**2. Remote sensing of water monitoring**

 Remote sensors capture the response of the electromagnetic interaction with water (**Figure 1**). Absorption and scattering are inherent optical properties (IOP) of water; and variations in IOP change the reflectance of water which is captured by a remote sensing sensor, and this is known as the apparent optical properties (AOP) of water (**Figure 2**). Reflection, absorption, and transmittance of electromagnetic radiation are highly dependent on the concentrations, types, and presence of substances in water. Total absorption is the sum of absorption by phytoplankton (microalgae), non-algal pigments (NAP), color dissolved organic matter (CDOM), and absorption by water, whereas light scattering by water is mainly controlled by suspended sediments (SS) present in water. Hence, ocean color represents the responses in , green, and red region, and data can be used to estimate the concentrations of water constituents [7].

 Generally, clear water has low reflectance in the visible spectrum and has no reflection in near infrared (NIR) region, as it is absorbed by clear water. However, high reflectance measurements in red (600–700 nm) and NIR region (750–1400 nm) show a strong correlation with SS concentrations. A high concentration of suspended sediments blocks the transmittance to and from lower depths and therefore increases reflectance from the water surface. Similarly, high concentrations of chlorophyll (a photosynthetic pigment in phytoplankton and macroalgae) in water cause high reflectance in the green region (500–600 nm) and high absorption in the blue and red regions due to photosynthetic activity (**Figure 2**).

A portion of absorbed incident energy by the earth's features is also re-emitted in the thermal infrared region of the electromagnetic spectrum. Many satellite sensors such as MODIS, VIIRS, the advanced very high-resolution radiometer (AVHRR), and the sea and land surface temperature radiometer (SLSTR) measure the emitted thermal energy to determine sea surface temperature (SST). SST is an important parameter for understanding ocean water circulation. In case of large oil spills, these data can be effective for pinpointing the oil spilled areas, as they appear cooler than water surface due to their lower emissivity [31].

#### **Figure 1.**

*Interaction of light with the water surface. a is absorption (aph, absorption by phytoplankton; anap, absorption by non-algal pigments; aCDOM, absorption by color dissolved organic matter; and aw, absorption by water), b is backscattering (bb, backward scattering; bf, forward scattering), Rrs is remote sensing reflectance recorded by sensor, Ed is downwelling irradiance, Lu is upwelling radiance, and Lw is water-leaving radiance [32].* 

#### **Figure 2.**

*Reflectance (Rrs) by clear water (blue), water with chlorophyll content (green), water with CDOM (black), and sedimented water (orange) [32].* 

Fluorescence is another type of energy emitted by a substance when it comes to a lower energy level from a higher energy level. The emitted energy is in a longer wavelength than the excitation wavelength. Algae absorb visible light for the photosynthesis process and emit excessive energy in the form of fluorescence signal (681 nm, the fluorescence band) when chlorophyll molecule comes to the nonexcitation state during the photosynthesis process. The fluorescence can be detected by optical sensors with fine spectral resolution in the far-red and NIR and has a potential source for monitoring changes in the photosynthesis process in plants. Furthermore, in laser fluorometry, laser light is used to excite molecules [33]. This technique is common to detect oil and chemical spills [31].

### **2.1 Remote sensing platforms and sensors for water monitoring**

There are now several remote sensing platforms for monitoring water pollutants, and they can be categorized into two types: airborne and spaceborne.

### *2.1.1 Airborne sensors*

An aircraft flies at relatively low altitudes (a few hundred meters to a few kilometers above the surface); therefore, the acquired data always have higher levels of detail. Airborne data are particularly useful for real-time monitoring of oil and chemical spills. Four common airborne sensors used for spill surveillance [34] are listed below:

i. Infrared/ultraviolet line scan (IR/UVLS)

ii. Side-looking airborne radar (SLAR)

iii. Microwave radiometer (MWR)

iv.Laser fluorosensor (LF)

#### *Detection and Monitoring of Marine Pollution Using Remote Sensing Technologies DOI: http://dx.doi.org/10.5772/intechopen.81657*

Airborne hyperspectral sensors with fine spatial resolution are able to capture detailed spectral variations. Therefore, they help to select the appropriate spectral region to study a specific water quality parameter, design satellite sensors, and improve already developed algorithms. Some airborne hyperspectral sensors particularly useful for studying coastal/river water quality are described in **Table 1**.
