OIL SPILL SCIENCE chapter 7 – laser fluorosensors

OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors OIL SPILL SCIENCE chapter 7 – laser fluorosensors

Laser Fluorosensors

Carl E Brown

Chapter Outline

7.1 Principles of Operation 171

7.2 Oil Classification 175

7.3 Existing Operational

Units

179

7.4 Aircraft Requirements 180

7.1 PRINCIPLES OF OPERATION

7.1.1 Active versus Passive Sensors

Passive sensors are those that measure naturally available energy such as that produced by the sun.1 Passive sensors can only be used when the naturally occurring energy is available, that is, during periods of daylight when the sun is illuminating the earth Passive sensors cannot therefore be used during periods

of darkness, which is at night Some naturally emitted energy such as thermal infrared energy can be detected during the day or night, providing there is enough energy to be detected

Active sensors, on the other hand, provide their own energy or source of excitation for illumination The sensor illuminates or excites the target to be investigated The energy or radiation reflected from that target is then detected and measured by the active sensor The main advantage of active sensors is the ability to obtain measurements anytime day or night Furthermore, active sensors can be used at wavelengths that are not provided by the sun, such as the microwave region It should be noted that active remote sensors require the generation of a large amount of energy in order to adequately illuminate the targets Examples of active remote sensors include synthetic aperture radars (SARs) and laser fluorosensors

7.1.2 Sensor Features

Laser fluorosensors are the only sensors that detect a primary characteristic of oil, namely, the unique oil fluorescence spectral signature Other generic sensors rely

Oil Spill Science and Technology DOI: 10.1016/B978-1-85617-943-0.10007-3

Ó 2011 Elsevier Inc All rights reserved. 171

on secondary characteristics of oil such as the reflection of light of various wavelengths, scattering of microwaves, and emission of infrared energy Laser fluorosensors are active sensors that take advantage of the fact that certain compounds in petroleum oils absorb ultraviolet light and become electronically excited This excitation is rapidly removed through the process

of fluorescence emission, primarily in the visible region of the spectrum Since very few other compounds show this tendency, fluorescence is a strong indication of the presence of oil Natural fluorescing substances, such as chlorophyll, fluoresce at sufficiently different wavelengths than oil to avoid confusion As different types of oil yield slightly different fluorescent inten-sities and spectral signatures, it is possible to differentiate between classes of oil under ideal conditions.2-10

7.1.2.1 Excitation Source and Wavelength Selection

Most laser fluorosensors used for oil spill detection employ a laser excitation source operating in the ultraviolet region of 300 to 355 nm.4The fluorescence response of crude oil when excited with an ultraviolet laser ranges from 400 to

650 nm, with peak centers in the 480 nm region A typical laser fluorosensor system with excimer laser and scanner is shown inFigure 7.1 These excitation wavelengths are a compromise in that they excite all three classes of oil with reasonable efficiency (shorter wavelength lasers would excite lighter oils effi-ciently but would be rather poor at exciting crude and heavy refined oils) There

FIGURE 7.1 Scanning Laser Environmental Airborne Fluorosensor (SLEAF) (Environment Canada)

are several reasonably priced, commercially available ultraviolet lasers in the 300e355 nm region, including the XeCl excimer laser (308 nm), the nitrogen laser (337 nm), the XeF excimer laser at 351 nm, and the frequency-tripled Nd:YAG at 355 nm With excitation in this wavelength region, there exists

a broad organic matter fluorescent return, centered at 420 nm This is referred to

as Gelbstoff or yellow matter, which must be accounted for While Gelbstoff disappears if the oil thickness is greater than 10e20 mm (i.e., where the oil is optically thick), it can be an interfering signal when attempting to detect thin films of light oils on water Chlorophyll yields a sharp peak at 685 nm

Another phenomenon, known as Raman scattering, involves energy transfer between the incident light and the water molecules When the incident ultra-violet light interacts with the water molecules, Raman scattering occurs The water molecules absorb some of the energy as rotational vibrational energy and emit light at wavelengths that are the sum or difference between the incident radiation and the vibration rotational energy of the molecule The Raman signal for water occurs at 344 nm when the incident wavelength is 308 nm (XeCl laser) The water Raman signal is useful for maintaining wavelength calibration

of the fluorosensor in operation, but has also been used in a limited way to estimate oil thickness because the strong absorption by oil on the surface will suppress the water Raman signal in proportion to thickness,11 where trans-mittance¼ EXP ( thickness  absorption coefficient) The point at which the Raman signal is entirely suppressed depends on the type of oil, since each oil has a different absorption coefficient The Raman signal suppression has led to estimates of sensor detection limits of about 0.05 to 0.1mm.12Details of the use

of Raman scattering to measure oil slick thickness can be found in the early work of Hoge and Swift13and the recent studies by Patsayeva et al.14

7.1.2.2 Detection System

The detection systems in most laser fluorosensor systems usually involve the collection of laser-induced fluorescence by a telescope and the focusing of the fluorescence onto the entrance slit of a grating spectrometer and then onto either photomultiplier tubes or intensified diode-arrays The fluorescence spectrum is then recorded at a number of selected wavelengths or over a wide spectral range covering the ultraviolet through the visible

7.1.2.3 Range-Gating

The majority of modern laser fluorosensors are equipped with range-gated detection systems Range-gating is simply the turning on of the detection system at precisely the time at which the laser-induced fluorescence is expected

to return to the laser fluorosensor This is accomplished by turning the detection system on and off at a precise time based on the known altitude To accomplish this, the timing of the laser pulse is monitored prior to exiting the aircraft and the elastic backscatter from the surface is then monitored to determine the

precise aircraft altitude, which is then used to control the range-gating elec-tronics This allows the detector to observe only the fluorescence induced by the excitation laser and neglect the majority of the background solar radiation

7.1.2.4 Field of ViewdFixed versus Scanning Systems

As noted earlier, active sensors need to deliver sufficient energy to the target, the surface of the earth containing oil contamination, to excite sufficient fluorescence to allow for the detection and classification of the oil With most airborne laser fluorosensor systems, this means illuminating a field of view (FOV) of about 1 3 mrad, giving a footprint on the surface of about 0.1 m 0.3 m at 100 m altitude This does not allow for a large amount of the surface to be interrogated by each laser pulse With higher-powered ultraviolet lasers, one can fly at higher altitudes and enlarge the footprint of the sensor The repetition rate of the laser and the ground speed of the aircraft are also major factors in the sampling of the surface where the oil contamination is being examined At ground speeds of 100e140 knots (nautical miles) at a laser repetition rate of 100 Hz, a fluorescence spectrum is collected approximately every 60 cm along the flight path (at 100 m altitude)

Some laser fluorosensors only “look” directly below the aircraft and collect fluorescence spectra in a straight line; this is referred to as a “fixed” FOV system As spilled oil often piles up in narrow bands at the high-tide line, detection of this oil with a fixed FOV system is not optimal This means that the oil might not be detected because the sensor is striking the surface of the earth

on either side of the high-tide line To compensate for this tendency of the oil to accumulate in a narrow band, it is preferable to change the laser FOV by employing a scanner The scanner can either be moved in a conical (circular) fashion or back and forth across the surface to increase the likelihood of striking the oil contamination There are conical scanning laser fluorosensor systems that have been developed in Germany15and Canada.16An example of

a conical scanner is shown inFigure 7.2

7.1.3 Pros/Cons

Laser fluorosensors are capable of detecting oil and related petroleum products

in complex marine, coastal, and terrestrial environments These sensors are extremely sensitive and can discriminate between oiled and unoiled naturally occurring substances such as kelp and seaweed It is under these circumstances that the laser fluorosensor can aid in the direction of oil spill countermeasures

by discriminating between contaminated and clean areas in the marine and terrestrial environment

Laser fluorosensors are the only sensors that detect a primary characteristic of oil, namely, the unique oil fluorescence spectral signature Other generic sensors rely on secondary characteristics of oil such as the reflection of light of various wavelengths, scattering of microwaves, and emission of infrared energy

At the current time, laser fluorosensors are very large, heavy, and power-hungry systems These characteristics necessitate the use of large multi-engined aircraft to house the systems These conditions will remain until much smaller diode-pumped solid-state lasers are developed in the ultraviolet region

of the electomagnetic spectrum This development has lagged behind that of solid-state lasers in the visible region and might be another decade in coming to fruition

7.2 OIL CLASSIFICATION

7.2.1 Real-Time Analysis

One of the benefits of modern laser fluorosensors is the ability to detect and classify oil contamination in real time This availability of real-time oil contamination is essential for rapid oil spill response and environmental damage mitigation A recent analysis of oil spill detection algorithms for laser fluorosensors has been undertaken by Jha et al.17

In earlier fluorosensors like the LEAF system, Pearson correlation coef-ficients were calculated to determine the presence of oil contamination and to broadly classify the petroleum products.18 Standard reference fluorescence spectra for light refined, crude, and heavy refined classes of oil, along with

a standard water reference spectrum, were stored in the LEAF data analysis computer Correlation coefficients were calculated for the live spectrum versus the three broad classes of petroleum products and water at the rate of

100 Hz When the value of the correlation coefficient versus a class of petroleum product was above a certain level and greater than the correlation with the water spectrum, the live spectrum was identified as being of that class of petroleum

FIGURE 7.2 Scanner unit (narrow/wide swath) SLEAF system (Environment Canada)

With modern computer technology, it is now possible to analyze a large amount of laser fluorosensor data in real time For example, with the Scanning Laser Environmental Airborne Fluorosensor (SLEAF), it is possible to analyze fluorescence data at a rate of nearly 400 Hz and display oil detections along with the flight path of the aircraft on a geo-referenced map output.8,19With the SLEAF system, fluorescence spectra are analyzed in real time to determine the presence or lack of oil in the sensor field-of-view Principal component analysis20 is used to classify the oil class as light, medium, or heavy and to estimate the extent of oil coverage in the field of view as clean, light, moderate,

or heavy

7.2.2 Sensor Outputs

As noted earlier, a high volume of fluorescence spectral data can be analyzed

in real time In most laser fluorosensor systems, the fluorescence data is geo-referenced (i.e., the location of the oil contamination is well known) and the data can be presented in either spectral or map display outputs While displays

of spectral data are important for the sensor operator to verify the proper operation of the sensor, they are of little use to the spill responder What is essential for the spill responder is knowledge of the location of the oil contamination so that spill response equipment can be rapidly deployed to the spill scene and cleanup operations undertaken The positive identification of oil contamination afforded through the use of laser fluorosensors is one of their main advantages

7.2.2.1 Spectral Data

The display of spectral data is essential for the effective operation of modern laser fluorosensors Laser fluorosensor display monitors often include a repre-sentation of sensor parameters such as laser pulse energy, operating altitude, laser backscatter energy, reference spectra, and live or real-time fluorescence spectra The observation of each of these parameters is extremely useful to the sensor operator In particular, real-time spectra are useful for providing an indication of the interaction of the laser beam with the surface By observing the live spectrum, the operator has an indication of the water clarity through observation of the water Raman scattering spectrum A trained sensor operator can easily recognize the presence of oil contamination by the characteristic spectrum of light refined, crude, or heavy refined petroleum products Figure 7.3shows typical laser-induced fluorescence spectra of a crude oil The lack of a proper spectral signature can indicate a problem with the fluorosensor system such as low laser power, low laser backscatter signal, incorrect range-gate timing, or laser misalignment It is impossible to display all of the spectral data collected with a high sampling rate laser fluorosensor system; a subsample

of the spectral data is all that is needed for an experienced operator to monitor system operations

7.2.2.2 Map Display

An efficient map display of oil contamination location(s) is essential for the rapid mitigation of the environmental effects of spilled oil Displays of oil contami-nation superimposed over aircraft flight lines are useful for spill responders who participate in oil spill remote sensing overflights.Figure 7.4shows the operator’s

FIGURE 7.3 Laser-induced fluorescence spectradlight crude oildSLEAF system.

FIGURE 7.4 Operator’s monitor for, sensor parameters, and map display of SLEAF system (Environment Canada)

display from Environment Canada’s SLEAF system Figure 7.5 shows the operator’s display with areas of oil contamination illustrated as colored bars perpendicular to the flight path Similar information is presented inFigure 7.6, which overlays the scanner pattern on the flight path along with the oil

FIGURE 7.5 Oil contamination indicated with colored bars perpendicular to aircraft flight path

on SLEAF system map display (Environment Canada)

FIGURE 7.6 Scanner pattern superimposed on aircraft flight path; colors are an indication of oil classification, SLEAF system (Environment Canada)

contamination information (different colors for clean, light, medium, or heavy oil classifications) Spill response organizations and personnel are not interested

in sensor parameters or spectral data What is needed are geo-referenced maps showing oil locations These maps, or at least the geo-referenced oil contami-nation locations, should be in a format that can be transmitted electronically and

be compatible with commonly used geographical information systems (GIS) These maps will help in the rapid and efficient deployment of spill response resources and equipment to the location(s) where oil contamination is the heaviest and the cleanup of contamination is most needed

7.3 EXISTING OPERATIONAL UNITS

7.3.1 Airborne

There are a number of operational airborne laser fluorosensor units operating around the globe Some of these are research and development units that have progressed to become operational spill response sensors such as the SLEAF system.21Other laser fluorosensors have been combined with other sensors in commercially available sensor packages, while others are standalone systems

or remain as research instruments There are a number of recent reviews

of airborne laser fluorosensor systems in the literature, including those by Samberg,10as well as Brown and Fingas.18, 22

There are a number of commercially available airborne laser fluorosensors in the marketplace and a few examples are provided here The first is the Fluorescent LiDAR Spectrometer (FLS-AU) developed by Laser Diagnostics Instruments International Incorporated The FLS series of LiDARs is designed for pollution monitoring of terrestrial, river, lake, and ocean targets, oil and gas pipeline leak detection, and oil exploration.23The second example is the laser fluorosensor system developed by Optimare Sensorsysteme AG as part of the MEDUSA system.24MEDUSA is a flexible real-time data acquisition and processing system that combines a number of sensor technologies for the detection, mapping, quantification, and classification of marine pollution MEDUSA incorporates

a number of unique sensor systems, for example, laser fluorosensors, infrared/ ultraviolet (IR/UV) line scanners, microwave radiometers, radar systems, camera systems, as well as the corresponding processing software The final example

is the Swedish Space Corporation’s MSS 6000 Maritime Surveillance System that can be tailored to integrate with a number of sensors, including a selection of side-looking airborne radar (SLAR), IR/UV cameras, microwave radiometers, forward-looking infrared (FLIR), and laser fluorosensors.25

7.3.2 Ship-Borne

A small number of ship-borne laser fluorosensors have been developed Most ship-borne laser fluorosensors are research and development technologies, although there have been recent commercial developments Two examples of

ship-borne laser fluorosensors are the FLIDAR (Fluorescence Lidar) developed

by the research group at the Istituto di Ricerca sulle Onde Elettromagnetiche

“Nello Carrarra” IROE-CNR26and the compact lidar system developed by the Japanese Ship Research Institute.27The FLIDAR system incorporates an XeCl excimer laser, a 12 spectral channel detection system, and a conical scanner to direct the ultraviolet laser beam onto the surface of the ocean alongside the marine vessel onto which the system is mounted The compact lidar system at the Japanese Ship Research Institute is a frequency-tripled Nd:YAG laser coupled to an intensified CCD camera and uses a series of optical band-pass filters One commercially available ship-borne laser fluorosensor system is the FLS-S (Fluorescence LiDAR SystemeShip-borne) developed by Laser Diag-nostics Instruments International Incorporated The FLS-S is designed to detect, measure, and map natural Dissolved Organic Matter (DOM), oil pollution, photosynthetic algae, and other contaminants in water.23

7.4 AIRCRAFT REQUIREMENTS

The combination of large-size, heavyweight, and demanding power require-ments for the ultraviolet lasers detailed below necessitate the use of midsized fixed-wing propeller or turboprop aircraft for laser fluorosensor system installation Typical aircraft housing laser fluorosensors have included the Dornier 228-212 (see Figure 7.7), Douglas DC-3, CASA C-295, P-3B, and Beech B-99.18

7.4.1 Power

The high-powered excimer lasers often employed in airborne laser fluo-rosensors for oil spill detection require a significant amount of power This power is typically supplied in the form of 3-phase 208 VAC at 400 Hz for the excimer laser Additional power is required for systems such as the laser

FIGURE 7.7 Dornier 228-212 twin turboprop aircraft.