Q measurements for PhaseX overtones

Linear stacking procedures are used to retrieve the attenuation of 91 modes belonging to the 3rd, 4th and 5th Rayleigh overtones branches in the 80–160 s period range, and contributing to the so-called PhaseX wave group. Our data show in general slightly less attenuation than expected from available models. Data space inversion shows that, when combined with previously measured fundamental modeQ's, this new dataset improves resolution significantly in the 1000–2000 km depth range. Based on this remark, we carry out a number of parameter space inversions. Our results suggest a narrow (80–200 km) zone of high attenuation (Q =75–90), low attenuation in the intermediate mantle (670–1500 km); (Q 350), and lower values in the deeper mantle (Q 200).     

Frequency-dependence of velocity and attenuation of elastic waves in granite under uniaxial compression

Velocity as well as attenuation factorQ ^–1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 _f , where _f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ ^–1 increases. Particularly, the change ofQ ^–1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ ^–1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ ^–1 is approximated asf ⁿ in the present frequency range, the exponentn takes a value from 0.63 to 0.77.     

Laboratory measurements of spatial fluctuation and attenuation of elastic waves by scattering due to random heterogeneities

To study the effects of strong scattering on elastic waves, spatial fluctuation and scattering attenuation ofP waves were examined by laboratory experiments for 2-D models of random media approximately characterized by a triangular correlation function in the range of 2<ka<33, wherek is the wave number anda is the correlation distance of the heterogeneities, i.e., the heterogeneity size. The results obtained are as follows: (1) Forka>10, both the intensity and the correlation distance of the amplitude fluctuation are approximate for any phase of theP-wave train. The correlation distance nearly agrees with the heterogeneity size. These fluctuation properties are quite consistent with the theoretical prediction by the forward-scattering approximation. (2) For 3<ka<6, the fluctuation intensity becomes stronger in later phases of theP-wave train. This shows that scattering is approximately isotropic, and therefore, the scattered energy increases with time within theP-wave train. The correlation distance of the amplitude fluctuation disagrees with the heterogeneity size, and it shows a frequency-dependent property decreasing from 7a to 4a with the increase ofka from 3 to 6. These properties for 3<ka<6 have not yet been predicted theoretically. (3) Forka<3, though the fluctuation is considerably smaller compared with that ofka>10 and 3<ka<6, the fluctuation property is considered similar to that of 3<ka<6. (4) The observed scattering attenuation,Q ^–1, increases withka forka<3, has a peak aroundka=35, and then decreases withka. (5) When _min = 15° and = 0.075, the theoreticalQ ^–1 curve, predicted by the approximate theory of Wu, roughly matches the observedQ ^–1 values, where _min is the minimum scattering angle measured from the propagation direction of theP waves and is the rms of fractional velocity fluctuation. This suggests that the energy scattered in the range of >15° is lost from theP waves, while the energy scattered in the range of <15° is retained; and that the approximate theory overestimates by about three times the value of the model media used owing to the neglect of multiple scattering. (6) When the size of velocity heterogeneities responsible for forward scattering at 3<ka<6 is estimated from the _min value of 15° on the basis of Wu's theory, it nearly agrees with the correlation distance for the initial phase of theP-wave train.     

模拟黄河水的声衰减测量

对两种含沙量(8.28千克/米~3和31.56千克/米~3)的模拟黄河水进行了在不同频率(3.2—20千赫)下的声衰减测量。结果表明:声衰减系数与频率之间基本上成线性关系,其斜率与含沙量有关。     

MFSK水声通信接收系统中的幅度均衡设计

为了抗幅度随频率的衰减，提出了一种基于多频移频健控（MFSK）调制方式的水声通信接收系统中的幅度均衡技术，介绍了构成幅度均衡电路的基本原理以及在厦门港浅海域中的实验结果，实验结果表明，该技术能有效克服上系统中的信号民幅度随率的衰减问题。使接收到的图像清晰易读。     

Acoustic imaging of an inhomogeneous sediment matrix

This paper presents and applies a new acoustic imaging approach to quantitatively determine geophysical parameters of complex inhomogeneous sediment layers. The concept is called an Acoustic Sub-seabed Interrogator or ASI and it is based on optimising the criteria of temporal and spatial resolution and of coherence by utilizing broadband, narrow beam calibrated sources and fixed receivers on a stationary platform.The ASI concept was experimentally tested over a complex pseudo-glacial till built at a scale of 20:1. Compressional velocity, attenuation losses and thickness values were easily attained using the ASI's coherent spatial scanning approach and through a discrete application of the Hilbert transform in the post processing of data. The simulations provided indepth insight into the model's physical and spatial variations and by statistical distribution emphasized the depositional processes involved in creating the layer. It is believed that the fundamental internal information elucidated by the ASI for the pseudo-till structure and illustrated in the resultant acoustic core cannot be attained using current marine geophysical and geotechnical techniques.     

珊瑚礁坪波浪的衰减特性分析

珊瑚礁海底与一般的礁石或泥沙海底结构相差很大，波浪衰减特性也相差较大。根据实测海浪资料，讨论波浪在礁坪上传播的衰减特性和波浪能量的转移。分析结果表明，在相对水深为0．0613—0．0867之间，波浪传播单位距离(100m)的波高衰减系数为22．09％—46．56％，平均值为31．35％；波能衰减系数为33．74％—53．23％，平均值为43．61％。波浪在礁坪上传播过程中，高频损失的能量多于低频，谱能量向低频转移。应用统计方法得到波高、波能衰减系数与相对水深D/L关系的拟合方程。     

CDOM and its contribution to the underwater light climate of a shallow, microtidal estuary in south-western Australia

Light attenuation (K_d) of photosynthetically active radiation (PAR) by chromophoric dissolved organic matter (CDOM), total suspended solids (TSS) and chlorophyll a (Chl a) were measured at nine stations along an estuarine gradient in the Swan River, Western Australia, over 15 months. There were strong spatial gradients associated with the marine-freshwater transition along the 32 km of estuary sampled, as well as seasonal gradients mainly associated with rainfall, 80% of which occurs between May and September. CDOM absorbances at 440 nm reached a maximum of 10.9 m⁻¹ with the freshwater inflow but concentrations of suspended matter remained low throughout the sampling period (1.0–21.0 mg l⁻¹) under the diurnal tides of the estuary. CDOM was the dominant constituent of K_d and a stepwise multiple regression showed that 66% (p < 0.0001) of the variation in K_d can be explained by CDOM and an additional 8% (p < 0.0001) by TSS. As a consequence of this result, analysis into the influence of river discharge rates on CDOM absorbance levels was examined for 2002 using data collected during this study, and for 2000 and 2001 using historical dissolved organic carbon (DOC) and river discharge data. The outcome of this analysis infers that greater river discharge rates result in increased CDOM absorbances in the Swan River estuary.     

In-water algorithms for ADEOS/OCTS

In-water algorithms for OCTS standard products were developed using in situ data and installed for operationally processing at NASDA/EOC. This paper describes the in-water algorithms Version 1.0 for chlorophylla concentration, pigment concentration, and attenuation coefficient at a wavelength of 490 nm. The selected OCTS standard algorithms (Ver. 1.0) are as follows:      

高浓度悬浮泥沙的声学观测

在分析高浓度悬浮泥沙的声衰减机理和进行实验研究的基础上,研制了两种型号的超声波观测仪器(UBD-500/1500超声重度计)用于悬浮泥沙浓度剖面的连续和实时观测,并且已经在长江口航道的浮泥探测、黄河小浪底水库的泥沙观测中得到应用.讨论了测量原理和仪器的设计、标定等问题.这种基于声衰减测量原理的观测仪器的主要优点是标定简便和稳定,含沙量观测范围为10～800kg/m3,标定后的测量误差可达±5%(F.S).