Measurement And Modeling On The Underwater Noise Radiated From Ships In The Area Of Zhoushan Archipelago

The long-range detection technique of underwater sonar is developing toward the low frequency domain.One of the difficulties is lacking of understanding on the low-frequency ocean ambient noise field,especially fundamental research on the characteristics of shipping noise.At present there is no absolutely scientific theoretical basis and effective processing method at domestic and overseas.During the past decades,the impact of shipping noise on marine ecology has always attracted more and more interest of global researchers.Therefore,the research on the shipping noise not only has an important significance for national defense,but also has a special significance in protecting ocean ecology.This thesis is organized generally like this: based on the abundant measured data of shipping noise,the source level models of merchant ships were proposed with noise mechanism methodology and statistical regularity,respectively.Combined with the realistic geoacoustic parameters and real-time AIS data,the spatial distribution of shipping noise in the area of zhoushan archipelago was predicted.Firstly,the propagation regularity at shallow water was studied and discussed.Although a number of measurement standards exist internationally,most of them describe measurement procedures for dedicated measurement ranges,the depth in situ or well-controlled measurement conditions for cooperation ships.For shallow water areas,non-ideal measurement conditions,the transmission loss(TL)is easily affected by the Lloyd's mirror effect(LM)and geoacoustic parameters.Hence,the source level is depicted using “monopole method”.To obtain accurate transmission loss,the Normal Modes(NM)and Finite Element Method(FEM)are used at low frequencies,and Ray Method(RM)is used at high frequencies.The research results show that the calculated TL averaged from 50 Hz to 1 kHz in third-octave has a stronger correlation with horizontal received distance with a mean slope of 16.7.Furthermore,several factors influencing the transmission loss were analyzed.It is demonstrated that the bottom type is the largest factor influencing the TL among all uncertainties,which is followed by the source depth.Other factors,including the range with 10 m change,constant Sound Speed Profile(SSP)vs real SSP and receiving depth with 1 m change,have little influence on the TL within 2.0 dB.Secondly,the source level model based on noise mechanism was proposed for the cooperation ships.Considering the main characteristics of three kinds of underwater radiated noise sources,these noise sources are depicted as functions about speed and frequency.On this basis,the modeling methodology of source level is proposed,that is to say,finding 17 parameters including three cut-off frequencies such as the patial measured results are best matched with estimated results using model.To verify the modeling methodology,we carried out detailed measurements on the underwater radiated noise of a cooperation ship at the South China Sea.By analyzing the noise data,some low-frequency characteristics are obtained,and the background noise is indispensable information which dominates the signal to noise ratio,especially for shallow water.Combining with the measured data,a more reasonable source level model is obtained by solving the constrained optimization problem using sequence quadratic program(SQP).This model not only realizes the coincidence with the measured results,but also makes the noise sources be separated.Thirdly,the ship source level empirical model for opportunistic ships was estabished.In order to establish a more reliable model,we carried out measurement at least one day in each month during two and half years.The measuring method for opportunistic ships is adopted,and the receiving point is deployed near the sea bottom.To guarantee the accuracy of measurement,the sensitivity of hydrophone is checked and the distance error according to AIS data is analyzed.By analyzing the noise data one by one,an ensemble radiated noise of 57 merchant ships was picked out.On this basis,the transmission loss corresponding the closet distance for each ship observed was estimated,in which the realistic sound speed profile(SSP)and geoacoustic parameters and source depth are also considered.The results show that a prominent “hump” is presented at 63 Hz,and there are some tones present in the spectrogram,which makes the SSLs of 57 merchant ships at frequencies of 500 Hz and 630 Hz be lifted deviating from the mean decaying trend of SSL.Besides,the source levels of merchant ships are predicted using RANDI-3 model and Ross' s model,respectively.The results show that estimation performance using RANDI-3 model is better than that using Ross' s model,but both models need to be improved.Inspired by the methodology of RANDI-3 model,we firstly obtained the mean source level along with frequency.Then we found the relationship between source level and speed or ship length at frequency domains of 50 ~ 100 Hz and 100 Hz~1 kHz,respectively.The results show that the observed dependences of the SSL on ship speed and length are close to the trends suggested in Ross' s model in the 100 Hz ~ 1 kHz frequency band.In the 50 ~ 100 Hz band,the SL appears to increase with 38(±2)times the logarithm of the ship speed V,which is lower than that the 50 lgV trend in Ross' s model.On this basis,a modified source level model is proposed,which shows that the source level estimations using modified model are much closer to the measured results than that with RANDI-3 model at frequencies below 200 Hz.Finally,on the basis of ship source level empirical model,the shipping noise in the area of Zhoushan Archipelago was predicted using N × 2D fast arithmetic.At first,the computational grids were meshed according to the realistic bottom topography.Besides,the horizontal distribution of shipping noise field produced by point sources located at different positions along the lane was discussed.The global flow chart of shipping noise prediction and the spectrogram at fixed point were also given.The spectrogram shows that the shapes of cumulative distribution functions of noise level at different frequencies are consistent basically.With increasing of the frequency,the cumulative distribution function shifts to the left.Then the source level of each ship was predicted using modified model combining with the basic information of each ship in this area.According to the principle of energy superposition,the horizontal noise field distribution produced by all merchant ships with receiving depth of 10 m in the whole area at different frequencies was obtained.The primary characteristic of noise pattern is that with the increasing of analyzing frequency,the noise level near the point source becomes weaker,and the propagation range becomes wider.The sound propagation will be different according to the different positions due to the shielding effect of islands and different bottom topography.Futhermore,each noise contribution from the individual ship at different range and bearing was presented.These research results may provide reference value for realizing shipping noise fast prediction at complex waters near islands and reefs.