水中油对水体上行辐亮度光谱的影响

The Effect of Oil-in-Water on the Upward Radiance Spectrum in Seawater

目前,学者主要关注利用遥感技术探测海面油膜。然而,经海洋物理过程或人为喷洒化学分散剂处理形成的水中油对海洋生境也具有危害作用。水体上行辐亮度是水色传感器的重要信号源,通过分析含油水体的上行辐亮度光谱特征,探索快速有效地遥测水中油的方法对保护海洋生境具有重要意义。基于大连港海域现场实测数据及Hydrolight模拟含油水体水下光场,通过分析上行辐亮度随波长、水深及太阳天顶角的变化特征,剖析水中油对上行辐亮度光谱的影响及水中油的敏感光谱特性。结果表明水中油的主要波谱响应区间位于可见光波段(380~760nm)。随着水中油浓度的增加,上行辐亮度光谱峰值有逐渐向长波方向移动及蓝光波段辐亮度量值逐渐降低的趋势,这些变化处于水色遥感的探测光谱范畴,为利用水色遥感技术探测水中油提供了光谱依据。其次,上行辐亮度随水深逐级递减,并在接近水体下界面前不降反升的现象说明刚好在水面之上的上行辐亮度由各深度水体组分的后向散射及下界面的反射共同贡献,再经水汽界面上行透射而得,属于水体辐射传输的核心机理。这与水面油膜通过油类物质改变海表反射率而产生与自然海表不同反射光谱的探测机理具有本质上的差别。再者,与含水中油水体后向散射产生的上行辐亮度相比,海表对太阳光的反射属于强信号,会掩盖水体组分信息。水色卫星搭载的水色传感器具有一定的侧摆能力,能避开太阳辐射反射信号并接收到含水中油水体的上行辐亮度;水色卫星的当地过境时间一般为10至14点,且水色传感器具有高信噪比特征,满足含水中油水体的暗像元探测要求。该研究揭示了水色遥感探测含水中油水体的光谱和机理依据,表明可以视水中油为一种新的水体组分,基于光在水体中的辐射传输过程,开展含水中油水体的水色遥感反演研究。

Currently,oil slick detection at a large scale by remote sensing is widely concerned.However,oil-in-water,which is subject to seawater and chemical dispersant’s physical process,can also harm marine environment and organisms.The upward radiance of seawater is the key signal of watercolor sensors,so finding the approaches to detecting oil-in-water by analyzing the feature of the upward radiance spectrum for oil-in-water waters is also significant for the marine ecosystems.A simulation,which parameters were from the ground-based measurement at Dalian Port,was made by Hydro light for the computation of the underwater light field of seawater,which contains oil-in-water.To reveal the feature of the upward radiance as a function of wavelength,water depth and zenith angle,and to analyze the effect of oil-in-water on the upward radiance spectra and its response spectra in seawater medium.The simulation shows that oil-in-water mainly affects the visible spectrum.The peak of the upward radiance spectrum shifts from 417.5 to 442.5~472.5 nm and the value of the radiance decreases as the content of oil-in-water increases.It is the primary spectrum basis for detecting oil-in-water by the ocean color remote sensing techniques,for these responses belong to the detection domain of watercolor remote sensing.Besides,the upward radiance curves decrease with increasing water depth,then increase near the bottom.It indicates that the upward radiance just above the sea surface is contributed by the backscattering of water components at depth,the reflection of the bottom and the upward transmission through the air-water surface,following the radiative transfer model.It is different from the detection mechanism of oil slicks by changing the reflection ratio of the nature sea surface.Furthermore,compared with seawater’s upward radiance,which contains oil-in-water,the reflection of direct solar radiation is powerful enough to shield the upward radiance.The swing capability of satellite-borne ocean color sensors can exclude the direct solar reflection to sense the upward spectral signals of oil-in-water.Meanwhile,the high signal to noise ratio of ocean color sensors and the transit time of ocean color satellites from 10 am to 2 pm also meet the detection requirement of dark pixels for waters containing oil-in-water.This paper indicates the spectral basis and the radiative transfer mechanism to detect oil-in-water by ocean color remote sensing techniques.It shows that the oil-in-water can be regarded as a new water component to develop the remote sensing inversion model to retrieve oil-in-water by the radiative transfer procedure of light in water media.