Airborne LiDAR Methods Applied to Riverine Environments - Fluvial Remote Sensing for Science and Management - page 141

7.2.2 LiDARfor aquaticsurfaces where P s denotes the power returned by the water sur-

face, and P bsc ( d ) denotes the power returned by the
water column at a given depth d . Consequently, the
total water column distribution P bsc is computed through
Equation (7.1):

LiDAR sensors designed for surveys over aquatic areas are
called bathymetric hydrographic LiDARs, or ALB for Air-
borne LiDAR Bathymeters. These LiDARs were primarily
developed to survey across the land-water boundary in
near-coastal areas, but since 2005, there has been an
increasing use of these systems in riverine environments.

P bsc = P bsc ( d ) . dd (7.3)

P b denotes the power returned from the bottom; P bg is the
background power coming from sunlight or backscatter
in the air; and P N is the power from system noise.
More detailed formulations can be found in Tulldahl
and Steinvall (2004). A number of authors expound on
these complex physical equations and describe each of
the interdependent components (e.g., Guenther, 1985;
Jurand et al., 1989; Tulldahl and Steinvall, 1999, 2004;
Zege et al., 2004).
The first condition controlling the shape of each com-
ponent is the typically Gaussian form of the transmitted
pulse P T ( t ) itself. Indeed, the LiDAR waveform and com-
ponent forms result from the convolution product of
P T ( t ), with the range of target backscatter properties for
targets encountered by the laser corresponding to time t .
Optical properties such as scattering and attenuation
increase the diameter of the pulse and bias towards the

7.2.2.1 Physical equations

Compared to LiDARs used for terrestrial surfaces, bathy-
metric LiDARs use a green wavelength (532 nm) to
penetrate the water surface with minimal optical atten-
uation. The relevant portion of the bathymetric LiDAR
signal corresponds to a back-scattered waveform, which
usually contains two peaks, typically indicating water sur-
face and bottom reflections. A schematic description of
this system is shown in Figure 7.2a
When considering only the green signal, the foundation
equations of the collected signal power P by the receptor
for an emitted pulse can be stated as a sum of four
dependent components (Guenther, 1985; Jurand et al.,
1989), as in Equation (7.1):

P = P s + P bsc + P b + P bg + P n (7.2)

P bg

H

Afov Ao

P s

Air

P bsc

P

Water

D

Aw

P b

Soil

Figure 7.2a A scheme of the bathymetric LiDAR system (green laser) and main components convolving to the LiDAR waveform.
Courtesy of Airborne Hydrography AB.