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Using radar to detect oil in ice and snow Ground penetrating radar has emerged as a valuable tool in locating spilled oil in Arctic onshore and offshore conditions Alan Bailey Petroleum News
In recent years the use of portable radar equipment, referred to as ground penetrating radar, has become a valuable technique for detecting oil that has been spilled in or under ice and snow in Arctic regions. And at the Dec. 18 meeting of the Alaska Geological Society, Esther Babcock, a geoscientist with GeoTek Alaska Inc., described how radar technology combined with modern computer systems can enable people to detect an otherwise hidden oil layer just a few centimeters thick.
Babcock commented that an awareness of the ground penetrating properties of radar came about literally by accident when, in the 1950s, an aircraft crashed in Greenland, with a suspected fault in the aircraft’s radar altimeter being blamed for the pilot flying the aircraft into the ice that covers the land. It turned out that the radar signal had actually penetrated the ice cover, thus resulting in a false indication of the aircraft’s altitude relative to the ice surface.
However, the practical uses of ground penetrating radar particularly moved ahead in the mid-2000s, thanks to advances in the technology that the technique requires, Babcock said.
Reflected radio waves GPR works in a similar manner to seismic surveying, but uses radio waves rather than sound waves to locate subsurface objects and structures. A radio wave is directed into the subsurface, while a surface radio receiver detects echoes of the electromagnetic radiation, reflected from those subsurface features. Computer processing of detected echoes, done in a very similar manner to that involved in seismic data processing, enables images of the subsurface to be constructed.
Reflections of the radio waves occur where there is a boundary between materials with contrasting electrical properties, say between a layer of snow and a layer of ice.
With radio waves having relatively high frequencies and short wavelengths, GPR data can resolve subsurface features to relatively fine levels of detail. However, while a seismic survey can penetrate to depths of thousands of feet, a GPR survey can only image near-surface features. Babcock said that the radio signals in a GPR survey cannot penetrate seawater and are blocked by materials such as water-saturated clay.
Optimum frequency In fact, the optimum frequency to use in a GPR survey becomes a tradeoff between the resolution of subsurface detail and the depth penetration - the higher the frequency, the finer the resolution but the shallower the maximum penetration depth. In practice, when viewing a GPR image of a subsurface layer, the top and bottom surfaces of the layer tend to merge and become indistinguishable as the layer thickness thins to about one-eighth of the wavelength of the radio waves used in the survey, Babcock said. However, through appropriate data processing, it is possible to infer layer thicknesses, even at the limits of GPR detection, using characteristics such as the amplitude, phase and frequency of the reflected signals, she said.
And this matters in an Arctic oil spill response situation, given the tendency for an oil pool to spread out over or under an ice sheet, to form a thin oil layer. A radio frequency of 500 megahertz, a frequency that is often employed in an Arctic GPR survey, can enable a layer 4 to 5 centimeters thick to be detected, Babcock said. And data processing can enable resolutions finer than that, she said.
Successful tests Measurements of the electrical properties of snow, ice and oil indicate that it should be possible to locate the boundaries between these materials using GPR surveys. And tests in the field at Svalbard and in a tank at the U.S. Army Corps of Engineers’ cold regions test facility have shown that the technique does indeed work. People have successfully demonstrated the use of the technique to image oil in snow, under snow, in ice, or under ice, Babcock said.
However, testing of the technique has revealed that the electrical properties of sea ice tend to be anisotropic. In other words, they are different in different directions. This appears to happen because sea currents cause ice crystals to align as they form, with the salts that precipitate during ice crystallization forming aligned brine channels. Dealing with this issue in a GPR survey involves running the survey with the radio signals polarized in two perpendicular directions and then combining the results during the subsequent data processing, Babcock said. Practical to use A GPR survey involves the use of compact, easily portable equipment. And, at Svalbard, a successful test was conducted using a helicopter survey, with the helicopter moving slowly a few meters above the ground. Unlike other oil detection approaches, GPR enables measurements of layer thicknesses over a continuous area, with a few boreholes used to ground truth the GPR data and tie the data together.
The use of GPR can present benefits in terms of reduced data collection times, reduced exposure of personnel to Arctic conditions and the need for fewer people out on the ice, Babcock commented.
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