Description |
This project positively detected oil trapped in and under ice with two
completely independent technologies, both of which have potential for
further development and large-scale field testing. In many respects (limited
size of spills, lack of natural cracks and fractures in the ice), the design
of this test program represents a worst-case scenario, compared with the
expected characteristics of a real spill under sea ice. In this context, the
results reported here represent a significant breakthrough, especially when
viewed against decades of previous work, resulting in few if any practical
solutions to the oil-in-ice detection problem. There is a worldwide need to develop a practical remote sensing system to
detect and map oil in ice. Such systems will facilitate leak detection and
improve spill response capabilities for oil and gas operations in Arctic
regions. This project presents results from tests in November 2004 on a 35
cm (14 in) thick sea ice sheet grown at the Cold Regions Research and
Engineering Laboratory (CRREL) in Hanover, NH. Two independent technologies
were evaluated: high-frequency pulsed Ground Penetrating Radar (GPR), and an
ethane gas sensor. The objective was to establish whether off-the-shelf
technologies and sensors could detect oil under solid ice.
Fresh South Louisiana crude was injected inside six plastic skirts frozen
into the smooth ice. Spill volumes ranged from 49 to 188 liters (13 to 50
gal), representing nominal oil film thickness from 8 to 30 mm (0.3 to 1.2
in). The six spills included an equal mix of trapped oil within the ice
sheet and free oil under the ice sheet. A seventh spill was made in rubble
ice with a rough undersurface. Analysis of the saturated headspace vapor for
the oils used indicated that the ethane concentration ranged from 5000 ppmv
before the test to only about 3000 ppmv at the conclusion of the field test.
The radar group completed a series of 2D and 3D experiments, utilizing
two radar systems, each with three antenna configurations, ranging from 450
MHz to 1200 MHz. Radar results show a clear reflection from the ice/water
interface in both the smooth ice and rough ice areas over the full range of
antenna frequencies (including airborne runs up to three meters above the
ice surface). At frequencies above 800 MHz, researchers observed clear, well
defined frequency, phase, and amplitude anomalies where oil was known to be
present at the ice/water interface and trapped within the ice. The agreement
of experimental results with initial modeling indicates the potential to
accurately predict GPR response to a variety of arctic spill scenarios and
radar parameters. Overall, the results clearly demonstrate the potential for
detecting oil under sea ice with GPR.
The LightTouch™ ethane gas sensor uses a Tuneable Diode Laser
Spectrometer (TDLS), that can measure real-time concentrations to an
accuracy of ~50 parts per trillion, approximately 200 times better than gas
chromatographic measurements. Results show measurable, but very low, levels
of ethane flux being transmitted through the ice sheet within the oiled
areas. These measurements were made 2-3 days after the last four spills
(under the maximum ice thickness) and 9-13 days following the initial three
spills (under thinner ice). Although the ethane flux from oil trapped under
these artificial, test-tank conditions was extremely small, the ice coring
data demonstrated that the oil and light gases, such as ethane, had
penetrated nearly to the surface of the ice within the 14 day program
duration (initial spill to final day of testing). Given longer times and
natural conditions, where tectonic forces would provide additional migration
pathways, it appears likely that an airborne LightTouch™ detection system
would be capable of detecting ethane emissions associated with a real oil
spill.
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