Influence of forward and multiple light scatter on the measurement of beam attenuation in highly scattering marine environments

Abstract

Using three-dimensional Monte Carlo radiative transfer simulations, we examine the effect of beam transmissometer geometry on the relative error in the measurement of the beam-attenuation coefficient in an aquatic environment characterized by intense light scattering, especially within submerged bubble clouds entrained by surface-wave breaking. We discuss the forward-scattering error associated with the detection of photons scattered at small angles (< 1 degrees) and the multiple-scattering error associated with the detection of photons scattered more than once along the path length of the instrument. Several scattering phase functions describing bubble clouds at different bubble void fractions in the water are considered. Owing to forward-scattering error, a beam-attenuation meter (beam transmissometer) with a half-angle of receiver acceptance of 1.0 degrees and a path length of 0.1 m can underestimate the true beam attenuation within the bubble cloud by more than 50%. For bubble clouds with a beam attenuation of as much as 100 m(-1), the multiple-scattering error is no more than a few percent. These results are compared with simulations for some example phase functions that are representative of other scattering regimes found in natural waters. The forward-scattering error for the Petzold phase function of turbid waters is 16% for a typical instrument geometry, whereas for the Henyey-Greenstein phase function with the asymmetry parameter of 0.7 and 0.9 the error range is 8-28%.

Signal of single scattering albedo in water leaving polarization

“…As a first step the all of non-trivial elements of the Mueller matrix for the bubble size distributions (measured acoustically at the Scripps Institution of Oceanography, see [13,14]) were calculated. We have used a normalized form of the Mueller matrix (relative to the element S 11 ), which therefore by definition equal to unity (S 11 = 1).…”

Spectral variations of light scattering by marine particles in coastal waters, from the visible to the near infrared

“…Errors associated to beam attenuation or b p measurements when using an ac-9 sensor (10-cm path length) in highly scattering marine environments have been estimated by Piskozub et al (2004). On the basis of Monte Carlo simulations, these authors showed that errors on b p only increase from about 18% to 25% when b p increases from 0.1 to 100 m 21 .…”

The influence of schlieren on in situ optical measurements used for particle characterization

“…This is coincident with c values of 30-45 m -1 , corresponding to a beam transmission of 14% to 22%. Such low transmission values can be due to bubbles (Piskozub et al 2004). However, as no bubbles were observed in the DFC images, it is unlikely that bubbles influenced the measurements.…”

“…For decades, optical instruments measuring light scattering and/or light transmission in situ have been used to derive information about the nature of the underwater light field (Beardsley Jr. et al 1970;Jerlov 1976;Voss 1992;Bogucki et al 2004;Piskozub et al 2004;Agrawal 2005;Bogucki and Domaradzki 2005), suspended particle size and concentration (Jones and Wills 1956;Spinrad et al 1983;Bale and Morris 1987;Agrawal and Traykovski 2001;Boss et al 2001a,b;Fugate and Friedrichs 2003;Ellis et al 2004), suspended particulate matter dynamics (Pak 1983; Thorpe and White 1988;McPheeShaw et al 2004), and to classify water masses (Jerlov 1976;McKee et al 1999). Correct interpretations of the data collected by these instruments rely on knowledge of their behavior through a range of conditions that may exist in the waters being studied.…”