percentcorrelation
with the simplenodulecoveragemadefrom the SeaBeamdata.
During acamerarun, thetowedinstrumentis flown approximately 10 m abovethe bottom. This renderspictureswhich
cover about 35 m2 of seafloor, thus giving a clue to nodule
sizes.From the photographsand from the 1/4 m2 box cores
(photo D), the averagenodule was found to be 4 to 8 cm in
diameter,with a flattenedand irregular cauliflower-likeshape,
as opposedto the rounder nodules found in other areas
(Heezenand Hollister,1971;BischoffandPiper,1979).
The first resultsfrom the Deep Tow multifrequency,near(Spiesset al.,
measurements
bottom acousticbackscattering
1984)also reinforcethe belief that SeaBeam is well suited to
a
H
J
a
k
o
o
z
s
r
0
r
s
2
0
2
AMPLT
I U D Ex ] 0 0
5
3
0
3
Frc. 8. Histogram of peak amplitude data distribution from the
data shown in Figure 5. 20 percent of the values fall in the
2 900-3.000 A/D units range due to saturation in the data
acqulsrtlonsystem.
5
997
nodulecoveragein this prospectivemining
map out manganese
area.Figure 11 showsthe normal-incidencereflectivity differpatchanda barrenone,
encebetweena denselynodule-covered
of the DeepTow backscatter
asmeasuredat the six frequencies
system.The optimum frequencyfor this site appears to be
between15 kHz (16 dB) and 9 kHz (13 dB). The operating
frequencyof the SeaBeamsystem(l?kHz) falls exactlyhalfway
At 12kHz equation(5) yieldsa
betweenthesetwo frequencies.
meannoduleradiusof 2 cm (1.6cm at 15 kHz). Althoughthe
nodulesof this site are poorly approximatedby spheres,the
foregoingresultsindicatethat the SeaBearnfrequencyis near
However,do not presumethat
optimumfor the areasurveyed.
l2kHz will be optimumfor all typesof nodules,or that any 12
kHz sonarwould providethe sameresults,sinceSeaBeamis a
systemwith2l degreeresolution.As
roll- and pitch-stabilized
pointedout earlier,this high resolutionhas a significanteffect
fluctuations
levelsand associated
on observedbackscattered
due to the sizeof the observedarea.In comparison,normal
echosounderstypicallyhavea beamwidth between30 and 60
and a bandwidthwiderthan SeaBeam's,resultingin a
degrees
ratio. Both the Deep Tow
substantiallylower signal-to-noise
of the nodule
and the SeaBeamdata point to the patchiness
somepatchesbeinglessthan a kilometerin extent.It
coverage,
systemwhichspansanarea
is doubtfulwhetheran unstabilized
beam at 4 500 m) can resolvesuch
2.4 km wide (30-degree
patches.
Note that the valueobtainedfor mud-to-nodulereflectivity
differencewith SeaBeam is lower than thoseobtainedwith
DeepTow at 9 and 15kHz. Eventhoughthe frequencydependenceof mud-to-nodulereflectivitymight not be linear,it is
to expectthe valueat 12kHz to be betweenthoseat
reasonable
in the
thedifferences
9 and 15kHz. To explainthis discrepancy,
geometryof measurement
betweenthe two systemsshouldbe
Indeed.for SeaBeamthe areaof the first Fresnel
considered.
zoneis approximately33 m wide at 4 500 m depth,which is
almostone order of magnitudelargerthan the diameterof the
first Fresnelzone(4 m at 15 kHz) observedwith Deep Tow
whentheinstrumentis 80 m offthe bottom.However,the Deep
Tow acousticdata havebeenaveragedover spatialdistances
with SeaBeam'snormal incidence
along track commensurate
presumably
footprint.Moreover,the beamwidth dependence
cancelsout when the reflectivitydifferencebetweennodule and
mud is measuredwith the sameinstrument.In the reflective
modelthe geometricargumentis thenruledout.To accountfor
the Sea Beam'smud-to-nodulereflectivitydifferencebeing
lowerthan thoseof the two neighboringDeepTow frequencies
(9,15kHz),I thereforeneedto reconsider
the reflectivitymodel.
over a 200
nodulearea,the local roughness
In this manganese
or absence
m horizontalextentis primarilydueto the presence
I
of nodules.From the bottom photographsand coresamples,
estimate the rms roughnessto be about 2 cm and spatial
to be 10cm or less.At 9 and 15 kHz,
dimensionsof roughness
80 m off the bottom, the diameterof the first Fresnelzone(5.2
is then still substantiallylargerthan the
and 4 m, respectively)
and ro : 0.75and
spatialdimensionsofthe bottom roughness
As mentionedin the "Background"section,
1.25,respectively.
Ko = I correspondsto the ill-definedboundarybetweenreflection and scattering.
If I now take the differencebetweenthe mud-to-nodulereflectivity difference(16 dB) measuredat 15 kHz and the computedbottom lossfor the mud (17.6dB),I obtaina bottom loss
a smoothlayerof chert(a
for nodulesof 1.6dB. In comparison,
de Moustier
998
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Sea Beam Acoustic Backscaltering Data
sedimentary rock having an acoustic impedance almost 4 times
greater than the estimated impedance of a nodule) has a bottom
loss of 3 dB (Tyce et al., 1980).The estimateof the mud bottom
loss is perhaps questionable.However, it is reasonable to believe that this value is within 2 dB of the real one. Moreover, the
nodules are porous and do not form a continuous slab, but
rather are scattered at variable distances from each other.
Therefore the bottom loss ofsuch an assemblageis expectedto
be greater than the 3 dB given for a chert layer. Clearly, a
simple reflectivemodel is insuflicient to explain the high value
found at 15 kHz.
A more realistic result is obtained by consideringthe nodules
as a thin layer separating water from mud. In this case, the
reflection coeflicient at normal incidence (Clay and Medwin,
1977, chap.2) is given by
R:
nl + Rl + 2R1R2cos($)
I + ( R 1 R 2 ) 2+ 2 R 1 R 2c o s ( Q ) '
,
4nfh
a: c"*'
(6)
999
where R, is the water-nodule reflection coelficient fas given by
equation (3)1,Rz is the nodule-mud reflection coeffrcient,/is the
acoustic frequency, lr is the thickness of the thin layer, C^oois
the compressional wave speed in nodules. Using p : 1.37
g/cm3 and C : | 507m/s for mud (Hamilton, 1970b),h : 4 cm
for the thickness of the thin layer, and the values given in the
"Background" sectionfor the other parameters,yields a bottom
loss of$.3 dB at 15 kHz. Also with these values,the reflection
coeffrcient R of equation (6) reaches a maximum at 15 kHz.
Therefore, this thinJayer model agrees with the frequency dependence shown in the Deep Tow data (Figure 11), but does
not match the mud-to-nodule reflectivity diflerence measured
at 15 kHz, especiallysince(as mentioned above) the nodules do
not form a continuous slab. A scatteringmodel should then be
consideredfor nodules. However, it requires knowledge of the
ensonified areas and therefore of the beam patterns of the Deep
Tow backscattering assembly. These beam patterns were not
available at the time of this writing, precluding this analysis.
Ftc. 10. Photographic ground^tnrth: (a) dense nodule coverage representative of the dot patterned areas in Figure 9; (b) intermediate. coverage representative of the brick and hatched patterns; (c) bare mud representative of peak amplitude valuds 1 500 A/D
units and lower; (d) ll4 m2 box core number 358 (Figurie 6) giviirg clues to representative nodule iizes for the a.ea.
de Moustier
1000
Further analysis of the data presented here, on a scatterlng
rather than a reflective basis,and investigation of the angular
dependenceof acoustic backscatteringfrom the Sea Beam and
the Deep Tow data should help explain the processesat work
here.
Further use of Sea Beam's acoustic returns
Using only the peak amplitude of the specular return of the
16 Sea Beam preformed beams to map out normal incidence
bottom reflectivity is a nice flrst-order application. It yields
good results over this particular manganesenodule field. Clearly, more can be expected from the Sea Beam acoustic data.
Work is currently in progressto model the angular dependence
of SeaBeam acousticbackscatterover various types of seafloor
and to relate the data to the geologic characteristics of the
bottom (Patterson, 1967).Following work done by Clay and
Leong (1973) and recently by Stanton (1984) the variations in
the shape of the echo envelope can also be used to predict
bottom roughness.According to Stanton, his statisticalanalysis
works best for amplitudes of sea floor roughness less than a
quarter of the acousticwavelength.
Since the Sea Beam acoustic wavelength is about 12 cm and
since I estimatedthe local rms bottom roughnessto be about 2
cm, the data from this manganesenodule area qualify for such a
statistical analysis which could prove useful for discrimination
betweenvarious bottom substrateswith similar reflectivity patterns.
CONCLUSIONS
In a recent monograph on polymetallic nodules (Rapport de
l'Acad6mie des Sciences,Paris, 1984)the Sea Beam systemwas
deemedan essentialtool for any large-scalenodule prospection
ACKNOWLEDGMENTS
N O R M A LI N C I D E N C E
2 0 / 2 1J U N E8 3
22 5s 02 30GMT
m
a
I
o
-
l
F
(,)
U
J
il
4s
'
since detailed knowledge of the bathymetry is a prerequisiteto
nodule mining.
The data presentedhere bring a new dimension to the Sea
Beam multibeam echo-soundingsystem.In addition to contour
mapping, I have shown that it is possible to detect changesin
reflectivity along the track by using the amplitude of the nearspecular acoustic return. This works particularly well over a
manganesenodule fleld with little reliel since nodules and mud
provide a definite acousticcontrast (11 dB or more). Although a
scatteringmodel would seemmore appropriate than the simple
reflective one used here, the normal-incidence backscattering
data gathered with the Deep Tow multifrequency acoustic
array confirm that high reflectivity contrast between nodules
and mud should be expectedat Sea Beam operating frequency
(12 kHz). This frequency seemsto be near optimum for this
manganesenodule site. Bottom photographs taken throughout
the site agreesurprisingly well with the simple nodule coverage
prediction made from SeaBeam acousticdata.
In spite of dynamic range limitations evidencedin my data, I
believethat nodule coverageassessmentcan be performed very
effectivelywith a Sea Beam system while contour mapping the
topography of a prospective mining site. In essence,areas
where amplitude saturation occurred correspond to areas of
denser nodule coverage with strong reflectivity. Photographic
ground truth can be obtained with an unmanned, deep-diving
free vehicle, with towed vehicle camera and sonar systems,or
even with conventional bottom cameras.
More extensiveprocessingof Sea Beam's detected echoesis
in progress with the intent of assessingwhether the backscatteredreturns separatedby Sea Beam's high angular resolution can be correlated with seafloor geologic characteristics.
This includes identification of the reflectivity and roughness
characteristicsofdifferent bottom types: sediments,rocks, rock
outcrops, manganesenodules,etc. Initial resultshave been very
encouraging, as suggestedboth in this paper and in ongoing
work on various tvpes of seafloor.
'?rro,rrir?
110
160
Frc. 11. Frequency dependenceof relative reflectivity, as measured with the Deep Tow multifrequency acoustic array. Each
point representsthe differencein reflectivity between a densely
nodule covered patch and bare mud at the acoustic frequency
indicated. For the data shown in this figure, the transmit pulse
lengths at the various frequenciesare:0.9 ms (4.5 and 9 kHz);
1.0ms (30 and 100 kHz), and 1.1ms (15 and 60 kHz). The Deep
Tow instruction packageis approximately 80 m offthe bottom.
The work reported here involved the cooperative efforts of
many people including the captains and crews of the R/V
Thomas Washington and Meluille. Special thanks go to F. V.
Pavlicek for his support during the development of the Sea
Beam acoustic data acquisition system which made this work
possible.I am especiallyindebted to Dr. F. N. Spiessand Dr. R.
C. Tyce for their valuable suggestionsand critical reviews of
this paper and also grateful to J. Griflith for the art work, R.
Hagen for the final editing, and A. Plueddemann for helpful
comments.The Sea Beam work was supported by the Offrce of
Naval Research, Contract No. N00014-79-C-0472.Shiptime
and the Deep Tow work were funded through grants from the
National Science Foundation and the National Oceanic and
Atmospheric Administration. Contribution of the ScrippsInstitution of Oceanography,new series.
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