Mobile NMR leaves the field behind
Jul 19, 2002
Engineers and geologists often use mobile nuclear magnetic resonance detectors to inspect roads, buildings and geological features, but the Earth’s magnetic field can impair the accuracy of these tools. Now Thilo Brill and colleagues of Schlumberger-Doll Research in the US have developed a technique that promises to eliminate the effects of background magnetic fields, leading to more accurate measurements (T Brill et al 2002 Science 297 369).
In conventional nuclear magnetic resonance – or NMR – a detector fires magnetic field pulses at a material to align the spins of certain nuclei in it. When the magnetic field is switched off between pulses, these nuclei emit ‘echoes’ that show how abundant they are in the sample. These detectors use an array of magnets to create a strong, uniform magnetic field over a significant volume into which the sample is placed.
But mobile NMR detectors cannot generate such uniform magnetic fields, which means that these devices have smaller sensitive regions and are influenced by the Earth’s magnetic field. These effects make it harder to interpret the echoes and can reduce the accuracy of the measurements.
A more promising technique for mobile devices is based on rapid reversals of the magnetic field applied to the sample. This technique enlarges the detection area of the device, but the echoes it produces only last a few milliseconds because the Earth’s magnetic field quickly misaligns the nuclear spins. This has hindered attempts to develop the technique into a practical tool.
To combat this problem, Brill and co-workers have now developed a technique that eliminates the effect of the Earth’s magnetic field by keeping the nuclei aligned for longer. Using two sets of coils set at right angles to each other, the team slowly rotated the orientation of the magnetic field and – at the same time – rapidly switched its direction.
They found that the slowly rotating field cancelled out the misalignment introduced by the Earth’s magnetic field each time the applied magnetic field switched direction. Since this technique does not depend on the ‘Larmor frequency’ of the nuclei under study, the researchers say it could also be used to detect different nuclei in a single sample.
About the author
Katie Pennicott is Editor of PhysicsWeb