Optical Measurements of Suspended Sediments

  • Madihah Jafar Sidik


Sunlight in the ocean water is important for primary productivity, however, in coastal seas, suspended sediments (SPM) limit the availability of sunlight. The suspended sediments scatter and absorb light. Some of the scattered light travel back (backscattered light) to space and can be used in remote sensing of suspended sediments. The absorption of light by suspended particles gives coastal seas their characteristic green colour (suspended sediments in coastal water absorb blue light and only green light of the visible spectrum left in the water column). In this thesis, we evaluate two very different optical methods for measuring the suspended sediment concentration: (i) a standard remote sensing algorithm that relies on backscattering light, and (ii) a colour scale in which perceived colour of the sea is given a number (the Forel-Ule index). Satellite measurements of the mineral component of suspended solid (MSS) have been compared to measurements at a mooring in Liverpool Bay over the period 2003-2010. The mooring is equipped with an optical backscatter sensor (OBS), which has been calibrated against filtered samples to provide estimates of SPM at 10-minute intervals. There are generally good agreement between MSS from the satellite and SPM from the mooring, however, there is a clear seasonal variation in the ratio of MSS/SPM, from about 1.5 in January to 0.5 in June. Part of this variation can bee explained by the sediment being more organic in summer but effect is unlikely to explain the change by a factor of 3 in the ratio of satellite to insitu suspended materials. We interpret most of the observed variation as being due to seasonal changes in the backscattering to total scattering ratio. This ratio is high in winter when the particles are mostly mineral. The satellite, being most sensitive to backscattered light, over-estimates SPM in winter and the reverse is true in summer. Allowing for seasonal changes in mass-specific backscattering coefficient (bb*) which found in this study (higher in winter with 0.0074 m2 g-1 and lower during summer with 0.0031 m2 g-1 will lead to improvements in remote sensing of SPM (satellite SPM algorithm), which assuming a constant bb* value (0.0058 m2 g-1) by Gohin et al ., 2005 over a year. The colour of the sea expressed by the Forel-Ule index (FUI) has been calculated from MODIS satellite imagery of northwest Europe over the period 2003 to 2014. The calculations have been successfully tested against a small observational data set. The measurements of FUI and associated water properties have been made at a coastal location (the Menai Strait), for a period of one year. There is a consistent relationship in these data sets between the FUI and the concentration of SPM, which can best be represented by (for example in the Irish Sea) FUI =6.025log (SPM)+1.99. The colour of the sea, observed from space undergoes seasonal changes consistent with the known variation of SPM in these waters. The sea is greenest in winter when storms increase the SPM concentration. Spatial variation in FUI largely follows water depth. A quantitative comparison between SPM from the satellite (based on backscattering) and that based on the FUI (based on water colour) in Liverpool Bay is in a good agreement in a low concentration but not in higher concentration (more than 15 mg l-1) with the RMS is 3.69.


Original languageEnglish
Awarding Institution
  • David Bowers (Supervisor)
Thesis sponsors
  • Universiti Malaysia Saba
  • Malaysian Ministry of Higher Education
Award dateJan 2016