In order to understand how photons from a laser beam are scattered as they move from air into turbid water, we ran an AO3D simulation (Figures 11-15). The source was a 0.1-meter radius laser beam at a height of 100 m, directed downward at a zenith angle of 150 degrees, pointing North, with a Southerly wind of 10 m/s. The atmosphere is a polluted marine aerosol (scattering coefficient 2.7x 10-4 m-1, absorption = 0), the water is from the Snohomish River data (scattering coefficient 2.84 m-1, absorption coefficient 0.25 m-1). Figure 11 shows the logarithm of the relative strength of the downwelling photon density at the point in space. For illustration purposes, only downward moving photons are shown. It is remarkable how rapidly the downwelling light is extinguished. Including upwelling photons complicates the illustration, but is included in the simulation output. The occurrence of photons outside the main laser beam entering the water outside the beam is due to atmospheric scattering between the laser and the ocean surface.
Figure 11. Downwelling photon density (upwelling photons not shown), normalized to beam intensity, for 443 nm, 10,000 photons, flat sea surface (wind speed = 0). Color scale is base-10 logarithmic and sea level at z = 0 m.
Figure 12. Downwelling photon density (upwelling photons not shown), normalized to beam intensity, for 443 nm, 10,000 photons, C&M sea surface (wind speed = 10 m/s). Color scale is base-10 logarithmic and sea level at z = 0 m.
photon density at water surface
(depth of 0 m).
Same conditions as for
Figure 12, but with 100,000 photons.
The beam is 0.1 m radius, as initialized, the “purple”
photons are those
forward scattered within the atmosphere.
Again, only downwelling photons are shown.
Slider at right on actual Matlab AO3D application allows
select depth of cross-section display.
Grid is offset to North , allowing deeper layers to be
shown on the same
axis ranges, because the beam has a 30-degree nadir angle.
Figure 14. Same conditions as for Figure 13, but with depth of 2 m.
Figure 15. Same conditions as for Figure 13, but with depth of 4 m.