Inversion Of Sea Bottom Geoacoustic Parameters Using Underwater Explosion At Fixed Sea Depth

It is of great significance to invert acoustic parameters of sea bottom in shallow water since the bottom, sea surface and sea water work together to influence sound propagation. This paper provides the geoacoustic inversion results using broadband data from underwater explosion at fixed depth during the Acoustics Experiment in the South China Sea in 2014. Considering sensitivity differences of acoustic parameters of sediments to physical parameters of sound field as well as geoacoustic inversion methods, different approaches are used respectively for different acoustic parameters of sediments. Firstly, acoustic impedance can be achieved by vertical bottom reflective coefficients coming from sound reflected from sea bottom divided by directly arrived sound.Secondly, sound velocity and density can also be achieved by combining Hamilton empirical equation with 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. Thirdly, the thickness of sea bottom sediment can be achieved by correlation analysis of the direct arriving sound and the sound reflected from sea bottom. The thickness equals the inverted sound speed of sediment multiplied by half of the interval between main peak and second peak of correlation diagram.Different to sea bottom,acoustic parameters of sediment samples are measured in experiment condition with 23℃room temperature and standard atmospheric pressure 1-atm.As a result,acoustic parameters especially sound velocity need to be corrected to match different temperature and pressure.Sound velocity is corrected successfully,in this paper,according to the fact that porewater of sediment critically determines sound velocity and the ratio of the sediment sound velocity and the porewater sound velocity is relatively stable although the two velocities change greatly with varying temperature and pressure.Compared with global searching algorithm in parameter space,it is easier,faster and more efficient of invert sound speed and density using drawing(交点法)method.Sediment sampling experiment and correlated theories concerning parameter measurement of sample are also introduced in detail.Inversions results were compared with gravity core and data from Hamilton both corrected for laboratory condition to sea bottom condition(especially temperature and pressure) and the results appear to be well consistent. In details, in terms of acoustic impedance whose unit is-1-25××scmg10, the inverted, the gravity core and the Hamilton results are respectively 2.0656, 2.0460 and 2.2380. In terms of sound velocity, whose unit is-1sm ×, the inverted, the gravity core and the Hamilton results are respectively 1482.6, 1467.5 and 1502.8. In terms of sediment density whose unit is3cmg-×, the inverted, the gravity core and the Hamilton results are respectively 1.3932, 1.4000 and 1.4890. In terms of sediment thickness whose unit is m, the inverted results is 1.366 while acoustic impedance change greatly in the depth of 1.4~1.6 layer according to samples analysis.