The INTIFANTE’00 sea trial: preliminary sourcelocalization and ocean tomography data analysis

S.M. Jesus1, E. Coelho2, J. Onofre2, P. Picco3, C. Soares1 and C. Lopes1

1 SiPLAB-FCT, Universidade do Algarve, PT-8000 Faro, Portugal2 Instituto Hidrografico, PT-1296 Lisboa, Portugal

3 ENEA, S. Teresa, 19100 - La Spezia, Italy.

Abstract— The INTIFANTE’00 sea trial was a multidisci-plinary experiment including testing of an autonomous sur-face vehicle, underwater communications, source localiza-tion and acoustic ocean tomography. The results shown herewill concentrate on the source localization and ocean tomo-graphy data sets. The data gathered during a 24 hour runalong a range independent track shows strong oceanographicfeatures, possibly due to internal tide signature, both on thetemperature data, as measured on the thermistor chain col-located with a vertical line array(VLA), and on the acousticdata. A range dependent track between 120 and 60 m wa-ter depth, shows a highly variable channel impulse responsealong time and range when the source was moving outwardsfrom the VLA. In another acoustic track, the source wasnavigatated across a underwater canyon where the energywas rapidly distributed over a deep acoustic channel withsound trapped well below the thermocline. Good agree-ment between the modeled and measured channel responsesrepresents the first step towards matched-field processing-like methods such as source localization and tracking andocean tomography.

Keywords— Shallow water, source localization, ocean to-mography.

I. Introduction

Sound propagation in the ocean is highly dependent onthe environmental characteristics of the propagation me-dia between the source and the receiver. Since the oceanicenvironment is continuously changing in time and space,using acoustics for underwater communication and sonardetection are very challenging tasks. Conversely, the inter-action between sound waves and the environment allows forretrieving environmental information from the analysis ofthe emitted and received signals - this is acoustic tomogra-phy [1], [2]. Therefore, being able to predict the acousticbehaviour of a given environment is the key to current ad-vances in the usage of acoustics for ocean exploration.

In 1997, the Portuguese Foundation for Science andTechnology (FCT)1 has financed two initiatives in marinetechnology, namely projects INTIMATE2 and INFANTE3.The former aims at developing and testing ocean tomogra-phy techniques for estimating internal tides in the conti-

This work was supported under projects ATOMS, contract PD-CTM/P/MAR/15296/1999, FCT, Portugal and TOMPACO, CNR,Italy.

1Fundacao para a Ciencia e a Tecnologia, Ministry of Science andTechnology, Portugal.

2“Internal Tide Measurements with Acoustic TOmography Exper-iments”

3“Development of Vehicles and Advanced Systems for SubmarineInspection”

nental platform. The later aims at developing autonomousunderwater vehicles (AUVs) and includes a component oftesting methods and algorithms for improving the capabili-ties of the underwater communication channel between thesurface and a submerged vehicle.

The INTIFANTE’004 sea trial was carried out in thevicinity of Setubal, situated approximately 50 km to thesouth of Lisbon, in Portugal, during the period from 9 to29 October, 2000. The leading institutions were the Insti-tuto Hidrografico, that carried out the oceanographic ob-servations and managed the research vessel NRP D. CarlosI, SiPLAB/UALG that provided the acoustic data acquisi-tion system and the emitted source signal control and IST,that was in charge of the high frequency data communi-cations testing. Other collaborating/participating institu-tions were the NATO SACLANT Undersea Research Cen-tre with the loan of the acoustic sound source and the EnteNazionale per l’Energia ed l’Ambiente (ENEA) that parti-cipated in the hydrological measurements.

This sea trial served a number of specific purposes underthe leading projects INTIMATE and INFANTE, namely toacquire data for testing the Time-Reversal Mirror (TRM)principle for underwater communications at low-frequency(Event 1), internal tide acoustic tomography through a25 hours observation of continous transmissions (Event 2),source localization and tracking over strong environmentalvariabilities, (Event 3) and over a mild range dependentenvironment (Event 4). The other events 5 and 6 were con-cernned with ocean acoustic tomography using broadbandnoise sources and ships of opportunity as ocean tomogra-phy source signals.

The work presented in this paper concentrates on theocean tomography and source localization objectives overthe different bottom topography conditions and with vari-ous types of emitted signals, including the noise sources.

This paper is organized as follows: section 2 givesan overview of the INTIFANTE’00 sea trial including adescription of the environmental characteristics, such asbathymetry, bottom properties and hydrology, as weel asthe overall experiment geometry. Section 3 describes theacoustic data gathered at the VLA and shows some forwardmodelling and matched-field results. Some conclusions aredrawn in section 4.

4INTIFANTE is a madeup acronym from INTImate and inFANTE.

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II. The INTIFANTE’00 sea trial

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INTIFANTE’99 Area

LISBON

Sesimbra

Setubal

Espichel Cape

Fig. 1. Localization of the INTIFANTE’00 experimental site

The INTIFANTE’00 sea trial took place nearby thePeninsula of Troia, approximately 50 km south from Lisbonin Portugal, from 9 to 29 October 2000 (see figure 1). Theregion is characterized by a relatively uniform continentalplatform with depth varying from 60 to 140 m with variousbottom types, crossed by a deep underwater canyon with asteep depth variation from 120 to 500 m. The bottom to-pography of the area and the acoustic transmission tracksare shown in figure 2.

Fig. 2. Bathymetry of the INTIFANTE’00 sea trial area, verticalline array location (�) and acoustic tracks.

The vertical line array (VLA) is the center of three acous-tic legs along which acoustic data was trasmitted at variousranges and depths. The acoustic source ship was eitherstopped or moving along the tracks. The NW acoustictrack is nearly range independent and parallel to the plat-form edge with a water depth of approximately 120 m. The

NE acoustic track is perpendicular to shore and is charac-terized by a slow range dependency both in terms of waterdepth varying from 120 to 60 m and in terms of bottomcharacteristics that were changing from a thin sand layerto mud passing over large rock patches. The SE track ispassing over a 500 m deep underwater canyon that pene-trates the continental platform edge to an extent of a fewkilometers. The water depth is varying from 120 to 500 min only approximately 400 m of horizontal distance at bothedges of the canyon.

Fig. 3. XBT temperature profiles during INTIFANTE’00.

The XBT temperature profiles measured at various loca-tions during the experiment are shown in figure 3. There isa strong thermocline gradient of approximately 3 ◦C start-ing at 10 m depth and extending over 50 m. Even if thetemperature field is largely undersampled, a semidiurnaleffect can be clearly seen from figure 4.

Fig. 4. XBT temperature profiles along time during INTIFANTE’00.

Other temperature data was recorded on the thermistorsensors colocated with the VLA. These data were transmit-ted and analysed online with the acoustic data on boardNRP D. Carlos I. The recordings obtained during event

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1 for nearly 24 hours is shown in figure 5. Despite severaldata interruptions due to battery change and RF transmis-sion drop outs a clear semidiurnal pattern can be clearlynoticed. Note that the time scale is oversampled while thedepth sampling interval is equal to the thermistor spacing,i.e., 8 m and spans the VLA acoustic aperture.

Fig. 5. Temperature field recorded at the VLA thermistors duringEvent 2.

As an overview of the technical aspects involved in theexperiment, it can be referred that acoustic signals weretransmitted with an HX90 acoustic projector from onboardNRP D. Carlos I and received on a moored 16 hydrophone-4m spacing VLA (see figure 6). The acoustic aperture ofthe VLA was located between 30 and 90 m in a 120 m watercolumn. The acoustic signals received in the VLA weretransmitted via an high-speed RF link to the research shipNRP D. Carlos I, processed, monitored and stored. Varioussignals were emitted by the sound projector ranging fromlinear frequency modulated (LFM) sweeps in the band 200-800 Hz to broadband pseudorandom noise sequences.

Fig. 6. Vertical Line Array(VLA) structure.

III. Acoustic data

A. Transmitted signals

The acoustic source power spectrum has several reso-nance peaks in the frequency band of interest (figure 7)which introduce a frequency dependent amplitude modu-lation. That frequency dependent amplitude modulationwas compensated for in the transmission of the pseudoran-dom noise sequences (code C2) in the band 100 - 2200 Hz.The 170 and 250-800 Hz-2 second duration LFM’s, codesA3 and A6 respectively, were not source spectrum com-pensated. During most part of the experiment the soundsource was between 60 and 70 m depth.

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Fig. 7. HX90 acoustic source sensitivity

Table I contains the VLA hydrophone’s depth as esti-mated from the top and bottom depth sensors.

TABLE I

Hydrophone array depth

Hyd. Depth Hyd. Depth# (m) # (m)1 32 9 642 36 10 683 40 11 724 44 12 765 48 13 806 52 14 847 56 15 888 60 16 92

The VLA received signals were GPS-synchronized withthe emitted source signals which lead to a perfect timealignement and a very accurate estimate of the absolutetravel time. This is an essential requirement for ocean to-mography inversion of the water column properties.

B. Range independent NW track

The module of the matched-filtering of the received timesignals with the emitted signals (pulse compression) givesan estimate of the channel impulse response and is a clear

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indicator of the time variability of the propagation channel.The pulse compression result for the 25 hours run along theNW range-independent track is shown in the right plot offigure 8 while the plot on the lefthandside of the same fig-ure shows the estimated source range from the first arrivalpeak. Each estimated channel response is obtained fromthe averaging of 10 individual snapshots, which acountsfor approximately 100 seconds worth of data. The pulsecompressed signal plot shows a quite stable arrival patternwith arrivals separated in up to 12 resolved packets. Oscil-lations in the late arrivals are strongly correlated with theexternal (baroclinic) tide while the early arrivals packet isstrongly correlated with the temperature profile variabilityin the water column [3].

Fig. 8. Pulse compressed arrival patterns during Event 2 with leadingedge synchronization (right) and estimated source-VLA range (left).

C. Range dependent NE track

During this track the acoustic source was first towedaway from the VLA on a mild range dependent track emit-ting LFM codes A3 and A6, up to 5.3 km range. On theway back to the VLA along the same track the acousticsource was emitting a source spectrum compensated pseu-dorandom broadband noise sequence - code C2. Figure 9shows the pulse compressed signals of the successive ar-rivals while the acoustic source was approaching the VLA.Possibly due to the source movement and the 100 sec-

onds averaging the estimated channel impulse response isblurred during the middle portion of the track.

Fig. 9. Pulse compressed arrival patterns during Event 5 with leadingedge synchronization (left) and estimated source-VLA range (right).

IV. Modelled data

A. The range independent NW track

The first objective when performing either source loca-lization or ocean acoustic tomography is to match the re-ceived and the modeled acoustic fields. This is generallycalled forward modelling and represents an important - ifnot the most important - step towards a successful fieldinversion. Figure 10 represents the assumed model for therange independent NW track.

This model was used with the propagation code C-Snap[4] in a range independent mode to produce the arrivalpattern shown in figure 11 as a comparison with the arrivalpattern measured experimentally. It can be clearly seenthat the agreement is quite good for the two curves with aclear alignement of the arrivals. This result shows that themodel gives an appropriate description and a good matchof the real environment, but it does not tell how that matchchanges through time and space.

That is presented in figure 12 that shows the correlationbetween the modelled and measured arrival patterns forthree hydrophones and during the whole duration of the

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0

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Fig. 10. Physical model for the range independent NW track.

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Fig. 11. Arrival pattern during the range independent NW trackon hydrophone 8 (66 m depth) at 5.2 km range: measured (bottomcurve) and modelled (top curve).

recording (20 hours). The correlation is quite high duringall track with a mean value of approximately 0.8.

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Fig. 12. Correlation between the modelled and the measured arrivalpatterns along 20 hours of the range independent NW track for hy-drophones 1 (top), 8 (middle) and 16 (bottom).

B. The range dependent NE track

An attempt of forward modelling with pseudorandomnoise was made for the end portion of Event 5 (shown infigure 9), while the source range was approximately 1 kmand the physical model is that given in figure 10. The bestattempt is shown in figure 13.

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Fig. 13. Arrival pattern during the range dependent NE track onhydrophone 1 (32 m depth) at 0.95 km range: measured (bottomcurve) and modelled (top curve).

While the source was moving at a constant range pat-tern around the VLA during approximately 30 minutes thecorrelation between the modelled arrival pattern and thedata estimated arrival pattern is given in figure 14. A re-latively good match was obtained even at this short rangewhere most of the energy is concentrated on a few hori-zontal paths, which are most of the time unresolved anddifficult to accurately estimate.

291 291.005 291.01 291.015 291.02 291.025 291.030.6

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Fig. 14. Correlation between the modelled and the measured arrivalpatterns along 30 minutes of the range dependent NE track for hy-drophone 1.

C. The across-canyon SE track

This propagation track represents a much more chal-lenging problem for the acoustic model. Figure 15shows the physical model that was used to represent the

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strongly range dependent propagation across the underwa-ter canyon. The transmission loss plot across the canyon(not shown) shows that a large portion of the energy is ac-tually sucked at the canyon border and the acoustic fieldstructure is not reconstructed across the canyon, on thereceiver side.

0

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th (m

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1506 1515 m/s

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VA

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430

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Fig. 15. Physical model for the range dependent SE across-canyontrack.

Figure 16 shows the model predicted and the measuredarrival pattern at 5 km range for hydrophone 8 locatedat 60 m depth. Visually the agreement is not so good asfor the range independent case but represents neverthelessa correlation of approximately 0.8. The variation of thatcorrelation along time is shown in figure 17.

3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6 3.65 3.7 3.750

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Nor

m. a

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itude

Fig. 16. Arrival pattern during the across canyon SE track on hy-drophone 8 (60 m depth) at 5 km range: measured (bottom curve)and modelled (top curve).

V. Conclusion

The data gathered during a 24 hour run along the rangeindependent track shows strong oceanographic features,possibly due to internal tide signature, both on the tem-perature data, as measured on the thermistor chain collo-cated with the VLA, and on the acoustic data. The rangedependent track between 120 and 60 m water depth, showsa highly variable channel impulse response along time andrange when the source was moving outwards from the VLA.Such strong variations are amplified during the source na-

291 291.2 291.4 291.6 291.8 292 292.20.45

0.5

0.55

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Julian Time

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rela

tion

Fig. 17. Correlation between the modelled and the measured arrivalpatterns along the across canyon range SE track for hydrophone 8 at60 m depth.

vigation over the underwater canyon where the energy wasrapidly distributed over a deep acoustic channel with soundtrapped well below the thermocline. Good agreement bet-ween the modelled and measured channel response on indi-vidual sensors along time was obtained for all tracks, fromthe range independent to the strongly range dependent, asa preliminary requirement to matched-field inversion, gi-ving good perspectives for source localization/tracking andocean tomography inversion.

Acknowledgment

The authors would like to thank the NATO SACLANTUndersea Research Centre for the loan of the acousticsound source and the participation of the technician En-rico Muzi during the sea trial preparation.

References

[1] W. Munk and C. Wunsch, “Ocean Acoustic Tomography: ascheme for large scale monitoring”, Deep-Sea Research, Vol. 26A,pp. 123-161, 1979.

[2] W. Munk, P. Worcester and C. Wunsch, Ocean Acoustic Tomo-graphy, Cambridge Monographs on Mechanics, New York, USA,1995.

[3] O.C. Rodrıguez and S.M. Jesus, ”Physical limitations of traveltime based shallow water tomography”, J. of Acoust. Soc. ofAm., Vol.108(6), p.2816-2822, 2000.

[4] C.M. Ferla, M.B. Porter e F.B. Jensen, “C-SNAP: Cou-pled SACLANTCEN nnormal mode propagation loss model”,SACLANTCEN SM-274, Saclant Undersea Research Centre, LaSpezia, Italy, 1993.

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