INDIAN OCEAN HYDROACOUSTIC WAVE PROPAGATION CHARACTERISTICS

Transcription

1 ABSTRACT INDIAN OCEAN HYDROACOUSTIC WAVE PROPAGATION CHARACTERISTICS Pierre-Franck Pierchia, Pierre-Mathieu Dordain CEA/DIF Département Anlaye, Surveillance, Environnement, France Sponor Commiariat à l Energie Atomique (CEA), France The channeling efficiency of the Deep Sound Channel (often referred a the Sofar channel) allow long range propagation of hydroacoutic wave over a few thouand of kilometer. Strong T-wave, referring to a third arrival on eimic wave, are commonly oberved on underwater receiver (hydrophone tation) and on coatal receiver (T-phae tation), when an oceanic earthquake or an underwater exploion occur, even for mall event. Conequently, to inure the verification of the Comprehenive Nuclear-Tet-Ban Treaty (CTBT), the hydroacoutic network of the International Monitoring Sytem (IMS) ue five T-phae tation and ix hydrophone. At the end of 2001, three hydrophone tation -HA1 at Cape Leeuwin, HA4 at Crozet, and HA8 at Diego Garcia will continuouly end their data to the IMS. Thee data will alo be available at National Data Center. Then, uing thee data it will be poible 1) to refine the network detection capability 2) to etimate the network localization preciion and 3) to etimate the tranmiion lo of the hydroacoutic propagation and the hydroacoutic-to-eimic converion at the T-phae tation. To prepare thi evaluation, we are tudying the underwater propagation in the region of the Indian Ocean and in the South Atlantic Ocean uing modeling approache. The firt part of thi paper give a general view of the variation of the bathymetry, the ound peed propagation and the Sofar channel axi in the Indian Ocean. In particular, it i hown that there i a trong ound peed profile variation, from the North to South Indian Ocean, due to a cold water front coming from the Sub-Antarctic Ocean. In a econd part, three area are defined in the region of the Indian Ocean and in the Antarctic Ocean. In each of them, a typical ound peed profile ha been conidered to etimate numerically the underwater tranmiion lo characteritic. The underwater blockage effect due to underwater eamount are alo invetigated in thi part. In the three region previouly defined, thi approach allow u to propoe a baic formulation of the tranmiion lo (TL in db) : TL( f, z, r) = A ( f, z ) + 10 log( r) + B( f, z ) + α( f ) ( r 1) o where f, z, r are repectively the frequency, the depth, the ditance of the underwater ource andα i the aborption coefficient. Thee law could give a good and imple characterization of the tranmiion lo and can be ued to etimate quickly the ource charge weight from an hydroacoutic record of an underwater blat. In thi preliminary tudy, Ao and B are etimated numerically uing a parabolic equation technique in the frequency domain. In the dicuion, we invetigate the number of record point and ource localization which are neceary to provide a good etimation of A and B. o 1

2 Objective: At the end of 2001, three hydrophone tation (Figure 1). -HA1 at Cape Leeuwin, HA4 at Crozet, HA8 at Diego Garcia - will continuouly end their data to the IMS. Thee data will alo be available at National Data Center. Then, uing thee data it will be poible 1) to refine the network detection capability 2) to etimate the network localization preciion and 3) to etimate the tranmiion lo. The objective of thi reearch i to tudy the underwater propagation in the region of the Indian Ocean uing modeling approache to prepare an experimental evaluation of the IMS hydroacoutic network in the Indian Ocean. We are in particular invetigating the tranmiion lo and blockage effect due to ea mount or underwater plateau. HA8 HA1 HA4 Figure 1 : Indian Ocean map. Three hydrophone tation participate to the International Monitoring Sytem in the Indian Ocean -HA1 at Cape Leeuwin, HA4 at Crozet, HA8 at Diego Garcia. Reearch accomplihed : The firt part of thi paper give a general view of the variation of the bathymetry, the ound peed propagation and the Sofar channel axi in the Indian Ocean. In the econd part, we invetigate numerically the underwater tranmiion lo. 2

3 Overview of the Indian Ocean main characteritic Hydroacoutic wave propagation in an ocean i mainly controlled by the velocity variation with repect to depth and range. It i well known that the velocity increae with temperature, alinity and depth. In the Indian Ocean, band of equal temperature run eat-wet. Water temperature i the highet along the equator, becaue there Earth i mot warmed by olar radiation, and become cooler toward Antarctica (Figure 2). Becaue the temperature decreae with depth, the velocity decay rapidly with depth down to a minimum at the Sofar channel axi. Below thi depth, the velocity increae quite linearly with the hydroacoutic preure. Due to thi variation, band of equal ound peed run eat-wet and there i a trong ound peed variation on the Sofar channel axi, from about 1500 m/ in the North to 1455 m/ in South Indian Ocean. In our tudy, we have conidered three different type of ound peed profile (Figure 2). Hydroacoutic wave propagation in an ocean i alo controlled by the bathymetry variation (Figure 1) along the wave path. The Indian Ocean bain i the hallowet of the two other major bain becaue it i the younget. Thi bain ha five important ridge that may block hydroacoutic wave propagation. It ha three active mid-ocean ridge : the Southwet Indian Ridge, the Southeat Indian Ridge and the Mid-Indian Ridge where there i the tronget eimic activity and two inactive volcanic ridge : the Chago-Laccadive Plateau and the Ninetyeat Ridge. The large Kerguelen Plateau ha alo ome regional eimic activity. Figure 2 : Ocean urface temperature (from Gro et al., 1996). In the Indian Ocean, band of equal temperature run eat-wet. Water temperature are the highet along the equator, becaue there Earth i mot warmed by olar radiation, and become cooler toward Antarctica. Note the preence of a cold water front at the latitude 40. Figure 3 : Ocean Indian ound peed variation. The Sofar channel axi depth decreae from North (blue profile) to South (black profile) to reach the ea urface. Note a trong ound peed variation on the Sofar channel axi, from about 1500 m/ in the North to 1455 m/ in the South. 3

4 Tranmiion lo etimation from modeling Our trategy After an underwater exploion occur, only a portion of energy i trapped in the Sofar channel (Pierchia et al, 1998) allowing propagation of wave over large ditance without any reflection loe at ea flour and ea urface. A we will ee in thi paper, the trapped energy flux depend on the Sofar channel characteritic, the ource depth and the ource frequency. It i well known, that the tranmiion lo of thi energy i mainly due to the geometrical attenuation, blockage effect and aborption. To invetigate the role played by the ource and Sofar characteritic a well a the blockage effect, the baic tranmiion lo formulation (TL in db ref. 1 m): Lo (db re 1 m) Depth (m) Flat bottom Range (km) Lo (db re 1 m) Depth (m) Sea mount Range (km) Figure 4 : Two cae of underwater propagation uing the profile n 2. A 10 Hz iotropic ource i located on the Sofar channel axi at 800 m in a flat bottom ocean in the firt cae and in front of a ea mount in the econd cae. Thee two color image of the tranmiion lo have been computed by the 4

5 code RAMS. In the firt cae, the tranmiion lo (in db ref. 1 m) can be approximated by TL = log(r) wherea in the econd cae we obtain TL = log(r) TL f, z, r) = A ( f, z ) + 10 log( r) + B( f, z ) (1) ( o a been conidered. The ource may vary with depth and frequency and the receiver i alway conidered to be on the Sofar channel axi. f, z, r are repectively the frequency (in Hertz), the depth (in meter), the ditance (in meter) of the underwater ource. A0 repreent the trapped energy at 1m in Sofar channel. A0 decreae when the trapped energy increae. The econd term 10 log(r) repreent the attenuation due to the cylindrical preading. The econd parameter B repreent additional attenuation due to blockage effect. In thi preliminary tudy, A0 and B were numerically etimated in the three previou region we defined. The code RAMS i ued to olve the Helmoltz equation uing the parabolic equation approximation in the frequency domain (Collin, 1993). In a firt tep, we computed the tranmiion lo in the three region. Then, in a econd tep, A0 and B are computed minimizing the difference between the numerical etimation and our formulation (1). Numerical reult Table 1 and 2 repreent the numerical etimation of A0 in two region located in the north of the cold water front. In thee two region, A0 i evaluated with a tandard deviation error lower than 5 db. In both cae, when the ource i on the Sofar channel axi repectively at 1500 m (profile n 1) and 800 m (profile n 2), A0 reache a minimum equal to 38 db and 35 db. We re-find the well known property that the tranmiion lo i minimum when the ource i on the Sofar Channel axi. When the ource goe cloer to the Sofar channel limit near the urface (at 100 m) or the ea bottom (at 5000 m), A0 and therefore the tranmiion lo can increae up to 13 db at 10 Hz. Thi additional lo i all the more important that the frequency i low. In the frequency range 5 to 15 Hz, thi additional lo i alway higher than 10 db. An under evaluation of 10 db of the level would lead to an under charge weight etimation of a factor 10. Table 3 give the reult we obtained in the Antarctic Ocean. In thi region, becaue the minimum ound peed zone i at the ea urface, there are reflection loe. A0 mut be interpreted cautiouly becaue our modeling do not take into account of thee loe. Variation A0 of with the ource location and frequency Profile n 1 Z=100 m Z=500 m Z=800 m Z=1500 m Z=5000 m 5 Hz 53 db 42 db 41 db 38 db 57 db 10 Hz 51 db 42 db 41 db 38 db 49 db 15 Hz 50 db 42 db 41 db 38 db 48 db Tab. 1 : A 0 etimation in the region n 1 at the North of the Indian Ocean. The minimum ound peed zone i cloe to 1500 m. Profile n 2 Z=100 m Z=500 m Z=800 m Z=1500 m Z=5000 m 5 Hz 52 db 39 db 35 db 39 db 57 db 10 Hz 48 db 39 db 35 db 40 db 49 db 15 Hz 46 db 39 db 36 db 41 db 48 db Tab. 2 : A 0 etimation in the region n 2 at the South of the Indian Ocean. The minimum ound peed zone i cloe to 800 m. 5

6 Profile n 3 Z=100 m Z=500 m Z=800 m Z=1500 m Z=5000 m 5 Hz 45 db 35 db 35 db 38 db 53 db 10 Hz 38 db 33 db 38 db 44 db 48 db 15 Hz 35 db 37 db 40 db 39 db 47 db Tab. 3 : A 0 etimation in the region n 3 in the Antarctic Ocean. Thee reult demontrate that it i important to know the variation of the tranmiion lo with the ource depth variation in each region of the Indian Ocean. They have been obtain conidering a flat bottom ocean. A it ha been aid in a previou part the Indian Ocean bain ha five important ridge that could generate hydroacoutic wave blockage effect. A an illutration of thee effect, Figure 4, give two cae of underwater propagation uing the profile n 2. A 10 Hz iotropic ource i located on the Sofar channel axi in a flat ocean bottom in the firt cae and in a front of a ea mount in the econd cae. In the firt example, the tranmiion lo can be approximated by TL = log(r) wherea in the econd example we obtain TL = log(r) The additional increaed of 15 db i due the blockage effect of the ea mount. Figure 5 illutrate an other example of underwater blockage due to a ea mount. The tranmiion lo after the ea mount i compared with the tranmiion lo in a flat ocean ea bottom. In thi example, it i hown that the addition lo increae with the frequency decayed from 5 db at 5 Hz to 15 db at 15 Hz. An under etimation of 15 db of the ource level could lead to an under charge weight evaluation of a factor 30. Figure 5 : Blockage effect due to an underwater ea mount. The ource i located in front of the ea mount on the Sofar channel axi. The figure on the right repreent the tranmiion lo variation with range and frequency behind the ea mount. 6

7 Dicuion We invetigated numerically the number of record point and ource localization which are neceary to provide a good etimation of A0 and B. In thi paper, the tranmiion lo wa computed every 500 m by the code RAMS. Then in a econd tep, A0 wa evaluated to minimize the difference between the numerical etimation and our formulation (1). In a numerical parameterized tudy, we found that a ditance between point maller than 400 km and more than three point of record are needed to inure an acceptable error lower than 5 db on the A0 calculation. Concluion and Recommendation : A it ha been recommended at the Tahiti Hydroacoutic Workhop (organized by CTBTO and CEA/DASE in September 1999), we believe that it i neceary to calibrate the hydroacoutic network of the International Monitoring Sytem in the Indian Ocean. Thi tudy demontrate that the tranmiion lo may vary with the ource depth and from one region of the Indian Ocean to an other. Thi i the reaon why we believe that the tranmiion lo hould be tudied carefully in particular to validate model calculation ued to etimated ource level. During the definition of the calibration experiment, we recommend to alo invetigate ource geometry to analyze tranmiion lo phenomena and blockage effect. Key Word : T-phae, hydroacoutic, Indian Ocean propagation, tranmiion lo Reference : Collin, M.D., A higher-order energy-conerving parabolic equation for range-dependent ocean depth, ound peed, and denity, J. Acout. Soc. Am., 101, , CTBTO/PTS, CEA/DASE, Informal workhop on hydroacoutic, French Polyneia Tahiti, Conference proceeding, September Gro, M. G., Gro, E.. Oceaography, eventh edition, NJ : Prentice Hall, Levitu, S., Boyer, T. Antonov, J., Burgett, R., and Conkright, M., World Ocean Atla 1994, NOAA/NESDIS, Silver Spring, Maryland, Pierchia, P.-F, Rodrigue, D., Virieux J., Gaffet S., Detection of underwater exploion from very long range record, Ocean 98, Nice, Conference proceeding, vol. 2, ,