Broad Band Properties Of Sound Speed Dispersion And Attenuation In Sandy Sediments

For the presented large quantity of sound speed and attenuation data collected from in-situ measurements in seafloor sediments,sound speed at low frequencies(below 1 kHz)and attenuation at high frequencies(above 100 kHz)are inconsistent with Biot-Stoll model predictions,and this problem has not been clearly explained yet.These data can not be used to validate the applicability of acoustic models of sediments,nor can they accurately reflect the real acoustic characteristics of sediments.Considering the uncertainties of marine environments and the composition heterogeneity of seafloor sediments,it is of great significance to carry the investigation of sound speed dispersion and attenuation in seafloor sediments under controlled conditions in laboratory.According to the sediment scale requirement for direct acoustic parameters measurement in sediments,three experimental platforms were designed separately in laboratory,which highly reduce the influence of ocean environmental uncertainties.Sound speed and attenuation of sandy sediments were effectively collected in the frequency bands of 30-170 kHz,2-120 kHz and 300Hz-3kHz,which are very important for model validating and acoustic characteristics research of sediments.Incorporated with several measured characteristic parameters of sediment acoustic models,the models were validated.The one-to-one correspondence between sound speed peaks and attenuation peaks were observed for the first time from laboratory measurements and the effect of gas bubbles on acoustic properties of sediments was revealed.The specific research contents are as following:1.Study about sound speed dispersion and attenuation from 30 to 170 kHzA high-frequency experimental platform was built in a glass water tank,and a sandy sediment sample was degassed by a high-temperature boiling method.Sound speed and attenuation of the sandy sediment in the band of 30-170 kHz were obtained by narrowband measurement and broadband measurement,respectively.In order to reduce the influence of bandwidth limitation of the transmitting transducer and the uneven transmitting voltage response in its operating frequency band on the measurements,a method for measuring sound speed and attenuation in sandy sediments using the transmitting voltage response to compensate for the driving signals was proposed.Both the numerical analysis and the experimental results show that the spectral peak frequency shift of single frequency short pulse acoustic signals and bandwidth narrowing of broadband pulse acoustic signals were effectively solved.The fluctuations of broadband sound speed and attenuation data are obviously weakened,the bandwidth of narrowband measurement results is effectively maintained,and the results of narrowband and broadband measurement become more consistent,which denotes the improvement of data accuracy.The compensation method lays a solid foundation for the reliable acquisition of sound speed and attenuation data of sediments and the study of sediment acoustic properties.The predictions of Biot-Stoll theory and effective density fluid approximation model(EDFM)were compared with the measured data by the inversion of model parameters.The results show that the measured sound speed at frequencies above 50 kHz agrees well with the model prediction,but the measured attenuation above 30 kHz is higher than the model prediction.2.Study about sound speed dispersion and attenuation from 2 to 120 kHzAccording to the same requirement of sediment acoustic experiments,an experimental platform was designed in the 2-120 kHz frequency range in a laboratory channel tank.The sediment sample of about 3.15m3 was also degassed by the boiling method.Transmission measurement method was used,the sound speed as well as coefficient of attenuation in sandy sediments over the frequency range of 2-120 kHz were obtained by different signal processing methods.The experimental results show that a small amount of bubbles still exist in the sediment,which results in the peaks of sound speed and attenuation in sandy sediment at some frequencies.Through numerical analysis of the Anderson&Hampton model,the corrected Biot-Stoll model and the corrected EDFM for gassy sediments,the results show that the frequency of sound speed peak is always greater than that of the corresponding attenuation peak,which agrees well with the characteristics of experimental data.With the bubble volume fraction increases,the transition zones of sound speed and attenuation increase gradually near the gas bubble resonance frequency,while sound speed gradually decreases and attenuation gradually increases well below the gas bubble resonance frequency.Further analysis of the measurement results in the 10-120 kHz band reveals that there are four one-to-one correspondence between sound speed peaks and attenuation peaks,which is the first clear observation of this phenomenon in a laboratory tank.Five continuous modified Gaussian functions were used to represent the bubble size distribution.The rationality of the corrected EDFM was validated using both sound speed and attenuation data for the first time.And the optimal bubble size distribution in gassy sediment was inverted from the measured sound speed and attenuation.The sound speed and attenuation data obtained for the first time in this frequency band provide extremely important data support for the study of acoustic characteristics and the validating of acoustic models of gassy sediments.3.Study about sound speed dispersion from 300 Hz to 3 kHzAccording to the same requirement of sediment acoustic experiments,a low-frequency band(300Hz-3kHz)experimental platform was designed in a laboratory channel tank.The design dimensions of sandy sediment are 4.1m long,2.44m wide and 1.13m high.Sound speed in sandy sediment was acquired by using a double-hydrophone method and a multi-hydrophone inversion method in the bounded space in the frequency band of 300Hz-3kHz.Through the continuous variation experiment of the horizontal distance between a sound source and a hydrophone,the correctness of the acoustic wave propagation path and the reliability of the sound speed data were verified.The sound speed(112-121m/s)obtained by direct measurement agree well with the sound speed(79-142m/s)obtained by inversion.But sound speed measured at different sediment areas is significantly different.At the acoustic frequency well below the gas bubble resonance frequency in sandy sediment,porous water and gas bubbles were regarded as an effective uniform fluid based on the effective medium theory in this thesis.On this basis,the density and the bulk elastic modulus of porous water in EDFM were replaced by the effective density and the effective bulk modulus of the effective uniform fluid.The numerical analysis indicates that the existence of gas bubbles will cause a significant decrease of the low-frequency sound speed in gassy sediment.By analyzing the bubble volume fraction in different sediment areas,the reliability of sound speed data was verified again,and the phenomenon of sound speed inhomogeneity in the gassy sediment was explained.