JOURNAL OF GEOPHYSICAL RESEARCH.VOL.91. NO 83. PAGES 3407_3424,MARCH 10,1986

BathymetricArtifacts in SeaBeam Data: How to RecognizeThem andWhat CausesThem

CszusrrnNde MousneRAND M,A.nrrN C. KlerNnocr

ScrippsInstitution o.f ,University o.f California, San Diego, La Jolla

Sea Beam multibeam bathymetric data have greatly advanced understandingof the deep seafloor However, severaltypes of bathymetricartifacts have been identifiedin SeaBeam's con- toured output. Surveyswith many overlappingswaths and digital recordingon magnetictape of Beam's 16 acousticreturns made it possibleto evaluate actual system performance. The artifactsare not due to the contouringalgorithm used Rather,they resultfrom errorsin echo detectionand processing.These errors are due to internal factorssuch as side lobe interference, bottom{racking gate malfunctions,or external interferencefrom other sound sources(e g., 3.5 kHz echosounders or seismicsound sources) Althoughmany artifacts are obviously spurious and wouldbe disregarded,some (particularly the"omega'effects described in this paper)are more sub- tle and couldmislead the unwaryobserver Artifactsobserved could be mistakenfor volcaniccon- structs,abyssal hill trends,hydrothermal mounds, slump blocks,or channelsand could seriously affect volcanic,tectonic, or sedimentologicalinterpretations. Misinterpretation of theseartifacts may resultin positioningerrors when seafloor is usedto navigatethe ship Considering thesepossible geological misinterpretations, a clear understanding of the SeaBeam system's capa- bilitiesand limitationsis deemedessential.

l. INrnooucrroN tion errorsare mostlyrelated to somechpracteristics of the oceanbottom (e.9.,type of substrateor suddenchange in The Sea Beam bathymetricsurvey systemis a multi- slope) or to interferencefrom other sound sourcesrun- beam echo sounderdeveloped by the GeneralInstrument ning in parallel with Sea Beam (mostly subbottom Corporationto producenear-real-time high-resolution con- profilers:3.5-kHz echo sounder and seismicsources). toured swath charts of the seafloordown to maximum This paperdescribes several artifacts discovered in Sea oceandepth (ll km). Since 1977when the first system Beamdata and discussesthe associatedpossible geological becameoperational aboard the French RlY Jean Charcot, misinterpretationsWe suggesta number of solutionsto nine other systemshave been installedaboard research improve data quality. We also considerthe existenceof vesselsfrom the United States,Germany, japan, and Aus- relatedartifacts in similar multibeamecho sounders (e.g., tralia. the SonarArray Sounding System (SASS) [Glenn,1970]). SeaBeam systems have provenextremely useful in the study of the geomorphologyof the oceanfloor and have 2. Be,crcnouxo: SnRBEeu SysreH,r madepossible striking discoveries of featureswhich would not have been detectedwith conventionalsingle-point Beforegoing into a detailedexplanation of the problems depth sounders [e.g., Macdonaldand Fox, 1983',Lonsdale, found in the SIO system,we briefly review Sea Beam's 19831.However, after 3 yearsof experiencewith the sys- generalframework for the readerunfamiliar with the sys- tem installed aboard the R/V Thomas Washingtonof Ihe tem. Further discussionof the systemare found in the Scripps Institution of Oceanography(SIO), we have works by Renard and Allenou [1979] and Farr ll980l. discovereda number of artifactsin Sea Beam's contoured Becausea clear understandingof Sea Beam's acoustic output. Their artificialnature has been demonstratedby geometryand echoprocessing methods is a prerequisiteto comparingoverlapping Sea Beam swathsand by analyzing analyzeits bathymetric output, we have included in the digitizedraw acousticdata. During four cruisesaboard the appendicesrelevant technical information not availablein RlY ThomasWashington we used a data acquisitionsystem the literature. (MPL) developedby SIO's Marine PhysicalLaboratory to As illustratedby the simplifiedblock diagramin Figure record digitallythe acousticreturns from Sea Beams' 16 I, the Sea Beam system uses a multibeam narrow beam preformed beams on magnetic tape, This data set has echo sounderand an echo processor(EP) to generatein enabled us to determine the c auses of these artifacts. near-realtime, contour maps of the oceanfloor, A 20- They do not stem from the vagariesof the contouring element projectorarray mounted along the ship's keel algorithm used; rather they are the result of errors in echo sendsout a 7-ms pulseof 12.158kHz at intervalsthat are processing. detection and In our experience,such detec- integralmultiples of 1 s. The transmissionperiod is usu- ally determined by an analog graphic recorder. The receivingunit lies athwartshipsand consistsof 40 line- Copyright1986 by theAmerican Geophysical Union hydrophonearrays whose long axes are oriented fore-aft. Papernumber 585427. The resultingtransmit/receive geometry is illustratedin 0148-0227/86/ 0058-5427$05 00 Figure 2. In Figure 2, vertical crosssections of theoretical

3407 3408 de MoUSTIERAND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA r- --_l DATA MPL I acoutsrT|oNsYsrEM I 40 I HYOROPHONES I TEST I SI GNAL GENERATOR

E.P BEAM FORMING RECEIVERS MATRIX (4x4O) (r6)

vERTICAL, A/D CONVERTER PORT,STED RECEIVERS CONTROL VERTICAL AND REFERENCE TIM ING GYRO T RANSMITTER TCH DIGITAL PI SIGNAL SERVO PITCH LOGGER COMPE NS ATOR GEN€RATOR l2'158 kHz I DISPLAY I {*; TAL PLOTTER ROLL HEADING I INPUT I NPUT

PROJECTOR I ARRAY (20) I I (NBES) I ECHOPROCESSOR Fig. 1. Block diagramof the SeaBeam system showing the narrow beamecho sounder and the echo processor. The positionof the MPL dala acquisitionsystem is shownfor reference. beam patterns are shown for both the projector (Figure ingly numberedfrom the center out (l-8) on port and 2a) and the receiver (Figure 2b) arrays. The transmit starboard.In Figure 3 the ridge of synchronousreturns beam pattern spans 54' athwartshipsby 2 2/3" in the (seearrow labeled "side lobe') correspondsto energyfrom fore-aft direction. It is pitch stabilizedwithin a rangeof the near-speculardirection (tallest return) entering the + 10" of pitch. There is no pitch compensationfor the side lobesof the other beams. The 16 bottom return sig- receive beam pattern, instead it spans 20' in the fore-aft nals form a parabola,indicating a flat portion of seafloor. direction to accommodatepitch angles of + 10". The These data have been digitized and recordedon magnetic athwartshipsbeam width is 2 2/3'. SeaBeam receives with tape with a separatedata acquisitionsystem (Figure l) 16 fixed preformedbeams obtained by electronicallysteer- built around an LSI 1ll23 minicomputer,in an experi- ing this 20' x 2 2/3' beam at athwartshipsintervals of ment conductedby MPL to measureacoustic backscatter 22/3" belween+20o of incidence.In this configuration from the deep seafloor.They have proven invaluable to there is no beam alongthe ship's verticalaxis; rather two evaluate the performance of the EP becauseSea Beam of the beamspoint at I 1/3" on eitherside of this axis. only retainsdepths and cross-trackhorizontal distances. The acousticenergy received at the ship comesfrom the In the Sea Beam computer, 16 such waveforms are intersectionof the transmit and receivebeam patterns. simultaneouslydigitized at a frequencyof 300 Hz per This appearsin Figure 2c as 16 squares2 2/3' on a side. waveform. This corresponds to one digitization cycle Figure 2c is only meant to illustrate the angular relation- every 3.33 ms or 2.5 m of slant rangeassuming a sound ship betweenthe main lobe of the transmittedbeam pat- velocity of 1500 m/s. Consequently,slant range and tern and the main lobes of the 16 preformed beams. therefore depth determinationresolution is limited to 2.5 Actual footprints are not rectanglesor squares;they are m. While it digitizesthe acousticdata, the computeralso ellipseswhose areasincrease away from vertical incidence. performs severalecho processingtasks. For eachdigitiza- Sincedepths are ideallydetermined at the centerof each tion cycle these tasks are receivergain correction,refrac- of the preformed beams, the maximum swath width tion correction, roll compensation, detection threshold correspondsto 730/oof the water depth. level computation, and echo detection. Automatic The beam-formingoperation described above generates bottom-trackinggates (one for each beam) determine a 16 acoustic signals. These are sent to the EP receivers time window during which a return is expectedon any one where they are filtered, rectified, amplified, and beam based on previous sounding history. A return is transferredto the SeaBeam computer (Figure 1). Figure 3 detected if it falls within the gates and lies above the shows a typical output of the 16 EP receivers. Each threshold. waveformcorresponds to a preformed beam and is accord- In general, the threshold level is computed to ride de MoUSTIERAND KLEINRocK: BATHYMETRIcARTFAcTs IN SEA BEAM DATA 3409

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Fig. 2. Sea Beam transmit/receivegeometry Computed beam pattern cross sectionsin the athwartshipsvertical planecentered on the array and in the verticalplane passing through the ship's fore-aft axis are shown for (a) the projectorarray and (D) the receiverarray. The effectof Dolph-Chebyshevamplitude shading is alsoillustrated. (c) A summarycartoon showing the angularrelationship between the main lobe of the transmittedbeam pattern and thoseof the l6 oreformedbeams.

SIDELOBE f eorroM RETURNS

5.a 6.O I o.Z 6.2 secon 6.6 ds 6.a

Fig 3. Acoustic signal envelopesof the 16 preformed beams at the output of the Sea Beam echo processor receivers. The time axis represenlsseconds after transmission. The vertical axis in volts representsthe voltage equivalentof the sound pressurelevel at the receiverarray, correctedfor acoustictransmission losses in the by a time-variedgain (TVG). No roll compensation,recording gain, or receivergain correctionshave been appliedto the data at this stage. Such data are recordeddigitally on magnetictape every transmissioncycle, along with time,TVG, andship's roll. 3410 deMousnER nNo Krerunocr: Bersyrrltrttc ARTrFAcrsrN Sne BpelraDere above the noise, above the side lobe responseto a strong return is presenton one of the 16 preformed beamsor, if specularreturn, and above potential noise bursts interfer- the manualthreshold is set high enough,in lossof data. ing with bottom echo detection. A manual threshold can On most ships equippedwith SeaBeam the bathymetry also be enteredby the SeaBeam operator. As we shall see data are mergedwith ship navigation(transit satellitenavi- in the following sections,thresholding and gating are two gation and deadreckoning, or NAVSTAR Global Position- critical operationsin the echo processing.A more detailed ing System navigation when available) by another com- descriptionof Sea Beam's echo processingmay be found puter aboardthe ship and recontouredalong the ship's in AppendixB. track on a 30 inches digital plotter with a delay time of For each roll-compensatedbeam having sufficientsignal about 2 min. This gives the surveyor the ability to to noiseratio, a slantrange R is calculatedby computing effectively control ship navigation and track spacing by the center of mass of all the detectedsignal samplesfor looking at the contours plotted. A second stage of data that beam and by multiplying the correspondingarrival postprocessing,done on ship or ashore,consists of adjust- time by 750 m/s. Knowingthe slantrange R and the sta- ing the navigation to fit correspondingcontour lines on bilizedbeam angle V, a simplecalculation yields the depth adjacenttracks and of regriddingthe entire data set to pro- Z and the cross-trackhorizontal distance I: duce a map. When navigation comes from the Global PositioningSystem there is virtually no need for adjust- /- ments. These operationsusually smooth the raw Sea Z : R cosv Y : YR tinv' Ln Beam soundingsby averagingalong track over a certain number of transmission cycles (often five) to produce more even grid spacingalong versus acrossthe ship's whereCo is the mean sound velocityobtained by averag- thus removingmost of the jitter apparenton near- ing the values of the sound velocity profile from the sur- track, plots. However,when system cause face to the averagebottom depth (in uncorrectedmeters) real-timeswath errors bad soundings,the resulting fictitious bathymetry will and Cn is the nominal sound velocity in water (1500 not averageout as we shall show in the following m/s). The depth Z is given in uncorrectedmeters refer- often Therefore in order to assessthe validity of encedto a sound velocityof 1500 m/s. The cross-track sections. an investigatorneeds to horizontal distance I/ is a true distancebecause it is suspiciousSea Beam bathymetry, the and use any informa- correctedfor both refractionand travel time. refer to raw data corroborating tion available.When only the raw dataare available,as is Finally,ne (Z,X) coordinatesfor eachvalidated beam the case,such assessmentrequires a clear under- are output as a cross-trackbottom profile on the EP often standingof the processingperformed by the Sea Beam cathoderay tube (CRT) display.The depthsare alsoused computeron the digitizedacoustic signals. to updatethe bottom-trackinggates on eachbeam for the next transmissioncycle. 3 Snl Bnnu BarHyMETRrcARTrFAcrs: The depthsand cross-trackhorizontal distances for each ExeuplEs, ExpleNetroNs,AND GEoLoGTcAL IMpLrcATroNS transmissioncycle are logged on a magnetic storage medium (diskor tape)along with time and ship'sheading, SeaBeam has beenused extensively in the pastseveral as well as output in near-realtime (- I min delay) on yearsto studythe morphology,tectonics, volcanology, and paper as a contour chart by an 1I inchesdigital swath sedimentologyof the seafloor. So many SeaBeam surveys plotter. In the following,we will refer to the (2,)t) data have been run that a completelist is too largefor inclu- asthe raw SeaBeam data. sion here; thereforewe only referencesome of the more The Sea Beam echo-processingsequence outlined here recent works. Bathymetric charts produced from Sea will vary dependingon the EP mode chosenby the opera- Beam data have been used as basemaps for more detailed tor. Three modesare available.Mode I is essentiallya studiesusing deeplytowed instrumentpackages such as start-up mode during which no data loggingor contour MPL's Deep-Tow lSpiessand Lonsdale,1982', Spiess et al., plottingare performed The EP displaysthe verticalbeam 1984,Hey et al., this issuel and mannedsubmersibles such depth and the CRT showsunprocessed echoes on the 16 as Woods Hole OceanographicInstitute's (WHOI's) preformedbeams The detectionthreshold used in mode DSRV l/vin and DSRV Cyana of the Institut Frangaisde I is the highestof the noisethreshold, the sidelobe thres- Recherche pour I'Exploitation de la Mer (IFREMER) hold, or the thresholdentered by the operator.Mode 2 is (formerly CentreNational pour I'Exploitationdes Oc6ans a semiautomaticEP operationwith data logging and con- (CNEXO)) le.g., Ballard and Franchetequ, 1983l, Fran- tour plotting. The CRT displaysprocessed data in the cheteauand Ballard, 19831. Many surveyscovering fairly form of a cross-trackbottom depth profile,but the opera- large areas (hundreds of square kilometers) with nearly tor controlsthe trackinggates' width and center. Mode 3 total coverage have lead to valuable insights into the is a completelyautomatic version of mode 2. It is the processesat spreadingcenters le.g., Hey el a/., this issue; mode in which the EP usuallyoperates during bathymetric Crane et al., 1985 Macdonald et al., 1984 Mammerickx, survey work. A very importantand poorly documented 1984; Lonsdale,19831, transform faults [e.9., Gallo et al., difference exists between the detection threshold level 7984, Detrick et al., 79841 trenches [e.9., Shipley and determinationof mode 1 and that of modes 2 and 3. In Moore, 1985;Lewis et al., 19841,microplates le.g., Hey et modes 2 and 3, a nonzero thresholdlevel input by the al 1985, Nqar and Hey, this issuel, [e.g., For- operator supercedesany other threshold computation It nari et al., 19841,and systems [e.g., is thereforeimperative that the manual thresholdbe set to Lewis et al., 19841. Sea Beam's regional depiction of the zero when in mode 2 or 3. Failureto do so resultsin the seafloorin theseareas has been extremelyuseful. EP tracking the side lobe responseany time a specular In most cases,the finer-scaleSea Beam bathymetry is deMousrrER eNo Kt-etNlnocr:BetHvvetntc Anrrrecrs IN Spe BBelt Dere 3411

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sFt&Y f,*ruYnsts*ffiil{ Fig 4 Echo processorCRT displayshowing a cross-trackbottom profile(solid trace) characleristic of a "tunnel" effect The dashedtraces represent the upperand lowerpositions of the automaticbottom-tracking gates. The vert- icalscale is 200 m per division The horizontalscale is compressedto accommodatea receptionbeam width span- ning 80' (40"actual beam width with + 20' for roll).

dependableand is reproducibleon overlappingswaths apparentslope fluctuations on seafloordipping perpendicu- However,given the existenceof the bathymetricartifacts lar to the ship's track, and it typicallyaffects only a few discussedin this paper,investigators should be cautious beams A more seriousproblem occurs when the seafloor when studyingbathymetric details on the scaleof hun- surveyedis relativelyflat and Sea Beam rendersit as a dreds to thousandsof meters. This is particularlyimpor- trough (the "tunnel" effectlSmith, 19831). This is seenas tant for surveys of large ileas where bathymetricand a concave-uparc on the cross-trackCRT bottom profilein structuraldata are interpolatedbetween widely separated Figure 4 To understandthis artifact, considerthe Sea SeaBeam swaths. Misinterpreting any of theseartifacts as Beamacoustic data shown in Figure5 which is identicalin true bathymetricfeatures could also result in positioning format to Figure3. Note that the sidelobe level is much errorswhen the vesselis navigatedby comparisonof real- higherin Figure5 than in Figure3 If the EP is in mode time bathymetrywith compiledcharts. 3 and a nonzeromanual threshold level has beenentered In the followingwe discussthree typesof bathymetric by the SeaBeam operator, the systemdoes not calculatea artifacts resulting from echo-processingerrors. These noiseor a sidelobe threshold. It thereforetracks the side errorsare due to internalfactors such as sidelobe interfer- lobe responsewhen presentand when abovethe manual ence or malfunctionof the bottom-trackinggates or to thresholdlevel. Arrival timesare then synchronouson all externalinterference from other sound sources. In each beams as if coming from a concave-uphorizontal half case we present evidence of artifacts through Sea Beam cylinder. Figure 6 shows an example of the resulting data samples,explain their cause,and indicatetheir geo- bathymetry. logicalimplications. The apparentrelief of the "tunnel"walls in this example rangesfrom 40 m to 100 m, althoughtheoretically it may 3.1. SideLobe Interference be as much as 6oloof the water depth. The actualseafloor morphology in this area is not preciselyknown because Renardand Allenou [1979] recognized side lobe interfer- the MPL acousticdata acquisition system was not available enceas a potentialproblem in SeaBeam (e.g.,Figure 21 during this survey. This area is believedto be generally in their paper). The interferenceis characterizedby small flat with indications of roughly north-south abyssalhill 3412 de MoUsTIER AND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA

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Fig. 5. Acotrsticsignal envelopes of the 16 preformedbeams at the output of the SeaBeam EP receivers The for- mat is identicalto that of Figure 3. The ridge of synchronousreturns due to side lobe responseis much more pro- nouncedin this figurethan in Figure 3 becauseit is due to a strongernear-specular return (starboardbeam 1, which is clippedin thisfigure) trends. Such "tunnels' might be mistakenfor troughs receivedat the ,indicating a highly reflective betweenabyssal hills or submarinechannels, but investi- seafloor. Second,the EP receiver outputs were found to gators would recognize them as artificial because the saturateat 8.5 V ratherthan the specifiedmaximum out- trough axesfollow the ship track, independentof course put of l0 Y lde Moustier,198541, As a result the peak changes. amplitude on the specularreturn is clipped (starboard The "tunnel" effect can also occur when a zero manual beam I in Figure 5). The side lobe thresholdlevel com- thresholdhas beenentered, even thoughthe systemcom- puted on a clippedpeak only partiallyremoves the side putes a noise and a side lobe threshold.In the example lobe response,and the remainingportions of side lobe given in Figure 5, we identify two processeswhich com- responsebias the centerof masscalculation in their direc- bine to defeatthe side lobe rejectionscheme outlined in tion given the comparativelylow signalto noise ratio of the appendices.First, a very strong specularreturn was the real backscatteredbottom returns. Eventually,the

Fig. 6. The "tunnel" effect. Four portionsof SeaBeam swathsadjusted for navigationare shown here to illustrate the effectsof a non-zeromanual threshold when the echo processorruns in mode 2 or 3. SeaBeam's rendition of the bathymetryis seenas a trough approximatelycentered on the ship'strack This trough persiststhrough changes in thb ship's course,thereby indicatingits artificialnature. Fictitiousgullies also appearon slopesupdip as well as downdip (best seen in the upper right section of this figure). Contour interval is 10 m, and tick marks point downhill. de MoUsTIER AND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA 3413

J'C K.FlZ I NTERFERENCE t'> ^ BorroM gJ nrrunus

6;2 6;4 6.6 I seco n ds 6;g

Fig. 7. Evidenceof a 3.5-kHzecho sounder transmitting during a SeaBeam reception cycle The corresponding noiseburst appearsas a synchronousridge acrossall 16 preformedbeams. The format is the sameas that of Figure 3. A noise burst ridge differs from a side lobe responseridge in that the levelsof the peaksare more or lesscon- stant for the former, while a marked differencein level exists betweenthe specularreturn and its corresponding side lobe response(Figures 3 and 5) The differencesin level seenin this figure are due to differencesin receiver eainswhich were not corrected. systemtracks the side lobe responseinstead of the bot- 3.5-kHzecho sounder interference as seenin the acoustic tom, creatinga troughlikefeature. The limiting caseis data. It appearsas a synchronousridge across the 16 pre- that of a mirrorlike hard surface from which there is no formed beams. This is a classicalexample of a noise backscatter. In this case, one would see only a strong burst. As for the side lobe interference,the dynamic specularreturn and a synchronousridge of side lobe thresholdingused to reject such noise bursts has side returns. However, most of the time the bottom offers effects which produce fictitious bathymetry, examplesof some roughnesson the scaleof SeaBeam's l2-cm acous- which can be seenin Figure 8. The portion of the noise tic wavelength,and the signalto noise ratio of the back- burst which is well separatqdfrom the actual bottom scatteredreturns is sufficientto track the bottom correctly. returns are effectivelyrejected by dynamic thresholdingor It is importantto note that the prerequisitefor sidelobe by gating. However,where signal and noiseburst overlap, interferenceis a strong near-specularreturn on any one of cancelingthe noise burst also cancelspart of the signal the preformed beams. The bottom does not necessarily and skewsthe centerof masscalculation for that return. have to be flat (e.8.,Renard and Allenou's[1979] exam- As the noiseburst slowly progresses through the reception ple). In cases where the side lobe responseis well cycleover a number of pings,bathymetric peaks appear on separatedfrom the bottom return (e.g., port beam 8 in the contouredoutput in the directionof the beamswhich Figure5), it usuallyfalls outside the trackinggates. When point away from specular incidence. Thp near-specular the side lobe and the actual bottom returns are close directionsare not affectedas much since pulse stretching togetheror overlapping,as is usuallythe caseon returns is minimal,thereby reducing the marginfor error. adjacentto the near-specularreturn (e.g., port beam I in The bathymetric peaks are typically short wavelength Figure 5), the system has no way of differentiatinB (hundreds of meters) and may vary in amplitude from betweenside lobe responseand bottom return. Rejecting tens to hundreds of meters. The more pronounced of the side lobe responsewill most likely cancelsome of the these peaksare often clearlyspurious and extremelysteep bottom return, resulting in a slightly erroneous depth and sharp; such features are geologicallyu4likely, and determination. Likewise the computeddepth is in error if investigators will readily disregard them. Smaller- the side.lobe response is not rejected. The errors are amplitudegrtifacts are lessobvious and might be mistaken small (- 5 m) for beams oriented in the near-specular for small volcanic cones or large hydrothermal mounds. directions,and increaseaway from specularincidence due One common, although not ubiquitous,characteristic of to the lengtheningof the backscatteredreturn signaldura- these artifacts is the simultaneous occurrenceof more tion (pulsestretching) with both beam angleand depth. than one in different parts of the swath. Investigators awareof the potentialfor thesephenomena are unlikely to 3.2. InterferenceFrom ExternalSound Sources misinterpret them. Seismicsound sourcessuch as water guns produce similar effects, but no observableinterfer- External sound sourcesintefere with the SeaBeam sys- enceshave been reportedwith air guns, probablybecause tem when they transmit while Sea Beam is receiving they do not output enough acousticenergy in the 12-kHz echoesfrom the seafloor. Figure 7 showsan exampleof a frequencyband [Szltlr, 1983]. 3414 deMousrrER eNo Klr,Irnocr: BetHvtr,trtntc ARTIFACTstN Sne BBntr,tDere

Fig 8 Examplesof contouredswath plots showing the resultsof externalsound source interference Artifacts can be recognizedas individualpeaks on one or both sidesof the ship'strack (centerline in all threeplots). Contour intervalis l0 m. and tick markspoint downhill

A specialcase of interferencefrom external sound pinger have also been noted during dredgingor coring sourcesexists for l2-kHz bottom transponders.Figure 9 operations. showsan exampleof such interferencewith evidenceof a 3.3"Omegd'Effects and Data Gaps transpondertrace on the correspondinganalog center beam depth profile. The flat sedimentarybottom over Most SeaBeam users are awareof the possibilityof side which this datawas taken illustrates the progressionof the lobe or externalsound sourceinterference in the system. interference.The interferenceenters the outer beams' A lesser known and more insidious artifact has been tracking gates while falling outside those of the near- found to occur on sloping bottoms producingcontours specularbeams. This is evidencedin Figure9a by a cen- resemblingthe capitalGreek letter omega(Q) [Kleinrock tral ridgefollowed by two small moundson either side of et al., 19841or data gaps. They are generallycharacterized the ship's track, The small mounds would be difficultto by an arcuateplateau followed by a steep, curvilinear identify as artificial,were it not for evidencefrom the ana- scarp. They occur within a singleSea Beam swath,com- log record (Figure 9b) which shows the transpondertrace monly near the center, and have lateral dimensionsof intersecting the center beam depth profile at the hundredsto thousandsof meters. The plateausmay be correspondingtime. Due to their small size, these peaked(Plate le) or flat (Plate 1d). (Plate 1 is shown artifactswould probablynot be consideredvery significant, here in blackand white The color versioncan be found although some might mistake them for satellitecones or in the separatecolor sectionin this issue.)The scarpmay hydrothermalmounds. be semicircular(Plate ld the classic.f) shape)or irregular The situationof this exampleis uncommonbecause the (Platel/). "Omegas'sometimes evolve into or are asso- ship was maneuveringat about 1.5 knots over a bottom ciatedwith datagaps (Plate lb and 19) and can be created transpondernetwork while towing the Deep-Towinstru- on sides of seamounts(Plate lc) as well as relatively ment package. However, it may become more common straightscarps. with the Sea Beam system installed on WHOI's R/V Plate la showsthe problemclearly. In this case,the Atlantis II, the mother ship for the manned DSRV l/ytn same portion of seafloor has been surveyed in three which is often navigatedusing l2-kHz transponders. At different directions. The arrows indicate the direction of normal surveyspeeds (-10 knots) this artifactwould be ship travel. In Plate la , sections1 and 2, Sea Beam's greatlyreduced. Similar artifactsdue to interferencefrom rendition of the bathymetryis nearly identicalfor opposite the direct or the bottom bounced sienal of a l2-kHz ship coursesin the along slope direction. The bathymetry deMousrrER ero Krr,rNnocr: Betuvl.tBtnlc Anttpects tN Sre Bnev Dere 3415

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Fig 9. Interferencefrom a 12-kHz bottom-mooredtransponder during a Deep-Towsurvey (a) Sea Beam near- real-timecontoured output Contourinterval is 10 m, and tick marks point downhill The centerline represents the ship'strack On this line, shortticks above the ship'strack are spaced2 min apart,long ticks refer to informa- tion at the top of the plot (time Ihour/min/s], ship'sheading, contour interval in meters),and shortticks centered on the track refer to centerbeam depth in metersindicated at the bottom. (b) Analoggraphic recorder output displayingSea Beam's center beam depth profile and the traceof the 12-kHztransponder. The horizontalscale is matchedin time to that of Figure 9a The artifactcan be seenin Figure 9a as the two small moundson either side of the ship'strack is markedlydifferent when the ship track runs downdip sion cycles. While inspectingthe raw SeaBeam data, we (acrossa slopein the down hill direction)(Plate la , sec- noticed unrealisticvariations in depth from one ping to tion 3). In all the coloredcontour plots shown, contour the next as well as missingsoundings in the data of Plate lineshave beensmoothed by averagingover five transmis- 1a, section3. No evidenceof externalinterference was 3416 de MoUSTIERAND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA

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xl

xi xi

b

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CONTOUR INTERVAL20m COLORCHANGE INTERVAL100 m 5 PING AVERAGE Ikm

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Plate I "Omegas"and gaps. (The color version and a completedescription of this figure can be found in the separatecolor sectionin this issue)

found in the acousticdata. After computing depths and gatesdo not open fast enough upon a sudden changeof cross-trackdistances from this acousticdata, we contoured bottom slope. As a result, data are lost for points falling them using the same postprocessingsoftware used outsidethe gates. Second,when going downdipacross a throughout Plate l. The resultingbathymetry shown in slope the fore-aft transmit beam pattern geometry (Figure Plate la, section4 matchesthat seenin Plate la , sections 2a ) is such that acousticenergy from the side lobesmay I and 2. Sea Beam was clearly in error when the ship ensonifythe slopein the speculardirection. Althoughthis track ran downdip. transmitted energy is about 25 dB lower than that A combination of three factors may be responsiblefor transmittedin the main lobe in the true verticaldirection, this artifact. First of all, the automatic bottom-tracking it becomessignificant due to the angular dependenceof de MoUsTIER AND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA 341'7

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EARLY r^ H AttitlvAL)1L I

-:== 3,2 ,t'4 J.6 4'o """o7d"u'o 4.2

Fig. 10. Evidenceof transmitbeam patternside lobe interferencein the digitizedacoustic data Early arrivals correspondingto near-specularreturns from transmittedside lobe energyare bestseen on starboardbeams 1-3. Thesedata correspond to the "omega'effect shown in Plate le The format is the sameas that of Figure 3.

backscattering.Measurements have shown that one can pattern point in the speculardirection as in this case. expect a drop of 10-15 dB in the acousticbackscatter Becausethe receivingbeam pattern is 20' wide in the between normal (specular) incidence and 20' incidence fore-aft direction (Figure 2b), earlyreturns are received lPatterson1969, Urick,19831. For a flat bottomthe acous- as seen in Figure 10. Given the proper thresholdand tic backscatterdue to side lobe transmittedenergy is negli- sufficientlynarrow trackinggates (which is the casewhen giblecompared to the returnsdue to the main beamin the the bottom has remainedrelatively flat for some time), speculardirection. This is no longertrue when the bot- the systemtends to track these early returns,creating a tom slopeis suchthat the sidelobes of the transmitbeam plateaulikefeature. At somepoint the signalto noiseratio

SHIP TRAVEL r+>

SEABEAM 9ATHYMETRY \ \ I AUTOMAT IC \\ TRACKING TRUE GATES BOTTOM

ALONGTRACK BOTTOMPROFILE

ATHWARTSHIPS BOTTOMPROFI LE

Fig 11. Cartoons of bottom profiles associatedwith "omegas"and gaps. (a) Along-track bottom profile of an "omega''artifact Sea Beam's rendition of the bathymetry,shown by the dashedline, is a plateaufollowed by a steepscarp. (b), (c) Bottom-trackinggates conditions as would be seen on the echo processor'sCRT. In Figure 1lb the athwartshipsbottom profile (solid line) lies inside the tracking gates (dashedlines). This is the normal mode of operationwhen the echo Processoris in mode 3. It is also what one would see were an "omega"effect present. In Figure 1lc the athwartshipsbottom profile lies partiallyoutside the trackinggates, and the correspond- ing data pointsare lost. This situationis characteristicof data gaps. 3418 de MoUSTIERAND KLEINROCK:BATHYMETRIC ARTIFACTS IN SEA BEAM DATA

CALIBRATION SIGNAL (7

BOTTOM RETURNS(7

@

EARLY ARRIVAL S

3.4 3'6;--- J.Ct Z,- n ,u"ord" e " 4.2 4.4

Fig. 12. Receivers'gain calibration. The raisedsignals seen at the end of eachol the 16 preformedbeam returns representcalibration signals injected into eachreceiver to determineits gainsetting Here one can appreciatethe usefulnessof the echoprocessor gain correction which brings all receiversto a commongain level The formatof this fieureis identicalto that of Fieure3

of thesereturns becomes too low, and the systemfails to effectson slopesbetween 30' and 45" fot ship speedsof detectan echo until the trackinggates open wide enough about 10 knots, but similar though subtlerartifacts seem to recoverthe real bottom. Hencea sharpdrop in depth to appearon gentlerslopes, perhaps as low as 15o. resultsat the end of the plateauas shown in the cartoon The tracking gates and the transmit/receiveacoustic of Figure114 . geometryare the two main factors contributingto the Considertwo possibletracking gate conditions: a normal "omega' effect. A third factor is relatedto the half-hour conditionwhere the instantaneousbottom profile is con- calibrationof the EP receivers.In severalinstances, we tained within the gates (Figure llD) and a condition found that this calibrationoccurred immediately prior to where part of the profile falls outsidethe gates (Figure an "omega"effect. Figurel2 showsthe onsetof a receiver llc). The latterproduces a datagap as seenin Plateslb gain calibrationsequence just at the end of a reception and lg where the onset of an "omega"effect immediately cycle The following transmissioncycle showed only the precedesthe gap. Apparently, the dip of the bottom calibrationsignals. Inspectionof the raw Sea Beam data increasedtoo rapidly for the "omega" effect to develop showedthat no data had been loggedfor these two cycles fully, and a gap appearedbecause the gatessimply could even though one would have expectedthe first (Figure not open fast enough. Such gaps exist in Sea Beam data 12) to havebeen processed by the EP. As a consequence, on updip as well as downdipship tracks;however, in our data are lost for two transmissioncycles every half hour. data we have seen"omega" effectsonly for downdip ship Moreover the trackinggates are not updatedduring this tracks. This was confirmed at sea when an observed time. A coincidentalincrease in bottom slopeputs the EP "omega"effect on a downdip track was immediatelyresur- in a difficult bottom-trackingsituation which, given the veyed updip, and no "omega"was detected. The most appropriateslope angle, ship direction,and signalto noise likely explanationfor this asymmetrycomes from the fact ratio in the acousticreturns, generatesan "omega"effect. that the gatesare alwayslagging upon a suddenchange in Of all the artifactsdiscussed here, "omega' effectsare bottom slope. Downdip, the gates track from the left in the most likely to misleadinvestigators because they often Figure 10, and they are therefore likely to track early appearas geologicallyplausible volcanic, tectonic, or even returns. Updip,the gatestrack from the right in Figurel0 depositional(mass-wasting) features. An "omega" on the so they have a better chance to track bottom returns side of a seamountas in Plate lc, could possiblybe mis- insteadof early arrivals. Also the gateshave more time to taken for a flank or satelliteconstruct. Irregular"omega"- adjust at the baseof rising slopesdue to the accumulation type artifacts (Plate I appearingon what are actually of talus. We cannot specify a slope range for which an relatively straight scarps might be misinterpreted as "omega"effect occursbecause this effect varieswith ship's changesor variationsin structuraltrend. This could result speedand dependson the side lobe level on the transmit in errors in determining the tectonic characterand evolu- beam pattern of the SeaBeam systemconsidered. As ship tion of an area. Other"omegas' (Plate le) might be mis- speedis reduced,the tracking gates have more transmis- taken for volcanic constructionalfeatures and incorrect sion cyclesto adjust to a sudden drop in slope, and the conclusionscould be reached regarding post-tectonicor "omega" effect is less likely. Our data shows "omega" syntectonic volcanism on scarps such as de MoUSTIER AND KLEINROCK: BATHYMETRIC ARTIFACTS IN SEA BEAM DATA

iV<- *, t 20 $r+ 50' :r-

t 9r :: ? E t. 20 30' t ! !: F t ri E

i-F ' a :.' = Zo _ SLOPE PARAMETER 3Oo SLOPEPARAMETER = l5c Io'N A = oBsERVED oMEGA ARTTFACT 95"30', 95010'w 95030', 95010'w

Fig. 13. Bathymetricgradient charts of SeaBeam data from Galapagos95 5"W propagatingrift survey(modified from Hey et al , lLhis issuel), All areaswhere five-ping-averagedSea Beam data exhibit slopesgreater than the slopeparameter (specified in the lower right of eachplot in degreesfrom the horizontal)are darkened (a) Slope parameter: l5'. Datacoverage and tectonicfabric (mostly E-W, seeHey et al , [thisissue] lor discussion)of this ruggedterrain are visible in this plot. (b) Slopeparameter : 30". Trianglesshow locations ol featuresappearing on this plot that areassociated with "omega"artifacts seen in 20-mcontour plots

walls, rift valley walls, pseudofaultwalls, abyssalhill overall tectonic structure. The " omega" effect was scarps,caldera walls, etc. In addition, some " omegas" discoveredwhile analyzingthis densedata set with several might be mistakenfor serpentinitediapirs, while others overlappingswaths and the "omegas"shown in PlateI are (for example,at trenchesor submarinecanyons) could be examples of artifacts that Hey and coworkers removed erroneouslyidentified as slump blocks or other mass- from their data. In eight casesfor which we initially wastingdeposits. When dealingon scalesof hundredsof suspectedthe "omega' bathymetryto be false and then metersto severalkilometers, failing to recognize"omegas" studiedthe acousticdata, our suspicionswere confirmed. as artifactscould lead to errors in geologicunderstanding By checkingthe raw Sea Beam data, we have identified becausethey might suggestunexpected volcanism or tec- eight others. We then estimated the probability of tonism in supposedlyinactive areas. The implicationsof encountering"omegas" on downdiptracks over fairly steep thesepossible misinterpretations are very important. slopes. We have visually examined the computer- Suspiciousfeatures which have the characteristicshape generatedSea Beam 20-m contourplots and identifiedall of "omegas"have been observedin data from every Sea the "omegas"which we feel confident are artifacts (many Beam survey we have investigatedthus far. Many geo- questionable examples were also found but were not physicalsurveys are run orthogonalto the tectonic fabric includedin the exercise).Triangles in Figure l3D mark becauseimportant variations in magnetic,seismic, gravity, the locations where features on this plot are associated and bathymetric data often are found in cross-strike with "omegas"seen in the contour plots. Knowing the profiles. Unfortunately, because"omegas" are found on directionof ship's travel, we were able to distinguishon downdip tracks, this type of survey pattern increasesthe Figure l3 those downdip slope crossingswhich have probability of occurrenceof these artifacts.In an effort to "omega" artifacts and those which do not. For approxi- quantify this probability,we have analyzeddata from such mately 5&1, of all occurrenceswhere the ship steamed a survey (Figure l3). Figure 13 shows Sea Beam data downdip acrossslopes which Sea Beam detectedas being from the propagatingrift at 95.5"W on the Cocos-Nazca greater than 30", "omega" artifacts are present. Though spreadingcenter lHey et ol., this issuel. In Figure 13, all we realizethat these estimatesare rough, the salientpoint areaswhere the gradientof the bathymetry,as detectedby is clear: "omega"artifacts can be very common in conven- Sea Beam, exceedsa specifiedslope are darkened. Figure tional surveys which run perpendicularto the tectonic 13a is included mainly to show the data density and the fabric. As mentionedabove, the frequencyof occurrence de MoUSTIER AND KLEINROCK: BATHYMETRIC ARTIFACTS IN SEA BEAM DATA of these artifacts may vary with each Sea Beam system of an operator, and it will not correct the effect pointed dependingon the side lobe level of its transmit beam pat- out by Renardand Allenou ll9l9l. In generalthe current tern, as well as with ship'sspeed. side lobe responsesuppression technique suffers from the Our detailedanalysis of these artifactshas concentrated saturation in the EP receivers. A simple modificationof on the SIO SeaBeam system because of the availabilityof the detectionamplifiers may solvethe saturationproblem, its acousticdata. But we have observed"omega'-like but use of logarithmicdetection amplifiers to increasethe featuresin bathymetricdata collectedwith systemsaboard EP receivers'dynamic rangeseems desirable lde Moustier, the R/V's Conrad (Lamont-Doherty GeologicalObserva- 19854l. At present,there is no way to controlthe perfor- lory), Surveyor(National Oceanographicand Atmospheric manceof the amplitudeshading in the receiverarray. As Administration), and Jean Charcat (IFREMER). Sea an example,we changedthe shadingcoefficient by 30ozoon Beam investigators who see suspicious features with four arrayelements in the computedbeam pattern of Fig- characteristicssimilar to the "omegas"shown here would ure 2b. It brought the side lobe level from 30 to 23 dB be prudent to survey the sites with crossing Sea Beam below the main lobe. This may not appear to be swaths for confirmation before attributing them great significantsince the side lobe thresholdcomputation is significanceor planninghigher-resolution studies. If such basedon a valueof 12 dB belowa peakamplitude, which featuresare not recognizedas poteptiallyimportant until is approximatelythe side lobe level of an unshadedarray after the survey, the raw Sea Beam data should be (Figure2b). However,we believethat ensuring optimum checked,looking for unrealisticdepth changesfrom one performanceof the array amplitude shading can only ping to the next and for missingdata points which indicate benefit any subsequentside lobe response rejection that the systemlost trackingof the bottom during that scheme. time, As the EP works on the rectifiedenvelope of the return Although we have only analyzeddata from the Sea signals,it has no way of differentiatingbetween side lobe Beamsystem, we believesimilar multibeam echo sounders responseand actualbottom return when the two overlap. might outputthe sameartifacts. The U.S.Navy SASSsys- To tell them apartrequires phase information which is not tem hasbeen in operationsince 1965, and someof its data availableto the EP in the currentmode of operation.One has been declassifiedfor use on the Mid-Atlantic Ridge way to deal with this problem would be to heterodyne Rift Valley lPhillips and Fleming, 1918; Ballard and van (multiply by an externaloscillator frequency and filter in Andel, 19'771and on the GalapagosRift at 86'W [van the desiredfrequency band lClay and Medwin, 1977D each Andeland Ballard,1979 Craneand Ballard,1980l. Com- of the l6 preformed beamchannels to obtain l6 channels parison of SASS bathymetry with Deep-Tow bathymetry of complexdata (32 channelsof real data). Thesecould lCrane, 7978, van Andel and Ballard, 19791seems to indi- be digitizedand processedas currentlydone in the EP. It cate that "omega'-likeartifacts exist in SASSdata. Data would then be possibleto applyadvanced adaptive filtering gapsand onsetsof "omegas"similar to thoseof Plate lb techniqueslMcCool and Widrow, 19111to effectivelycan- are also apparent in the work by Phillips and Fleming cel side lobe responseas well as noisebursts while retain- 11978,Figure 3Dl. ing the real bottom return signal. Of course, such a scheme might be hampered by the processingtime 4. Possrst-sCoRRpcrroNs required,and it needsto be tested.

Depths and cross-trackhorizontal distances cannot be 4.2. PossibleCorrections.for Interference recomputed as a postprocessingoperation unless the From ExternalSound Sources acousticdata are digitized and recorded on tape as was done for our data. Therefore investigatorsdiscovering Wheneverthe 3.5-kHzecho sounder is run in conjunc- fictitiousbathymetry in their data have no alternativebut tion with Sea Beam aboard the R/V ThomasLl'ashington, to disregard the portion of data affected. Also, these the analoggraphic recorder is usedto displayboth the Sea artifactsoccur too infrequentlyto warrant recordingof the Beam centerbeam profile and the 3.5-kHzecho sounder acousticdata on a routine basis. Rather than relying on outgoingpulse. Interferenceoccurs when the correspond- data reprocessing,it seemsmore sensibleto deal with the ing signaltraces intersect. To preventthis, it is necessary problems at their source. In the following, we suggesta to phasedelay the 3.5-kHzoutgoing pulse enough to keep problems number of solutions to the discussedin the ore- the two tracesseparated lsmith, 19831. This methodis not vioussection. entirelyreliable since it requiresan operator A more reli- able method consistsof using a simple electroniccircuit 4.1. PossibleCorrectionsfor Side Lobe Interference which gates out 3 5-kHz transmissionwhenever a Sea Beamreception cycle is in progress.Such a devicereport- When a "tunnel" effect developsduring a survey, it is edly works well on the R/V Conrad [Tyce, 19841. Unfor- common practiceto switch the EP from mode 3 to mode 2 tunately, this device will not prevent interference from lsmith, 19831. As a result, the automatic tracking gates l2-kHz transponders,pingers, or seismicsound sources. open to their maximum (upper and lower limits of the The latter usually cannot be phasedelayed for mechanical CRT display)according to parametersset by the SeaBeam reasons(constant pulse energy requirement)as well as operator. A subsequent return to mode 3 resumes data postprocessingreasons (constant firing rate require- automatic tracking. This method has proven effective in ment). A solutionwhich takesinto accountthe transmis- dealing with the "tunnel' effect which is essentiallyside sion rate requirementsof all possibleunderwater sound lobe responserelated. However, it relies on the vigilance sourcesavailable aboard a ship can be implementedon the a

deMousrIER euo Kre,tNnocr: Betuvlvlrtntc ARTIFACTsIl See Beeu Dern 3421

shipboard computer. With a knowledgeof the water a surveyand in describingabyssal morphology. This paper depth,the computerwould decidethe bestfiring sequence attempts to make the scientific community awarq of a necessaryto keep the sound sourcesfrom interfering with number of bathymetric artifacts observed in Sea Beam each other (J. L. Abbott, SIO, personalcommunication, data which, if unrecognized, might lead to geological 1984). misinterpretations.We have shown that artifactsdue to externalsound sources (e.g., subbottom profilers) or inter- 4.3. PossibleCorrections for " Omegd' nal side lobe interferencecan usuallybe clearlyidentified Effectsand Data Gaps as resultingfrom spuriousdata. In most of thesecases, correctiveaction can be takenin realtime whilesurveying. "Omegas'and data gapsare dealtwith in the sameway We also discusseda more insidiousartifact (the "omega" the "tunnel" effect is, by switchingthe EP from mode 3 to effect)which is virtuallyimpossible to detectin real time 2 and backagain. However,there is no way to detectan for lack of warning In addition,such artifactscommonly "omega"effect in real time since the cross-trackbottom found when steamingdowndip over slopesgreater than profileson the CRT appearto be within the gates,and by 30' may appearas geologicallyplausible volcanic, tectonic the time evidenceof it is seenon the swathplot it is usu- or sedimentaryfeatures. When navigationis basedon ally too late to correctanything. The automatictracking seafloor morphology, failure to recognizebathymetric gate software was modified by the manufacturer in artifactsmay leadto positioningerrors October1984 on the systeminstalled aboard the German In order to explain the causesof these artifacts,we R/Y Polarstern(W.Capell, GeneralInstrument Corpora- have analyzedSea Beam'secho detectionand processing tion, personnelcommunication, 1984). The changescon- techniques. Errors have been found to relate to the sistedof increasingthe minimum allowablewidth for each methodsof sidelobe rejection,automatic bottom-tracking, gateand enablinga fasterrate of changeof the gatesfrom and automatic receiver gain calibration. These grrors ping to ping. Sea Beam systemsinstalled since October result in incorrectdepth determinationswhich causethe 1984 benefit from this modificationwhich has proven artifactsobserved. Because Sea Beam only retainsdepths effectivein substantiallyreducing the problemsof data and cross-trackhorizontal distancesfrom the received gapsand "omega"effects. However,the wideningof the acousticsignals, investigators have no alternativebut to bottom-trackinggates tends to decreasethe depthdetermi- disregard the bathymetric artifacts they identify. A nationaccuracy on the outer beamsbecause the remaining number of correctionsare proposedto preventsuch data sidelobe responseis no longergated out on thosebeams. disposal: improved side lobe control in the Becauseof the side effectsof the EP receivers'calibra- transmit/receiveacoustic geometry, extension of the EP tion, mentionedin section3.3, we recommendthat an receivers'dynamic range, side lobe and noiseburst rejec- additionalchange be madeto allowthe gatesto widendur- tion through advanced adaptive filtering techniques, ing a calibrationcycle. This way a coincidentalincrease of improved bottom-tracking gate operation, delayed bottom slopewill easilybe accommodatedby the EP upon receivers'gain calibrationto allow for completionof the return to a normal receptioncycle. Also, it would be use- receptioncycle in progress,and computer-coordinatedsig- ful to have the half hour calibration,which is triggered nal transmissionfor all activesound sources during a sur- upon interrupt from the Sea Beam computerclock, wait vey. for the completionof the transmissioncycle in progress, Recently,presentation formats for SeaBeam data have and avoid situationssuch as that of Figure 12. Data extendedbeyond contour maps to includegray-tone and would then be lost for only one transmissioncycle, and colorshaded relief maps lEdwardset al , 19841,and bathy- the updatingof the trackinggates would be more reliable. metric gradientcharts IHey et d1.,this issuel. Thesefor- Finally,as for the receiverarray, some measureof the mats are very valuablein interpretingthe data; however, performanceof the projector array amplitude shading " omegas" and other bathymetric artifacts will persist seemsnecessary. At present,the system teststhe perfor- becausethe errors are in the raw Sea Beam data. not in manceof the poweramplifiers on an all or nothing (blown the contouringalgorithm employed. fuse) basis. Tyce ll984l reported deviations from the manufacturer'sspecifications by as much as 4&zoon the outputs of four projectorelements for the systeminstalled APPENDIXA: aboardthe R/V Conrad. For comparisona changeof 40oto Ssa BBeu Acousrrc GnovErnv in the shading coefficientsof four elements in the com- puted beam pattern (Figure 2a ) moved the side lobe level In the following,the beam widths are calculatedat the from 30 to 22 dB below the main lobe. Such levelswill half power point of the beam patterns. The transmitted definitelyenhance the signalto noiseratio of earlyspecu- beam pattern spans 54' athwartshipsby 2 2/3' in the lar returnsdiscussed in section3.3 (Figurel0), increasing fore-aft direction. It is pitch stabilizedto ensurevertical the probabilityof "omegas." projection by phasing the outputs of the 20 power amplifiersrelative to a pitch anglesupplied by the vertical 5. CoNcl-usIoNs referencegyroscope (Figure l) within the limits of + 10" of pitch. As shownin the computedbeam pattern (Figure In conclusion,we would like to stressthe importanceof 2a), the projectorarray is designedfor sidp lobe attenua- a clear understandingof the capabilitiesand limitations of tion 30 dB down from the main lobe and grating lobes the SeaBeam systemwhen analyzingits output. We fully appearat 55' on the fore-aft axis. The side lobe level is recognizethe value of SeaBeam bathymetryin conducting controlled by amplitude shading the output of the 20 J+LL de MousrIERnxo Klprunocr: Bntnyurtntc ARTlFAcrstN See Benrr,rDere

power amplifiers(Figure 1) using the Dolph-Chebyshev have a common gain at any one time. The gainsof the amplitude shading method for acoustic arrays lDolph, individualreceivers are automaticallycalibrated by the EP 1946;Riblet and Dolph, 19471.Since the array is contained every half hour by inputting a common voltagethrough in a housing,the actualside lobe level is 25-26dB down the beamline drivers (Figure 1) and digitizingthe output from the main lobe lDolph 1946; Renqrd and Allenou of the receivers. 19791. Propercontrol of the sidelobes on the transmitted The refractioncorrection uses valuesof a sound velo- beam in the fore-aft direction is crucial for adequateper- city versuswater depth profile, enteredat the beginningof formanceof the systemwhen the ship'strack runs down- a surveyby the SeaBeam operator, and Snell'slaw to cal- dip (acrossa slope in the downhill direction). In this culatethe receptionangle @ for eachbeam with respectto geometry,weakly attenuated side lobesensonify the slope the ship'svertical. The soundvelocity profile is measured at near-normalincidence in the fore-aft direction. The with an expendablebathythermograph cast for the first correspondingbottom returns are receivedearlier than few hundred metersand extendedto the maximum bot- thosedue to verticalprojection in the main lobe,and they tom depth in the survey areausing valuesfrom Carter's disruptthe echo-processingand bottom-trackingfunctions. tablesof sound velocityin the oceanlCartet', 19801 . The designof the receivingarray yields a beam pattern The roll compensationuses the ship'sinstantaneous roll which is 2 2/3" athwartshipsby 20'in the fore-aftdirec- angleB given by the vertical referencegyroscope to refer- tion (Figure 2b). The 20' beam width in the fore-aft encethe receptionangle@ to the true vertical:@ : @+8. directionis meant to accommodatepitch anglesof + l0', A set of stabilizedbeams angles V spaced2 2/3o apartare as no pitch stabilizationis performed on the receiving then created.The amplitudesof the stabilizedbeams are beams. Sea Beam generates16 preformedbeams fixed linearly interpolatedbetween those of the two adjacent with respectto the ship'svertical by electronicallysteering preformedbeams with receptionangleOl andO111 This such2 2/3' beamsat intervalsof 2 2/3'athwartshipsfrom yields 15 stabilizedbeams each 2 2/3" wide, fixed in a 20" incidence on port to 20" on starboard. Dolph- verticalplane athwartships with one beamaligned with the Chebyshevamplitude shadingof the output of the 40 true vertical. As provisionhas been made for + 20" of preamplifiers(Figure l) attenuatesthe side lobes 30 dB ship'sroll, thereare 31 possiblestabilized beam positions below the main lobe (Figure2D). For the samereasons between-t-40'. Occasionally,one of the preformedbeam given for the projectorarray, the actualside lobe level anglesO will lie on the true vertical(V: 0), and there may be somewhat higher. Renard and Allenou [19791 may be l6 stablizedbeams measureda valueof 28 dB on two preformedbeams The A set of bottom-trackinggates determines the detection acousticdata we have recordedindicate a mean side lobe time window during which a bottom echo is expected level of 25 dB below the main lobe on 10 preformed basedon previoussounding history. The trackinggate is beamsfor the SIO system[de Moustier,1985r]. Proper an essentialfeature of the EP becauseit conditionsproper side lobe level control is importantfor the receivingarray echosignal detection and thereforereliable depth determi- because each of the preformed beams has side lobes nation. Each beam has its own trackinggate. It is cen- orientedin the directionof the main lobe of all the other teredon the averagedepth for that beamusing depth his- beams. A strong return coming into the main lobe of a tory over the lastfive transmissioncycles (pings) weighted particularbeam will thereforebe receivedby all the side decreasinglyinto the past The gate width is determined lobespointing in the samedirection. by the observed ping-to-pingdepth fluctuations with allowancefor variationsin signal duration due to beam APPENDIXB: angle, bottom slope, and beam width. As a result, the SEnBp,nv Ecuo Pnocpssrxc gatesare narrowerfor the near-verticalbeams than for the outer beams.A constantvalue (20 m) is added to the Informationconcerning Sea Beam'secho processingis width of eachgate as a safetymargin to ensurethat the containedin the Sea Beam software technical manual echo signaldoes not fall outsidethe gate. Bottom echoes IGeneralInstrument Corporation, 19817. In the following, falling outside the tracking gates are not taken into we give, with the manufacturer'spermission, an overview accountby the EP which usuallywill not computea depth of the main featuresof the echo-processingsoftware. We and a cross-trackdistance for the correspondingbeams for emphasizethe featuresimportant to understandthe causes lack of signalto noiseratio. This situationcreates a data of the bathymetricartifacts discussed in this paper. gap Sinceonly 15 (occasionally16) of the possible3l During eachtransmission cycle, l6 bottom returns(e.g., stabilizedbeams bear data, gate settings for null or unused Figure 3) are digitized by Sea Beam's analog-to-digital beamsare interpolatedor extrapolatedlrom thoseof adja- converter at a frequency of 300 Hz for each beam. For cent beams. Finally,the gatesettings are smoothedacross each conversion cycle the Sea Beam computer performs all 3l possiblebeams. The analog-to-digitalconversion the following operations: receivergain correction,refrac- startsat the onset of the gate with the shallowestsetting tion correction, roll compensation,detection threshold (earliesttime). The conversionstops when the deepest level determination, and signal detection for each of the gate has been reached. roll-compensatedbeams. The detectionthreshold level determinationis a very The receivergain correctionconsists of multiplyingthe critical operationin the echo processing.It is adjusted digitized signal voltage for each beam by an amplitude every conversioncycle and is thereforea dynamic process multiplication factor to compensatefor differencesamong taking several parametersinto consideration: (1) the receivers.Because the roll compensationinvolves interpo- manual thresholdlevel input by the Sea Beam operator, lation between beams, it is important that all 16 signals (2) the backgroundnoise level of the receivers,(3) the .lellousllt:R,r'io KLF-I.trr.rxr. Il,rtt*'rrt tttr Anttr,r( fs tl.tSt,l lltlv Dlte 3423 receivers' side lobe response, and (4) potential noise Ballard,R D and i Francheteau,Geologic processes of the mid- bursts interfering with the bottom echo detection. In gen- oceanridge and their relationto sulfidedeposition, in Hydroth- ermal Processesat SeafloorSpreading Centers, pp l7-26, Plenum, eral, the thresholdlevel is computedto ride above the New York, 1983. noise and above the side lobe response. For reference, Carter, D i. T., Echo-SoundingCorrection Tables, Hydrographic the noiselevel measuredon datasimilar to that of Figure Department,Ministry of Defence,Somerset, U K , 1980 3 is usually around 20 mV. The side lobe thresholdis Clay, C S and H. Medwin, AcousticalOceanography: Principles and Applicarions,p 128,iohn Wiley,New York, 1977 as one-fourththe amplitude (12 dB down) of computed Crane, K Structureand tectonicsof the GalapagosInner Rift, (Figure , the highestof the 16 signalsat any one time 3) 86'10'W,J. Geol.,86, 715-730, 1978 A noise burst appearsas a synchronousridge similar to Crane,K. and R D Ballard,The GalapagosRiit at 86'W', Mor- the side lobe ridgeof Figure3, but the amplitudesof the phological waveforms, 4, Structure and morphology of individualpeaks are more or less constanton all beams hydrothermalfields,./. Geophys. Res., 85, 1443-1454,1980 Aikman, R. Embley,S Hammond,A Malahoff, (e.g., Crane, K., F, Figure 7). By comparingthe maximum amplitude and i Lupton, The distributionof geothermalfields on the with the median amplitudeacross all beamsat any one iuan de FucaRidge, J. Geophys.Res., 90, 727-744,1985. time, the software is able to recognizea noise burst. de Moustier,C., Inferenceof manganesenodule coverage from When a noiseburst is detected,and when the correspond- Sea Beam acoustic backscatteringdata, Geophysics,-50, 989- 1001,1985a ing thresholdlevel is higherthan both the noiseand the de Moustier, C , Beyond Bathymetry,mapping acoustic back- side lobe thresholds,the 16 amplitudesare rejected.Oth- scatteringfrom the deepseafloor with SeaBeam, "/. Acoust. Soc. erwise,the higherof the noiseor the sidelobe thresholds Amer , (in pres), 1985b will be usedas the detectionthreshold. With this method, Detrick, R. S., P. J. Fox, K Kastens,W, B. F Ryan,L Mayer, however, cancelingside lobe responseor noise bursts and J KarsonMid-Atlantic Ridge Rift Valley,A SeaBeam sur- vey of the Kane FractureZone and the adjacent,EOS, Trans. when they overlapwith a bottom return resultsin cancel- AGU.6s.1006.1984 lation of the correspondingpart ol the bottom return. Dolph, C L , A current distributionof broadsidearrays which Also, becauseof saturationin the EP receivers'amplifiers, optimizesthe relationshipbetween beam width and side-lobe side lobe rejectionis only partiallyachieved in caseswhen level,Ploc. Insr. Radio Eng., 34,335-348, 1946 Edwards,M H, R E Arvidson,and E A Guiness,Digital the specularreturn is clipped. This resultsin both echo imageprocessing of SeaBeam bathymetricdata for structural detectionand depth computationerrors. Finally,for each studies of seamountsnear the East PacificRise, ,/. Geophys. conversioncycle, a signalsample is detectedif it is above Res..89. 11108-11116. 1984 the detectionthreshold and within the bottom-tracking Farr, H. K , Multibeambathymetric sonar: SeaBeam and Hydro- gates. chart,Mar. Geol, 4,77-93,1980 Fornari,D i., W B F Ryan, and P i Fox, The evolutionof Once the analog-to-digitalconversion sequencehas cratersand calderason young seamounts:Insights from Sea been completedon all beams, the next set of echo- MARC I and Sea Beam sonar surveysof a small processingoperations is done once per transmissioncycle. groupnear the axisof the EastPacific Rise at - 10"N,J. Geo- The signallevel of eachdetected beam is integratedover phys.Res.,89, 11069-11084, 1984 i and R. D Ballard,The East PacificRise near (within the gatesand Francheteau, the durationof the detectedreturn 2l'N, l3'N and 20'S; inferencesfor alongstrrke variability of abovethe threshold). If the resultingenergy in the return axialprocesses, Earth Planet Sci Lett, 64,93-116,1983. is below a prescribedminimum, the beam is deemed Gallo,D i,P i Fox,andi A Madsen,Themorphotectonic invalidduq to poor signalto noise rario[Farr,1980]. For signatureof fast-slippingridge-transform-ridge systems: A syn- thesis of Sea Beam bathymetry, EOS Trans.AGU, 65, 1103, a valid beam,a slantrange is calculatedby computingthe 1984. centerof massof all the detectedsignal samples for that General lnstrument Corporation,Sea Beambathymetric swvey sys- beam, and by multiplyingthe correspondingarrival time tem, TechnicalManual, Vol.2, Westwood,Mass, 1981. by 750 m/s. Depth and cross-trackhorizontal distances Glenn, M F , Introducingan operationalmulti-beam array sonar, arethen calculatedas describedin section2. Inr. Hydrogr.Rev., 47( l), 35-39,1970 Hey, R. N., M C Kleinrock,S P Miller, T M Atwater,and R C Searle,Sea Beam/Deep-Tow investigation of an activeoce- anic propagatingrift system,J. Geophys.Res., (this issue) AcknowledgmenlsThe work reportedhere would not havebeen Hey, R N., D. F Naar, M C Kleinrock,W. i. PhippsMorgan. possiblewithout the cooperative efforts of thecaptain and crew of E Morales,and i G. Schilling,Microplate tectonics along a the R/V ThomasWashington. We wishto thankW Capellfrom superfastseafloor spreading system near EasterIsland, Nalare, patiencein answeringour GeneralInstrument Corporation for his 3 1 7, 320-325, 1985 numerousquestions; R. N. Hey,P F. Lonsdale,i. L Abbott,T. Kleinrock,M C., R N Hey, and C de Moustier,The "Omega' H Shipley,and P. C. Henkartfor helpfulcomments and data deceptionin Sea Beam data, EOS Trans.Am. Geophys.Union, samples;R. C. Tycefor initiatingthe SeaBeam acoustic back- 65, 1103,1984. scatteringexperiment at MPL;and F. V Pavlicekfor hissupport Lewis. S. D, i W Ladd, T, R Bruns, D E Hayes,and R. duringthe developmentof theSea Beam acoustic data acquisition Von Huene,Growth patternsof submarinecanyons and slope system.We areindebted to R N. Hey,K Crane.i A Hilde- basins,Eastern Aleutian Trench, Alaska,COS Trans.Am. Geo- brand,and S P. Miller for their valuablesuggestions and critical phys.Union, 6J, 1104,1984. reviewof the manuscript.We are alsograteful to i Barronand I-onsdale,P F., Overlappingrift zones at the 5 5'S offset of the E Ford for typingand editingand i Griffith for the art work Res.,88, 9393-9406,1983 (con- EastPacific Rise, J. Geophys. For their support,we thankthe Officeof NavalResearch Macdonald,K C and P. i Fox, Overlappingspreading centers: A tract N00014-79-C-0472)and the NationalScience Foundation new kind of accretion geometry on the East Pacific Rise, (grantOCE-8109927). Nature,301. 55-58.1983 Macdonald,K C, i C. Sempere,and P. i. Fox, EastPacific Rise RenEnnlces from Siqueirosto Orozco fracturezones: Along-strike con- tinuity of axial neo-volcaniczone and structureand evolution Ballard,R D andT H. van Andel,Morphology and tectonics of of overlappingspreading centers, J. Geophys.Res., 89, 6049' the innerrift valleyat lat 36"50Non the Mid-AtlanticRidge, 6069,1984 88.507-530. 1977. Mammenckx,i., Morphologyof propagatingspreading centers: a

3424 (tel\'l()ustn R.csD KLEI.'Rrrr: llettnvl-tRI(. ARltAc'rs t:i St,n llt-,*"t [),rre

New and old, J. Geophys.Res., 89, 18l7-1828,1984. Spiess,F. N. and P. F. Lonsdale,Deep-Tow rise crestexploration McCool, i. M. and B. Widrdw, Principlesand applicationsof adap- techniques,Mar Technol.J., 16,67-75,1982. tive filters: A tutorial review, NUC TP 530,Nav. OceanSyst. Spiess,F. N., R. Hessler,G Wilson, M. Weydert, and P. Rude, Cent.,San Diego, CA., 1977 Echo I cruise report, SIO Ref. 84-3, Scripps Institution of Naar, D. F and R N. Hey, Fast rift propagationalong the East Oceanography,La Jolla,Calif, 1984. PacificRise near EasterIsland, i. Geophys.Res., (this issue). Tyce, R. C., Seafloor mappingaboard R/V CONRAD - The first Patterson,R. B., Relationshipsbetween acoustic backscatter and year of SeaBeam, EOS Trans.AGU,6t 1103,1984 geologicalcharacteristics of the deepocean floor, J. Acoust.Soc. Urick, R. 1., Principleso.f UnderwaterSound, 3rd edition, Mccraw- Am., 46,756-761,1969. Hitt. New York. 1983. Phillips,i. D. and A. S Fleming, Multibeam sonar study of the van Andel, T. H. and R. D. Ballard,The GalapagosRift at 86'W, MAR Rift Valley 36-37'N, Map Series MC-19, Geol. Soc. qf 2, Volcanismstructure and evolution of the Rift valley,J. Geo- Amer.,Boulder, Colo , 1978 phys. Res.,84, 5390-5406,1979. Renard,V. and i. P. Allenou, SEA BEAM multi-beamecho- soundingin "iean Charcof'. Description,evaluation and first C. de Moustier, Marine PhysicalLaboratory, A-005, Scripps results,l?l. Hydrog.Rev., 56(l), 35-67,1979. Institution of Oceanography,University of California,San Diego, Riblet, H. L. and C. L. Dolph, Discussionon a currentdistribu- La Jolla,CA92093. tion of broadside arrays which optimizes the relationship M. C. Kleinrock, GeologicalResearch Division, A.-020,Scripps betweenbeam width and side-lobelevel, Proc.Inst. RadioEng., Institution of Oceanography,University of California,San Diego, 35.489-492.1947. La Jolla.CA 92093 Shipley,T. H. and G. F. Moore, Sedimentaccretion and subduc- tion in the Middle America Trench, OJI InternationalSeminar on the .formation of margins, Ocean Research Institute, Universityof Tokyo, Tokyo, Japan,1985. (Receivedjanuary 4, 1985; Smith, S. M., Sea Beam Operator Manual, SIO Reference83-7, revisedAugust 2,1985; ScrippsInstitution of Oceanography,La Jolla,Calif., 1983. acceptedAugust 7, 1985.) dc VlousrttiRANt) KLEtNRocK:[]A f llYf"tlirRICARTll'Ac'rs I),r S[A I]l,AMDATA 3889

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xi

b

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CONTOUR INTERVAL 20m COLOR CHANGE INTERVALlO0m 5 PING AVERAGE ikm

I siup DiREcuoN ++REPROCESSED DATA

Ptate1. lde Moustierand Kleinrockl"Omegad' and gaps. Tick marks point downhill. Contouredsections not starred are original SeaBeam data. Contouredsections marked with a star are the result of reprocessingthe acousticdata recordeddigitally with the MPL system. Our simptifiedecho-processing technique does not include ray-bending corrections,and arrival times are determinedby the first arrival above a presetthreshold. The thresholdlevel is selectedafter visualinspection of the roll-compensatedacoustic data. Recomputeddepths and cross-trackdistances are thereforein uncorrectedmeters referencedto a sound velocity of 1500m/s. Although crude, this processjng method sufficesto prove the fictitiouscharacter of SeaBeam's contoured bathymetry shown in Plates\a, (sectiOn ,, lb, ld, le, lf and lg. We do not show a recomputedversion of Plate lc becausethe correspondingacoustic data wasonly recordedevery five pings. This wasenough to confirm the "omega"effect, but contour resolutionwas seri- ously degradedby the five ping decimation.