LASER REMOTE SENSING OF COASTAL AND TERRESTRIAL POLLUTION BY FLS-LIDAR

Abstract

The paper presents recent results of field experiments with airborne fluorescent lidar FLS-A based on excimer laser and multichannel detector. Helicopter installation was used to monitor water and land areas of oil transportation and storage. Techniques for real-time data acquisition and process- ing are described based on compiled and systematized library of LIF spectra of various underlying surfaces. The field tests have proved the ability of FLS-A lidar to distinguish the fluorescence of minor oil pollution on different spectral backgrounds usually recorded in airborne measurements. Further lidar developments are discussed. METHODS The FLS-lidar series is the family of laser remote sensing systems developed by Laser Diagnostic Instruments (LDI). These lidars were primarily developed for monitoring marine environments. Natural aquatic and terrestrial targets containing organic pollution and vegetation have been stud- ied in numerous field experiments. The specificity of lidar applications in coastal shallow waters has been investigated with FLS-lidars operating at tuneable and fixed sensing wavelengths. Addi- tional requirements on analytical lidar technologies have been determined due to specific meas- urement conditions: small depths of sensed water column; typically lower water transparency and higher amounts of suspended solids; effects of bottom and shore underlying surfaces on lidar echo signals. Further developments and field trials have shown that extending their application to diag- nostics of chemicals on different types of underlying surfaces (water, soil, industrial sites, land and agricultural vegetation, etc.) is highly feasible. The entire set of an FLS lidar includes a basic laser source with an integrated module of sensing wavelength converters, beam-transmitting and receiving telescopes, a highly sensitive, gated hy- per-spectral optical receiver based on a flat-field polychromator, control electronics / processors, and operator's console. The FLS modular design enables easy replacement of the system's com- patible units and change of the overall lidar configuration depending on the application. During several years of operating in the field, FLS lidars have been used on trucks, ships, airplanes, and helicopters to study and / or monitor various aquatic and terrestrial targets. The FLS-A (airborne) lidar is based on excimer laser with non-stable resonator. A UV beam ex- pander is used to decrease the beam divergence and to fulfil eye safety requirements. A gated intensified linear CCD - camera coupled with polychromator provides simultaneous read-out of Laser Induced Fluorescence (LIF) spectra and background subtraction (ambient and dark signals) as well as an optional signal accumulation feature to increase the signal-to-noise ratio. Due to the hyper-spectral detection system used in FLS, high-resolution LIF spectra can be obtained per laser pulse during both day and night operations. It allows a detailed analysis of spectral shapes of lidar echo signal to be performed. Switching of diffraction grating position enables spectral recording in two spectral ranges. Technical specs of FLS-A are shown in Table 1. Due to its moderate weight and dimensions the lidar can be operated on board of small airplanes or helicopters at the sensing distances ranging from 50 to 300 m. FLS-A lidar operation is fully automated due to integrated control and data processing software, which operates in unattended