Classification And Change Analysis Of Coral Reef Geomorphology Based On Sentinel-2 Multitemporal Image

The importance of coral reefs in the ocean is self-evident.The South China Sea area is the key area and the primary support for the realization of my country's maritime power strategy.A large number of coral reefs grow around it.Surveying the distribution of coral reefs and geomorphological conditions will facilitate the implementation of the maritime power strategy and promote the country's geographical monitoring process.In this paper,some islands and reefs in the Yongle Atoll in the South China Sea are used as the research area.Sentinel-2 satellite remote sensing image data is selected to classify the coral reef geomorphology and extract area information.Analyze the correlation of water environment parameters,coral reef bleaching,and El Nino and typhoons with coral reef geomorphological changes.The specific work is as follows:(1)Extract and analyze the coral reef geomorphology in the study area.First,pre-process the four Sentinel-2 satellite image data acquired from 2015.12 to 2019.12;then,based on previous experience and the actual situation of the research area,formulate a coral reef geomorphological classification system and select a classification sample set;second,use K-Nearest Neighbor,Decision Tree,Random Forest,AdaBoost,Gradient Boosting,Gaussian NB,Linear Discriminant Analysis and Quadratic Discriminant Analysis classify the coral reef geomorphology of the same time image,and select the optimal classifier for the rest of the time according to the classification accuracy Facies data classification;finally extract the information on the area and distribution of coral reef geomorphology in each phase of the study area.(2)The correlation analysis of multi-parameters and coral reef geomorphological changes.First,collect the Chl-a concentration,POC concentration and PAR data released by NASA respectively,NOAA releases coral reef bleaching monitoring products-SST,SSTA,HS,DHW and BAA data,and El Nino and typhoon meteorological information data in the study area during this period;Then,perform statistical analysis on various types of data in turn;and then analyze the correlation between various types of data and coral reef geomorphological changes;and finally get the parameters that are highly correlated with coral reef geomorphological changes and the impact of each parameter on coral reef geomorphological changes.This study obtained the following results:(1)Coral reef geomorphological classification and statistical analysis.The overall classification accuracy of AdaBoost classifier,Random Forest classifier,K-Nearest Neighbor classifier,Gradient Boosting classifier and Decision Tree classifier are all greater than 80%.The overall classification accuracy of the AdaBoost classifier is the highest,reaching 87.984%,and the Kappa coefficient is 0.847;from 2015.12 to 2019.12,the geomorphology below the study area changed greatly.Such as,the reef level outside the study area has increased by 0.8708km2,the biological sparse zone has decreased by 1.3355km2,the biological dense zone has increased by 0.7093km2,the reef development zone has increased by 0.4853km2,the lagoon slope has decreased by 0.501km2,and the point reef area has decreased 1.0739km2,the area of coral sand increased by 0.8884km2.(2)Correlation between changes in water environment parameters and changes in coral reef geomorphology.There are high positive correlations between the reef front,outer reef flat,bio-dense zone,reef pit development zone and coral sand and Chl-a concentration and POC concentration.There is a negative correlation between the reef ridge,bio-sparse zone,lagoon slope and point reef and Chl-a concentration and POC concentration.Indicating that when the above-mentioned various types of geomorphological increase or decrease,the concentration of Chl-a and POC in the water environment will also increase.According to this analysis,changes in water environment parameters are related to changes in coral reef geomorphology.(3)Correlation between coral reef bleaching and coral reef geomorphological changes.The area increase of the outer reef flats,reef pit development zones and coral sands in the coral reef geomorphology of the study area is positively correlated with the interannual changes of SST and SSTA;the decrease in the area of reef ridges,biological sparse belts and spot reefs is related to the year of SST and SSTA The international change is negatively correlated.According to the analysis of coral reef bleaching monitoring products provided by NOAA,various degrees of bleaching occurred in the study area from 2016 to 2019.Coral reef bleaching caused an increase in geomorphological areas such as outer reef flats,reef pit development zones and coral sands,resulting in reef ridges and biological sparse zones The area of Hedian Reef decreased.Based on this analysis,the coral reef bleaching caused by the rise of SST in the study area has an impact on the coral reef geomorphology.(4)The impact of El Nino and typhoon on coral reef geomorphological changes.El Nino and typhoon indirectly affect coral reef geomorphology through changes in SST and Chl-a.For example,from 2015.12 to 2016.05,under the influence of El Nino,SST increased in the study area,causing coral reefs to bleach,resulting in an increase in geomorphological areas such as outer reef flats,reef development zones,and coral sands,resulting in reef ridges,biological sparse zones,and spot reef areas.Decrease;4 typhoons occurred in 2017,which reduced the average annual SST in 201 7 and the concentration of Chl-a increased significantly.Among them,the concentration of Chl-a and the reef front,outer reef flat,bio-dense zone,reef pit development zone and coral The increase in sand area is positively correlated.