OPTIMISATION OF TRANSMISSION PREDICTIONS FOR A SONAR PERFORMANCE MODEL FOR SHALLOW OCEAN REGIONS

Transcription

1 OPTIMISATION OF TRANSMISSION PREDICTIONS FOR A SONAR PERFORMANCE MODEL FOR SHALLOW OCEAN REGIONS Adrian D. Jones*, Janice S. Sendt, Z. Yong Zhang*, Paul A. Clarke* and Jarrad R. Exelby* *Maritime Oerations Division, Defence Science and Technology Organisation Australia, South Australia 5111 adrian.jones@dsto.defence.gov.au Thales Underwater Systems Australia, New South Wales 2116 Janice.Sendt@au.thalesgrou.com 1 INTRODUCTION The Maritime Oerations Division (MOD) of DSTO has been conducting research directed at assessment and imrovement of sonar erformance rediction tools for range deendent ocean environments. In conjunction with this work, MOD has on-going rogrammes of research on benchmark testing of acoustic models and on the acoustic roerties of the seafloor in shallow ocean regions. In a related activity, MOD has been conducting research into techniques for the raid determination of seafloor roerties and, with Thales Underwater Systems (TUS), has been investigating the otential alication to sonar range rediction models for undersea warfare (USW) and anti-surface warfare (ASuW) alications. MOD has a considerable body of at-sea data to aly to the assessment of candidate models, with transmission loss (TL) and, in some cases, accomanying high resolution bathymetry for a number of sites. This aer describes recent rogress in the assessment of transmission redictions obtained by a develomental sonar range rediction model for ocean sites corresonding to MOD s holdings of TL data, and resents comarisons of measured TL, range-deendent TL redictions based on a seafloor database, and redictions of rangedeendent TL based on seafloor roerties inferred by MOD s raid assessment technique inut to the RAM 1 (Range-deendent Acoustic Model) acoustic roagation model. In the last-mentioned case, an MOD algorithm has been used to derive equivalent fluid seabed arameters for the RAM model from the in-situ data. In earlier work, comarison between at-sea measurements and redictions of TL had been reorted for locations known as site #2 2,3,4 and site #3 3. This aer reorts a comarison for a transmission track at a different ocean site, known for resent uroses as site AE. 1.1 Sonar Performance Prediction Model Ideally, a sonar erformance rediction model used for USW and ASuW will rovide detection range redictions for sonar sensor systems for realistic ocean environments located at a user defined latitude and longitude. Tyically, it achieves its urose by accessing aroriate internal global databases and sulying the necessary arameters to run its

2 acoustic models. These databases often include bathymetry, wave height, wind seed, sound seed, sediment thickness and sediment roerties. This aer addresses some recent advances in a joint DSTO/TUS Pty task for assessment of candidate acoustic transmission models and databases. Recent work has been three-fold: (i) benchmarking range deendent low-frequency acoustic transmission loss models; (ii) investigating databases of seafloor roerties and (iii) investigating the inclusion of in-situ assessment techniques. Asects of this assessment which are addressed in this aer include the comarison of transmission redictions based on an historical database of seafloor roerties with transmission data which was measured at a shallow water site. Also, an assessment is made of the otential for a MOD in-situ technique to infer seafloor reflectivity at shallow grazing angles and rovide inut to the transmission model for regions in which existing holdings of seafloor roerties are sarse. 1.2 In Situ Determination of Seafloor Reflectivity The MOD technique for seafloor roerties inversion, using sectral variability of short-range transmission in shallow water, has been described elsewhere 5, but is reviewed briefly below. The MOD technique is based on the receit of broadband signals at medium ranges (r = 2 to 4 km aroximately), and the summation of all multi-ath arrivals at a single receiver. As exlained by Jones and Bartel 6, the rate of variability of the received signal amlitude, as a function of frequency, is related to the geometry of the transmission situation, the seed of sound in the ocean, and the seafloor bottom loss. In articular, the frequency scale of transmission amlitude variability may be linked to an assumed function of seafloor bottom loss. By inverting the relationshi, the characteristics of the seafloor bottom loss versus grazing angle function may be estimated to an accuracy sufficient for sonar rediction models. The technique assumes multi-ath transmission by straight ray-aths as occur with an isovelocity ocean. MOD has, however, established that for the short measurement ranges used, any ocean refractive effects have a minimal imact on the function of the technique. For this work, it has been assumed that the bottom loss (in db) versus grazing angle function is linear for shallow grazing angles these being of greatest significance for shallow water transmission. This assumtion has been found to be reasonable, based on simulations carried out at MOD (also see, eg. equation 5.7 in Etter 7 ). In turn, this has led to a direct relationshi between the assumed bottom loss versus grazing angle function β db radian and the sectral variability arameter f h. As exlained elsewhere 6, f h is defined as the frequency dislacement at which the normalised autocorrelation of the amlitude of the sound channel frequency resonse, ρ ( f ), falls to 0.5. This normalised autocorrelation is carried out as ( f ) ( f + f ) ( f ) ρ ( f ) = (1) 2 ( f ) ( f ) 2 where equation (1) imlies that the autocorrelation is carried out on the zero-mean sound f f. For ractical imlementations of the ressure modulus, that is, on ( ) ( ) technique, either an imulsive transient or swet tone signal source may be used. 2

3 2 SITE DESCRIPTION AND DATA The data resented in this aer were obtained by MOD in a shallow ocean region. For resent uroses, the transmission track is known as AE. The on-site trial activity included shi-based deloyment of Mk 64 SUS (Signals, Underwater Sound) charges, with received sonar signals being recorded on modified Mk 41B sonobuoy receivers. Both the signal sources and sonobuoy receivers were deloyed at 18 m deth. On-site activity included the collection of echo-sounder bathymetry data at aroximately every 1 km this showed the track to have a near uniform deth of about 58 m. Other data available from the site include Sound Seed Profile (SSP) at start-of-track and end-of-track. The SSPs at the start and at the end of the track AE are shown in Figure 1. These SSPs are indicative of a strong downward gradient (downward refraction) for which significant acoustic interaction with the seafloor is exected. Further, as source and receiver were at 18 m, these rofiles indicate that the surface ducting conditions, which are strongly linked to high frequency transmission henomena, were likely to be highly variable along the track, and that in retrosect, the two sets of sound seed data are insufficient to describe the rangedeendent refraction conditions which clearly existed Start SSP End SSP Deth (m) Figure 1: Sound seed (m/s) Sound Seed Profiles at start and end of Track AE 3 TRANSMISSION LOSS DATA AT SHALLOW SITE In order to achieve an otimum sonar erformance rediction for shallow water sites, it is necessary to describe the seafloor with a high degree of accuracy. At most sites however, detailed surveying of the seafloor geoacoustic roerties has not been carried out, and the best available historical data is based on geohysical data in the form of surface grab samles, a much lesser number of seafloor drilled cores, and sediment thickness data based on measurements or estimates. A seafloor database, based on such available data, was used in this study. At some sites, MOD has obtained measurements of received signal transmission, and at some of these sites inferences of seafloor geoacoustic roerties have been made based on analysis of head-wave data. By such techniques, comressional sound seed and thickness of a surficial sediment layer may be made, and deending on the data in question, further

4 layers may be resolved and/or basement comressional sound seed may be obtained. Further, by re-rocessing short-range SUS charge data, using the unique MOD technique discussed in section 1.2, seafloor secular reflectivity may be obtained directly. The otential value of this latter technique as an adjunct to a sonar range rediction model is under resent investigation. Progress in this work is illustrated below. 3.1 Historical Descrition of Seafloor at Site For track AE, the seafloor database has the descrition for the sediment as shown in Table 1. The sediment at the site is described as sand-silt-clay. This descrition, and the values within Table 1, are consistent with the mean grain size φ = 6.4 as determined from a sediment samle at the start of the track. Table 1: Seafloor Parameters for Track AE Comressional sound seed c 15 m s Comressional attenuation α db λ Density ρ 16 kg Sediment thickness 259 m Transmission loss data redicted using the RAM model, and using the above inut arameters, will be resented in section Seafloor Proerties via MOD In-Situ Technique The MOD in-situ technique is based on the determination of the frequency variability of transmission as observed by use of a broadband signal source, as caused by the multi-ath combination of significant arrivals. For this work an imulse source is ideal and a Mk 64 SUS is a suitable aroximation of such a source. The signal which was used for this study was that of a single Mk 64 SUS received at 2.2 km range, as shown in Figure 2. The derived broadband sectrum is shown in Figure 3. Note that this has been based on the ms of data which includes the initial arrivals and recedes the section of the signal due to the bubble ulses. The bubble-ulse section of the SUS signal has been excluded from this analysis as it generated sectral comonents associated with the bubble harmonics and its inclusion would dominate the true oceanic transfer function, which Figure 3 aroximates. Note, also, that as 2 the time searation between arrivals is aroximately t = 4nD ( rc) seconds (at range r in an isovelocity ocean of deth D and sound seed c), where n is the order number of the arrival family and corresonds with the number of bottom bounces, the eriod of ms ermits inclusion of families of arrivals with u to 4 bottom bounces, sufficient for the oceanic transfer function to be defined. 3 m

5 Figure 2: Mk 64 SUS signal received at 2.2 km along Track AE Figure 3: Sectrum from first 0.04 s of Mk 64 SUS received 2.2 km along Track AE The data shown in Figure 3 were rocessed using equation (1) to determine the sectral variability arameter f h for each octave band of frequencies. Based on these values, the MOD inversion technique was used to derive the values of bottom loss versus grazing angle, as db/radian, for the different octave ranges. Derived values were: 6.57 db/radian for 125 Hz octave band, 6.52 db/radian for 2 Hz band and 8.52 db/radian for 0 Hz band. These values, together with an assumed relationshi for reflection hase angle, were used as inuts to an associated MOD rocess that yielded a set of arameters for a fluid seabed which had an equivalent acoustic effect at shallow grazing angles. These derived geoacoustic arameters were, in turn, used as inuts to the RAM model and used with ocean deth data and measured sound seed data to obtain long-range transmission loss values. The redictions of transmission loss so obtained are shown in Figures 4, 5 and 6.

6 Note that the MOD inversion technique is based on the assumtion that the ocean transmission channel is highly multi-modal, with resultant robust statistics from multi-ath, or multi-modal, signal combinations. Also, ractical imlementations of the technique do imly that there is sufficient variation in signal amlitude over the san of an octave in frequency for a value of f h to be extracted. In the case of the resent data, the number of modes of transmission at about 63 Hz is not exected to be large. Further, there is little variation of amlitude with frequency across the octave band centred at 63 Hz. As a result, a lower limit for alication of the MOD technique to the site AE is 125 Hz, and so no data at lower frequencies is resented in this aer. 3.3 Transmission Loss Data Transmission loss measurements obtained using the received Mk 64 SUS charge signals gathered during the MOD trial are given in Figures 4, 5 and 6 for one-third octave bands centred at 125 Hz, 2 Hz and 0 Hz, resectively. Three-way comarisons have been carried out between: (i) TL data measured by MOD at secified ranges; (ii) TL redicted using seafloor data from an historical database (labelled Seafloor Database ); (iii) TL redicted by MOD using the RAM model with the seafloor described as a fluid which is equivalent to the inverted seafloor reflectivity at shallow angles of incidence (bold dashed line in each of Figures 4, 5 and 6). The MOD redictions used the SSP at the start of the track; redictions based on the seafloor database used the start SSP to 15 km and the end SSP from 15 km to km. For ease of comarison with the measured one-third octave TL data, the MOD redictions were each obtained by averaging TL values obtained at 21 frequencies equisaced over each resective one-third octave. Here, the single frequency data were averaged using incoherent summation at m range intervals. Incoherent summation was used since, in the general sense, TL values at neighbouring frequencies cannot be combined coherently unless the sectral content and hase relationshi of the emitted signal is known for a articular set of data. The TL data based on the seafloor database are at the single frequency corresonding with the centre of each resective band. The comarisons of transmission loss values are shown in Figures 4, 5 and 6 for 125 Hz, 2 Hz and 0 Hz resectively. Seafloor Database 3.0 db/rad 6.57 db/rad 12.0 db/rad Figure 4: TL measured & redicted, Track AE range-deendent, 125 Hz

7 Seafloor Database 3.0 db/rad 6.52 db/rad 12.0 db/rad Figure 5: TL measured & redicted, Track AE range-deendent, 2 Hz Seafloor Database 3.0 db/rad 8.52 db/rad 12.0 db/rad Figure 6: TL measured & redicted, Track AE range-deendent, 0 Hz The data in Figures 4, 5 and 6 show very good agreement between measured transmission loss values and the data redicted using the MOD inverted seafloor reflectivity and do seem to imly that the MOD technique is valid for this site. The TL redictions based on the seafloor database are accurate at 125 Hz but slightly further from the measurement at 2 Hz (Figure 5) and 0 Hz (Figure 6). This generally good agreement is resumed due to the fact that a reasonable quantity of seafloor data for this site exists in the historical record. In order to illustrate the sensitivity of the derived TL data to any errors in the inversion of seafloor reflectivity, MOD carried out redictions of TL for seafloors about half as reflective (β = 3 db/radian), and twice as reflective (β = 12 db/radian), as that obtained by the inversion data for the site. These redictions are shown in Figures 4, 5 and 6. Clearly, at the lower frequencies of 125 Hz and 2 Hz, a correct descrition of seafloor reflectivity is critical to accurate TL redictions. To illustrate the increased relevance of the Sound Seed Profile for TL redictions at higher frequencies, Figure 7 indicates the sensitivity of redictions to a change in the SSP at 2 Hz. Here, the TL aroriate to a seafloor of 6.52 db/radian bottom loss is derived for an

8 isovelocity ocean, and for the standard MOD case (SSP corresonding with start-of-track). The redicted TL values in Figure 7 show that at 2 Hz, the recise nature of the SSP is indeed most relevant. Although not shown here, at 125 Hz the redicted TL is much less sensitive to SSP. In turn, this does suggest that the slight underrediction of TL at 2 Hz obtained using either the MOD-inverted reflectivity data, or the geoacoustic data obtained from the seafloor database, may be an artefact of the limited samling of the SSP with range along the transmission track (only two SSPs taken) db/rad 6.52 db/rad Isovelocity Figure 7: TL measured & redicted, Track AE range-deendent, 2 Hz 3.4 Transmission Loss Based on Head-wave - Derived Parameters The MOD at-sea measurements included the collection of Mk 64 SUS data at a large number of range values. These data were sufficient for an alternative analysis of the seafloor based on the receit of head-waves, and in this way seabed layering was determined in earlier work at MOD. The following data were estimated: comressional wave seed in a single seafloor layer; the thickness of this layer; comressional wave seed in the basement. The sediment density was estimated from the velocity-density relationshi of Richardson and Briggs 8. Values for attenuation in the sediment layer were determined using the emirical relations of Hamilton 9 for estimates of orosity obtained using the velocity-orosity relationshi of Richardson and Briggs 8. The basement density was estimated from velocity-density relations of Hamilton 10. The value for comressional attenuation in the basement was based on the work of Vasilev and Gurevich 11. The derived geoacoustic roerties are shown in Table 2. Table 2: Seafloor Geoacoustic Parameters for Track AE from Head-wave Analysis Layer comressional sound seed c 16 m s Layer comressional attenuation α db λ Layer density ρ 1956 kg Layer thickness Basement comressional sound seed Basement comressional attenuation 595 m c 63 m s α 0.2 db λ Basement density ρ 2610 kg 3 m 3 m

9 Predictions of TL obtained using the geoacoustic arameters shown in Table 2 are shown in Figures 8 and 9 for 125 Hz and 2 Hz resectively, along with the RAM redictions of TL based on the MOD-inverted seafloor reflectivity. Here, the former data were obtained from the RAM model run at the single frequency corresonding with the centre of each resective band. Clearly, at 125 Hz there is good agreement with measured TL, however, at 2 Hz the agreement is not as good as that obtained by use of the MOD inversion technique db/rad Using headwave data Figure 8: TL measured & redicted, Track AE range-deendent, 125 Hz 6.52 db/rad Using headwave data Figure 9: TL measured & redicted, Track AE range-deendent, 2 Hz 4 CONCLUSIONS Based on the data resented in this aer, it does aear that sonar erformance rediction models may be best emloyed in a way in which collated or gridded databases of the best available historical information are sulemented, judiciously, by the inut of additional quality data describing the seafloor reflectivity for the local region. Further, it does aear that the local Sound Seed Profile should be closely samled in range. For the shallow ocean site in question, it does aear that the seafloor database under investigation rovides a very good redictive caability. Further, the data resented do suggest that the MOD in-situ seafloor inversion technique is a viable adjunct to redictions of sonar range erformance obtained for shallow waters.

10 REFERENCES 1. M. D. Collins User s Guide for RAM Versions 1.0 and 1.0, anonymous ft@ram.nrl.navy.mil 2. A.D. Jones, P.A. Clarke, D.W. Bartel and J.S. Sendt, February 2000, Transmission Loss Inferred from a Seafloor Database Comarison with Data, Proceedings of UDT Pacific 2000, Darling Harbour, Australia 3. J. S. Sendt, A. D. Jones, P. A. Clarke and J. R. Exelby, November 2001, Inference of Inut Parameters for Range Deendent Transmission Models from Data, Proceedings of UDT Hawaii 2001, Hawaii 4. A. D. Jones, J. S. Sendt, P. A. Clarke and J. R. Exelby, 2002, Seafloor Data for Oerational Predictions of Transmission Loss in Shallow Ocean Areas, Acoustics Australia, Vol., No. 1, Aril 2002, A. D. Jones, D. W. Bartel, P. A. Clarke, and G. J. Day, February 2000, Acoustic Inversion for Seafloor Reflectivity in Shallow Water Environment, Proceedings of UDT Pacific 2000, Australia 6. A. D. Jones and D. W. Bartel, February 1998, Sectral Variability in a Shallow Water Environment, resented at UDT Pacific 98 Conference, Darling Harbour, Sydney 7 P. C. Etter, 1996, Underwater Acoustic Modeling Princiles, Techniques and Alications, 2 nd edition, E & FN Son 8. M. D. Richardson and K. B. Briggs, 1993, On the use of acoustic imedance values to determine sediment roerties, in Acoustic Classification and Maing of the Seabed, ed. N. G. Pace and D. N. Langhorne, Institute of Acoustics, University of Bath, E. L. Hamilton, 1972, Comressional-wave attenuation in marine sediments, Geohysics, 37, E. L. Hamilton, 1978, Sound velocity-density relations in sea-floor sediments and rocks, J. Acoust. Soc. Am., 63, Yu. I. Vasilev and G. I. Gurevich, 1962, On the ratio between attenuation decrements and roagation velocities of longitudinal and transverse waves, Bull. Acad. Sci. USSR, Ser. Geohys. (English Translation), No. 12,