In-situ Acoustic Properties Analyse Of The Sediment In HangZhou Bay And Surface Low-velocity Layer Study

Acoustic characteristics and physical characteristics of seabed sediment is one of the hot issues of the current international forefront of marine science and research. Especially in situ acoustic detection technology is the forefront of current international seabed sediment acoustic science. Domestic research also relatively less. The paper rely on national "863" plan "submarine in situ acoustics measurement system", has studyed in detail in-situ multi-frequency acoustic detection technology, data processing, acoustic inversion and numerical simulaition in the Hangzhou Bay, the new model application, low-speed inshore seabed sediments Formation factors and acoustic characteristics, the middle scale of Hangzhou Bay, the relations between acoustic properties and mechanical parameters.1,Deployed the sediment in-situ acoustic measurements, geological samples, deck testing such as marine data acquisition in Jintang waterways in Hangzhou Bay and Zhapu two areas and carried out in situ P-wave velocity measurements of gas-bearing sediments and obtained the original data in Xiazhu Lake.2,Have set up a viable new speed picking-up method: sonic speed picking-up method based on maximum and minimun correlation. The method can obtain the correct speed values throuth the phase analyses when velocity anomaly.3,There are sharp pulse signal and analogous valid signal two noise signal. Sharp impulse noise can be eliminated throughFourier Transform, but because of the high degree of self-similarity between analogous valid signal and valid signal, it is difficult to remove through Fourier Transform, but it can seriously affect the acoustic velocity extraction. The wavelet analysis method was first introduced to the in-situ acoustic data processing, and has been very good to eliminate the effect of signal interference.4,Analyzed the inversion results of a variety of theoretical models in Jintang waterways of Hangzhou Bay and discussed the effects of different models in sediment acoustic measurement, and found: Biot and BISQ model Fit very good to velocity dispersion, and from 103 to 106 Hz can be seen as the transitional zone from high-frequency velocity limit to low-frequency velocity limit, and these are consistent with other domestic and international basic research results; The Biot model better fit the sound attenuation in the low frequency, and the high-frequency attenuation inversion is slightly lower than the measured values, and and BISQ model better fit in high-frequency attenuation, and in the low frequency band, measured attenuation slightly higher than BISQ inversion; Results from the Biot inversion indicates: in this region, bulk modulus of sediment particles is small, the sediment is loose, porosity is large and moisture is high, which are just the reasons of the low velocity in the region. Results from the BISQ inversion indicates: in this region, bulk modulus of sediment particles is small, squirt flow length is large, and the coupling density is high, which is due to the larger porosity and the sediments with a lot of fluid features; Contrast Biot and BISQ inversion results, we can see that the penetration rate inversed by BISQ is smaller than results inversed by Biot. As the pore and moisture content of sediments in this area is high, so it should be smaller permeability and BISQ inversion results should be more scientific.The inner particle shear model fit acoustic attenuation more closely with the measured linear attenuation changes with frequency, but the effect of sonic speed fitting is not very satisfactory. From the above it can be seen that various acoustic model are certain flaws for the actual in-situ measurement results. This is mainly because the complex and ever-changing reality of the sediment can not be fully simulated. This is consistent with the results which show that the sonic velocity and attenuation changing with frequency is diversiform.5,Based on BISQ theory, simulate waves spread of the in-situ acoustic measurement using high-staggered grid finite difference numerical simulateion. Because of the only P-wave source, there are only fast and slow P-wave components in the seabed sediment. But because the velocity of slow P-wave is much slower than of fast P-wave, so usually in the in-situ measurement, within the intresting time the receiver receive only fast P-wave. In addition, by numerical simulation discuss the different performance of the received sonic pulse when source and receiver are in different locations.6,Introduce and enduce the BICSQS model, and for the first time, the model is applied to the inversion of in situ acoustic measurements. BICSQS model can explain the difference between different results of sediment acoustic measurement.7,For The first time, study the formation, impact factors and acoustic characteristics of the shallow inshore low-speed layer. And combining with the acoustic dispersion in two-phase media, has redefined the conception of low-speed layer.Through numerical simulation, found that the penetration rate is no contribution, and gas content in the porosity, sediment particles mineral composition, Biot curvature factor of the sediment structure have important implications to the formation of low-speed layer. And have discussed the effects of these parameters to sonic velocity and attenuation.The model can be explained the different manifestations of the acoustic characteristics of low-speed layer. Make in situ acoustic data inversion using the model in Hangzhou Bay and Xiazhu Lake.Hangzhou Bay inversion results show that: the Hangzhou Bay sediments are saturated water and non-free gas sediments; in the sediment particles, the main mineral composition is clay mineral and quartz mineral composition is only 25% -35%; Biot curvature factor is great, the distance between porosity is about two times of the straight line distance, that is due to the Stackable loose of shallow surface sediments; Mineral composition and sedimentary structure are important reasons led to low speed in the region. Xiazhu Lake inversion results show that: the regional sediment's pore contains 0.3 of free gas, and mineral composition of the sediment contains little quartz content, which also contribute to low sonic speed; And found that the theory attenuation values are slightly lower than the measured values, and this should because that the model does not consider the relative movement between free gas and pore fluid and the gas bubble breakdown and overflow due to free gas pressure changes in the process of acoustic propagation.8,Re-processing in-situ acoustic data based on the new method, and combining physical and mechanical parameters of the sediment, then found a new shallow middle scale model: low - high speed - low - high speed model. And because of the smaller scale, apart from the sea into and the sea retrea in big regiont, local scope of the current campaign could also result in sediment varied and different acoustic model. Examine in the Hangzhou Bay the sediment acoustic properties and the mechanical properties, found that: Wet density, porosity, water content, liquid limit, plastic limit are good correlation with P-wave velocity. Particle density, internal friction angle and cohesion are poor correlation with the P-wave velocity; The non-linear regression formula is different with study area; Multiple regression correlation coefficient is about 5-10% higher than the single parameter regression equation; Density and porosity, water content and porosity, water content and liquid limit are better correlation with P-wave speed; Density, porosity and water content have played a very important role to the P-wave velocity change; And for practical applications, two or three regression has been sufficiently represent the relations between acoustic characteristics and sediment physical and mechanical parameter in experience.In this paper, a more systematic study of seafloor sediments in situ acoustic measurement, processing, inversion model, acoustic wave numerical simulation, analysis of the theoretical low-speed layer models and relatios between acoustic and physical and mechanical parameters of sediments in Hangzhou Bay. As fewer domestic-related research, this paper developed China's research in the field to a certain extent, and study of low-speed layer, data processing, model inversion applicability and BICSQS application have some pioneering. The study has significance to basic theory of the submarine acoustics, inshore marine engineering, marine military affairs and marine geological disasters.