We use marine transducers to determine whether acoustic methods are practical to identify the presence of crude oil trapped beneath sea ice. Here we present the modeled results and compare the theoretical response to a field test conducted in March, 2006. There is a clear contrast in acoustic velocity values between typical sea ice, average density crude oil, and salt water near freezing temperatures. Our selected field results match the expected response for amplitude and travel time values, however to obtain a clear signal from the ice/fluid interface and sea bed, we must alter the ice surface. In situ sea ice conditions often consist of a layer of snow and/or trapped air above the solid ice surface. We removed this layer and coated the ice surface with a thin layer of sea ice. Once prepared, we obtained consistent reflections from the ice/oil, ice/water, and water/sea bed interfaces.
As a result of increased offshore oil and gas exploration, development and production planned for arctic regions, we are modeling and testing geophysical methods for the remote sensing and surveillance of oil in and under ice. Although we have successfully demonstrated the use of radar for imaging through sea ice to identify the presence of crude oil (Bradford et al., 2005), we also tested the efficacy of acoustic methods for similar objectives. The benefit of acoustic methods compared to radar methods to image in and through sea ice is the potential presence of conductive brine that may attenuate radar signals and the potential to image with shorter wavelengths with acoustic methods.
Seismic reflection methods have shown promise in imaging through sea ice (e.g., Jones and Kwan, 1984; Jones et al., 1986). The primary challenge in acquisition comes from the need to couple the source and receiver to the ice surface. Once achieved, seismic frequencies upwards of 200 kHz that produce cm-scale wavelengths have been documented to penetrate sea ice (e.g., Jones et al., 1986). Although these earlier studies concluded that simple reflection methods to identify amplitude anomalies due to the presence of oil under sea ice was not possible, improvements in acquisition and processing technologies have prompted us to revisit the potential use of seismic methods to address this problem. Here, we discuss the physical properties of each material involved and we model the response of the presence of oil under sea ice under various conditions.
