Dogan%20et%20al%20-%20Seabed%20and%20Sediment%20-%202015 (1).doc

At central frequencies of 26 30 khz noise events

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dominant noise events occurred. At central frequencies of 26-30 kHz, noise events occurred at 35 kHz or greater. The majority of the input frequencies used (30-100 kHz) has shown noise events at less than 10 kHz and greater than 300 kHz. Note that different receiver devices out of the selected receiver pairs may have noise distributed to different frequencies; however it was ensured that the major part of the ‘useful’ energy is retained by applying the same appropriate cut-off frequencies to both receiver signals. Fifth-order Butterworth band-pass filter were used, the band-width of which (varying for each insonifying frequency) was computed from power spectral density. Fig. 3 shows an example of filtered signals recorded at a pair of receivers. The compressional wave velocity of the sediment is evaluated performing time correlation of the signals. To determine the attenuation, the steady part of the signals are identified, then the attenuation is obtained by comparing the decrease Vol. 37. Pt.1 2015
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Proceedings of the Institute of Acoustics of wave amplitudes in these steady parts. 5 RESULTS In situ and laboratory transmission measurements were carried out using the same materials and methods for the intertidal site named Mercury (N 50° 52’ 56”, W 001°18’ 34). In situ attenuation results are shown in Figure 4. The solid line in Fig. 4a represents results from the first hydrophone pair (hydrophone 1 and 2) and the solid line in Fig. 4b from the second hydrophone pair (hydrophones 2 and 3). The first hydrophone pair measured an average P -wave sound speed of 1323 m/s and a standard deviation 110 m/s (not shown here). If a linear dependence of attenuation with frequency were to be fitted to these data, then the value of K P would equal to 0.32 dB/m/kHz. The second hydrophone pair measured an average sound speed of 1250 m/s with a standard deviation of 240 m/s. Assuming, again, a linear dependence of attenuation with frequency, the value of K P equals 1.1 dB/m/kHz. The values of K P serve as baselines for comparing attenuation trends between in situ measurements and previous literature. (a) (b) Figure 4: Attenuation profiles as a function of frequency: measured attenuation profiles (original data), processed data used in the inversion algorithm (filtered) and output of the inversion for bubble population (fitted) . a) Attenuation data from receiver pair 1-2 and b) attenuation data from receiver pair 2-3 . The inverted bubble populations from the Mercury site are shown in Fig. 5 for hydrophone pair 1-2 and 2-3. A void fraction of 0.66 % is predicted for the pair 1-2 and for hydrophone pair 2-3 a void fraction value of 0.17 %. The regularization parameter λ was determined via the L-curve as 0.0478 for the receiver pair 1-2 and as 2.33 10 -7 for receiver pair 2-3. Vol. 37. Pt.1 2015
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Proceedings of the Institute of Acoustics Figure 5: Bubble population results for the Mercury site inverted from receiver pairs 1-2 and 2-3.
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  • Fall '19
  • Frequency, Sediment, Hydrophone, Institute of Acoustics

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