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Bathymetric lidar has been widely used for ocean floor mapping. By identifying two distinctive return peaks, one from the water surface and the other from the bottom, the water depth can be estimated. In addition to bathymetry, it is also possible to estimate the optical properties of the water by analyzing the lidar return waveform. Only the few systems (e.g. Optech's SHOALS and CZMIL systems) that have good radiometric calibration demonstrate the capability to produce the water's inherent optical properties and bottom reflectance. As the laser pulse propagates through the water, it is scattered by the water constituents. The directional distribution of scattered radiant power is determined by the volume scattering function. Only the backscattering within a very narrow solid angle around the 180° scattering angle travels back to the detector. During the two-way travel it experiences the same optical interaction (absorption and scattering) with the water constituents. Thus, the lidar return waveform between the surface and bottom peak contains information about the vertical distribution of the water attenuation coefficient and the backscattering coefficient in the form of the rate of change of the return power. One challenge is how to estimate the inherent attenuation from the apparent attenuation. In this research we propose a technique to estimate the true water attenuation coefficient from the total system attenuation. We use a lidar waveform simulator that solves the irradiance distribution on the beam crosssection using an analytical Fourier transform of the radiance based on a single-scattering approximation.

Estimation of water column optical properties and seafloor reflectance (532 nm) is demonstrated using recent SHOALS data collected at Fort Lauderdale, Florida (November, 2003). To facilitate this work, the first radiometric calibrations of SHOALS were performed. These calibrations permit a direct normalization of recorded data by converting digitized counts at the output of the SHOALS receivers to input optical power. For estimation of environmental parameters, this normalization is required to compensate for the logarithmic compression of the signals and the finite frequency of the bandpass of the detector/amplifier. After normalization, the SHOALS data are used to estimate the backscattering coefficient, the beam attenuation coefficient, the single-scattering albedo, the VSF asymmetry, and seafloor reflectance by fitting simulated waveforms to actual waveforms measured by the SHOALS APD and PMT receivers. The resulting estimates of these water column optical properties are compared to in-situ measurements acquired at the time of the airborne data collections. Images of green laser bottom reflectance are also presented and compared to reflectance estimated from simultaneously acquired passive spectral data.

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