Coherent marine seismoacoustics: New approaches to reconstructing bottom layer structures in shelf water areas

Results from the theoretical development and experimental study of the coherent seismoacoustic sounding of the sea floor under natural conditions are discussed. Ways of solving the inverse problem of estimating the geoacoustic parameters of the bottom layer by layer are proposed, and their effectiveness and robustness are analyzed numerically. Practical means of seismoacoustic profiling and reconstructing the parameters of individual layers at the sea floors of the shallow water areas using coherent hydroacoustic radiating elements of original design are demonstrated.     

Dispersion dependences of elastic waves in an ice-covered shallow sea

The dispersion dependences of acoustic waves propagating under conditions close to an ice-covered shallow sea are investigated. A dispersion equation is derived for a layered medium consisting of an elastic layer (the ice cover), a liquid layer (the water column), and a homogeneous elastic half-space (the bottom). The possibilities of estimating certain parameters of the studied layered structure from an analysis of the dispersion curves are discussed. The results of numerical calculations are compared with the data of a full-scale experiment on detecting seismoacoustic signals in an ice-covered sea region.     

Experimental investigation of potentialities of seismoacoustic sea-bottom sounding using coherent pulse signals

We describe the results of experimental investigations of the seismoacoustical sounding of the bottom structure of the Caspian Sea. They were obtained using a ship towed hydroacoustic emitter of LFM pulse signals in several frequency ranges of frequency band from 100 to 1000 Hz. Based on the high coherence and relatively high frequencies of emitted signals, the results point to feasibility of substantial improvement in noise immunity and resolution of sounding the bottom rocks?? structure at depths of up to 1000 m thanks to combined application of a series of procedures of coherent processing of incoming signals. The processing involves matched filtering of individual pulses, coherent accumulation of pulse trains within the horizontally uniform bottom area, and adaptive path accumulation of pulses accounted for inclination of individual reflecting layers. The resulting gain in noise immunity came to about 30 dB, which points to possibility of efficient use of relatively low-power (up to 100 W) coherent sources for seismoacoustic sounding of sea bottom at minimal damage to local ecology.     

Numerical modeling of the wave field in seismic profiling of the seafloor

The seismoacoustic field produced by an omnidirectional sound source located near the bottom of a shallow-water sea is numerically modeled at frequencies lower than 100 Hz. The main types of waves that are excited and scattered in the fluid and the layered bottom medium are represented in the form of wave hodographs on the distance-arrival time plane. A possibility to solve some problems of acoustic tomography of the bottom is demonstrated, in particular, the problem of determining the thickness of the bottom sediment layers and the velocities of the longitudinal and transverse waves propagating in them. By varying the elastic parameters of the layered bottom model, typical changes in the wave field are analyzed and the possibility to predict the presence of oil-saturated layers in the seafloor is established.     

Reconstruction of the Sea Bottom Parameters for Coherent Seismoacoustic Sounding: I. Decision Rules

We have developed an algorithm for reconstructing the geoacoustic parameters of bottom layers using parametric models of the formation of signals reflected from a layered halfspace during coherent sounding of the bottom of a sea shelf. We propose a method for layer-by-layer reconstruction that makes it possible to efficiently search for a solution in the multiparameter space with limited a priori data.     

Surface-to-volume wave conversion in shallow water with a gently sloping bottom

Marine sediments support seismoacoustic surface waves, which can propagate along the seafloor, in deep and shallow water, and even onshore. Because of the strong attenuation of compressional and especially shear waves in the sediments, the surface waves can significantly contribute to the acoustic field far from the shore only through their coupling with volume waves in the water. We theoretically study the excitation of acoustic normal modes by seismoacoustic surface waves in a shallow-water waveguide with a sloping bottom consisting of unconsolidated marine sediments. It is found that the coupling primarily occurs in the vicinity of a modal cutoff. The effects of geoacoustic parameters and stratification of soft marine sediments on the efficiency of surface-to-volume conversion are investigated. Published in Russian in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 6, pp. 809–816. The article was translated by the author.     

Use of aircraft noise for acoustic monitoring of shallow water areas

Acoustic noise in the sea water area, excited by aircraft and helicopter motion, which can be used as useful signals for sounding the water column and shallow water bottom, was analyzed. Promising application of various technical means and methods for acoustic monitoring using aircraft noise was demonstrated.     

Surface-to-volume wave conversion in shallow water with a corrugated bottom

Acoustic transmission between points onshore or in very shallow water and points in deep water is strongly influenced by the shear rigidity of marine sediments, which control the parameters and the very existence of seismoacoustic surface waves. Previously, it was found that coupling between acoustic modes and the seismoacoustic surface waves is normally weak, although not negligible in the case of a gently sloping seafloor and soft sediments. In this paper, the previous work is extended by accounting for the small-scale roughness of the seafloor. The significant role of roughness in coupling between volume and surface waves is demonstrated. The combined effect of bottom topography, roughness, and wave attenuation in soft marine sediments on the sound propagation between points in shallow and deep water is discussed. Published in Russian in Akusticheskiĭ Zhurnal, 2008, Vol. 54, No. 3, pp. 400–407. The article was translated by the author.     

Estimating the acoustic characteristics of surface layers of the sea bottom using four-component vector-scalar receivers

It is shown that scalar, horizontal, and vertical vector receivers efficiently split modes of different numbers, which makes it possible to analyze the mode structure and estimate the characteristics of surface layers of a shallow sea bottom. To analyze the mode structure of propagating pulses from a towed pneumatic source, Winger transform was applied, with which seven modes were isolated by vertical vector receivers, whereas the scalar receivers and horizontal vector receivers isolated only three modes. It is established that the use of four-component vector-scalar receivers makes it possible to increase the accuracy in estimating the parameters of a layered bottom model.     

Dependence of the Mean Intensity of a Low-Frequency Acoustic Field on the Bottom Parameters of a Shallow Sea with Random Volumetric Water-Layer Inhomogeneities

The study is devoted to statistical modeling of low-frequency acoustic signal propagation in a twodimensionally inhomogeneous random shallow sea with a thermocline and differing penetrability of the bottom. Calculations are performed using the local-mode representation of the solution in the one-way propagation approximation. Plots are presented for the behavior of the mean acoustic field intensity for different sound velocity and density values in the bottom. It is shown that the earlier described effect of a decrease in propagation losses in a model randomly inhomogeneous shallow sea with an absorbing bottom significantly depends on the parameters of bottom sediments and is more strongly manifested for bottom boundaries with greater penetrability.     

Debye analysis applied to multiple reflections and attenuation of electromagnetic plane waves in a stratified sea substratum

Investigation of the wave problem related to multiple reflections and attenuation in marine controlled source electromagnetic is important for geophysical sounding of the sea substratum. An approach towards this goal is to study the scattering of obliquely incident plane waves by a four-layered earth: sea, sea bottom sedimentary rock, hydrocarbon reservoir, and lower sedimentary rock. The approach employs Debye theory in the frequency domain and assumes that each layer is a planar lossy medium. Explicit expressions show that the secondary waves related to the sea substratum layers multireflections carry useful information about their features.     

Methods for measuring bistatic characteristics of sound scattering by the ocean bottom and surface

It is very interesting to measure the bistatic characteristics of sound scattering by the ocean bottom and surface for the development of present-day hydrolocation net-centric schemes. Possible methods for measuring scattering bistatic characteristics are evaluated. The angular characteristics of reverberation related to illumination of a water area by active linearly frequency-modulated signals are studied using linear passive arrays to test a measuring scheme in a shallow water area with a depth of ~20 m in the 1–3 kHz frequency range. The bistatic sound-scattering characteristics in the water area are calculated based on the measurements. The obtained characteristics are compared with the known data on the backscattering of sound.     

Experimental testing of a hardware and software complex for remote measurement of current velocities and temperatures in shallow sea water areas

We describe a technique and hardware-software implementation for remote monitoring of sea currents and temperature using data from pulsed hydroacoustic sounding on stationary horizontal tracks in shallow sea water areas. As sounding signals, we used complex phase-manipulated signals based on pseudorandom M-sequences. The complex was tested in Posiet Bay in the Korea Strait in the Sea of Japan.     

双基地冰-水界面混响强度的理论预报

针对北极冰层冰水界面具有小尺度粗糙界面的情况,给出由冰层造成的混响平均强度的理论预报公式。首先将北极冰层等效为具有粗糙界面的弹性介质,并且采用小粗糙度微扰理论在精细拟合粗糙度谱的基础上,建立粗糙冰层的三维散射强度模型,然后建立三维双基地声呐几何模型确定有效散射区域,最后给出双基地冰下混响平均强度的估算公式,并在所拟合的粗糙度谱的情况下,利用该预报公式计算双基地声呐配置参数和海冰物理及声学特性对冰下混响强度的影响算例。数值仿真表明该公式能够估算出北极冰下双基地声呐产生的混响平均强度,并且分析出了时延、基线长度等声呐配置参数和冰层声速比、密度比等海冰物理参数会对混响强度造成的影响。声呐的配置参数主要影响散射的有效面积,海冰的物理参数则影响着冰层的散射强度。其中,冰层声速比不仅影响混响强度的大小,还会影响混响强度随时间的衰减速度。      

一种低声速沉积层海底参数声学反演方法

软泥底环境下沉积层参数的声学反演是国际水声领域的一个研究热点.浅海中,当高声速基底和海水之间存在一层低声速(小于海水声速)的沉积层时,小掠射角情况下不同频率声传播损失会出现周期性增大现象.基于此现象,提出一种适用于低声速沉积层的海底参数声学反演方法.首先,推导给出小掠射角情况下传播损失周期增大的频率间隔与沉积层声速、厚度及近海底海水声速之间的解析表达式;其次,利用一次黄海实验中软泥底环境下的宽带声传播信号,提取了小掠射角下传播损失增大的频率周期;再次,把该解析表达式作为约束条件,结合Hamilton密度与声速的经验公式,采用匹配场处理反演给出沉积层的声速、密度、厚度及基底的声速、密度;然后,利用声传播损失数据反演得到泥底环境下不同频率的声衰减系数,通过拟合发现泥底声衰减系数随频率近似呈线性关系;最后,给出了双层海底模型和半无限大海底模型等效性的讨论.反演结果为低声速沉积层海底声传播规律研究与应用提供了海底声学参数.     

Monitoring of anthropogenic noise on the shelf of Sakhalin Island during seismic survey research

The paper describes the technique and gives the results of acoustic-noise and seismoacoustic-signal-parameter measurements on a northeast shelf of Sakhalin Island, generated during seismoacoustic research at the licensed Chaivinskii site. The aim of measurements was acoustic control of the water area round an emitting vessel. Results of field measurements and 3-D simulations of seismoacoustic signal propagations on sea and land are presented.     

Reverberation of Wideband Signals in Shallow Water when Using Sound Focusing

Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area.     

一种基于单水听器的浅海水下声源被动测距方法

针对浅海波导中宽带脉冲声源的被动测距问题,本文在群延迟理论的基础上,与warping变换处理相结合,提出了一种适用于浅海波导中宽带声源的单水听器被动测距方法.利用warping变换可以实现对脉冲声源接收信号各阶简正波的分离提取,对分离后的简正波进行时频分析处理可以得到各阶简正波到达时刻和频率之间的关系,即各阶简正波的频散曲线,从而得到任意两阶简正波到达接收水听器的时延差.海底相移参数P是描述海底地声参数的一个重要参量,包含了海底地声参数信息,在海底环境参数未知而P已知的情况下,利用P和简正波水平波数之间的关系可以求得任意两阶简正波的?S_(g,mn)(群慢差).根据群延迟理论,利用得到的任意两阶简正波的时延和?S_(g,mn)可实现利用单水听器对水下声源进行被动测距.本文提出的测距方法测量简单、计算方便,具有较强的实用意义.数值仿真和海上实验数据处理结果的测距误差都在10%以内,证明了该方法的有效性.     

Azimuthal dependence of reverberation from a harmonic signal propagating in shallow water

The problem of scattering from a rough sea surface is considered for a harmonic low-frequency (200–400 Hz) acoustic signal propagating in a shallow water area. An experimental study of the azimuthal reverberation dependence is carried out on the basis of the bistatic location scheme with the use of linear phased arrays. By comparison with calculations, the formation of the frequency-angular characteristics of reverberation is investigated for various wind directions with respect to the path of acoustic signal propagation.