摘要
利用后散射强度 (ABS)估计悬浮沉积物浓度原理 ,根据Rayleigh散射理论 ,给出了利用现场粒径分析仪与声学多普勒流速剖面仪 (ADCP)同步测量悬浮沉积物浓度的粒径修正方法 ,并推导出了一种新的、用于对ABS进行粒径修正的等效粒径计算公式 ( 9)。利用这一公式并结合LISST 1 0 0所测的粒径分布信息 ,可以对ADCP所测的ABS进行粒径修正 ,其修正方法见式 ( 1 0 )。结合利物浦湾和Anglesey周边海域的现场采样、测量及其分析结果 ,对上述粒径修正方法进行了验证。分析结果表明 ,采用上述公式进行粒径修正后 ,ADCP测量悬浮沉积物浓度的精度有较大程度的提高 ,线性拟合的相关系数从 0 .65— 0 .71提高到 0 .78— 0 .88。
Measurements of suspended sediment load are crucial to the study of dynamics of suspended sediment. Over last decade, much work has been carried out in exploring the potential of ADCP (Acoustic Doppler Current Profiler) to estimate SSC (Suspended Sediment Concentration), due to its commercial availability and widespread application in measuring current velocity. One of the great advantages of the ADCP is its capability of measurement throughout the whole water column for a long period and the combination of measurements of vertical profiles of fluid velocities and back scatter intensities. This method not only improves temporal and spatial resolutions of SSC estimates, which are essential to the study of suspended sediment dynamics, but also can directly evaluate profiles of suspended sediment flux, and hence transport from just one instrument. Analysis of acoustic backscatter data with suspended sediment concentrations, however, must take account of the fact that ADCP observation may represent concurrent changes in concentration and morphology of suspended sediment without discrimination. Detailed knowledge of the characteristics of the suspended sediment, therefore, is essential for obtaining an accurate measurement. Variable nature of sound scatterers and some other physical parameters in ocean makes it difficult for in-situ estimation of all relevant parameters to inverse acoustic back scatter signals to accurate measurements of suspended sediment concentration with high resolution. Accurate measurements can’t be obtained without particle size correction due to particle size dependence of ABS (Acoustic Backscattering) measurement. Based on Rayleigh Scattering Theory, the equivalent particle radius as well as the calibration equation was derived for particle size correction of ABS. When the wavelength of incident sound is much greater than the diameter of scatters, Rayleigh scattering function applies, which means that the backscattering cross section is proportional to the fourth power of the scatter radius. Therefore, in the water where scatters are widely and randomly spaced, the backscattering cross section of the individuals simply adds to form the total backscattering cross section. Under the assumption that the scatters are homogeneous except their radius, the total backscattering intensity can be further simplified to be proportional to the sum of the third power of particle radius of every size band multiplied by its mass concentration. The volume concentration and particle size distribution can be obtained from in-situ particle size spectrum analyzer. With these data, we can defined an equivalent particle radius which could be used to correct ABS measured by ADCP to inverse SSC.A series of field experiments were conducted respectively to help understand the acoustic measurements of SSC by ADCP and verify the correction scheme mentioned above in Liverpool Bay and coastal water of Anglesey, by deploying a 600kHz RDI ADCP near the seabed. Vertical profiles were obtained with a Sea-Bird CTD fitted with a LISST-100 and water sampling bottles. Water samples from CTD-mounted bottles were used to measure SSC by filtering a known volume of the water sample and by differential weighing of the dried filters, while the measurements from LISST100 were used to analyze PSD (Particle Size Distribution) of the suspended sediment and obtain equivalent particle radius for ABS correction. To secure an accurate inversion result, time average should be applied to ADCP echo intensities before further analysis to reduce the random fluctuation due to the random position of the suspended scatters within the ensonified region of the water. Data analysis of those field tests showed considerable improvements of the measurements after applying the particle size correction formula, with the correlation coefficients growing from 0.65—0.71 before the correction to 0.78—0.88 after the correction.
出处
《海洋与湖沼》
CAS
CSCD
北大核心
2004年第5期385-392,共8页
Oceanologia Et Limnologia Sinica
基金
中国科学院重大科技合作项目
H4 2 0 32 6 0 2号
中国科学院海洋研究所知识创新领域前沿项目
L84 0 32 6 0 9号
中国科学院留学基金项目
2 0 0 2- 2 0 0 3
关键词
ADCP
悬浮沉积物
粒径修正
后散射强度
ADCP, Suspended sediment concentration, Particle size correction, ABS
