Light Detection and Ranging, or LiDAR, is a remote sensing technology that uses laser pulses to measure distance. The system emits rapid pulses of light, typically in the near-infrared spectrum, and calculates the time it takes for those pulses to reflect back to a sensor. This time-of-flight measurement allows for the creation of highly accurate, three-dimensional maps of the Earth’s surface. However, a standard LiDAR system cannot penetrate opaque ground surfaces, such as soil, rock, or concrete, to reveal buried objects or features.
The Physics of Light and Opaque Surfaces
The limitation of LiDAR stems directly from the physics of how light interacts with dense, opaque materials. Standard terrestrial LiDAR systems primarily utilize near-infrared wavelengths, often around 1064 nanometers, to map surfaces. When this light encounters soil, it is immediately subjected to absorption and scattering.
The light pulse is quickly absorbed by the ground materials, including mineral grains, organic matter, and moisture content. Water is a strong absorber of near-infrared light, meaning wet or damp soil drastically reduces any potential for penetration. The remaining light is scattered by the dense, irregularly shaped particles that make up the soil matrix.
Due to this rapid absorption and scattering, the light energy is attenuated almost instantly upon contact with the ground surface. For typical soil conditions, the “skin depth,” or the distance the light can travel before being completely extinguished, is limited to only a few centimeters at most.
Distinguishing Subsurface Mapping Technologies
Because LiDAR is limited to mapping surfaces, Ground Penetrating Radar (GPR) is the primary technology used for non-invasive underground mapping. Unlike LiDAR, which uses high-frequency light waves, GPR transmits lower-frequency electromagnetic energy in the form of radio waves.
Radio waves interact with materials differently than light, allowing them to pass through non-conductive mediums like dry soil, rock, and pavement. The GPR system listens for reflections of these waves, which occur when the wave encounters a boundary between materials with different dielectric properties. These boundaries can include interfaces between soil layers, bedrock, or buried objects.
The effective depth and resolution of GPR are determined by the frequency of the antenna used. Lower-frequency antennas can penetrate deeper into the ground but provide a lower-resolution image. Conversely, higher-frequency antennas offer finer detail for shallow targets but cannot reach as deep.
LiDAR’s Interaction with Water and Submerged Features
An exception to LiDAR’s inability to penetrate solid materials occurs in aquatic environments. Bathymetric LiDAR systems are designed to map shallow water bodies, such as coastlines, rivers, and lakes. These specialized systems use a different wavelength of light, typically in the green spectrum around 532 nanometers.
The green wavelength is selected because it experiences less absorption and scattering when traveling through clear water compared to the near-infrared light used for land mapping. A portion of the green laser pulse reflects off the water surface, while the rest travels through the water column to the seabed. By measuring the time difference between the surface return and the bottom return, the system calculates the water depth.
The depth penetration of bathymetric LiDAR is highly dependent on water clarity and can reach up to 25 meters in clear conditions. Turbidity, caused by suspended sediments or algae, quickly scatters the green light and limits the effective mapping depth. This shows that LiDAR’s ability to penetrate a medium relies entirely on the material’s composition and the precise wavelength of light employed.