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Photoacoustic detection and monitoring of oil spill
Bescond, Christophe; Krüger, Silvio E.; Levesque, Daniel; Brosseau,
Charles
Photoacoustic detection and monitoring of oil spill
Christophe Bescond, Silvio E. Kruger, Daniel Levesque and Charles Brosseau
National Research Council Canada, Boucherville (Qc), Canada
Corresponding Author: christophe.bescond@cnrc-nrc.gc.ca
Abstract: The detection and monitoring of oil spills in marine environment are crucial to respond rapidly and efficiently and it is especially important in ice-covered areas. Detection and quantitative characterization of the affected areas as well as monitoring of remediation measures are critical for optimized cleaning operation and minimal environment impact. In the recent years, techniques of oil spill detection from under the ice with Remote Operated Vehicle (ROV) or Autonomous Underwater Vehicle (AUV), have been explored and have shown promising results. These techniques are based on ultrasonic or sonar technologies to quantify the oil volume and on optical techniques to obtain a chemical signature of the oil presence. In this paper we present a new promising technique based on photoacoustics for detection and sizing of oil spill under the ice, encapsulated within ice, or on open water. The technique has the advantage to provide signal in the presence of oil and no signal in its absence. It is also much less sensitive to alignment compared to ultrasonic and sonar techniques. Experimental results on detection of oil under the ice are presented and discussed. A first prototype with a scanning unit that can be operated in ROV is also presented. The solution proposed should be especially useful as a tool for emergency response, but should also be suitable when operated in AUV for monitoring high risky areas due to navigation, transportation or oil exploration and production.
INTRODUCTION
Oil spill can cause a major marine environmental disaster with serious long-term and large scale impacts. Global climate change is resulting in diminishing sea ice in the Arctic, which in turn is generating more demand for marine transport including commercial, tourism and community services and the potential of generating more offshore hydrocarbon developments. This will engender more risk of an oil spill in the Arctic. When oil spill occurs in such ice-covered sea region, detection and monitoring is very challenging and of prime importance to respond rapidly and effectively to protect the exceptional vulnerability of Arctic marine ecosystems and Indigenous cultures.
To detect and monitor oil in open water or on top surface of the sea, remote sensing from aerial vehicle or satellite can be efficient and a rapid solution to deploy. Technologies can be based on optical or radar technologies [1, 2], or on unconventional techniques such as long distance laser ultrasonics from an airplane [3]. When oil spill occurs in ice-covered regions, oil can be in open water, in frazil ice, in snow, under the ice or encapsulated in ice with extreme environmental conditions. Safely detecting, monitoring and tracking can be challenging. In ice-covered sea region, two approaches can be envisaged, from above or from under water as presented in JIP report [4]. From above the sea, techniques such as camera, infrared sensors, fluorosensors, radar and ground penetrating radar can be installed in aerial vehicle or techniques can be based on satellite imagery. From below the sea, remote sensing technologies such as camera, fluorescence, sonar or ultrasonic technologies have been tested to detect and monitor oil under ice or encapsulated in ice. The most promising approach for underwater detection is to use a Remote Operated Vehicle (ROV) or an Autonomous Underwater Vehicle (AUV). Instrumented ROV appears to be the ideal solution for emergency response to monitor oil spill when detected and during remediation, recovery or burning. Instrumented AUV appears to be the ideal solution for detection of oil spill under ice when surveying large ice-covered sea regions.
To monitor and quantify oil thickness and volume reconstruction from under water, sonar and ultrasonic techniques have been tested with a certain success [5-7]. One difficulty is sensitivity of alignment of the sensor with the water-oil interface. In this study, we present a new technique that is particularly adapted to oil under ice or encapsulated in ice. The technique is based on photoacoustics and is found much less sensitive to misalignment. The technique will be first described with its advantages in comparison to conventional ultrasonic or sonar techniques. Small scale laboratory
experiments will be presented with image processing results based on Total Focusing Method (TFM). Also, a first underwater prototype using the photoacoustic technique will be presented as well as a large scale laboratory experiment to demonstrate the potential of this novel technique for oil spill detection and monitoring.
NEW APPROACH FOR OIL SPILL DETECTION BASED ON PHOTOACOUSTICS
In Fig. 1, a simple sketch presents the problem of oil spill detection and monitoring in ice-covered region with instrumented under water vehicles. See JIP report for more details including the cases of open water and frazil detection [4]. In the figure, oil spill under ice and encapsulated in ice are shown, illustrating the complexity of oil spill monitoring that is oil thickness assessment or volume characterization. Also in the figure, instrumented AUV for long range detection missions and instrumented ROV for small to mid-range emergency missions are illustrated with instruments such as camera, sonar and laser fluorometer. AUV is designed with minimum communication, usually low bandwidth acoustics, which is incompatible with oil spill instrument data transfer. AUV has also in board energy so minimum energy for instruments. As illustrated in Fig. 1, ROV has an umbilicus for vehicle guidance, power supply and real time data transfer. Therefore, developments in this study have been conducted primarily for ROV and emergency response deployment. AUV implementation is nevertheless feasible and should also be a perspective of interest to survey oil spill risky areas in Arctic. Monitoring being critical for emergency response in such complex ice-covered region, it is also natural to focus first with ROV implementation of a new technique, that we believe is more adapted than techniques commonly used for oil spill monitoring.
FIGURE 1. Sketch of oil detection in ice-covered region with instrumented AUV and ROV.
The new approach proposed for oil spill detection and monitoring is based on photoacoustics and is illustrated in Fig. 2. A pulsed laser with transparent or almost transparent wavelength in water and ice is used under water and is upward firing in the direction of the floating ice. In this study, the chosen wavelength is 532 nm, with a solid-state Nd:YAG laser frequency-doubled to produce green light. At such wavelength, oil is not transparent and the laser is absorbed over a short penetration depth that generates ultrasonic waves. For detection, an upward looking ultrasonic transducer at a certain distance below the ice is used as receiver. As shown in Fig. 2, when the pulsed laser reaches a water to oil interface, or an ice to oil interface for encapsulated oil, divergent ultrasonic waves are generated and propagate backward and forward like with a volumetric point like source. In the case of water-oil generation, an ultrasonic beam propagates in water towards the sensor and another ultrasonic beam propagates in the oil upward to the oil/ice interface and is reflected downward to the sensor in water. In the case of ice-oil generation, an ultrasonic beam propagates downward in the ice and then refracted in water to the sensor, and a second beam propagates upward to the oil/ice interface and is reflected downward to the ice and to the sensor in water.
In the absence of oil as in the left of Fig. 2, no laser absorption occurs since the laser wavelength is transparent in ice and no ultrasonic beam is generated. This is a major benefit of this underwater photoacoustic technique in comparison to conventional sonar or ultrasonic technique for oil spill detection. Indeed, this technique provides an
on/off signal since ultrasonic waves are generated only in the presence of a thin absorbing layer of oil. With sonar or ultrasonic techniques, emitters upward looking the ice will generate acoustic waves that will be reflected by either water-ice or water-oil interface. So with these techniques, a downward ultrasonic beam always exists and signal interpretation for the presence or absence of oil can be complex.
FIGURE 2. Principle of the proposed oil spill photoacoustic detection technique.
Another benefit of this underwater photoacoustic technique is the fact that the acoustic source is generated at the first oil interface. With conventional sonar or ultrasonic technique, the wave propagates upward from the source to the ice or oil and after reflection, downward to the detector. Sonar or ultrasonic beam is very sensitive to alignment between the emitter, the reflector and the detector. With the photoacoustic technique, since generated ultrasonic beams are produced at the first oil interface, the technique is less sensitive to alignment for detection, oil thickness measurement and volume reconstruction.
SMALL SCALE LABORATORY EXPERIMENTS
To validate the underwater photoacoustic technique for oil spill detection and monitoring, experiments have been conducted in a small tank as shown in Fig. 3. The small coil permits to refrigerate the salted water to conduct experiments with ice. Experiments have been performed with oil below a plexiglass target but also below a real ice target. The plexiglass target and the ice block are shown in Fig. 4. The plexiglass target has entrapped oil within a cavity with various thickness steps, ¼ inch, ¾ inch and 1 ¼ inch and a 56o slope ramp. The plexiglass cavity is
closed by a film to prevent oil leakage in the tank. For the ice target, the oil is placed into a plastic bag. These two targets simulate oil spill below the ice in small scale laboratory experiments.
FIGURE 4. Plexiglass target (left) and ice target (right) with entrapped oil.
For the experiment as shown in Fig. 5, the tank is filled with water and the target is placed on the top of the water column with the oil facing the bottom of the tank. For detection a 5 MHz immersed ultrasonic transducer is placed on the bottom of the water column, looking upward and is fixed to a scanning translation table to perform acquisition at various positions along a direction parallel to the tank bottom. For generation, the pulsed laser is outside the tank and a mirror is placed below the tank in order to fire onto the target. The mirror is also fixed to a scanning translation table to generate ultrasound at water-oil interface at various positions horizontally on the target. During the experiment, the laser beam is moved at successive positions with a step size of 1 mm along one line and for each laser position, acquisition is done with the transducer along a line parallel to the laser scan also with a step size of 1 mm. The ultrasonic data are recorded for all detector positions in order to perform volume reconstruction.
FIGURE 5. Setup of the small scale experiment.
As with laser ultrasonics, reconstruction methods like synthetic aperture focusing (SAFT) or the total focusing (TFM) [8, 9] can be used to get an improved image with better resolution. In the time domain, the TFM algorithm simply applies the delay-and-sum technique to the received signals. Figure 6 shows the geometry for reconstruction with ultrasonic propagation in oil and water media. The ray path starts with the ultrasound generated by the laser at the water/oil interface, diffracted from a point reflector as the back wall in oil and propagating backward to the transducer (scanned or from an array) at a certain distance in water. The exact time-distance relationship for the reconstruction is:
� = �1 ����+ �2 ����+ �3 ������ (1)
where pi are the path lengths in the figure and vi are the ultrasonic velocity in oil and water, with typical values of 1.35 and 1.48 mm/µs respectively. Due to refraction at the oil-water interface, use of this expression can be time consuming and different approximations have been proposed [10]. From geophysics, one approach is to use the so-called stacking or root-mean-square velocity [10, 11]. With reference to Fig. 6, the approximate relation used for TFM reconstruction then becomes:
� ≈ �1 ����+��0 2+(��−�)2 ����2
�
���2=
�/���������+|�+|���||/������������ (2) �0=�/����+ |��|/������FIGURE 6. Geometry of the ultrasound diffracted from a point reflector in oil.
In the small tank, experiments have been performed with transducer to target distance of 5 and 30 cm, giving similar results. For the plexigass target, positioned as in Fig. 5, the B-scans obtained from raw data and from TFM reconstruction are shown respectively in the left and right images of Fig. 7. The water and plexiglass are respectively at the bottom and top of the images. In the two B-scans, the first observed echo, at the bottom, corresponds to the water/oil interface where generation occurs. Such interface echo is strong only where oil is present due to laser absorption. This illustrates the on/off signal major benefit of this underwater photoacoustic technique in comparison to ultrasonic or sonar techniques. Also in the B-scans, the echoes from the various steps are clearly observed. However in the B-scan from raw data, the ramp is not properly imaged despite the presence of contributing echo signals. After TFM reconstruction, the oil region including the 56o slope ramp is perfectly imaged and the oil volume from this
moderately complex geometry can be determined.
In Fig. 8, the B-scan image obtained from raw data for the ice target with encapsulated oil (see Fig. 4) and simulating oil spill is shown. The water and ice are respectively at the bottom and top of the image. The water-oil interface where generation occurs is clearly visible with a strong echo. Note that, to the left and right of the figure, a strong first echo is observed that corresponds to a region where oil is encapsulated in ice. Also, on each side of the B-scan where no oil is present, no signal is observed in absence of absorbing media to generate ultrasound. The oil-ice interface on top of the encapsulated oil volume is also clearly visible and therefore the oil volume can be determined.
FIGURE 8. Ice target raw data result in the small tank.
FIRST PHOTOACOUSTIC PROTOTYPE AND IMPLEMENTATION IN LARGE
TANK
A first prototype for underwater photoacoustic oil spill detection and monitoring has been developed. This protoytype is not marinized at this stage and has to be operated outside water. The prototype, shown in Fig. 9, contains a pulsed laser (532 nm wavelength, 100 Hz pulse repetition rate, up to 25 mJ per pulse), an ultrasonic pulser/receiver with 16 parallel channels, a 2-axis galvanometer scanner, a camera, a I/O numeric/analog module, an Ethernet switch, an Optical to Ethernet modulus, an Ethernet to RS232 converter and power supplies. The system also includes 8 ultrasonic transducers and an umbilicus with a power line (110 V) and a dual mono-mode optical fiber for communication and data transfer.
FIGURE 9. First prototype for underwater photoacoustic oil spill detection.
For implementation and demonstration, the prototype has been placed under a large plastic tank with optical windows at its bottom for laser beam scanning and the camera. The large tank of 2 m height, 1.2 m width and a capacity of about 2000 L is shown in Fig. 10. In the right of the figure, the interior of the tank is shown with the optical windows and the eight transducers on the bottom. Also note that in the image, the pulse laser beam is visible.
With this prototype, 2D angular scan of the laser beam is performed with the galvanometer and optical mirrors while in the previous experiments, the optical scan was performed with a mirror placed on a translating table and moved along one direction with constant step size. Previously, the laser beam was well aligned with the normal to the target during optical scanning while, for this prototype, the laser beam is not systematically aligned with the target during scanning. For each incidence angle of the laser beam with the prototype, signals received from all transducers are recorded simultaneously with a fully parallel electronics.
FIGURE 10. Large tank for first prototype implementation and demonstration: outside (left), inside (right).
In the large tank, the target simulating oil spill under ice or encapsulated in ice is placed on top of the water column, as seen on the right of Fig. 11. The details of the two targets are also shown in Fig. 11. The first target, in left, is the same plexiglass target as before but with an additional half inch plate below the oil to simulate encapsulated oil in ice. The second target, in the center of Fig. 11, is a molded plastic target with a rounded cavity to simulate a geometry to be encountered in ice-covered region. This target is of larger size than previous targets, with larger volume of oil encapsulated. A plastic film was glued to the target to prevent oil leakage in the tank. Note that, an air bubble was entrapped in the oil volume after gluing the plastic film.
FIGURE 11. Plexiglass target geometry (left), hard plastic target (center), large tank with target (right).
In Fig. 12, the results obtained from raw data for the plexiglass target with encapsulated oil at a distance of 1.5 m from the ultrasonic transducers are shown. Note that a working distance of 1 to 2 m from ice bottom is envisioned for ROV oil spill monitoring during emergency response. In the figure, the amplitude C-scan is shown on the left and a B-scan is shown on the right. Again the on/off signal behavior of the photoacoustic technique provides excellent detectability of oil and simplifies signal processing and interpretation. In area with no oil, the amplitude C-scan is white and it is colored in areas with oil. The amplitude C-scan shows the first echo amplitude, therefore at the source. Such amplitude C-scan is delimiting precisely the oil surface, as illustrated in the sketch in the left bottom of Fig. 11. Absorbing media such as the oil cavity, the side-drilled hole used to inject oil and the O-ring to prevent oil leakage in the tank are well detected. Note that the amplitude color map obtained from one ultrasonic receiver to another varies because of the directivity response of the transducers.
For this experiment, no particular attention to alignment of the laser beam or ultrasonic receivers was carried out. For generation, alignment is not critical compared to sonar or ultrasonic techniques particularly for such a long working distance of 1.5 m. When the laser beam at oblique incidence reaches the oil interface, it is absorbed and generates divergent ultrasonic waves in the incident and the absorbing media. The main energy of the acoustic wave being normal to the interface in both media, it allows one imaging the oil with remote receivers in the incident medium. For detection, receiver directivity can limit the angular scan range but this can be optimized with the arrangement of multiple sensors.
Also the right of Fig. 12 shows a B-scan image for this experiment where all steps are observed. The contribution of the 56o slope ramp from the raw data is not observed and TFM reconstruction did not provide better image. This is
due to the large distance from the source to receiver, the limited length covered by the array of receivers, as well as the size of the target and the important slope angle. The B-scan is more difficult to interpret than in the previous scans, because it comes from an angular scan and the fact that oil is here encapsulated in plexiglass.
FIGURE 12. Plexiglass target amplitude C-scan (left) and B-scan (right) images obtained with the prototype in the large tank.
Figure 13 shows the results obtained for the hard plastic target with the amplitude C-scan (left), two B-scans (center) and the time-of-flight C-scan (right) images. The top B-scan in the figure corresponds to the blue dashed line sectional view in the C-scan at left, and the bottom B-scan corresponds to the black dashed line cross-sectional view in the middle of the target. Again the oil volume is very well delimited in the amplitude C-scan with an excellent contrast between areas with and without oil because of the on-off signal behavior of the technique. In the B-scans, the signal from water--oil interface at generation is very strong and the echo from oil cavity back wall is well defined. In the bottom B-scan corresponding to the centerline of the target, the air bubble is visible with a strong oil-air interface reflection. The right image of Fig. 13 shows the time-of-flight C-scan corresponding to oil thickness and obtained by monitoring the reflected echo from the oil cavity back wall and air bubble. Oil volume can then be assessed from such a C-scan image.
FIGURE 13. Hard plastic target amplitude C-scan (left), B-scans (center) and time–of-flight C-scan (right) images obtained with the prototype in the large tank.
Figure 14 presents fluorescence signature further obtained with the system in the absence (top left) and presence of oil (bottom left). This fluorescence signature was obtained by collecting and analyzing, with a spectrometer (200 nm to 720 nm wavelength), the light emitted by the target following laser firing. In the absence of oil, only the green wavelength (532 nm) is observed in the measured spectrum. In the presence of oil, light is emitted at wavelength higher than the source (532 nm). Note that an optical filter was used to reduce the collected emission at 532 mm. The fluorescence sensing technology provides a second true positive to the photoacoustic oil spill detection with minimum additional hardware since the same laser source is used. The fluorescence also provides a spectroscopic signature that could be exploited to identify the oil type and other absorbing media present.
As a final result in the right of Fig. 14, two B-scans obtained for open water oil spill conditions are shown. Oil was dispersed on top of the water column and tiny water waves were intentionally produced during the experiment. Excellent results are obtained and oil thickness measurement is achieved without any alignment issue due to the presence of waves.
FIGURE 14. Fluorescence signal with absence and presence of oil (top and bottom left) and B-scans for open water oil spill (right).
CONCLUSION
A novel and promising technique based on photoacoustics has been proposed and demonstrated for detection, sizing and monitoring of oil spill in marine ice-covered conditions. The technique combines underwater optical generation and underwater conventional ultrasonic detection. To image an oil spill area, a pulsed laser beam with
minimal absorption in water and ice is scanned, and ultrasonic waves are generated only when reaching an absorbing medium as oil. In the absence of oil, there is no ultrasonic generation and no signal is detected by the ultrasonic receivers, providing an on-off signal benefit for such underwater photoacoustic technique. This appears to be a major advantage compared to conventional underwater ultrasonic or sonar technique for oil spill detection. Moreover, the source of ultrasound being divergent and located on the first absorbing medium reached by the pulsed laser, this technique also appears to be more efficient for complex oil cavity imaging, so for oil spill volume assessment. For detection, multiple immersion ultrasonic transducers are used.
The technique has been demonstrated in a small tank with oil cavity under plexiglass and real ice. The use of the Total Focusing Method for volume reconstruction of oil spill with the photoacoustic technique has been demonstrated. In a large tank and using a first prototype, the potential of the technique for oil spill detection and monitoring with installation in a ROV for emergency response has been demonstrated. For complex oil spill volume under the ice or encapsulated in ice, the technique allows oil thickness mapping and volume assessment. With the same laser source, the technique can be combined with fluorescence to provide a second true positive signature of the oil presence and a spectroscopic signature that could be exploited to identify the oil type and other absorbing media present.
The next steps should consist in building a prototype to be embedded on an underwater vehicle as a ROV and tested in realistic conditions. This novel technique for oil spill detection and monitoring from underwater should be a promising tool for emergency response to map and evaluate oil volume before, during and after oil recovery and remediation. With its on-off signature, this photoacoustic technique should also offer great perspective to survey large and risky areas for potential oil spill in ice-covered sea regions using AUV.
ACKNOWLEDGMENTS
This work has been conducted within the NRC Arctic Program.
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