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Advantages of Parametric Acoustics for the Detection of the Dredging Level in Areas with Siltation Jens Wunderlich, Prof

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Advantages of Parametric Acoustics for the Detection of the Dredging Level in Areas with Siltation Jens Wunderlich, Prof 7th Workshop on Dredging and Surveying, Scheveningen (The Haag), June 07th-08th 2001 Advantages of parametric acoustics for the detection of the dredging level in areas with siltation Jens Wunderlich, Prof. Dr. Gert Wendt, Rostock University Keywords sediment echo sounder, parametric acoustics, dredging level, siltation, Abstract Dredging companies and local authorities are interested in exact information about the dredging area and the material conditions, like sediment structures, sediment types and sediment volumes especially in ship channels and harbours. To get these information several equipment for sediment echo sounding is used. But what are the differences? What are the advantages of non- linear echo sounders compared to linear ones, especially in dredging areas with siltation? These questions are briefly discussed in this contribution. After that some survey examples, using a parametric echo sounder system, are given. Echo Sounding and Echo Sounder Parameters Sediment echo sounding means v = const transmitting sound pulses in direction ∆h water surface to the bottom and receiving reflected y x signals from the bottom and sediment a x b layers, see fig. 1. z ∆Θ,∆Φ There are some important parameters noise reverberation to classify echo sounder systems: Θ • Directivity h • Frequency • Pulse length, pulse ringing ∆τ bottom echo • Pulse repetition rate • Beam steering and stabilizing bottom surface • Heave compensation objects • Real time signal processing l • Size, weight, mobility a,c = f(material) layer echo These parameters and their meanings for surveys shall be discussed briefly. layer borders Figure 1: Echo sounding principle Directivity The directivity of the transmitted sound beam depends on the transducer dimensions related to the sound frequency. There is a main lobe with the beam width Θ and side lobes. The sound at the border area of the main beam has a longer travel time than the sound in the centre of the beam. This makes the reflected signal longer than the transmitted signal. Particularly in deep- water areas refraction due to changes of the sound velocity may enlarge the sounded bottom area and that makes the echo signal longer, too. Longer signals will mean less vertical resolution. The horizontal resolution depends on the area that is hit by the sound beam. A great beam width means bad horizontal resolution and diffraction hyperboles occurs at small structures. A great beam width will also mean greater surface and volume reverberation. Another problem are side lobes that cause echoes from outside of the desired direction. To get echo prints with high spatial resolution you need directivity with small beam width and without side lobes. page 1 of 8 7th Workshop on Dredging and Surveying, Scheveningen (The Haag), June 07th-08th 2001 Frequency, pulse length, pulse ringing and pulse repetition rate The attenuation of sound inside the sediment layers is most important to the penetration depth that can be expected. The attenuation coefficient is proportional to the frequency and depends on type and structure of the sediment. For greater penetration depths lower frequencies should be used. On the other hand the vertical resolution depends on the pulse length of the transmitted signal. Shorter pulses will result in better resolution, which could mean using high frequencies. For different survey tasks different frequencies may be optimal, the echo sounding equipment therefore should be cover a wide frequency range. Short pulses can be transmitted only by systems with a high frequency bandwidth. The ringing effect at the end of the transmitted pulse depends on the transducer bandwidth. The ringing time should be as short as possible for echo sounding in shallow water. For safely detection of small sediment structures or buried objects the bottom area must be hit several times. Therefore the pulse repetition rate should be as high as possible. Beam steering and stabilizing, heave compensation Caused by rough sea the transducer may move during the survey in 6 directions. The most important unwanted motions are roll, pitch and heave. They should be compensated, if high- resolution echo prints are required. Therefore beam stabilizing should be possible, especially at greater water depths. Heave compensation is useful at shallow water areas, too. Fig. 2 illustrates the difference between echo prints with and without heave compensation. At slopes it is useful to direct the beam perpendicular to the bottom surface to make sure to get the best penetration. For that reason beam steering should be possible. Beam steering may also be used for widening the search area without losses in the horizontal resolution. Figure 2: Heave compensation example (left without; right with compensation) Real time signal processing and size, weight, mobility The echo prints should be calculated in real-time to get first survey results immediately. Thus makes a powerful real-time signal processing necessary. If you need mobile equipment to use on small, maybe different ships, the size and weight of the echo sounder system and in particular of the transducer will be important. The ideal echo sounder The ideal echo sounder for surveying should have • Small beam width and no side lobes • Wide frequency range • Short sound pulses without ringing • High pulse repetition rate • Beam steering and stabilizing as well as heave compensation possible • Echo prints calculated in real time • High mobility and therefore small and light equipment Some of these desires are only to fulfil using non-linear acoustics. page 2 of 8 7th Workshop on Dredging and Surveying, Scheveningen (The Haag), June 07th-08th 2001 Linear and Non-linear Acoustics Linear echo sounders generate the sound pulse of the desired frequency directly. The directi- vity depends on the ratio of the transducer dimension and the signal frequency. Therefore a good directivity at low frequencies requires large transducers. But such transducers are heavy and expensive. Parametric echo sounders transmit two signals of slightly different high frequencies at high sound pressures (primary frequencies f1 and f2). Because of non-linearities in the sound propagation at high pressures both signals interact and new frequencies are arising, see fig. 3. The difference frequency (∆f = f2 - f1) is low and penetrates the sea bottom. The primary frequencies may be used for exact deter- mination of water depth even in difficult situations, e.g. soft sediments. Figure 3: Non-linear acoustics principle Figure 4: Linear (left) and parametric transducer array (right) Fig. 4 shows a linear and a parametric transducer array. The linear transducer has an active sound area of approximately 0.7 m × 0.7 m and weights approx. 350 kg. The parametric transducer with comparable directivity has an active sound area of approximately 0.2 m × 0.2 m and weights approx. 15 kg. 1 1 4 kHz 100 kHz 0.9 8 kHz 0.9 4 kHz 0.8 12 kHz 0.8 8 kHz 0.7 0.7 12 kHz 0.6 0.6 0.5 0.5 0.4 0.4 Amplitude 0.3 Amplitude 0.3 0.2 0.2 0.1 0.1 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 Angle [°] Angle [°] Figure 5: Directivity pattern for linear (left; computed) and parametric transducer (right) Using parametric sound generation, the directivity for the difference frequency is similar to the primary frequency directivity and is nearly the same for different secondary frequencies. This is important if echo prints from different frequencies should be comparable, so for the calculation of sediment attenuation using multi-frequency signals. page 3 of 8 7th Workshop on Dredging and Surveying, Scheveningen (The Haag), June 07th-08th 2001 Fig. 5 shows some experimental data from a parametric transducer array with an active sound area of about 0.2 m × 0.2 m. All difference frequencies between 4 and 12 kHz (ratio 1:3) have nearly the same half power beam width as the primary frequency of about 100 kHz. The left side of fig. 5 shows the computed directivity of a linear transducer that is 10 times greater. In this case different radiated frequencies have different half power beam widths. Therefore the sounded area will be not the same at different frequencies and echo prints cannot be compared. 2 primary frequency secondary frequency 1.5 1 Amplitude 0.5 0 0 1 2 3 4 5 6 7 8 9 10 z / z0 Figure 6: Sound pressure at the beam axis (computed; circular piston transducer) The general sound pressure distribution using non-linear acoustics along the acoustical axis of a circular piston transducer is shown in fig. 6. There are no local minima or maxima in the near- field area for the difference frequency. The distance between the near field and the far field is longer for the parametric signal (approx. 5-10 times near-field length) than for the linear signal (approx. 3 times near-field length). The maximum sound pressure and the sound propagation depends on the near-field length of the primary frequency and the primary to difference frequency ratio. Due to the interaction length of the parametric transmitter, the decrease of the sound caused by geometrical losses and physical attenuation will be lower compared to linear echo sounders. Advantages of Parametric Systems for Surveying The properties of parametric systems described above causes a lot of advantages in surveying compared to linear echo sounders. Parametric systems have small beam width for the transmitted low frequent signal in spite of small transducer dimensions. The beam width only depends on the primary frequency related to the transducer aperture, even for the secondary frequency. There are no significant side lobes for the difference frequency and you will get a constant directivity for different secondary frequencies (fig. 5). Therefore the footprint is the same for different frequencies and you will get comparable results.
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