An a.c. bridge for the measurement of magnetic susceptibility of rocks in low fields

Pandit, Bhaskar Iqbal (1966) An a.c. bridge for the measurement of magnetic susceptibility of rocks in low fields. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (10MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

An a.c bridge for the measurement of the magnetic susceptibility of rocks in low fields has been constructed. The sensing unit is a double coil similar to the design by Bruckshaw and Robertson (1948), where two co-axial windings are connected in series opposition so that the net output emf is zero in the absence of a specimen. Following a treatment by Hall (1963) aimed at maximizing the sensitivity; the coil was designed for almost optimum proportions for the case when the specimen is placed on the coil axis at a bounding plane. The coil is 7.9 cms. thick, has an outer radius of 16.0 cms., and the inner radius of the inner winding is 4.Ocms. The inner and outer windings have 33,090 and 12,550 ±10 turns respectively. Use of relatively thick wire (AWG No. 26) resulted in a low Johnson noise level of 7.0x10⁻⁹ volts rms. The signal due to a specimen of volume susceptibility K = 1.0x10⁻⁵ cgs units placed 4.0 cms. beyond a bounding plane is 0.67μv rms, giving a signal-to-noise ratio of 95. -- Because of its large distributed capacitance the coil could be balanced adequately only at frequencies below 250 cps. The present frequency was 33.0 cps, and at this value the phase difference between the output emf's in the two windings could be made almost exactly 180° with the aid of 0.079μF capacitor placed across the outer winding. The output signal was amplified by a pre-amplifier and two narrow-band amplifiers connected in cascade, the total gain being 2700. To measure K ~ 1x10⁻⁷ cgs units would require a gain of about 10⁺⁵ but so far an increase in gain has been precluded by the presence in the bridge output of second and higher harmonics; moreover, the output signal at 'balance' has a serious instability with the time constant of the order of 10⁻¹ sec. The cause of instability has been traced to the oscillator and power amplifier in the input and the selective amplifiers in the output. -- The bridge was calibrated with rock specimens whose absolute volume susceptibility had been determined with an astatic magnetometer. With a calibration value of (7.3±0.2)x10⁻⁵ cgs units/cm potentiometer reading for a specimen at the coil center, the susceptibility of a fairly wide range of igneous rocks and the more strongly magnetic sedimentary rocks could be measured. For K ~ 1x10⁻³ the error in a single measurement is 6%, increasing to 25% for K ~ 8x10⁻⁵, the actual error being reducible through repeat measurements. -- Volume susceptibilities of 80 cylindrical specimens cut from 21 basalt samples from Southern Labrador were measured with the calibrated bridge. The mean value of K at two basalt exposures was (3.27±1.02)x10⁻³ and (9.74±0.19)x10⁻³ cgs units respectively where the quoted standard deviations are based on sample averages and, apart from error in the method are a measure of the inhomogeneous distribution of the chief ferromagnetic constituents in the rocks. -- The bridge was used to determine K in fields as low as 0.005 oe. rms. A significant decrease in K was observed in the case of three specimens when the magnetizing field was lowered from 0.02 to 0.005 oe. rms. -- The experimental variation of sensitivity with specimen positioning relative to the coil was compared in two cases with the theoretical variation and found to agree to less than 10% at the coil center.

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