Search

The paper develops a multiple matching attenuation method based on extended filtering in the curvelet domain, which combines the traditional Wiener filtering method with the matching attenuation method in curvelet domain. Firstly, the method uses the predicted multiple data to generate the Hilbert transform records, time derivative records and time derivative records of Hilbert transform. Then, the above records are transformed into the curvelet domain and multiple matching attenuation based on least squares extended filtering is performed. Finally, the attenuation results are transformed back into the time-space domain. Tests on the model data and field data show that the method proposed in the paper effectively suppress the multiples while preserving the primaries well. Furthermore, it has higher accuracy in eliminating multiple reflections, which is more suitable for the multiple attenuation tasks in the areas with complex structures compared to the time-space domain extended filtering method and the conventional curvelet transform method. [ABSTRACT FROM AUTHOR]

To accurately characterize the shear wave speed dispersion of seafloor sediments in the northern South China Sea, five types of sediments including silty clay, clayey silt, sandy silt, silty sand, and clayey sand were selected, on which the measurements of the shear wave speed at 0.5–2.0kHz and related physical properties were performed. Results reveal that the shear wave speed of sediments increases as the frequency increases, and the dispersion enhanced in the sediments in the order of silty clay, clayey silt, sandy silt, silty sand, and clayey sand, at a linear change rate of 0.727, 0.787, 3.32, 4.893, and 6.967 ms−1 kHz−1, respectively. Through regression analysis, linear and logarithmic regression equations for the correlation between shear wave speed and frequency were established for each sediment type and the determination coefficients of regression equations indicate that the correlation is closer to a logarithmic relationship. The Grain-Shearing (GS) and Biot-Stoll models were used to calculate the shear wave speed dispersion of the five sediment types, and the comparison between theoretical prediction and measured results of shear wave speeds shows that the GS model can more accurately describe the shear wave speed dispersion characteristics of these sediments in the frequency band of 0.5–2.0kHz. In the same band, the predictions obtained by using the Biot-Stoll model are significantly different from the measured data. [ABSTRACT FROM AUTHOR]

Three-hundred and thirty-one sediment cores, including six sediment types (clayey- and sandy-silt, silty- and sandy-clay, clayey- and silty-sand) were collected from the shallow and semi-deep seas of the South China Sea, and the P-wave velocities and physical properties of core sediments were measured under standard laboratory conditions. To eliminate the influence of environmental factors, the empirical sound speed ratio equations were established. Compared with several equations from literature, the porosity and wet bulk density empirical equations established in this paper agree well with Richardson and Briggs (2004)'s in-situ equations, which implies that our empirical equations can be used in the similar region of world's oceans under certain conditions and will be useful in areas lacking first-hand P-wave speed data. However, the mean grain size equations established in this study, similar to the previous studies, have low accuracy, which may be due to the different particle arrangements and degrees of compaction in sediments. The results also show that for different sediment types, the equation based on all sediment data is in good agreement with the measured data in the study area, as there are both siliciclastic and carbonate sediments on the studied seabed. It is suggested that appropriate empirical equations should be selected according to sediment types and sedimentary environment in future works, and the empirical equation of porosity or the two-parameter equation of porosity and grain size should be preferred. [ABSTRACT FROM AUTHOR]

Building empirical equations is an effective way to link the acoustic and physical properties of sediments. These equations play an important role in the prediction of sediments sound speeds required in underwater acoustics. Although many empirical equations coupling acoustic and physical properties have been developed over the past few decades, further confirmation of their applicability by obtaining large amounts of data, especially for equations based on in situ acoustic measurement techniques, is required. A sediment acoustic survey in the South Yellow Sea from 2009 to 2010 revealed statistical relationships between the in situ sound speed and sediment physical properties. To improve the comparability of these relationships with existing empirical equations, the present study calculated the ratio of the in situ sediment sound speed to the bottom seawater sound speed, and established the relationships between the sound speed ratio and the mean grain size, density and porosity of the sediment. The sound speed of seawater at in situ measurement stations was calculated using a perennially averaged seawater sound speed map by an interpolation method. Moreover, empirical relations between the index of impedance and the sound speed and the physical properties were established. The results confirmed that the existing empirical equations between the in situ sound speed ratio and the density and porosity have general suitability for application. This study also considered that a multiple-parameter equation coupling the sound speed ratio to both the porosity and the mean grain size may be more useful for predicting the sound speed than an equation coupling the sound speed ratio to the mean grain size. [ABSTRACT FROM AUTHOR]

In order to investigate the correlation between a sound velocity and sediment bulk properties and explore the influence of frequency dependence of the sound velocity on the prediction of the sediment properties by the sound velocity, a compressional wave velocity is measured at frequencies of 25-250 kHz on marine sediment samples collected from the Bohai Sea and the Yellow Sea in laboratory, together with the geotechnical parameters of sediments. The results indicate that the sound velocity ranges from 1.232 to 1.721 km/s for the collected sediment samples with a significant dispersion within the series measuring frequency. Poorly sorted sediments are highly dispersive nearly with a positive linear relationship. The porosity shows a better negative logarithmic correlation with the sound velocity compared with other geotechnical parameters. Generally, the sound velocity increases with the increasing of the average particle size, sand content, wet and dry bulk densities, and decreasing of the clay content, and water content. An important point should be demonstrated that the higher correlation can be obtained when the measuring frequency is low within the frequency ranges from 25 to 250 kHz since the inhomogeneity of sediment properties has a more remarkably influence on the laboratory sound velocity measurement at the high frequency. [ABSTRACT FROM AUTHOR]

The Bering Sea shelf and Chukchi Sea shelf are believed to hold enormous oil and gas reserves which have attracted a lot of geophysical surveys. For the interpretation of acoustic geophysical survey results, sediment sound velocity is one of the main parameters. On seven sediment cores collected from the Bering Sea and Chukchi Sea during the 5th Chinese National Arctic Research Expedition, sound velocity measurements were made at 35, 50, 100, 135, 150, 174, 200, and 250 kHz using eight separate pairs of ultrasonic transducers. The measured sound velocities range from 1 425.1 m/s to 1 606.4 m/s and are dispersive with the degrees of dispersion from 2.2% to 4.0% over a frequency range of 35-250 kHz. After the sound velocity measurements, the measurements of selected geotechnical properties and the Scanning Electron Microscopic observation of microstructure were also made on the sediment cores. The results show that the seafloor sediments are composed of silty sand, sandy silt, coarse silt, clayey silt, sand-silt-clay and silty clay. Aggregate and diatom debris is found in the seafloor sediments. Through comparative analysis of microphotographs and geotechnical properties, it is assumed that the large pore spaces between aggregates and the intraparticulate porosity of diatom debris increase the porosity of the seafloor sediments, and affect other geotechnical properties. The correlation analysis of sound velocity and geotechnical properties shows that the correlation of sound velocity with porosity and wet bulk density is extreme significant, while the correlation of sound velocity with clay content, mean grain size and organic content is not significant. The regression equations between porosity, wet bulk density and sound velocity based on best-fit polynomial are given. [ABSTRACT FROM AUTHOR]

To better understand the characteristics of the clay minerals in the southern Yellow Sea, the X-ray quantitative determinations have been carried out for the surface samples obtained from the Yellow Sea. With newly compiled clay mineral synoptic maps, the depositional processes were described for four main clay minerals (illite, chlorite, kaolinite and smectite). The analysis shows that most clay minerals are of terrigenous source with the Huanghe River acting as the major sediment supplier. Besides, the source of muddy sediments in the Yellow Sea was also discussed. As for the central Yellow Sea mud (CYSM), the sediments in its northern part mainly come from the Huanghe River, and those in the rest are of multi-origin. Very similarly, a large amount of sediments in the northern part of the southeastern Yellow Sea Mud (SEYSM) derive from the Keum River and Yeongsan River, while those in the southern part are of multi-origin. [ABSTRACT FROM AUTHOR]