Using fluorescence for oil spill detection and mapping

As oil exploration and development in Arctic waters expand, there is a recognized need to mitigate potential oil spills associated with the expansion of this activity. There is a developing requirement to remotely detect oil on, in, or under ice-covered waters during both summer and winter months. The Arctic is a challenging environment that includes extreme weather as well as extended months of no sunlight. The objective is to postulate a remote sensor that can rapidly and reliably detect and map oil under the polar ice.

Sensor deployment

There are two possible deployment platform-based standoff sensor architectures: airborne and AUV. The associated predicted performance and deployment scenarios for both are discussed here.

The airborne high-search-rate spectral fluorescence/reflectance lidar (SF/RL) has the potential to detect and geolocate oil beneath the Arctic ice as well as accurately measure ice thickness. The pod-mounted SF/RL sensor can be flown in both fixed and rotary-wing aircraft, either manned or unmanned. As the aircraft flies forward along its search track, the SF/RL spot is continuously scanned on the surface (snow, ice, or water) and generates an ice thickness and oil detection map that is geo-referenced to ground coordinates (Figure 1).

FIGURE 1. Airborne, high search rate SF/RL detects and geolocates oil beneath Arctic ice while mapping ice thickness. (Images provided by Qinetiq North America)

For a nominal aircraft altitude of 610 m (2,000 ft), a ground speed of 125 knots, and a scan angle of ±50° from nadir, the area coverage rate (ACR) is approximately 336 sq km/hr (130 sq miles/hr). Using state-of-the-art components, reliable oil detection is predicted through an average 4 in. of snow and 2.43 m (8 ft) of Arctic ice.

The AUV-borne sensor uses a smaller, low-power version of the SF/RL sensor, operating between the seafloor and the base of the ice, searching for oil trapped under the ice, settling on the seafloor, and/or drifting in the water column (Figure 2). The AUV-borne sensor sweeps out a 55-m (180-ft) swath, and the model predicts detection at a nominal depth of 23 m (75 ft) below the ice.

Illuminating oil

This sensor depends on the principle of fluorescence, whereby a small portion of the green illuminator beam (nominally less than 1%) is absorbed by the oil and converted to longer wavelengths of yellow to orange to red fluorescence. If there is no fluorescing material in the path, then there is no optical signal in the fluorescence spectral bands, and therefore no oil is reported as detected. The specific quantum yield for oil fluorescence is dependent on a number of factors, including excitation wavelength, type of crude oil, age of the oil, and concentration. The performance model assumes a conservative 0.5%.

The 532-nanometer (nm) green laser excitation wavelength was selected not only because of the compact and reliable short-pulse neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers available at that wavelength but also to minimize natural fluorescence of other material in the path. While the quantum yield at 532-nm excitation is less than the UV or blue, propagation through the ice and water is better at 532 nm, and there are fewer naturally fluorescing materials in the path to cause false signal returns and noise clutter. For example, chlorophyll within seawater has low absorption in the green and fluoresces at around 670 nm to 690 nm (where the ice/water attenuation is high).

Fundamental to the viability of either the airborne or the AUV-borne SF/RL sensor is the requirement for an extremely low false-alarm rate while maintaining a high search rate. Both the in-band laser reflectance return and the fluorescence spectral signature must be processed to enable sensitive detection and discrimination of any oil beneath or trapped in the ice. The SF/RL operates in a time-domain laser radar mode. This mode processes the fluorescence return and reflectance at both polarizations vs. time – the distance from the transmitter – to approximately 0.3 m (1 ft) resolution.

AUVs also could be used as the oil detection sensor platform, conducting a preplanned search pattern to cover a specified region of interest (Figure 2). The AUV payload is a smaller and simpler upward-looking version of the airborne SF/RL, mapping the subsurface of the ice sheet to detect oil present under or captured in the ice. The AUV sensor payload is composed of a small, 1.5-watt short-pulsed (less than 5 nanoseconds) Nd:YAG laser with a 2-in. diameter lidar receiver operating at a ±50° angle from nadir. At 23 m below the ice at 4 knots with a 30-Hz laser, the bottom of the ice is sampled every 3.5 m (11.5 ft) for an effective swath of 55 m for an ACR of 0.4 sq km/hr (0.15 sq miles/hr).

The concept of operations for the AUV search pattern would depend on many factors, including the local environmental condition (solid or broken ice cover), subsurface current at depth, and the nature of the oil spill.

FIGURE 2. An AUV-borne SF/RL sensor detects and geolocates oil beneath Arctic ice, and ARCDEEP enables high rate data exfiltration while at depth.

Data retrieval

The conventional way to retrieve sensor data from an AUV is to either physically retrieve the data from hard drives onboard the AUV or for the AUV to come to the surface and download the data via a radio frequency to a satellite or relay aircraft. However, collecting the data by physically downloading it from the AUV can interject unacceptable latency in the time to collect, process, and interpret this data. This delay could be critical in a time-sensitive oil spill scenario. Likewise, the ability of the AUV to come to the surface may be severely limited or prevented by the Arctic ice cover.

In the future, however, the AUV platform may have the ability to exfiltrate large amounts of data in a short amount of time directly through Arctic ice and snow to a relay aircraft. This can be done while the AUV operates at depth without delaying or disrupting the oil detection search mission. The Arctic relay communications, data exchange, and enhancement protocol (ARCDEEP) system could enable two-way blue-green laser communication data transfer between underwater assets – other AUVs, fixed nodes, or gateway buoys under the ice – and airborne platforms at 100 Kbps to 30 Mbps, depending on the environment, time of day, and season (Figure 2).

Acknowledgment
This article is based on OTC Paper 24590, which was presented at the Arctic Technology Conference in Houston in February 2014.