WATER COLOUR AND SUSPENDED SEDIMENTS: BAYS


[NRC-GSC/DFO]

LANDSAT false colour image of part of the Bay of Fundy
DID YOU KNOW? The first satellites to measure water colour were designed for land use. The early LANDSAT satellite series carried a multi-spectral scanner [MSS] measuring reflected visible light from the earth's surface. Land reflectance signals are more than a order of magnitude stronger than those from the ocean's surface. However, with very high levels of water turbidity a water signal is strong enough to be detected by the sensors designed for land use.

CAN SATELLITES SEE THE SEDIMENTS IN MY REGION?

Making a Secchi disc measurement


That depends on three factors.

The first two factors are linked. The land satellites detect the very high concentration levels of suspended sediments, they record data at a very high spatial resolution - between 100m to 20m and pass over an area just a few times per month. The ocean water colour satellites can detect the lower concentrations of suspended sediment, but at spatial resolutions of 1km and pass over a region several times per week. If your region is a small bay with a medium to low level of suspended sediments then you may need to consider obtaining imagery by aircraft. The concentration range is important because satellite algorithms are basically statistical fits and as such work best over a large dynamic range. The range suitable for optical-satellite processes may be different to the range being studied in connection with environmental processes. One of the simplest [and cheapest] methods of determining the optical range is to take Secchi disc [see above right] measurements throughout a region. The final determining factor of whether or not to use satellite data is the cost. In Canada satellite images are sold commercially and can cost from $750 to $3,000 per picture. For current costs see

http://www.rsi.ca/frame.htm

SATELLITE MAPS OF SUSPENDED SEDIMENTS

Satellite or aircraft imagery can show the spatial patterns of sediments. A time series of imagery may be able to show how sediment features (plumes or patches) are moving. But if a map of suspended sediment concentration is required then the imagery must be calibrated. One of the few examples of a calibrated map of suspended sediments is shown below.

To date, a "transferable" calibration for obtaining suspended sediments from satellite imagery does not exist. This means that for every different type of satellite which can "see" suspended sediments a new field experiment has to be conducted to produce an algorithm. Since the term "suspended sediment" is generic and can refer to a wide variety of material of varying composition and grain size, it also follows that algorithms developed for one location (sediment type) have rarely been successful when applied to another geographical region.

Example of aircraft flight lines and helicopter-satellite sampling sites.
Click on map to see full-size image

A series of experiments using aircraft and LANDSAT imagery have been conducted in those regions of the Bay of Fundy which have very high concentrations of suspended sediments [typically 10 to 1,000 mg/L]. In these regions the optical properties of the sediments may dominate over any contribution made by the other water constituents such as dissolved organics and biological pigments [see CZCS - INTERPRETING COASTAL WATER TURBIDITY IMAGES].

Aircraft false colour image of sediment plumes close to the CSS Dawson undertaking sampling.

Regions with such high concentrations of suspended sediments are often very dynamic. The Bay of Fundy experiences the highest tides in the world. This means that sediment concentration at any given location will change very rapidly as seen by the location of the ship [in boxed region] close to a large gradient in sediments. Whereas a large range in material is desirable for developing a satellite algorithm, the rapid changes in concentration levels make timing between in-situ and satellite sampling a critical issue. This problem was tackled for the Bay of Fundy by "grabbing" surface water sample along a line of strong sediment gradients, from a water sampler lowered from a helicopter.

Further experiments collected samples from launches deployed from a science vessel working at station at one end of the sediment gradient. The results of these "fast moving" unique data collection experiments was a set of data values from which sediment algorithms were derived.

Comparison between satellite and in-situ measured SSC.
Click on image to see full-size graph

The shift in the maximum wavelength with increasing sediment concentrations.
Click on image to see full-size graph

The sediments in the head of the Bay of Fundy are a striking red colour. The optical contribution to water reflectance for most lower concentration suspended sediments are thought to be solely from an increased scattering of light at all wavelengths with little or no shift in wavelength with increased concentrations. For the red sediments in the Bay of Fundy there is a marked shift in the maximum reflected wavelength. This optical characteristic may be a significant reason why satellite and aircraft calibration studies have been so successful for the Bay of Fundy, despite the fact that the region is so dynamic.

At lower concentrations (<10 mg/L) the satellite predictions were (the open circles in the figure to the right) all too high. These particular samples were collected during a biological bloom and so the water had a significant additional optical contribution from that biological bloom. In this case the presence of high, inshore, pigment concentrations interfered with the sediment prediction model - for the more typical reverse case see - CZCS - INTERPRETING COASTAL WATER TURBIDITY IMAGES].

REFERENCES:

Amos, C.L., and Alfoldi, T.T., 1989. The determination of suspended sediment concentration in a macrotidal system using Landsat data. Journal of Sedimentary Petrology, 49, 159-174.

Topliss, B.J. l984. Aircraft Multispectral Remote Sensing of Suspended Sediments in a Turbid Macrotidal Environment. Mathematical Geology, 16(7), 7l9735

Further details on marine sedimentology can be obtained from the WEB pages of the Marine Environmental Geoscience section of the Geological Survey of Canada, Atlantic.

http://agcwww.bio.ns.ca/menviro/meghome.html

Page Last Updated 03/04/1998