用定深爆炸声源反演海底声学参数

Inversion of sea bottom geoacoustic parameters using underwater explosion at fixed depth

根据2014年在南中国海开展声学试验的定深爆炸宽带声信号数据进行海底地声参数反演.考虑到不同海底声参数对不同声场物理参数的敏感程度不同以及不同海底声参数对不同反演方法的敏感程度亦不同,综合应用2种反演方法得到不同底质声参数:(1)根据接收的直达波和海底反射波计算得到关注海域的海底反射系数进而反演得到海底声阻抗;(2)实验海区的海底地形为大陆坡,选取Hamilton总结的关于沉积物声速与沉积物密度关系的经验公式,结合沉积物声阻抗与沉积物声速、沉积物密度的关系,进而反演得到沉积物声速和沉积物密度.沉积物声学参数的取样测量是在实验室条件下进行的,温度为23℃,大气压1×105Pa,由于沉积物孔隙海水是决定沉积物声速的关键且受温度压强变化的影响显著,本研究利用沉积物声速与孔隙海水声速的比值即使在温度压强变化的情况下较稳定的特点,可对沉积物声速在实验室条件和海底原位条件进行校正.校正到海底温度和压强后,反演结果与沉积物取样的实测结果和Hamilton总结的结果吻合得相当好:(1)声阻抗的反演结果为2.065 6×10~5g/(cm2·s),修正后的沉积物取样结果则为2.046 0×10~5g/(cm^2·s),Hamilton总结的结果为2.238 0×10~5g/(cm^2·s);(2)声速的反演结果为1 482.6m/s,修正后的沉积物取样结果为1 467.5 m/s,Hamilton总结的结果为1 502.8 m/s;(3)密度的反演结果为1.393 2 g/cm3,沉积物取样结果为1.400 0 g/cm^3,Hamilton总结的结果为1.489 0 g/cm3.

This paper provides the geoacoustic inversion results used broadband data from underwater explosion at fixed depth in an Acoustics Experiment in the South China Sea, 2014. Considering sensitivity differences of acous- tic parameters of sediments to physical parameters of sound field and geoaeoustie inversion methods, 2 approachesare used respectively for different acoustic parameters of sediments. Firstly, acoustic impedance can be achieved by vertical bottom reflective coefficients from sound reflected of sea bottom divided by directly arrived sound. Second- ly, sound velocity and density can also be achieved by combining Hamilton empirical equation to the relationship between acoustic impedance, sound velocity and density of sediments. The Hamilton empirical equation is about sound velocity and density of sediments and another key is to choose appropriate terrain which is continental terrace in this experiment. On different sea bottom, acoustic parameters of sediment samples are measured in laboratory conditions of temperature 23 ℃ and standard atmospheric pressure 1-atm. As a result, acoustic parameters especial- ly sound velocity need to be corrected to match different temperatures and pressures. Sound velocity is corrected successfully, in this paper, due to the fact that porewater of sediment critically determines sound velocity and the ratio of the sediment sound velocity to the porewater sound velocity is relatively stable even in the conditions that the temperature and pressure varies greatly. Inversion results appear to be well consistent when compared with in situ gravity core measurements and with data from Hamilton. In detail, in terms of acoustic impedance, the gravity core result after correction and the Hamilton conclusion are 2. 065 6 ×10^5, 2. 046 0 ×10^5 and 2. 238 0 ×10^5 g/（ cm2 ·s）, respectively. In terms of sound velocity, the gravity core result after correction and the Hamilton conclusion are 1 482.6, 1 467.5 and 1 502.8m/s, respectively. In terms of sediment density, the gravity core result after correc- tion and the Hamilton conclusion are 1. 393 2, 1. 400 0 and 1. 489 0 g/cm^3 , respectively.