High Frequency Acoustic Tomography Method At Coastal Estuary Areas

Acoustical techniques play an important role at marine environmental observations. High frequency Estuary Acoustic Tomography (EAT) provides an effective acoustical way for shallow water temperature, salinity, sound speed distribution and current field observations in estuaries and bays. In recent years, the applications of ocean acoustic tomography at coastal and marginal sea in the Asian-Pacific areas have received much attention. Most of the previous studies focused on middle frequency acoustic tomography observations at about10kilometer scale. However, there is little progress about high frequency acoustic tomography at coastal estuary areas. In this thesis, various topics about high frequency EAT, such as the theoretical study, computer simulation, instrument development, and field experiment, were all studied.First, the reciprocal transmission model at coastal estuary areas was introduced. Next, a grid based high spatial resolution reciprocal transmission inversion method in coastal estuary was proposed. Based on the surface temperature data obtained from remote sensing and the current field data from regional ocean model, theoretical studies of high spatial resolution EAT were employed to inverse two-dimensional temperature distribution and current field of the study sea area at Jiulong River estuary of Xiamen. The effects of acoustic stations number and travel-time measurement error to inversion accuracy were also discussed. The results showed that the grid based method can reconstruct the sound speed and temperature distribution and current field at40m spatial resolution with high accuracy.Furthermore, a high frequency (15kHz) EAT observation instrument was also developed. Based on the National Instrument data acquisition card and LabVIEW programming, this EAT observation instrument synchronized using GPS hardware timing method and at the accuracy of several microseconds. Thus the acoustical transmission time was measured very accurately.At last, a field experiment of high frequency acoustic tomography at Lujiang channel of Xiamen was implemented. In this experiment, the accuracy reciprocal transmission time was measured, and thus obtained the sound speed and the current velocity variance. This measurement result of tidal velocity also matched some previous results from field observations and modal studies. The length of PN code and its impacts on the Signal to Noise Ratio and current measurement accuracy were also discussed.In this thesis, the theoretical and experimental studies reflected that the high frequency EAT provides an effective way on high spatial resolution temperature, current, and other environmental parameters’ observations at coastal estuary areas.