Submerged-oil tracking by airborne hyperspectral fluorescent lidar

Although numerous remote-sensing systems recently reported on the scope of the Deepwater Horizon BP oil spill in the Gulf of Mexico, there was an obvious lack of detection capability for certain submerged-oil types that occurred after spill weathering and dispersant use. Undetectable movement of ‘hidden oil’ near the surface stymied skimming task forces and caused unpredictable oil sedimentation onshore.¹ Hyperspectral fluorescent-lidar-system^® (FLS)-series light detection and ranging (lidar) devices, which are based on laser-induced fluorescence (LIF) combined with spectral-fluorescence-signature (SFS) analysis, are capable of providing such detection, while expert-system advances now support near-real-time map updates for incident responders.

Laser remote sensing and its application to monitoring marine oil pollution have been explored extensively. Initial results of airborne LIF lidar used for oil detection at sea were already reported in 1986,² while use of multiple sensing wavelengths in LIF lidars was developed in 1991.³ It was subsequently reported that combined employment of LIF lidars and other on-board sensors increases reliability of airborne oil detection.^4,5 Initial airborne surveillance with hyperspectral LIF lidar (FLS-A) was reported in 1995.⁶ Airborne multiwavelength (FLS-AM) lidars (see Figure 1) were introduced for operational use in 2008.⁷ They are capable of detecting and measuring oil on land, on water surfaces, and up to several meters below the water level.⁸

Figure 1.(right) King Air 90 aircraft certified for carrying (left) fluorescent-lidar-system airborne multiwavelength lidar devices.

FLS uses a excimer laser that has been strengthened against wear and tear as its primary UV sensing source and to pump wavelength transformation. The secondary light source is typically a dye laser that uses a stimulated-Raman-scattering cell. The latter senses the object of interest at the most effective selective excitation or applies multiwavelength excitation to a remote water body. A beam shaper operates in eye-safe mode and adjusts the laser beam's footprint. The lidar's telescope is coupled with a flat-field polychromator and a gated linear multichannel detector (500 channels), which jointly constitute the hyperspectral receiving system.

FLS comprises an expert system, a global-positioning-system (GPS) receiver, a rangefinder, and a digital camera. GPS coordinates and metadata accompany every recorded LIF spectrum. Rangefinder information is used to calculate laser-spot positions and correct the intensity of LIF spectra as a function of altitude. Significant oil detections trigger real-time alarms and digital location photography. External devices may include inertial measurement units, parallel data storage, visualization displays, and communication channels.

We use a software expert system, which includes real-time and post-processing functions, to analyze LIF spectra by comparison with a reference LIF spectral library of individual organic compounds. The FLS-lidar technology has been under continuous development since 1990, in parallel with SFS, which has been realized in the Fluo-Imager^® analyzer.

We have demonstrated the sensitivity of airborne FLS lidars down to the sub-parts-per-million range for oil products in water at flight altitudes from 100 to 500m (see Figure 2). Figure 3 shows typical LIF spectra of water recorded by FLS-AM and airborne UV FLS (known as FLS-AU) lidars at an excitation wavelength of 308nm. Rayleigh scattering is cut off by an optical filter, and the peak at 344nm corresponds to Raman scattering in water. The signal-to-noise ratio of the Raman line is greater than 20. As in lidar applications, the Raman-scattering signal is used as an internal spectral benchmark. It confirms that the sensitivity level is good enough to detect low-level water contamination.

Figure 2.Spectral-fluorescence signatures (2D and 3D views) of (a) seawater exhibiting dissolved-organic-matter fluorescence, (b) gas oil, and (c) seawater polluted by this oil.

Figure 3.Example FLS spectra (in arbitrary units, a.u.). (a) and (b) Clean Baltic Sea water and sample polluted by ship motor oil (single laser shot, flight altitude 200m). (c) and (d) Clean Atlantic Ocean water and sample polluted by fuel oil (five combined laser shots, flight altitude 500m).

Our lidar systems have been tested on aircraft for detection of trace-level oil pollution in open seas and coastal areas, finding and mapping of oil spills, location and identification of submerged oil, profiling of dissolved organic matter in lakes and rivers, diagnostics of agriculture, industrial runoffs, and eutrophication, monitoring of accumulated oil pollution in port areas, and general environmental assessment of fresh-water quality.

The system was used in 2005 at the US Ohmsett facility to demonstrate its capability of detection of heavy oil floating below the water surface.⁸ The capability of mapping an oil spill on water and estimation of its volume has been confirmed in tests with controlled oil spills off the coast of Brittany (France) in 2004.⁹ In 2006, FLS-AM lidar was used to detect oil after several pollution incidents and a shipwreck in Estonian coastal waters.

The ability of airborne FLS lidar to detect and map subsurface oil can greatly expand oil mitigation and recovery options available to spill-response teams by directing them in near real-time to identified concentrations not detectable by other means on reasonable timescales. The sensor platform is portable and can be deployed in any number of aircraft types, either alone or in combination with other systems. Further FLS lidar development will address enhancement of in-flight data processing and real-time reporting to the ground data centers.

Technical and application development, as well as field work carried out by personnel of Laser Diagnostic Instruments in Estonia and Canada is gratefully acknowledged. The FLS series was developed by Laser Diagnostic Instruments of Estonia.