Study On In-Situ Acoustic Signal Extraction Technology For Seafloor Sediments

Seafloor surface sediments,as the interface between seawater and seabed,have unique acoustic properties.The acoustic properties of seafloor sediments in various sea areas and sedimentary environments are critical to the underwater acoustic environment and marine detection.It is prerequisite to obtain the accurate acoustic parameters of seafloor sediments for the study of acoustic properties.The in situ acoustic measurement technique of seafloor sediments is an effective method to obtain the acoustic parameters without changing the original state of sediments.The data measured by the in situ measurement system is calculated by extracting the time difference of the acoustic wave.Accurate interpretation of the travel time at which the acoustic wave arrives at the receiving transducer is the key to calculate the time difference of the sound wave and calculate the acoustic velocity.The difficulty to extract the arrival time of head wave is: the amplitude of head wave is very small,which is easy to be confused with noise.In the past,it was often done by artificial observation to select the starting point of the head wave.However,manual method to read the starting point is less efficient,and man-made operation prone to result in error data.In order to improve the reading efficiency and avoid such errors,it is necessary to design reasonable algorithms to obtain the time difference.In this paper,level threshold method,short window-long window energy ratio method and cross correlation method are used to extract the time difference.Based on the in-situ measurement data of seafloor sediments in Jiaozhou Bay and Qingdao offshore region,the effectiveness of several time-difference calculation methods was analyzed.Compared with the calculation results of three methods,it is found that the cross correlation method is not accurate enough.After ameliorating the method,the calculation results of the three methods are very close.Compared with the method of manual recognition,the window threshold method based on program calculation,long window-short window energy ratio method and cross correlation method can be more efficient extraction of acoustic time difference.The results show that the applicability of the three methods is verified by comparing the results of the three time difference extraction methods with water velocity calculated by the pool experiment and the empirical formula of seawater velocity.The results show that all three methods can be applied to the in situ acoustic signal extraction of seabed sediments.Using these methods,the in situ measurements of seabed sediments at 45 stations in Jiaozhou Bay and Qingdao offshore were processed.And the velocity distribution of the study area was obtained.According to the distribution law of sediments species,it is found that the sound velocity has a significant correlation with the sediment type.It is found that the velocity of the sediment increases with the increase of the mean grain size.