Mapping ocean surface features using biogenic slick-fields and SAR polarimetric decomposition techniques

Abstract

Polarimetric synthetic aperture radar (POLSAR) backscatter from the ocean contains a great deal of information about the physical surface itself. The authors focus on developing a method using polarimetric information and biogenic slick-fields to map selected ocean surface features. The spiral eddy surface features and slick-fields used in this study are those commonly found in waters of the Southern California Bight. To acquire a new dataset, NASA Jet Propulsion Laboratory (JPL) and the Naval Research Laboratory (NRL) jointly flew AIRSAR missions during April 2003 over waters of the Santa Monica Basin and the San Pedro Channel. Conventional single polarisation radar data indiscriminately image the slick-fields, in addition to all unwanted contributions, e.g. intensity modulations owing to the background wave-field and local air–sea interactions. Establishment of an intensity-based threshold to segment the slicks was found to be difficult; particularly under changing wind/wave conditions. In this study several feature and slick mapping algorithms are evaluated. Parameters of the entropy, anisotropy, alpha (〈H/A/α〉) Cloude–Pottier decomposition are used to determine the polarimetric scattering properties of both the biogenic slicks and the ambient ‘clean’ surface. Use of the Cloude–Pottier parameters allows segmentation of shorter Bragg wave effects from longer wave slope modulations. An algorithm combining the Cloude–Pottier decomposition parameters 〈H/A/α〉 with the complex Wishart classifier was evaluated and subsequently selected as the algorithm of choice when using slicks for the production of ocean feature maps. Spiral eddy ocean features, generated by current flow around the southern Channel Islands, were detected and mapped using biogenic slicks as markers for the presence and shape of the eddies.