Detecting a layer of oil under a meter of ice

Petroleum resources fuel the world economy, searching for these resources leads inexorably to ever more remote and challenging regions, including eventually the Arctic. If a spill or other failure occurs oil may float to the underside of, or be incorporated into, the seasonal sea ice. Searching for such spilled oil is technically challenging, [1]

With support from Exxon Mobil, ABQMR has been working for several years on a NMR based system capable of detecting a thin layer of crude oil beneath a meter of ice. The system should be transportable by helicopter and provide indications of the presence or absence of oil rapidly enough that a large region can be surveyed without refueling.

To that end we have recently constructed a 6m diameter, 350 kg, 4-turn, flat, figure-8 (gradiometer) coil and an earth's field NMR system capable of detecting liquid samples through ice and through a 1 m thick foam raft. The coil was constructed of 210-wire bundles of #11 AWG aluminum magnet wire. Figure 1

shows the raft and coil. The coil inductance is 300 μ H and its DC resistance is 50 m Ω which give it 5 ohm impedance at resonance and a relatively high Q. In fact, much of the dc-resistance is due to the interconnections, so it can be further reduced, if needed. It is easily driven by an audio amplifier and can be operated in the presence of culture noise due to the gradiometer design and narrow band response.

We use full- and half-passage frequency sweeps as described in [2] . We start the sweeps several kHz above resonance and sweep down to resonance. In the full-passage pulse we do a rapid 180 phase shift after a half-passage and then sweep the frequency back up. It is convenient that the inductive reactance increases with frequency which means that the current is automatically reduced for the off-resonance parts of the sweep which helps maintain the adiabatic condition ([3] ).

This type of operation at 2.2 kHz requires the system be operated broadband during transmission. Tuning capacitors are switched into the receiver circuit by relays. With ~18 Ampere (rms) current B₁ can exceed B₀ near the coil.

We have observed the water signal in as few as 5 scans. Figure 2 shows the signal after 512 scans.