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Section 8. Factors Affecting OBS Response

02468

Munsell Value (Black = 0)

0.0

0.2

0.4

0.6

0.8

1.0

Infrared Reflectance

Calcite

Bytownite

Actinolite

Magnetite

FIGURE 8.4-1. IR Reflectance of Minerals

8.5 Water Color

Several OBS users have been concerned that color from dissolved substances

in water samples (not colored particles discussed in the previous section)

produces erroneously low turbidity measurements. Although organic and

inorganic IR-absorbing dissolved matter has visible color, its effect on OBS

measurements is small unless the colored compounds are strongly absorbing at

the OBS wavelength (875 nm) and are present in very high concentrations.

Only effluents from mine-tailings appear to produce enough color to absorb

measurable IR. In river, estuary, and ocean environments concentrations of

colored materials are too low by at least a factor of ten to produce significant

errors.

8.6 Bubbles

Although bubbles efficiently scatter IR, monitoring in most natural

environments shows that OBS signals are not strongly affected by bubbles.

Bubbles and quartz particles backscatter nearly the same amount of light to

within a factor of approximately four, but most of the time bubble

concentrations are at least two orders of magnitude less than sand

concentrations in most environments. This means that sand will produce much

more backscatter than bubbles in most situations and bubble interference will

not be significant.

The scattering intensity of mineral particles, bubbles, and suspended organic

material are shown in FIGURE 8.6-1. OBS sensors detect IR backscattered

between 140

and 160

, and where the scattering intensities are nearly constant

with the scattering angle. Particle concentration has the most important effect

8-4

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Campbell OBS-3A Spezifikationen Seite 50