Passive Oil Slick thickness Measurement

By John Miller & Joel Hansen

FLIR Systems, Inc.

Wilsonville, OR

We have discovered bands that crude oil absorbs and transmits in the near and short wave Infrared. By employing a variable filter over all of these bands and moving said filter in the optical train, the detector, such as a thermal IR Imager or a Focal plane array (FPA) will be sensing bands from ”no transmission”, to ”some transmission” to ”completely transparent”. The resulting image will seem to have, crude oil appearing and disappearing. The rate at which the oil is sensed and not sensed can be used for relative (and potentially absolute) thickness indication. A ”Topo” map of the oil slick can be quickly generated by simple software. This applies to Airborne (rapidly mapping large spills), Ship borne (smaller spills and self-monitoring) and handheld as well as oil platforms.

A low cost easy and quick methodology to determine the relative thickness of an oil spill is presented. A rule of thumb in oil cleanup is that 90% of the oil is in 10% of the slick, often (e.g. Valdez) efforts are spent cleaning up the thin oil while the thick oil is ignored and causes environmental disaster. In the Deep Water Horizon Spill, fishing ships were sent out with sticks to crudely probe the slick and measure the thickness. This insitu procedure was costly time consuming and of little efficacy. Moreover it provided quantized point by point data which had to be mapped and contours estimated. The coast guard and corporate cleanup crews need a faster effective oil thickness estimator that can be airborne, handheld or ship-borne and produce instant images, and spill contour maps.

Brown et al. US 9,247,159, ”Infrared camera detection systems and methods” provides and detection method but does not provide analysis of the depth or thickness of the spill. There are other methods using lasers and microwaves, but they are all active, expensive, oversized and difficult to interpret. Some clean up operators have used standard single band infrared and visible cameras to humanly interpret the image and estimate oil thickness.

A series of laboratory tests indicated the transmission of crude oil varies from near zero in the visible band, which is why crude oil looks black to human eye, to completely transparent in the short wave infrared (SWIR). At some wavelengths it was not possible to see through the crude oil, while at other wavelengths crude oil appeared transparent, and other wavelengths formed only red or dim images.

I160037FL01 Passive Oil Thickness Measurement.pdf - Adobe Acrobat

In Near IR, Crude Oil does not penetrate (Left), yet Crude Oil appears transparent in the SWIR (Right).

A variable filter such as a circular variable filter (CVF) and others could be used in front of an InGaAs, or InSb based detection system to take advantage of this variations. One example embodiment would include a CVF in the optical path that rotates providing the FPA with an image from the near infrared through the SWIR. The rotational velocity of the filter would be timed to the FPA response, yielding a variable image across the FPA that would be combined in the temporal domain using software to create an image of an area at a given bandpass. This occurs from the near IR through SWIR.

Image processing would create a consistent image across the FPA in a given band. By temporally comparing the images, the appearance of the oil slick increases and decreases based on the slick’s thickness. The software can then produce a color coded image tied to GPS data or outline similar a topographic map. Such analysis can be done by an aircraft in a quick flyover and immediately down linked to the cleanup crews.

It is anticipated that an uncooled InGaAs or cooled InSb running at 60 Hertz, could provide data for a rough but accurate relative thickness topographical map, that could be generated in approximately 1/10 of a second giving 3 levels of thickness. A 100Hz FPA staring at a scene for a second, could yield a more accurate oil thickness map with 50 contours.

This technique can also be accomplished with a checkerboard filter on a FPA (similar to what GE did in the 1980’s), a linear variable filter, a filter wheel or electronically or otherwise variable filter.

Using available FLIR products it can be used in gimbals, ship sensors, handhelds, and security sensors on oil platforms and maritime vehicles.