## 1 Introduction  
In the field of ocean acoustics, the study of ocean bottom characteristics is a popular and important subject. The influence of the ocean bottom on sound transmission cannot be ignored except for the case of deep sea sound channel transmission. However, it is difficult to measure the various acoustic properties of the ocean bottom medium, and the laboratory analysis results of the sediment sampling may be different from the real parameters of the sea sediment. So far, the geoacoustic inversion method is still the most effective means to obtain the ocean bottom characteristics [1].  
Geoacoustic parameter inversion is generally based on a hypothetical geoacoustic model. The corresponding cost functions by the physical quantities closely related to geoacoustic parameters are established, such as transmission loss [2, 3, 4, 5], sound field spatial correlation [6, 7, 8], pulse waveform [2], ocean bottom reflection loss [9, 10, 11, 12], etc., and then the inversion results are obtained by the comparison between theoretical values and measured values through the grid method or multidimensional optimization algorithm. However, there may be many problems of insensitivity of some parameters and multi-valued results by using a single physical quantity for inversion. Therefore, the joint inversion method has been proposed [13; 14; 15]. The relatively sensitive physical quantities are selected to match different geoacoustic parameters, so that the geoacoustic parameters can be inverted independently. In recent years, in order to reduce the cost of inversion and the difficulty of experiments, the geoacoustic parameter inversion method with a single hydrophone is becoming more and more important, including extracting normal mode features from a single hydrophone using warping transform [16; 17; 18; 19], and obtaining the range-frequency interference structure of the sound field using towed sound sources, ship noise [20; 21; 22], etc. However, most of the work is concentrated in the shallow sea, and there are few studies on the geoacoustic parameter inversion in the deep sea.  
In previous studies, the range-frequency interference patterns of the sound field in deep water have been based on the ray theory when the source and the receiver are near the sea surface [20]. An inversion method of the sound speed and thickness of sediment was proposed by using the sound field interference period in the deep sea, when the sound source is at the sea surface and the hydrophone is at the ocean bottom [21]. On the basis of the work above, this paper found that there is a third interference structure appearing in the shadow zone which is caused by the ocean bottom reflection for high-speed seafloor sediment, when the transceiver depth is located at the sea surface. This paper analyzed the cause of the interference structure in the shadow zone with the high-speed ocean bottom reflection by the ray theory. Based on this phenomenon, a geoacoustic parameter inversion method using the spatial position matching of the interference fringe was proposed, and the corresponding extraction method of the fringe coordinates was designed in this paper. The proposed method was preliminarily verified by simulation experiments.  
Simulation results showed that the method in this paper could effectively extract the interference fringe coordinates. Combined with the grid search method, the relatively accurate and consistent sound speed and thickness of the sediment could be quickly obtained by matching the spatial position of the interference fringes in the shadow zone caused by the ocean bottom of the high-speed sediment.