mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface*

Investigations of the superfluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth.  However, the physicist’s tool box is versatile, and techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances.

The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt.  Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole.  Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall.  A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oilfield.

Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole.  Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside the sample, the temperature is as high as 175°C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s.  Apparatus with these specifications have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein.  They have also been used for geological and oceanographic studies in northern Alaska, and at the seafloor offshore Monterey, California.