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Owing to the decomposition of organic material and other reasons, the actual marine sediment contains gas bubbles, and the existence of gas bubbles will significantly affect the low-frequency acoustic characteristics of sediment. Therefore, it is significant to investigate the effect of gas bubbles on the low-frequency sound velocity in the sediment. Considering the uncontrollable environmental factors of field experiment, an experiment platform for obtaining acoustic characteristics of a large-scale gas-bearing unsaturated sandy sediment is constructed in the indoor water tank. Considering the long wavelength of low-frequency acoustic wave and the multipath interference in water tank, the transmitted acoustic signals are received by hydrophones which are buried in the unsaturated sediment. The sound velocity data (79-142 m/s) in the gas-bearing unsaturated sediment are acquired by using a multi-hydrophone inversion method in the bounded space for the first time in a 300-3000 Hz range, and the sound velocity data (112-121 m/s) are also acquired by using a double-hydrophone method in the same frequency range. The refraction experiments at different horizontal distances between the source and the hydrophones are conducted, which verifies the reliability of sound velocity data acquired by using the multi-hydrophone inversion method and the double-hydrophone method. At the acoustic frequency well below the resonance frequency of the largest bubble in the sediment, the pore water and the gas bubbles are regarded as an effective uniform fluid based on effective medium theory. On this basis, the density and the bulk elastic modulus of pore water in the effective density fluid model are replaced by the effective density and the effective bulk modulus of the effective uniform fluid, then a corrected effective density fluid model is proposed in gas-bearing unsaturated sediment. The numerical analysis indicates that when the gas bubble volume fraction is small (1%), a small increase in the gas bubble will cause a significant decrease in the effective bulk elastic modulus of sediment, but the density of pore water is much greater than the density of gas bubbles, the presence of a small number of gas bubbles hardly changes the density of pore fluid and certainly does not change the density of sediment, which results in a significant decrease at a low-frequency sound velocity in the gas-bearing unsaturated sediment. Furthermore, with the increase of gas bubble volume fraction, the sound velocity predicted by the corrected model gradually decreases, and the decreasing trend gradually becomes gentle. The corrected model reveals the effect of gas bubbles on the low-frequency acoustic characteristic of sediment. By analyzing the sensitivity of the predicted sound velocity to parameters of the model, the gas bubble volume fractions (1.07%-2.81%) of different areas are acquired by inversion according to the measured sound velocity distribution and the corrected model. In the future, it will provide a new method of obtaining the volume fraction and the distribution of gas bubbles in the sediment.
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Keywords:
- gas bubble volume fraction /
- sediment /
- low frequency /
- acoustic characteristic
[1] Turgut A, Yamamoto T 1990 J. Acoust. Soc. Am. 87 2376
[2] Maguer A, Bovio E, Fox W L J, Schmidt H 2000 J. Acoust. Soc. Am. 108 987
[3] Rosenfeld I, Carey W M, Cable P G, Siegmann W L 2001 IEEE J. Ocean. Eng. 26 809
[4] Stoll R D 2002 J. Acoust. Soc. Am. 111 785
[5] Williams K L, Jackson D R, Thorsos E I, Tang D J, Schock S G 2002 IEEE J. Ocean. Eng. 27 413
[6] Chotiros N P, Lyons A P, Osler J, Pace N G 2002 J. Acoust. Soc. Am. 112 1831
[7] Wilson P S, Reed A H, Wilbur J C, Roy R A 2007 J. Acoust. Soc. Am. 121 824
[8] Biot M A 1956 J. Acoust. Soc. Am. 28 168
[9] Biot M A 1956 J. Acoust. Soc. Am. 28 179
[10] Stoll R D, Kan T K 1981 J. Acoust. Soc. Am. 70 149
[11] Williams K L 2001 J. Acoust. Soc. Am. 110 2276
[12] Kimura M 2006 J. Acoust. Soc. Am. 120 699
[13] Kimura M 2008 J. Acoust. Soc. Am. 123 2542
[14] Anderson A L, Hampton L D 1980 J. Acoust. Soc. Am. 67 1865
[15] Anderson A L, Hampton L D 1980 J. Acoust. Soc. Am. 67 1890
[16] Lee K M, Ballard M S, Muir T G 2015 J. Acoust. Soc. Am. 138 1886
[17] Li H X, Tao C H, Lin F L, Zhou J P 2015 Acta Phys.Sin. 64 109101 (in Chinese) [李红星, 陶春辉, 刘富林, 周建平 2015 64 109101]
[18] Tth Z, Spiess V, Mogolln J M, Jensen J B 2014 J. Geophys. Res. Solid Earth 119 8577
[19] Ecker C, Dvorkin J, Nur A M 2000 Geophysics 65 565
[20] Ghosh R, Sain K, Ojha M 2010 Mar. Geophys. Res. 31 29
[21] Wilson P S, Reed A H, Wood W T, Roy R A 2008 J. Acoust. Soc. Am. 123 EL99
[22] Mavko G, Mukerji T, Dvorkin J 1998 The Rock Physics Handbook (New York:Cambridge University Press) pp110-112
[23] Wilkens R H, Richardson M D 1998 Cont. Shelf Res. 18 1859
[24] Schock S G 2004 IEEE J. Ocean. Eng. 29 1200
[25] Hovem J M, Ingram G D 1979 J. Acoust. Soc. Am. 66 1807
[26] Zheng G Y, Huang Y W, Hua J, Xu X Y, Wang F 2017 J. Acoust. Soc. Am. 141 EL32
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[1] Turgut A, Yamamoto T 1990 J. Acoust. Soc. Am. 87 2376
[2] Maguer A, Bovio E, Fox W L J, Schmidt H 2000 J. Acoust. Soc. Am. 108 987
[3] Rosenfeld I, Carey W M, Cable P G, Siegmann W L 2001 IEEE J. Ocean. Eng. 26 809
[4] Stoll R D 2002 J. Acoust. Soc. Am. 111 785
[5] Williams K L, Jackson D R, Thorsos E I, Tang D J, Schock S G 2002 IEEE J. Ocean. Eng. 27 413
[6] Chotiros N P, Lyons A P, Osler J, Pace N G 2002 J. Acoust. Soc. Am. 112 1831
[7] Wilson P S, Reed A H, Wilbur J C, Roy R A 2007 J. Acoust. Soc. Am. 121 824
[8] Biot M A 1956 J. Acoust. Soc. Am. 28 168
[9] Biot M A 1956 J. Acoust. Soc. Am. 28 179
[10] Stoll R D, Kan T K 1981 J. Acoust. Soc. Am. 70 149
[11] Williams K L 2001 J. Acoust. Soc. Am. 110 2276
[12] Kimura M 2006 J. Acoust. Soc. Am. 120 699
[13] Kimura M 2008 J. Acoust. Soc. Am. 123 2542
[14] Anderson A L, Hampton L D 1980 J. Acoust. Soc. Am. 67 1865
[15] Anderson A L, Hampton L D 1980 J. Acoust. Soc. Am. 67 1890
[16] Lee K M, Ballard M S, Muir T G 2015 J. Acoust. Soc. Am. 138 1886
[17] Li H X, Tao C H, Lin F L, Zhou J P 2015 Acta Phys.Sin. 64 109101 (in Chinese) [李红星, 陶春辉, 刘富林, 周建平 2015 64 109101]
[18] Tth Z, Spiess V, Mogolln J M, Jensen J B 2014 J. Geophys. Res. Solid Earth 119 8577
[19] Ecker C, Dvorkin J, Nur A M 2000 Geophysics 65 565
[20] Ghosh R, Sain K, Ojha M 2010 Mar. Geophys. Res. 31 29
[21] Wilson P S, Reed A H, Wood W T, Roy R A 2008 J. Acoust. Soc. Am. 123 EL99
[22] Mavko G, Mukerji T, Dvorkin J 1998 The Rock Physics Handbook (New York:Cambridge University Press) pp110-112
[23] Wilkens R H, Richardson M D 1998 Cont. Shelf Res. 18 1859
[24] Schock S G 2004 IEEE J. Ocean. Eng. 29 1200
[25] Hovem J M, Ingram G D 1979 J. Acoust. Soc. Am. 66 1807
[26] Zheng G Y, Huang Y W, Hua J, Xu X Y, Wang F 2017 J. Acoust. Soc. Am. 141 EL32
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