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Hindawi Publishing Corporation Advances in Acoustics and Vibration Volume 2012, Article ID 532458, 5 pages doi:10.1155/2012/532458 Research Article Seabed Identiﬁcation and Characterization Using Sonar Henry M. Manik Department of Marine Science and Technology, Faculty of Fisheries and Marine Sciences, Bogor Agricultural University, Kampus IPB, Darmaga, Bogor 16880, Indonesia Correspondence should be addressed to Henry M. Manik, firstname.lastname@example.org Received 5 May 2012; Accepted 27 August 2012 Academic Editor: Joseph CS Lai Copyright © 2012 Henry M. Manik. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Application of sonar technologies to bottom acoustics study has made signiﬁcant advances over recent decades. The sonar systems evolved from the simple analog single-beam and single-frequency systems to more sophisticated digital ones. In this paper, a quantiﬁed sonar system was applied to detect and quantify the bottom echoes. The increasing of mean diameter is accompanied by a higher backscattering strength. From this study, identiﬁcation and characterization using sonar is possible. 1. Introduction 2. Method Sonar technologies are most eﬀective and useful for sea- 2.1. Sonar Equation for Bottom Scattering. The bottom pro- bottom exploration. They are based mainly on the measure- jection is illustrated in Figure 1. The elemental backscattered ment, process, analysis, and interpretation of the characteris- power registered by the transducer is given by tics of signal reﬂected or scattered by the sea bottom. Sonar is 2 2 −4 4 dP = P r exp(−4αr)D S dS, also increasingly regarded as the remote-sensing tool that will (1) RB 0 provide the basis for identiﬁcation, classifying, and mapping where dP is elemental backscattered pressure signal from ocean resources. RB There are extensive literatures on the acoustic scattering a sea bottom, P is source pressure level, r is range, α is from the sea bottom [1, 2]. The focus has been on low- absorption coeﬃcient, D is directivity functions, and S frequency features in application such as subbottom clas- is bottom scattering. The elemental area dS is located at siﬁcation . Another feature of the sea-bottom scattering incidence angle θ, azimuthal angle Ψ, and range r, such that has been experimentally observed at a high frequency where dS = r tanθdθdφ. the transmitter and receiver are not colocated . This (2) method received contributions both from the bottom surface and subbottom echoes. Most of the data were at grazing The echo pressure amplitude of sea bottom is obtained by ◦ ◦ angles between 5 and 60 , but some data were collected for integration of (1): ◦ ◦ the interval between 1 and normal incidence (90 ). They 2 −2 P = P r exp(−4αr)ΦS , (3) obtained results similar to those of Urick . 0 S RB One of the acoustic methods to obtain bottom scattering is to use a quantiﬁed sonar system (QSS). The QSS can where Φ is equivalent beam angle for surface scattering measure echoes generated by reﬂection and scattering of 2π θ sounding pulses from the bottom. The observed echo is (4) Φ = D tanθdθdφ. primarily due to scattering from the water-bottom interface. 0 θ 1 2 Advances in Acoustics and Vibration dθ rdθ r sin θdφ dS dφ dφ Figure 1: Principle of bottom surface scattering. Pre TVG Bottom echo ampliﬁer ampliﬁer computation RB Sea bottom Figure 2: Simpliﬁed block diagram of quantiﬁed sonar system (QSS). Table 1: Integration limits θ and θ for two cases. 1 2 Scattering plane θ θ 1 2 Circular plane −1 0cos = (R ≤ r< R + cτ/2) r R R Circular ring −1 −1 cos = cos = (r ≥ R + cτ/2) t − cτ/2 r where K = P MG . K is transmitting and receiving TR o R TR factor and therefore Figure 3: Quantiﬁed sonar system. RB S = . (7) −2 ( ) K r exp −4αr Φ TR The length of pulse in sea water is cτ, and its leading and In decibel unit SS = 10 log S . Simpliﬁed block diagram of trailing edges make angles θ and θ as presented in Table 1. S 1 2 QSS is shown in Figure 2. Thesignalisampliﬁedtogive E = P MG , (5) RB RB R 2.2. Quantiﬁed Sonar System. Quantiﬁed sonar system used in this research was PCFF80 model manufacturer by where E is echo amplitude at preampliﬁer output (V ), RB CruzPro, Ltd. (Figure 3). The PCFF80 is a full-featured M is receiving sensitivity of transducer (V /μPa), and G dual frequency (50 and 200 kHz), high-resolution personal- is preampliﬁer gain (numeric). Combining (3)and (5)we computer-based color ﬁsh ﬁnder that runs under windows obtain 98, NT, 2000, XP, Vista and Win7 in both analog and DSP 2 2 −2 E = K r exp(−4αr)ΦS , (6) mode (digital signal processing). Communications Interface RB TR Advances in Acoustics and Vibration 3 106.568656 106.582144 106.595632 106.609120 106.622608 106.568656 106.582144 106.595632 106.609120 106.622608 Figure 4: Research location and bottom sampling point. between transducer and PC was conducted using RS-232 serial data. For data acquisition, QSS installed on the research vessel. Echo voltages were recorded on hard disc drive. The QSS was operated at a ping rate of about 40 per minute with a pulse duration of 0.4 ms and beam width of 8.5 . Calibration of QSS is a fundamental component for ensuring high-quality acoustical data. For this purpose, the acoustic system was calibrated with a 38.1 mm diameter of tungsten carbide sphere. The sphere was suspended under the boat at 0.5 m depth to obtain the transmitting and receiving factor (K ). The target strength of the sphere at the TR given frequencies were calculated following Miyanohanna et al. and Aoyama et al.. Figure 5: Collection of sediment sample. By deﬁnition, the target strength is given by the ratio between reﬂected sound intensity, I , from a target and the sound intensity transmitted towards the target, I ,referred i which produce same echo integral. Ψ is deﬁned mathemati- to 1 m distance. This is regarded as identical to the ratio cally as between the backscattering cross section, σ, for the target π 2π and the surface of a sphere with a 1 m radius . The target Ψ = b θ, φ sin(θ)dθ dφ. (10) strength can be expressed on a decibel form in the following 0 φ=0 way: In logarithmic unit, equivalent beam angle deﬁned as EBA = 10 log(Ψ) which is expressed in dB relative to 1 steradian. I σ ( ) TS = 10 log = 10 log dB . (8) The speciﬁcations of the QSS and calibration results are I 4π presented in Table 2. For simplicity, target strength of sphere was computed using 2.3. Survey Area. An acoustic survey was conducted in conjunction with oceanographic, ﬁsheries biology, and exploratory ﬁshing in the Seribu Island, North Jakarta TS = 10 log (dB),(9) Indonesia (Figure 4). where a is radius of sphere. 2.4. Bottom Sample Collection. Collection of bottom samples The equivalent beam angle Ψ is the solid angle which was accomplished with a system consisting of a sediment is measured in steradian at the apex of ideal conical beam sampler (Figure 5). The bottom sampler was lowered to the −5.751520 −5.743360 −5.735200 −5.727040 −5.718880 −5.751520 −5.743360 −5.735200 −5.727040 −5.718880 4 Advances in Acoustics and Vibration (a) (b) Figure 6: Underwater photography of sand (a) and clay (b). Table 2: Speciﬁcation of QSS and calibration data. bottom surfaces by using a diver and entrapped the sediment. The total time of operation for one collection was about 30 Parameters Quantity minutes. Frequency (kHz) 200 Bottom samples were separated into size component Beam width (deg) 8.5 using sieve separation and pipette settling procedures. Equivalent beam angle (dB re 1 sr) −19.0 Bottom material characterization was based on analysis of particles size distributions conducted during the research. Band width (kHz) 4 Pulse duration (ms) 0.4 Target strength of standard sphere (dB) −39.1 3. Experimental Results Absorption coeﬃcient (dB/km) 45.5 Transmitting and receiving factor (dB) 50.5 The bottom materials of sand, silt, and clay were determined using observed physical characteristics of the samples and mean diameter were calculated (Table 3). Bottom images from sand and clay and echogram for each bottom type were Table 3: Classiﬁcation of bottom type by particle diameter. presented in Figures 6 and 7. Figure 8 shows the bottom backscattering value for three bottom types. Figure 9 shows Bottom Sample Mean Sand (%) Silt (%) Clay (%) type point diameter the example echo for sand bottom. 1 289 95 2 3 Sand 2 305 96 3 1 4. Discussion 3 292 94 3 3 The quantiﬁed sonar system is useful to measure bot- 445 7 90 3 Silt tom backscattering (SS). We had derived bottom volume 552 4 92 4 backscattering strength (SV)from SS bottom. In this study 649 3 93 4 area, the increasing of mean diameter is accompanied by 710 3 90 7 a higher backscattering strength. The SS of sand is higher Clay 8 9 5914 than silt and clay by more than 10 dB. To some extent, 911 4 90 6 it was possible to relate SS to mean diameter suggesting the possibility of bottom-type classiﬁcation and character- ization. Character of the seabed (sediment type, grain-size distribution, porosity, sediment density, sediment velocity, roughness, etc.) are embedded in the sonar echoes from the the sediment. Physically, silt and clay have a higher porosity seabed. than sand. Acoustic-bottom interaction is too complex to The main reason for the higher backscattering strength describe by only frequency and mean diameter. The bottom with larger particle size is that the porosity of sand sedi- relief also determines the acoustic echo from the seabed. ment decreases as the grain size increases. As the porosity Because sound may penetrate into the sediments and the decreases, the density increases (less pore water, more subbottom, the echoes can also contain information about mineral constituent). As the density increases, the sediment the zone below the water-sediment interface. Increasingly, impedance increases, thus allowing more scattering from a sonar technologies are being used in the future to detect, higher impedance contrast between the overlying water and identify, characterize, and classify the sea bottom. Advances in Acoustics and Vibration 5 SV or SS (dB) SV or SS (dB) 0 3 0 −10 −10 4 2 −20 −20 3 1 −30 −30 −40 −40 −1 −50 −50 −2 −60 −60 −3 −70 −70 −80 −80 0 100 200 0 100 200 Ping number Ping number (a) (b) Figure 7: Echogram of sand (a), and clay (b). −10 Acknowledgments The author would like to thank the Directorate General −20 of Higher Education Ministry of Education and Culture Indonesia and Bogor Agricultural University for the Gradu- ate Research Grant Program. Research members are thanked −30 for ﬁeld data acquisition. He would like to express his very great appreciation to the reviewer for his valuable and constructive suggestions to this paper. −40 0 1020304050 References Ping number Sand  R. J. Urick, Principles of Underwater Sound for Engineers, Silt McGraw-Hill, 1967. Clay  H. Medwin and C. S. Clay, Fundamentals of Acoustical Oceanog- Figure 8: Bottom backscattering of sand (), silt (), and clay ( ). raphy, San Diego, Calif, USA, 1998.  P. C. Hines and G. J. Heald, Seabed Classiﬁcation Using Normal Incidence Backscatter Measurement in the 1–10 kHz Frequency Band, DREA, Ottawa, Canada, 2001.  K. L. Williams and D. R. Jackson, “Bistatic bottom scattering: model, experiments, and model/data comparison,” Journal of the Acoustical Society of America, vol. 103, no. 1, pp. 169–181, 2 First echo Sea bottom  Y. Miyanohanna, K. Ishii, and M. Furusawa, “Spheres to calibrate echo sounders at any frequency,” Nippon Suisan Second echo Gakkaishi, vol. 59, pp. 933–942, 1993.  C. Aoyama, E. Hamada, and M. Furusawa, “Total performance check of quantitative echo sounders by using echoes from sea bottom,” Nippon Suisan Gakkaishi, vol. 65, no. 1, pp. 78–85,  E. J. Simmonds and N. D. MacLennan, Fisheries Acoustics: Theory and Practice, Blackwell Science, 2005. −120 −100 −80 −60 −40 −20 0 20 Bottom backscattering (dB) SS SV Figure 9: Sand-bottom echo for one ping transmission. 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Published: Nov 5, 2012
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