Effects Of Remanent Magnetization Research Articles

The effects of remanent magnetization on magnetic flux leakage signals: Nestleroth, J.B.; Davis, R.J. Review of Progress in Quantitative Nondestructive Evaluation, Snowmass Village, Colorado (United States), 31 Jul. – 5 Aug. 1994. Vol. 14A, pp. 483–490. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1995) ISBN 0-306-45062-3

The effects of remanent magnetization on magnetic flux leakage signals: Nestleroth, J.B.; Davis, R.J. Review of Progress in Quantitative Nondestructive Evaluation, Snowmass Village, Colorado (United States), 31 Jul. – 5 Aug. 1994. Vol. 14A, pp. 483–490. Edited by D.O. Thompson and D.E. Chimenti. Plenum Press (1995) ISBN 0-306-45062-3

Magnetic flux leakage inspection of gas pipe-lines: the effects of remanent magnetization. Topical report, 1992–1994: Nestleroth, J.B.; Davis, R.J. Battelle, Columbus, Ohio (United States), PB95-226452/GAR, 28pp. (Apr. 1995)

The Magnetic Flux Leakage (MFL) Technique is the most commonly used technique to inspect large diameter transmission pipelines. A typical MFL inspection system uses permanent magnets to apply an axially oriented magnetic field to the ferromagnetic pipe material. Remanent magnetization affects the applied-magnetization because pipleline steels have sufficient retentivity to influence the magnetization of subsequent inspections. The remanent magnetization affects detection and characterization of pipeline corrosion in two ways. First, remanent magnetization changes the strength of the applied field level for subsequent inspection runs. Second, the remanent magnetization changes the flux leakage from corrosion defects, which affects defect detection and characterization of the defect geometry. Experimental data obtained from the GRI Pipeline Simulation Facility are used to illustrate the effect of remanent magnetization on flux leakage inspections.

Identifying remanent magnetization effects in magnetic data

Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.

Accounting for crustal magnetization in models of the core magnetic field

Remanent and induced magnetization occurs in crustal materials when they are below their Curie temperature, and we consider the problem of determining the magnetic field originating in the earth's core in the presence of such magnetization. Simple physical models of induced magnetization which have been proposed and which lead to deterministic effects account for only a small proportion of the high-degree internal field found using Magsat data, and thus a stochastic description appears more useful. We investigate the effect of remanent magnetization in the crust on satellite measurements of the core magnetic field by posing the question: if the crustal magnetization is correlated only on the shortest possible length-scale, and different components are uncorrelated everywhere, what is the correlation lengthscale at a radius above the earth's surface? Using an idea due to Parker (1988), we model the crust as a zero-mean, stationary, Gaussian random process. We show that the matrix of second-order statistics is proportional to the Gram matrix, which depends only on the inner-products of the appropriate Green's functions, and that at a typical satellite altitude of 400 km the data are correlated out to an angular separation of approximately 15°. Accurate and efficient means of calculating the matrix elements are given. This theory leads to a more conservative form for the correlation in the data than that previously given by Langel, Estes & Sabaka (1989), whilst not being incommensurate with the imprecisely known high-degree power spectrum. Previous studies examining the core field have treated satellite data as independent, and have given different orthogonal components equal weight. Both these assumptions are incorrect, and we show that the variance of measurements of the radial component of magnetic field due to the crust is expected to be approximately twice that in horizontal components. However, the size of the crustal effect is small compared to the random noise in the data, and may not lead to radically different results from those already published.

Magnetic field and remanent magnetization effects of basin‐forming impacts on the Moon

Maps of the distribution of lunar surface magnetic fields produced by the electron reflection method have shown that the largest observed concentrations of lunar crustal magnetization occur antipodal (diametrically opposite) to 4 relatively young large impact basins: Imbrium, Orientale, Serenitatis, and Crisium. A model is proposed here for the formation of these magnetization concentrations (or ‘magcons’) in which the partially ionized vapor cloud produced in a hypervelocity (> 10 km s−1) basin‐forming impact expands around the moon forcing a pre‐existing ambient magnetic field to be concentrated for a brief (< 1 day) time period in the antipodal zone. Acquisition of magnetic remanence during the period of compressed field amplification may occur by one of several mechanisms including shock remanence by impact of solid secondaries ejected from the basin‐forming event. The model implies that basin‐forming impacts have played a major role in determining the large‐scale distribution of crustal magnetization detectable from lunar orbit.

AN EXAMPLE OF THE INFLUENCE OF REMANENT MAGNETIZATION ON MAGNETIC INTENSITY MEASUREMENTS

Near Ironton, Missouri, measurements of the relative vertical intensity of the earth’s magnetic field were made along an 1,800‐ft line over an intrusive rhyolitic rock unit. Sharp variations of the order of 2,500 gammas were obtained along the sample line. These variations cannot be explained on the basis of induced magnetization alone. However, paleomagnetic measurements made on samples collected coincident to the line of magnetic measurements reveal great inhomogeneities in the magnetic properties: the magnetic susceptibility varies between [Formula: see text] and [Formula: see text] units; the ratio of the intensity of remanent magnetization to the intensity of induced magnetization varies from 0.21 to 26.91; the direction of remanent magnetization varies from sample to sample and is significantly different from the direction of the present geomagnetic field; and the polarity of the samples is both normal and reversed. A theoretical magnetic profile, calculated on the basis of the direction of total magnetization in the individual samples along the sample line, has the same gross features as the measured profile. The improved correlation which results upon consideration of the total magnetization properties of the rock unit emphasizes the necessity for considering the effects of remanent magnetization when interpreting magnetic intensity measurements.