小尺度海底沉积物的声衰减特性研究

为研究小尺度海底沉积物样品的声衰减特性,作者提出了用声学探针测量海底沉积物声波幅值的新方法,对沉积物样品扰动小,两个测量点的距离可小于波长,为海底沉积物微观声衰减测量提供了新手段。作者用小于波长的间隔逐点测量了沉积物的压缩波幅值,数据分析表明沿沉积物柱状样全长的声衰减满足指数衰减模型。目前主要用同轴差距衰减测量法获得海底沉积物声衰减数据,但该方法不能辨识声衰减模型,因此不同海区的测量结果难以建立联系。对此作者又提出用声吸收系数反演的幅值比与声衰减系数反演的R值(两种幅值比的比值)作评价依据,分析了垂直轴差距衰减测量法获得的南海海底沉积物声衰减测量数据,发现部分沉积物样品声衰减的R值远大于1,其声衰减不满足指数衰减模型。在声衰减满足指数衰减模型的条件下,用Hamilton的声衰减和频率经验公式预报的南海沉积物声衰减比与作者用声学探针测量海底沉积物所得的声衰减比对比,通过对R值分析得出Hamilton的声衰减和频率经验公式可以预报南海沉积物声衰减比的范围。作者提出的声学探针测量海底沉积物声衰减的方法的优点是既能获得声衰减数据又能辨识声衰减模型,不同海区测量的沉积物声衰减比可用R值建立联系。

Acoustic attenuation of seabed sediment at small scale

In order to study of sound attenuation characteristics of seabed sediment at small scale,an original method for measuring acoustic attenuation of sediment within a small-gap cross section is put forward.There are small disturbances in the sediment samples during the course of measurement by the method.The sound attenuation of seafloor sediment can be measured at two test points;the distance of them is less than one wavelength,which provides a method for finding out how microstructures work in wave attenuation of seabed sediment.Amplitudes of compression wave in sediment columns were measured point by point at the interval less than one wavelength in the study;and it is shown that negative exponential function is a type of acoustic attenuation model for sediment in its full length.Data of wave attenuation of seafloor sediments are mainly obtained by coaxial gap measurement method at present,but the sound attenuation model is not identified by using the method;so it is difficult to establish the relationship of results of wave attenuation of seafloor sediments taken from different sea areas.In this paper,an evaluated R,which result from the ratio of the amplitude inversed by the sound absorption coefficient divided by the ratio of the amplitude inversed by the sound attenuation coefficient(the ratio of two amplitude ratios),is proposed to be an evaluation criterion.After the data analysis of sound attenuation of seabed sediments from the South China Sea,which were obtained with measurement method of sound attenuation between the gap along the vertical axis,it was found that the evaluated R’s of sound attenuation from part of the sediment samples were much larger than one and their sound attenuations did not meet the exponential function of negative index.Under the conditions of the exponential function of negative index with which sound attenuation of seabed sediments was satisfied and in comparison with the evaluated R’s of sound attenuation of seabed sediments from the South China Sea predicted with Hamilton’s empirical formula of sound attenuation and frequency and the ones measured with acoustic probe in our project,it was found that the range of ratios of sound attenuation of seabed sediments from the South China Sea can be predicted by Hamilton’s empirical formula of sound attenuation and frequency.The advantage of measurement by the acoustic probe is in both accessing acoustic attenuation data and identifying sound attenuation model of seafloor sediments,and then the relationship of measured sound attenuation of seafloor sediment from different sea areas can be established with their evaluated R’s.