Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-8-A319
Timestamp: 2019-04-21 18:12:47+00:00

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The estimation of the bathymetry and the detection of targets located on the seabed of shallow waters using remote sensing techniques is of great interest for many environmental applications in coastal areas such as benthic habitat mapping, monitoring of seabed aquatic plants and the subsequent management of littoral zones. For that purpose, knowledge of the optical effects induced by the neighborhood of a given seabed target and by the water column itself is required to better interpret the subsurface upward radiance measured by satellite or shipborne radiometers. In this paper, the various sources of photons that contribute to the subsurface upward radiance are analyzed. In particular, the adjacency effects caused by the neighborhood of a given seabed target are quantified for three water turbidity conditions, namely clear, moderately turbid and turbid waters. Firstly, an analytical expression of the subsurface radiance is proposed in order to make explicit the radiance terms corresponding to these effects. Secondly, a sensitivity study is performed using radiative transfer modeling to determine the influence of the seabed adjacency effects on the upward signal with respect to various parameters such as the bathymetry or the bottom brightness. The results show that the highest contributions of the adjacency effects induced by the neighborhood of a seabed target to the subsurface radiance could reach 26%, 18% and 9% for clear, moderately turbid and turbid water conditions respectively. Therefore, the detection of a seabed target could be significantly biased if the seabed adjacency effects are ignored in the analysis of remote sensing measurements. Our results could be further used to improve the performance of inverse algorithms dedicated to the retrieval of bottom composition, water optical properties and/or bathymetry.
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Fig. 1 Sources of the photons that contribute to the subsurface upward radiance Lu: Ludp is the radiance due to photons coming from the water column without having previously interacted with the bottom of the ocean, LBdir is the direct radiance that represents the photons which directly comes from a seabed target pixel P up to the sea surface without being scattered, LPdif is the diffuse radiance coming from the seabed target only, LMdif is the diffuse radiance of the neighborhood of the seabed target (excluding the seabed target), LBdif is the radiance that represents the photons coming from all pixels located in the seabed (i.e., target + neighborhood of the target M) and that are scattered towards the direction of observation at the subsurface level (LBdif is in fact the sum of LPdif and LMdif), ρt is the reflectance of the seabed target pixel P, ρn↑ is the reflectance of a pixel in the neighborhood of the seabed target pixel P, ρenv↑(P) is the reflectance caused by all the pixels located at the seabed (i.e., the seabed target P and the neighborhood of the target M), Etot is the downward flux reaching the bottom.
Fig. 2 Representation of the photons that contribute to the environment function γenv for the diffuse upward radiance.
Fig. 3 Environment function Genv(R) (or δ↑) [Eq. (8)] at 550 nm, as a function of the radius R of a circular target P for bottom depth values of 1 m and 5 m and for the moderately turbid water condition.
Fig. 4 Representation of the two configurations that are used to model the bottom albedo: (a) a bright seabed target of reflectance ρc surrounded by dark pixels having a low bottom albedo ρd (case 1), (b) a dark seabed target of bottom albedo ρd surrounded by bright pixels having an albedo ρc (case 2).
Fig. 5 Relative variation of seabed contributions Δ (in %) at the subsurface level when accounting for or neglecting seabed heterogeneities (i.e., the weight of the seabed target is maximum when heterogeneities are neglected; δ↑ = 1) for three water turbidity conditions (a) clear water, (b) moderately turbid water, (c) turbid water. The results are presented for the following conditions: target size radius of 0.2 m, three bottom depth values (1 m, 5 m, 10 m) and two bottom albedo configurations (bright and dark seabed target/neighboring pixels as illustrated in Fig. 4).
Fig. 6 (a) Bottom reflectance spectra ρt of the seabed target (mix of the biogenic species Posidonia and Caulerpa Taxifolia) (red line) and its neighboring pixels (sand, blue line) ; Figs. 6(b)-6(d) show the terms δ↑ × ρt (red dotted line) and ρn↑ (blue dotted line) of the environment reflectance [Eq. (6)] for the clear water condition and for various bottom depth values: (b) 5 m, (c) 10 m, (d) 15 m.
Fig. 7 Representation of the various radiances that contribute to the subsurface upward radiance, namely LBdir, LPdif, LMdif and Ludp, for (a) clear water, (b) moderately turbid water and (c) turbid water. Representation of the ratio ΔAE = [LBdif(δ)-LBdif(δ = 1)]/Lu, which quantifies the seabed adjacency effect, for (d) clear water, (e) moderately turbid water and (f) turbid water. The bottom depth value is 5 m.
Fig. 8 Same as Fig. 7 but for a bottom depth value of 15 m and for the clear water case: (a) representation of the various radiances that contribute to the subsurface upward radiance, (b) ratio ΔAE.
Fig. 9 (a) Variations of the environment function Genv with respect to the seabed target radius for different values of the Junge exponent of the size distribution of the hydrosols: 3.5, 4 and 4.5, for H = 1 m and 5 m and for moderately turbid water (same as Fig. 3), (b) ratio of Genv calculated for two couples of Junge exponents: (3.5 and 4) and (4.5 and 4) for H = 5 m.
Table 1 Parameters used as inputs in the OSOAA model: aCDOM (440 nm) is the absorption coefficient of CDOM at 440 nm, the CHL concentration is in mg m−3, the SED concentration is in mg L−1, τw is the water optical depth at 550 nm.
Table 2 Bottom albedo values that are used for bright and dark pixels as inputs of the radiative transfer modeling at various wavelengths.
Table 3 Mean value of the relative variation of seabed contributions Δmean (in %); the seabed target size radius R value is 0.2 m; the bottom depth values are 1, 5 and 10 m for three water turbidity conditions (clear, moderately turbid and turbid waters). σ is the standard deviation (in %).
Table 4 Maximum values of the spectral ratio ΔAE for various bottom depths and for various water turbidity conditions.
Table 5 Mean value of the relative variation of seabed contributions Δmean (in %) as a function of the seabed target radius R for various bottom depth values H (1, 5 and 10 m) and for the moderately turbid water condition. σ is the standard variation (in %).
Parameters used as inputs in the OSOAA model: aCDOM (440 nm) is the absorption coefficient of CDOM at 440 nm, the CHL concentration is in mg m−3, the SED concentration is in mg L−1, τw is the water optical depth at 550 nm.
Bottom albedo values that are used for bright and dark pixels as inputs of the radiative transfer modeling at various wavelengths.
Mean value of the relative variation of seabed contributions Δmean (in %); the seabed target size radius R value is 0.2 m; the bottom depth values are 1, 5 and 10 m for three water turbidity conditions (clear, moderately turbid and turbid waters). σ is the standard deviation (in %).
Maximum values of the spectral ratio ΔAE for various bottom depths and for various water turbidity conditions.
Mean value of the relative variation of seabed contributions Δmean (in %) as a function of the seabed target radius R for various bottom depth values H (1, 5 and 10 m) and for the moderately turbid water condition. σ is the standard variation (in %).
ΔΑΕ Relative difference in the subsurface radiance when accounting for or neglecting seabed heterogeneities ΔAE = [LBdif(δ)-LBdif(δ = 1)]/Lu. ΔAE actually quantifies the influence of adjacency effects (AE) due to the bottom albedo of neighboring pixels on the upward radiance.

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