(1. Guangzhou Marine Geological Survey Guangzhou 510760; 2. Key Laboratory of Seabed Mineral Resources, Ministry of Land and Resources, Guangzhou 5 10760)

Brief introduction of the first author: Hwang Weng-Sing (1985—), master, assistant engineer, mainly engaged in remote sensing geology and structural geomorphology research, email: jackyhwxing @163.com.

In recent decades, with the development of coastal economy, environmental problems have become increasingly prominent, and coastal environmental geological problems have been paid more and more attention. Satellite remote sensing is widely used in coastal environmental geological survey because of its real-time, fast and efficient characteristics. These applications include coastal zone classification, coastline extraction, inshore water depth detection, inshore suspended sediment, sea surface temperature (SST), salinity (SSS), chlorophyll concentration inversion and other environmental geological contents. This paper briefly introduces the main principles, methods and shortcomings of these applications.

Keywords satellite remote sensing; Coastal environmental geological survey

1 preface

Coastal zone is an interactive zone between land and sea, and it is also an important area for human survival and development. Due to the change of natural environment and the interference of human activities, the environmental geological problems in coastal areas are increasingly prominent, mainly manifested in sea level rise, seawater intrusion, land subsidence, coastal erosion, storms, red tides and so on. Therefore, it is of great significance to carry out environmental geological survey in coastal areas.

Satellite remote sensing is a new technology developed in 1960s, which has the characteristics of macroscopic, rapid, dynamic and comprehensive. At present, it has been widely used in coastal geological survey-coastal water topography detection, coastal type identification, coastline change history, tidal flat evolution process, island reef distribution, channel change, sea surface temperature distribution, seawater salinity distribution, seawater suspended matter and chlorophyll distribution, current and wave conditions, etc. [1]. This paper mainly introduces the principles, methods and existing problems of coastal zone classification, coastline extraction, inshore water depth detection and inversion of inshore suspended sediment, sea surface temperature, salinity and chlorophyll concentration.

2 coastal zone type survey

Coastal zone type is one of the basic contents of coastal zone environmental geological survey. Different coastal zone types have different material composition, morphological characteristics and spatial distribution characteristics, which can generally be identified by the hue, shape, texture and shadow in satellite images and the spatial configuration relationship with related features.

When the surface sand body of sandy coast comes out of the surface, it has a strong reflection on visible light and is generally bright white. Near the water body, with the increase of water content, the reflection intensity of near-infrared band decreases rapidly, showing dark tone; In terms of spatial distribution, sandy coasts are generally open and flat, mostly distributed near estuaries and bays with abundant sand sources and relatively weak erosion. Muddy coast is mainly composed of silt and silt, with high water content, weak reflection in near infrared band and dark color, which is mostly distributed in closed bays and beaches. The bedrock coast is generally located in the headland, mostly an extension of the onshore mountains to the sea, completely isolated from the sea. Texture and hue are related to lithology, topography and vegetation.

The actual investigation shows that different coastal types cross each other. Take the bay near Tongguling Stone Park in Wenchang, Hainan as an example (Figure 1). Between the high tide level and the middle tide level, the surface layer is covered with sand. The middle tide level-low tide level, with a large number of bedrock reefs exposed, brings great difficulties to the description of coastal zone types, and further fine division puts forward higher requirements for the resolution and time phase (low tide level) of remote sensing images.

Figure 1 Bay near Wenchang Stone Park in Hainan

Figure 1 A bay near Stone Park in Hainan Province

3 coastline extraction

Coastline survey is also the basic content of coastal environmental geological survey. Through the interpretation of multi-period coastline, we can study the evolution history of coastline, which plays an important role in analyzing sea level fluctuation, port siltation and channel siltation, and can also provide reference for regional economic and environmental planning.

Generally speaking, in remote sensing images, the dividing line between seawater and land is very obvious, and this line is called waterline (Figure 1). The waterline is dynamic, and the fluctuation of tide is related to the acquisition time of the image. Coastline is the trace line of dividing line between sea water and land formed by high tide for many years.

The coastline of bedrock coast and artificial coast is steep, and the waterline can be directly used as the coastline when the surveying and mapping accuracy allows. Sandy coast and muddy coast, the coast is flat and wide, and the waterline often deviates from the coastline. Generally speaking, the waterline cannot be directly used as the coastline. In this case, the dividing line between beach mudflats and terrestrial vegetation is usually used as the coastline (Figure 1). Near large estuaries and deltas, beaches are open and landforms are complex, so it is difficult to determine their boundaries with terrestrial vegetation. Some scholars [2] put forward the method of tidal model to identify coastline. The basic idea is: firstly, extract the waterline of multi-period remote sensing images in the same area; Then, through the tidal model or the local measured tidal data, the waterline elevation values of each stage are calculated, and the topographic data of the coastal zone in the study area are constructed. Finally, the position of the maximum high tide line, that is, the coastline, is calculated according to the tidal model or tidal data. At present, the main problems faced by tidal model method are the lack of topographic data, the lack of image data and the difficulty in testing the accuracy of coastal zone.

In order to improve the interpretation efficiency of remote sensing images, in recent years, some researchers have tried to identify the coastline automatically. The recognition algorithms mainly include threshold method, edge detection operator method, active contour model method, object-oriented method and Markov field method [3]. At present, the coastline automatic identification technology is still in the exploration stage.

4 offshore bathymetry

Traditionally, bathymetry mainly depends on sonar echo measurement. However, the water depth near the coastal zone is shallow, and the waves and tides are strong, so it is difficult to use ships for sonar water depth measurement. Remote sensing is a good auxiliary means.

At present, there are two main methods to investigate water depth by satellite remote sensing: microwave remote sensing and optical remote sensing.

The penetration ability of microwave to seawater is very limited, which can only reach centimeter level, and it is impossible to directly detect the seabed topography. However, the interaction between ocean currents and underwater topography will cause sea surface fluctuations (waves), and microwave remote sensing has a good effect on waveform measurement, that is, microwave remote sensing can invert seabed topography by measuring waveforms. This method is obviously influenced by the direction and speed of ocean current and sea breeze in practical application [4], and the detection depth is limited [5].

Visible light has certain penetrating power to water (10 ~ 30m). If the water is clear enough, solar radiation can reach the bottom of shallow water and reflect back to the sensor. The brightness information received by the sensor includes water depth information. At present, there are three main methods for water depth inversion by using optical remote sensing [6,7]: one is a pure theoretical model, which mainly calculates theoretically according to the principle of remote sensing water depth and the spectral characteristics of water bodies. The main problems of this method are that it is difficult to obtain the optical parameters of water body and the calculation process is complicated, so it is difficult to popularize and use at present. The second is mathematical statistical model, which statistically analyzes the measured water depth data and the gray value of remote sensing image, fits the equation curve, and then extrapolates to calculate the water depth value. This method is simple and easy, but it can't guarantee the correlation between image gray value and water depth, and the calculation results are often not ideal. The third is the semi-empirical and semi-theoretical model, which is simulated mainly by simplifying the theoretical model and combining statistical data. This method combines the advantages of the first two methods and is widely used at present.

At present, the application of optical remote sensing in water depth survey has made some progress in clear water, but it is still in the exploration stage for coastal turbid water. The key technical problem is how to reduce the influence of suspended matter and sediment (sediment) color on the depth inversion model [6].

5 Investigation of nearshore water environment

In recent years, with the development of coastal social economy, the environmental problems in coastal areas have become more and more prominent, and the contents of coastal water environment investigation have been correspondingly increased in coastal geological survey [1], such as offshore suspended sediment investigation, sea surface temperature (SST), salinity (SSS), chlorophyll concentration, etc. Satellite remote sensing has also played an important role in the investigation of these projects.

5. 1 Remote Sensing of Suspended Sediment in Coastal Seawater

The temporal and spatial distribution of suspended sediment content in water is an important parameter to analyze the changes of erosion and deposition in estuaries and coasts, estimate the material flux of rivers into the sea and study the marine deposition rate. Therefore, it is of great significance to study suspended sediment in seawater.

At present, the most commonly used empirical model for quantitative inversion of suspended sediment by satellite remote sensing is to establish the relationship between field measured data and remote sensing reflectivity or normalized water emissivity. Common relationships are: linear relationship, logarithmic relationship, Gordon relationship, negative exponential relationship and so on. The main basis is that the spectral reflection curve of suspended sediment water body has the following characteristics: in general, the reflectivity of suspended sediment water body increases with the increase of suspended sediment concentration; The spectral curve of suspended sediment has two reflection peaks in yellow and near infrared bands [8]. When the suspended sediment concentration is low, the first peak is higher than the second peak, and with the increase of suspended sediment concentration, the second peak increases, and finally it is slightly higher than the first peak [9].

However, the reflection of suspended sediment is not only related to the concentration of suspended sediment, but also to the particle size, type and shape of suspended sediment. Therefore, the above relational models often have great limitations in popularization and application. In order to study a more operable and universal remote sensing algorithm for suspended sediment in water bodies, more calibration, verification and analysis models need to be developed.

5.2 Inversion of Coastal Seawater Surface Temperature

At present, AVHRR and MODIS are commonly used data sources in the global sea surface temperature (SST) survey, but the spatial resolution of these two data is kilometers, which can not meet the requirements of large-scale coastal SST survey. The thermal infrared bands of TM and ETM+ have high spatial resolution (120m and 90m, respectively), which have been widely used in the investigation of sea surface temperature in offshore waters and achieved good results [10- 13].

The main difficulty in retrieving sea surface temperature by Landsat is atmospheric correction, because TM and ETM+ data only have one thermal infrared band, and it is impossible to construct atmospheric correction equation by the difference of atmospheric absorption and emissivity in different bands, and there is often a lack of synchronous measured atmospheric isoline data and radiation transmission model.

5.3 Inversion of Coastal Chlorophyll Concentration

Chlorophyll concentration can be used to estimate the biomass and productivity of phytoplankton, and it is also an important parameter to reflect the eutrophication degree of water body [14]. In the open sea, the blue-green ratio method has achieved good results, and its application is relatively mature, but this method is not suitable for turbid coastal second-class water bodies. At present, the investigation of chlorophyll concentration in the second-class water body mostly adopts fluorescence method. The principle of fluorescence method is [15]: under the excitation of sunlight with the wavelength of 400 ~ 700 nm, phytoplankton can produce red radiation near the wavelength of 683nm, and the radiation intensity has a strong correlation with chlorophyll concentration, while atmospheric radiation and yellow matter and suspended sediment in seawater have little influence on the radiation peak. By measuring the radiation between 680nm and 660nm, the chlorophyll concentration can be inversed.

At present, there are three main problems in the fluorescence method [15]: first, the chlorophyll fluorescence process is complex and changeable, which needs further study in biology and ecology; Second, the fluorescence emitted by chlorophyll only accounts for 5% of the absorbed energy of chlorophyll, so it is difficult for the sensor to detect when the chlorophyll concentration is low; Thirdly, with the increase of chlorophyll concentration, the fluorescence peak of chlorophyll "redshifts", and the sensor channel is fixed, which will affect the accuracy of fluorescence peak radiation calculation.

5.4 Inversion of Surface Salinity of Coastal Seawater

Monitoring the change of coastal salinity is an important means for us to understand the ecological environment and physical processes of estuaries and coasts [16]. Traditionally, seawater salinity is mainly measured by taking water samples or using CTD, but this method has a large workload in the field and cannot obtain large-scale seawater surface salinity data at the same time.

At present, microwave remote sensing is mainly used to retrieve the surface salinity of seawater. The principle is that the change of seawater salinity will change the dielectric constant of seawater, and then change the microwave radiation characteristics. By measuring the microwave emissivity of the sea surface with a microwave radiometer, the surface salinity of seawater can be retrieved from the brightness temperature of the radiometer. At present, the electromagnetic wave commonly used to retrieve sea surface salinity is a band with a width of 20MHz centered on 1.4 13GHz. The main advantage of this band is that it belongs to the band protected by international treaties and used for radio astronomy research. There is no human signal interference, and it can be observed almost all day except for heavy rain [17]. The problem is that the spatial resolution of the sensor carried by the satellite is extremely low, which can not meet the requirements of offshore observation.

6 Conclusion and discussion

Remote sensing coastal environmental geological survey has the characteristics of high efficiency and low cost, and has been widely used at present. In some fields, such as coastal type survey and coastline survey, it is mature. The difficulty lies in the inshore water bodies, such as water depth measurement, suspended sediment measurement, temperature, salinity and chlorophyll concentration inversion, which are still in the exploration stage. The most important technical bottleneck is the sensor. The environmental geological survey of remote sensing coastal zone puts forward harsh "three highs" requirements for sensors (high spatial resolution, high spectral resolution and high temporal resolution). First of all, the coastal environmental geological survey takes the sea area 20 kilometers off the coastline and the land area 5 kilometers inside the coastline as the core investigation areas. Therefore, high spatial resolution is particularly important. The resolution of traditional water color satellites is mostly kilometers, which is difficult to meet the surveying and mapping accuracy requirements of 1: 10000 and larger-scale coastal surveys. Secondly, the coastal environmental geological survey is complex and diverse, including land and water, which involves surface temperature and salt, suspended sediment and underwater topography. In order to meet these requirements, it is necessary to have a high enough spectral resolution to effectively remove interference information and obtain an accurate spectral conduction model. Finally, the coastal zone is an area where lithosphere, biosphere, hydrosphere and atmosphere interact strongly, and it is also a concentrated residential area of human beings, which is strongly transformed by human beings and the environment changes rapidly. Without high time resolution, the sensor can not accurately grasp the law of environmental geological changes in coastal areas. As far as the current technology is concerned, it is the most effective solution to construct a small satellite constellation by using low-orbit high-resolution (spatial resolution and spectral resolution) sensors.

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Application of satellite remote sensing in coastal geological environment

Huang Wenxing 1, 20,000 Rongshen 1, 2

(1. Guangzhou Marine Geological Survey, Guangzhou, 510760; 2. State Key Laboratory of Marine Mineral Resources, Guangzhou, 5 10760)

Abstract: In recent decades, with the development of China's coastal economy, coastal environmental geological problems have attracted increasing attention. Satellite remote sensing is fast, real-time and efficient, which makes it widely used in coastal areas. Environmental investigation. These applications include coastal zone type classification, coastline extraction, water? Coastal zone depth measurement, suspended sediment detection, sea surface temperature (SST), sea surface salinity (SSS), chlorophyll concentration detection and other environmental geology. This paper introduces the principles and disadvantages of these methods. Keywords: satellite remote sensing; Coastal areas; Geo？ Environmental investigation