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OBS-5+ System
FIGURE 10-1. Effects of sediment size
10.2 NIR Reflectivity
The output of an OBS-5+ will increase with the NIR reflectivity of suspended
sediment independent of SSC. This can degrade accuracy when unknown
reflectivity changes occur during a monitoring campaign. For instance, when a
dredge cuts through a layer of oxidized, light-brown, reflective mud into an
underlying layer of black anoxic mud, the OBS-5+ will indicate that SSC of
mud stirred up by the cutter has dropped even when it has not. The relative
signal level per unit of SSC varies from a low of about one for dark, non-
reflective sediment to a high of about ten for white, reflective sediment
(FIGURE 10-2). In areas where sediment color changes with time, more than
one calibration curve may be required to measure SSC with an OBS-5+.
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