Red Tide Detection Method for HY-1D Coastal Zone Imager Based on U-Net Convolutional Neural Network

Existing red tide detection methods have mainly been developed for ocean color satellite data with low spatial resolution and high spectral resolution. Higher spatial resolution satellite images are required for red tides with fine scale and scattered distribution. However, red tide detection methods for ocean color satellite data cannot be directly applied to medium-high spatial resolution satellite data owing to the shortage of red tide responsive bands. Therefore, a new red tide detection method for medium-high spatial resolution satellite data is required. This study proposes the red tide detection U-Net (RDU-Net) model by considering the HY-1D Coastal Zone Imager (HY-1D CZI) as an example. RDU-Net employs the channel attention model to derive the inter-channel relationship of red tide information in order to reduce the influence of the marine environment on red tide detection. Moreover, the boundary and binary cross entropy (BBCE) loss function, which incorporates the boundary loss, is used to obtain clear and accurate red tide boundaries. In addition, a multi-feature dataset including the HY-1D CZI radiance and Normalized Difference Vegetation Index (NDVI) is employed to enhance the spectral difference between red tides and seawater and thus improve the accuracy of red tide detection. Experimental results show that RDU-Net can detect red tides accurately without a precedent threshold. Precision and Recall of 87.47% and 86.62%, respectively, are achieved, while the F1-score and Kappa are 0.87. Compared with the existing method, the F1-score is improved by 0.07-0.21. Furthermore, the proposed method can detect red tides accurately even under interference from clouds and fog, and it shows good performance in the case of red tide edges and scattered distribution areas. Moreover, it shows good applicability and can be successfully applied to other satellite data with high spatial resolution and large bandwidth, such as GF-1 Wide Field of View 2 (WFV2) images.