A Spinner Magnetometer for Susceptibility Anisotropy in Rocks

The study of rock fabric through the anisotropy of magnetic susceptibility requires instruments capable of high-precision measurement of weakly magnetic sedimentary rocks. The magnetometer here described is such an instrument. The rock specimen spins on a shaft in an air-gap in a closed magnetic circuit of permalloy with barium ferrite permanent magnets. The air-gap is designed to apply to the rotating specimen a very uniform flux density with a value of 0.115 tesla when the magnets are saturated. Magnetizing coils integral to the transducer are used to magnetize or demagnetize the permanent magnets. Coils in the specimen gap detect specimen magnetization parallel and perpendicular to the applied field. The magnetic circuit forms a balanced bridge and allows alternative modes of transduction. The design is such that Nyquist noise is the largest noise contributed by the magnetic circuit and is smaller than the Johnson noise from the detector coils. Measured noise from the completed transducer is only 4.7 dB above the Johnson noise. The limiting sensitivity is, however, set by the time variation of anisotropy signal from the shaft and empty specimen holder. This limit is at 1.5 × 10−9 mksu (1.2 × 10−10 emu/cm3), so that relative anisotropy of 0.01% can be detected in a weakly magnetic sandstone of bulk susceptibility 1.5 × 10−5 mksu. Calibration is discussed and sample measurements are presented. Construction of the transducer required solution of many technological problems, of which the greatest was encapsulation to suppress all vibration of parts without strain-induced loss of permeability of the permalloy. The more important of the solutions found are outlined briefly.