三门湾沿海声层析潮流观测实验

2009年9月6日至9日在三门湾进行了沿海声层析(Coastal Acoustic Tomography,CAT)潮流观测实验.实验由7台沿海声层析仪组网进行,并分别由渔船定点抛锚于7个站位.实验期间,还进行了定点ADCP(Acoustic Doppler Current Profiler)观测.通过建立逆模式对声传播时间差进行解析,引入权重因子,用L-curve法确定阻尼因子的最佳值,继而根据阻尼最小二乘法得到流速的最佳解.根据逆模式得到的流速分布可知该海区的潮流以半日潮(M2)为主,M2潮流椭圆呈东南-西北走向,潮流基本都是顺着水道,即涨潮为西北流向,退潮为东南流向.西北向与东南向最大流速分别为1.03m·s-1和1.09m·s-1.实验期间该区域的余流是从湾外流入湾内,平均流速约为0.05m·s-1.CAT与定点ADCP流速的东分量和北分量的均方差均小于0.18m·s-1.这样大面积的潮流和余流水平分布的同步观测,用传统观测手段很难实现.通过以上结果可以得出,沿海声层析技术可以作为一种新的测流方法对强潮海区进行大面积潮流观测,可在我国沿海的海洋环境监测等方面发挥重要作用.

An observational experiment of coastal acoustic tomography to map the structure of tidal currents in Sanmen Bay,China

Synchronous tidal current data are usually difficult to collect in regions disturbed by fishery activity, such as Sanmen Bay, China. Coastal Acoustic Tomography (CAT) is a promising technique to solve this problem, with which we measure tidal data can be recorded continuously even in busy coastal regions. To observe the tidal currents in Sanmen Bay using CAT, we set up seven CAT stations in the bay in September, 2009 (6^th to 9^th). The transducer was suspended at a depth of 3~5 m. Acoustic signals (5 kHz), modulated by the 10^th order M sequence were transmitted synchronously every 3 min. Meanwhile, the transducer received transmission signals from the six other stations to obtain the reciprocal travel time of each station pair. At night, an Acoustic Doppler Current Profiler (ADCP) was deployed to measure the current velocity and so verify the CAT results. In addition, conductivity-temperature-depth (CTD) casts were performed to generate the sound-speed profile. Inverse analysis was performed to the travel-time differences between each station pair, to obtain the horizontal distribution of the tidal currents. The inverse solution was obtained by the tapered least squares method, and the weighting factor (appearing in the inverse analysis) was determined using the L-curve method.The travel-time differences of the 21 station pairs show a typical semidiurnal oscillation. However, some of the data are missing due to an instrument failure. This failure also affects the subsequent inverse analysis. Missing data are interpolated using values predicted by harmonic analysis. The mean root mean square difference (RMSD) between the raw data and the predicted values is 0.49 ms, which indicates that the predicted values are very close to the raw data. The time series of the velocity fields (obtained by the inverse analysis) shows that the tidal currents flow mainly along the channel, i.e., toward northwest during the flood tide and southeast during the ebb tide. The maximum northwestward and southeastward current velocities are 1.03 m·s^-1 and 1.09 m·s^-1, respectively. The harmonic analysis of the tidal currents shows that the semidiurnal tidal constituent (M2) is dominant. The semi major axes of its tidal ellipses are directed southeast-northwest. The residual currents flow mainly northwestward, with an average velocity of 0.05 m·s^-1. The time series of the volume transport across the peripheral transects of the observed region during 5 periods of M2 shows that the mean inflow volume transport across all transects is 4735 m³·s^-1. This inflow volume transport is approximately equal to the outflow of 4371 m³·s^-1.The current velocities obtained by the inverse method agree with those obtained using the ADCP. The RMSD between the two measurements of the eastward and northward velocity components are 0.18 m·s^-1 and 0.16 m·s^-1, respectively. Thus, using CAT, we measured the continuous distribution of tidal currents over an area of about 58 km² in Sanmen Bay. The tidal and residual current structures we obtained supported previous numerical modeling results. Furthermore, our data agreed better with ADCP measurements than that data from previous CAT experiments, because we used the harmonic analysis method to interpolate the missing travel-time difference data. Our study suggests that CAT is a powerful tool to map tidal current structures. In addition, CAT could play an important role in the environmental monitoring of Chinese coastal regions.