Question

Which colour of EM spectrum is used for LiDAR bathymetry studies?

• A. Yellow
• B. UV
• C. Red
• D. Green

Hint:

• LiDAR bathymetry uses lasers to measure water depths.
• The laser wavelength must be able to penetrate water effectively.
• Green lasers (e.g., 532 nm) are commonly used because green light has good penetration in most water types (clear to moderately turbid).
• Red and infrared light are strongly absorbed by water and are not suitable for bathymetry.
• Blue light can also be used, especially in very clear deep ocean waters.

Answer 1

The Correct Option is D

LiDAR (Light Detection and Ranging) bathymetry is a technique used to measure the depth of water bodies by using laser pulses. For this application, the laser light must be able to penetrate water with minimal absorption and scattering to reach the seafloor or riverbed and then return to the sensor. The choice of laser wavelength (color) is critical for water penetration:

• Water absorbs different wavelengths of light to varying degrees.
• Red light is strongly absorbed by water, so it has very limited penetration depth.
• Near-infrared (NIR) light, commonly used for terrestrial LiDAR, is also strongly absorbed by water and is unsuitable for bathymetry.
• Ultraviolet (UV) light can also be absorbed and scattered significantly, and has safety concerns.
• Green light (typically around 532 nm) exhibits the best penetration in clear to moderately turbid coastal and inland waters. This is because water has a window of minimum absorption in the blue-green part of the spectrum. Green lasers are therefore commonly used in bathymetric LiDAR systems.
• Blue light can also penetrate water well, sometimes even better than green in very clear oceanic waters, but green lasers are often a good compromise for a range of water clarities and are readily available.

Option (a) Yellow light has moderate penetration. Option (b) UV light is generally not optimal. Option (c) Red light is poorly suited due to high absorption. Option (d) Green light is the standard choice for most bathymetric LiDAR systems due to its optimal water penetration characteristics in typical coastal and inland waters. \[ \boxed{\text{Green}} \]