The intervention of pollutants causes changes in the physical and chemical properties of water, changes in dielectric constant, and changes in the absorption and reflection of electromagnetic waves. Use satellite remote sensing or aerial remote sensing (including visible light, thermal infrared and microwave) to detect the transparency, surface temperature (SST), salinity, chlorophyll (algae), suspended solids, turbidity, etc. of large-area water bodies. Commonly used satellites are mainly Landsat's TM and MSS; France's SPOT satellite's HRV and NOAA series' AVHRR data sheets are a fast and effective method.
11.1.2.1 Satellite remote sensing monitoring of cyanobacteria blooms in Taihu Lake waters
Cyanobacteria blooms appear as green algae organisms accompanied by white foamy pollutants (algae blooms) gathered on the surface of the water body. The spectral characteristics of the coverage area (Figure 11.1.1) are significantly different from the normal water body of the surrounding lake. Due to the effect of cyanobacterial chlorophyll (Figure 11.1.1a), the cyanobacteria area has high light reflectivity in the Landsat TM2 segment (blue light). The reflectivity in the TM3 band (green light) decreases slightly, but is still higher than that of normal water, and the reflectivity in the TM4 band (red light) is the highest. Therefore, on the false-color composite images of TM2, TM3 and TM4, the cyanobacteria area appears crimson, which is obviously different from the normal lake water (dark blue, blue-black). In addition, the cyanobacteria aggregates are affected by the lake current and wind direction, and appear in strips. (Figure 11.1.1) texture.
The Landsat TM image on August 11, 1998 clearly reflects the intensity, location and distribution range of this blue-green algae outbreak in Taihu Lake (Figure 11.1.2), mainly in the western area of ??Taihu Lake, with a width of 0.5~1 km , along the shore of Huzuitou, Xuedianqiao Town, extending southward to the east of Yixing, about 20 kilometers long; the other is distributed along the coast of Wuxi City. Green algae can be seen on the shore, which is thick and dense, and can smell fishy. taste.
The blue algae outbreak in Taihu Lake is mainly the result of eutrophication of water bodies caused by urban and rural sewage discharge.
11.1.2.2 Remote sensing monitoring of suspended sediment content in water
Using satellite remote sensing to conduct quantitative research on suspended sediment in large areas of water is very advantageous. The reflectivity of water bodies with turbid sediments is higher than that of clean water bodies (Figure 11.1.3). Studies have shown that as the concentration of sediment in the water increases, the reflectivity increases, and a dividing point appears near 520 mm. The reflectivity in the long-wave direction is greater than that of normal water (Figure 11.1.3). As the concentration of suspended sediment increases, the boundary The point moves towards the long wave direction. When the sediment concentration is 0.1 mg/L, the boundary point is located at 570 nm; when the concentration reaches 0.5 mg/L, the boundary point moves to 690 nm. This is the basic sign of qualitative sediment content.
Figure 11.1.2 Distribution of cyanobacteria in Taihu Lake
Figure 11.1.3 Reflectance curve of yellow (containing sediment) abnormal area
Satellite remote sensing quantitative study of suspended mud in water body Sand started early, and many scholars put forward the optimal waveband and quantitative correlation calculation model through quantitative research. In 1974, Klemas applied satellite remote sensing to the Tolawa Sea waters. The study found that the suspended sediment content of the water body has a logarithmic relationship with the Landsat MSS brightness. After selecting the appropriate bands, it was found that the brightness data of the MSS-5 and TM3 bands are correlated with the suspended sediment content. The best correlation coefficient is more than 90%. Its best corresponding band is 0.6~0.8 μm.
In my country's Yellow River Estuary, Yangtze River Estuary, and Pearl River Estuary, Land satellite was used to study the sediment content of water bodies, and found that the sediment content has a linear relationship with satellite remote sensing light reflectance data.
11.1.2.3 Detection of sediments and abandoned materials in water
At the bottom of rivers, lakes and seas, suspended substances in the water and abandoned materials from human activities accumulated over the years are deposited, among which Some of them are pollutants, such as the long-term accumulated sewage discharge from some industrial and mining enterprises on the shore; the other part are tools, weapons and ammunition discarded by humans, such as accidental shipwrecks, military concealments, etc. People often need to judge and determine the location and distribution of these pollutants or abandoned objects, such as the thickness distribution of contaminated sediment, the burial depth and range of abandoned objects, etc. The most effective method for such detection is geophysical detection. The specific equipment required for detection depends on the characteristics of the object to be detected. The more commonly used ones are magnetic detectors, gravimeters, electrical detectors (electromagnetic meters) and seismometers. Field work requires a detection ship, which is available in winter. Under certain conditions, you can work directly on the ice.
During my country’s Anti-Japanese War and some subsequent military activities, a large number of weapons and ammunition were buried (some were hidden on land). It has not yet been identified. After long-term underground oxidation leaching, the surrounding Contamination of soil and groundwater is inevitable. Some are highly toxic. All incidents should be identified and dealt with as soon as possible to avoid future troubles.