| Sediment size
has the second largest effect on OBS measurements.
Size effects do not degrade OBS measurements when the sediment
size remains constant. If it changes unpredictably, however,
water turbidity and SSC will appear to change even though it
has not. Tests on OBS-3 sensors by the U.S. Navy Civil Engineering
Laboratory with ground-glass particles indicate that the response
of an OBS-3 sensor to angular clear material, expressed in Volts
per g per liter, declines in proportion to the grain diameter
raised to the power –0.6. The Navy results are relevant
to translucent angular particles of minerals like quartz and
calcite in the silt–sand size range. They illustrate the
general effect of particle area on an OBS signal. For example,
one gram of silt, with a grain size of 10 microns, suspended
in a liter of water (SSC = 1000 mg l-1) might produce
an OBS signal of 1 Volt, whereas a gram of sand with a grain
size of 100 microns would produce only a 0.25-Volt signal, with
other factors such as shape and mineral composition being the
same. Our experience is that variation of sediment size can
result in OBS sensitivity varying be more than a factor of 200.
The Navy tests and our own sediment calibrations underscore
the importance of sediment calibrations and restricting use
of OBS sensors to systems in which the material size does not
vary during a monitoring campaign. |
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Reference:
Conner, C.S. and A.M. De Visser. 1992. A Laboratory
Investigation of Particle Size Effects on an Optical Backscatterance
Sensor. Marine Geology, 108, pp.151-159.
Gibbs, R.J. & E. Wolanski. 1992. The Effects
of Flocs on Optical Backscattering Measurements of Suspended
Material Concentration. Marine Geology. Vol.
107, pp. 289-291.
Ludwig, K.A. & D. Hanes. 1990. A Laboratory
Evaluation of Optical Backscatterance Suspended Solids Sensors
Exposed to Sand-mud Mixtures. Marine Geology.
Vol. 94, pp. 173-179.
Sheldon, R.W., A. Prakask, and W.H. Sutcliffe, Jr. 1972.
The Size Distribution of Particles in the Ocean.
Limnology and Oceanography, Vol. XVII(3), pp.
327-340. |
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