Remanent Magnetization Of Rocks Research Articles

Using the integrated geophysical methods detecting active faults: A case study in Beijing, China

Active faults in urban have a potential damage to citizens, because they can induce not only earthquakes, but also damage pavements, utilities, homes, businesses, factories and other manmade structures because of the slow, secular and differential slippage. Consequently, the researches of detecting active faults are of great significance. This paper proposes a set of geophysical methods to detect active faults by an example in Beijing, including gravity, controlled source audio-frequency magnetotellurics(CSAMT), seismic reflection, DC resistivity and paleomagnetism (Natural remanent magnetization in rocks) to locate faults and discuss their activities. In proposed methods, gravity interpretation helps us obtain the distribution and characteristics of buried faults beneath the plain, the results of CSAMT, seismic reflection and DC resistivity reveal features and characteristics of faults from the deep to shallow part; paleomagnetism associated with radiocarbon dating help us analyze the fault slip rate; 3D seismic reflection interpretation shows the structure of two faults in the three-dimensional subsurface and the interaction of each other. Also, a few acquisition parameters, data processing methods and significant suggestions are mentioned.

Strategies to invert a suite of magnetic field anomalies due to remanent magnetization: an example from the Georgetown area of Queensland

Most magnetic field interpretation is based on the assumption that magnetization is in the same direction as the ambient geomagnetic field, and there is a widely held suspicion that anomalies cannot be inverted without this assumption. In this paper we aim to establish that the problems of uncertain magnetization direction can be largely overcome utilising both staged inversion and magnetic moment analysis (MMA). Induced magnetization has a single, known direction. Not only may a remanent magnetization direction be unknown, but it may also be variable across a wide range of scales. Palaeomagnetic studies address this variability with collection of multiple samples from multiple sites (generally outcrops, quarries or road-cuts) for hierarchical statistical analysis. For rapidly cooled volcanic or sub-volcanic bodies, magnetization directions may vary due to short-term changes in the geomagnetic field. For deeper, more slowly cooled bodies, or for bodies formed by multi-phase intrusions, magnetization direction may vary due to apparent polar wander. Furthermore, remanent magnetization of rocks with a complex thermal history may include over-prints of components of different direction acquired at different times, with variable contributions to the overall magnetization in different samples. Magnetization directions may also vary due to any post-acquisition tectonic rotations. The magnetic field generated by a body depends on the resultant of induced and remanent magnetizations, determined by the Koenigsberger (or ?Q?) ratio, which generally is also highly variable across a wide range of scales. To properly incorporate remanent magnetizations in a magnetic field interpretation we can not expect to just replace the geomagnetic field direction of an induced magnetization with a predetermined remanent magnetization direction. Rather, the interpretation of anomalies caused in substantial part by remanent magnetization requires flexible and adaptive methods. In this paper we firstly review the derivation of magnetization direction by MMA of vector components and the gradient tensor of the magnetic field. Then we investigate the combination of inversion and MMA in a study of the Newcastle Range Volcanics in north-west Queensland. This volcanic province has bodies with a wide range of shapes, sizes, complexities and settings, many with magnetizations dominated by remanence. Each anomaly presents an opportunity to determine magnetization direction with its own particular challenges. A palaeomagnetic study of the same rocks by Anderson et al (2003) provides data with which to validate and classify magnetization directions derived from the magnetic field interpretation.

On the identification of magnetostratigraphic polarity zones

Polarity zones of sedimentary sections reflect a pattern of alternating polarity of the geomagnetic field recorded by the remanent magnetization of rocks. Unfortunately, this pattern can have been modified by the variable sedimentation rate, which complicates the identification of polarity zones against the reference geomagnetic polarity time scale. To avoid this obstacle, the present paper suggests a transform applied to both the sequence of levels of polarity reversal horizons and the sequence of ages of polarity reversals before computing their cross-correlation. This transform usually reduces the impact of the variable sedimentation rate so that a sequence of more than eight polarity reversal horizons may be identified without biostratigraphic constraints. Numerical experiments involving random processes to simulate both the duration of polarity reversals and the sedimentation rate proved, however, that not all the parts of a hypothetical stratigraphic section spanning the past 165 Ma would be equally suitable for dating by magnetic polarity stratigraphy.

Possibilities of remote estimation of the natural remanent magnetization of rocks in the zones of geomagnetic field reversal

The possibilities of the remote estimation of the distribution character of the rock natural remanent magnetization in the zones of geomagnetic field reversal are considered. The Faraday effect—rotation of the signal polarization plane in the magnetic field—is used for this purpose in the process of radar impulse sounding of the magnetic field.

Magnetic study of metamorphosed sedimentary rocks of the Hatrurim formation, Israel

SUMMARY The Hatrurim formation is a ‘Mottled Zone’ complex of metamorphosed carbonate rocks, which is known as a complex subjected to combustion metamorphism (CM). Its magnetic characteristics and their relationships to the CM were studied using magnetic measurements by different techniques and on different scales. The magnetic features revealed are characterized by different spectral components and anisotropy determined at scales from hundreds of metres to a few centimetres. A very short wave of relatively high magnetization is associated with shallow small bodies of high-grade metamorphosed rocks of the Hatrurim formation. The magnetic susceptibility (κ )o fhigh-grade rocks is highly variable with marked heterogeneity and anisotropy. The frequency and magnitude of ground magnetic anomalies (up to 4000 nT) and the κ of high-grade rocks (up to 5600 × 10 −5 SI) are higher than those reported from other areas of CM. An approximate field method and conventional palaeomagnetic studies of oriented samples showed similar results, namely: (a) natural remanent magnetization of highgrade Mottled Zone rocks is heterogeneous and (b) the remanent magnetization of rocks within the Hatrurim basin usually exceeds their induced magnetization and has a similar direction. Relatively low and homogeneous magnetization is found in another type of slightly metamorphosed rock of the Hatrurim formation and in some cases also in adjacent outcrops of the underlying Mishash formation. This type appears to be the product of other sources not related to the CM process. Such sources can cause observed local aeromagnetic maxima that show some correlation with geological structure. The weak magnetization is also related to hydrothermally altered and weathered rocks of the Hatrurim formation. Thus, the magnetization of the Mottled Zone is due to a combination of near-surface/surface magnetic bodies (similar to the CM rocks in other regions) and magnetization induced by hidden magnetic sources possibly related to hydrocarbon fluxes or magmatic activity along the faults and fractures.

DOE funds Gizmos

The U.S. Department of Energy (DOE) will give $4.8 million to 21 U.S. universities for new instruments for energy‐related research, it was announced last week. The 24 successful proposals were selected from 188 applications. Four of the University Research Instrumentation Awards are designated for geoscience research.The minute sources of remanent magnetization in rocks will be investigated by John Geissman of the Univeristy of New Mexico in Albuquerque (UNM) with a magnetometer of special design, funded by a DOE grant of $115,000. Paleomagnetic measurements are generally based on a bulk determination of magnetization of 11 cm3 or so of rock. “But the size of particles that are the source of geologically stable remanent magnetism is very small,” says Geissman. His magnetometer is designed to analyze particles as small as a micron. It will be used with a scanning/transmission electron microscope (STEM) at UNM to explore where the small grains reside in rocks and to gain a fuller understanding of the paleomagnetism measured by conventional means. Future work may also involve using a laser to destroy selected magnetic phases and minimize the effects of parts of the magnetic mineralogy, permitting examination of any magnetic material that remains.

Effects of tectonic deformation on the remanent magnetization of rocks

Paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector. Deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis. However, the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known. Anisotropy of magnetic susceptibility (AMS) provides a quantitative basis for strain determination. The principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid. A linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types. If the correlation method used is valid, AMS observations can be used to establish the magnitudes and directions of the principal strains. Strain modifies bedding planes and realigns the grains that carry magnetic remanence. Compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain. The effects of strain on bedding can be corrected using passive marker theory. Attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive. During deformation linear and planar elements deflect away from the axis of maximum shortening. The passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter. Tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions. The passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix.

Petrologic evolution of plagioclase-rich cumulates from the Wichita Mountains, Oklahoma: Effects upon magnetic remanence properties

The Glen Mountains layered complex (GMLC) of the Wichita Mountains of Oklahoma is a coarse-grained analog to Layer 3 gabbros of the sea floor. The slowly cooled, mafic, layered complex developed in the site of an aulacogen, which is interpreted as a failed, intracontinental rift. The remanent magnetism of rocks of the layered complex is extremely stable due to the presence of magnetic inclusions in the major phases, primarily plagioclase. The early formation of plagioclase facilitated the formation of these magnetically important inclusions. Petrologic processes that promote the formation of magnetically stable mineralogic inclusions in coarsegrained gabbros should be operative in oceanic slow-spreading ridges. Therefore, the contribution of magnetization of Layer 3 gabbros should be greater in slower spreading ridges than in faster spreading ridges because of their differences in petrologic evolution.

Single-domain theory for the reversible effect of small uniaxial stress upon the remanent magnetization of rock

This paper on small uniaxial stress changing the remanent magnetization of rock is a companion to my previous paper on stress changing susceptibility, both phenomena being of current interest in attempts at earthquake forecasting.Theoretical expressions are derived (using rigorous energy-minimization but ignoring thermal activation) for reversible change in remanence parallel to the stress axis for samples containing single-domain grains of a ferromagnet with cubic magnetocrystalline anisotropy (K1 positive or negative) and anisotropic magnetostriction. The grains are assumed to be non-interacting and randomly oriented spheres or ellipsoids of revolution elongated along [Formula: see text], [Formula: see text], or [Formula: see text]. Also, approximate expressions are given for samples containing multidomain grains with very strongly pinned walls. Thermal (or chemical), anhysteretic, and saturation remanence are discussed.For remanence change perpendicular to the stress axis, one expects −1/2 the above expressions for change parallel to the stress axis, which is easily proven for thermal remanence.The expressions predict that for magnetite-bearing rock the decrease in thermal remanence along a 100 bar (1 × 104 kPa) compression axis should be 0.76% for spherical single-domain grains, 0.27% for 1.4 to 1 elongation along [Formula: see text], and 0.09% for great elongation along [Formula: see text]. The decrease for equidimensional multidomain grains with strongly-pinned walls should be ~0.38%. These are all much smaller than the corresponding estimates for susceptibility, but both remanence and susceptibility decreases should become larger and more comparable as titanium content increases.

Magnetization changes caused by burial and uplift

Theoretical blocking curves for hematite and magnetite are given over temperature ranges from room temperatures to Curie temperature and over time scales from 1 second to 10 9 years. In principle these curves predict how remanent magnetization in rocks is gradually lost during the heating accompanying burial and metamorphism, and how it is replaced by a new remanence during uplift and cooling. A series of experiments are described which give the decay of remanent magnetization and the build up of viscous magnetization during long-term heating at temperatures up to 400°C. Tests showed that in the selected samples minimal chemical changes occured. The treatment is considered to be analogous to the burial of rocks to 5–10 km for several millions of years. It is shown that burial magnetizations acquired in this way should be removed by standard magnetic cleaning procedures. A second series of experiments at somewhat higher temperatures (400–530°C) were carried out in order to observe the unblocking of magnetite and hematite in long-term heating experiments, and to compare the with the theoretically derived curves. In the second experiments there is a discrepancy with theory, and a compromise is proposed which indicates that very little, if any remanence can survive high greenschist facies metamorphism, and that the age of the remanence in high-grade metamorphic rocks is between the Rb—Sr whole-rock isochron and K—Ar mica ages. It is also argued that burial magnetizations are being generated in the present geomagnetic field in rocks at depths of the order of 10 km and are a potent source of magnetic anomalies over the continental crust.

An improved ballistic magnetometer for rock magnetic experiments

A quick, versatile and moderately sensitive instrument for measuring the effects of time, temperature and direct or alternating magnetic field on the remanent magnetization of rocks has been constructed. The instrument combines in one unit a ballistic magnetometer, a furnace, a solenoid and nulling coils. The ability to operate the solenoid at any sample temperature on ac as well as dc is a new feature; in addition the capabilities of the other components are significantly improved over previous instruments of the same type. The magnetometer, incorporating a galvanometer amplifier, reliably measures magnetizations of 10-5 e.m.u. per cm3 or larger. Experiments which can be performed without removing a sample from the instrument include: production and measurement of isothermal, viscous, anhysteretic and thermoremanent magnetizations; continuous thermal demagnetization without intermediate coolings; alternating field demagnetization and hysteresis measurements at ordinary or high temperatures.

Geophysical aspects of paleomagnetism

Abstract Paleomagnetism concerns the history of the earth's magnetic field as it has been recorded throughout geologic time by the remanent magnetism of rocks. It is no exaggeration to say that such studies have recently revolutionized our ideas in geology and geophysics. Field reversals, polar wander, continental drift, and sea floor spreading all essentially depend upon paleomagnetism for their demonstration. However, the theoretical basis for paleomagnetism has not developed with the same rapidity as has its application in geological thinking.

Local magnetic events associated with displacement along the San Andreas fault (California)

The piezomagnetic properties or stress-dependence of the magnetic susceptibility and remanent magnetization of rock indicates that there should be a 1% decrease in the net magnetization in the direction of an applied compressive stress of 100 kg/cm 2. Through proper observation of the local geomagnetic field, it may be possible to utilize these effects for the remote observation of changes in tectonic stress. In an attempt to observe such phenomena, a telemetering array of five rubidium magnetometers was set up in a differential manner and has been operating for over two years on the San Andreas fault in California. Local events in the geomagnetic field were indeed observed having occurred 10 to 20 h before local creep displacements of the fault and at no other time.

INTERPRETATION OF THE MAGNETIC ANOMALIES OVER SOME PRECAMBRIAN DIKES

In this paper, some examples of the influence of the remanent magnetism of rocks on magnetic fields are presented. In the study, a series of dikes were investigated since, in this case, the geometry is simple and the fields can be calculated with some precision. A large number of samples were collected and the remanent magnetization and susceptibility measured. Using this information the expected curves for ground vertical component magnetic profiles and airborne magnetic profiles were computed. These were then compared with the actual observed profiles. A good correlation was found when both remanent and induced magnetization were considered. In determining the remanent magnetization intensity, samples that had unusually high Q values were rejected because these probably represented local effects due to lightning strikes. In addition, the mean value of the magnetization was determined by taking into account the scatter of directions of the measured NRM’s. The agreement between observed and computed data was good.

Magnetism of the Earth's Crust and the Earth's Interior: ABSTRACT

Power spectrum analyses of the geomagnetic field over the earth's surface show that the field fluctuations of wave lengths shorter than about a thousand kilometers are connected mostly with the geological structure of the earth's crust. Possible interpretations of the relation between geological structure and the surface geomagnetic anomalies are demonstrated by referring to fairly detailed maps of geology, geomagnetic anomaly, and Bouger anomaly of the gravity field over Japan Islands as well as magnetic properties of various rocks. Special attention is drawn to the absolute importance of natural remanent magnetization of rocks for this kind of interpretation. It seems likely that geomagnetic anomalies of wave lengths from several hundred kilometers to several thousand kilometers are related to the distribution of continents, to the variation of thickness of the earth's crust, and to the undulation of isothermal surfaces in the earth's upper mantle. Geomagnetic field fluctuations of larger scale seem to be attributable to the electric eddy currents just beneath the surface of the earth's core. End_of_Article - Last_Page 354------------

Angular Dispersion Due to Random Magnetization

Summary The angular probability distribution generated by adding a set of randomly oriented vectors to a set of constant parallel vectors has been studied. If one random vector of constant length is added to each constant vector, the resultant angular probability distribution departs widely from the distribution used by Fisher as the basis for palaeomagnetic analysis. However, if n random vectors of equal length are added to each constant vector and if n > 6, the resulting angular probability distribution is nearly identical to that proposed by Fisher over most of the range of angular dispersions encountered in palaeomagnetic investigations. Equations are developed from which the mean intensity of random magnetization can be calculated, given the mean total intensity and the angular dispersion. These parameters, in turn, are related to the ratio of random to non-random magnetization for each ferromagnetic mineral grain in a rock specimen. I. Introduction Analysis of the natural remanent magnetization of a rock commonly reveals the presence of several components of magnetization, each due to a different magnetizing process occurring at a different time during the rock’s history. Even under ideal conditions two components are present, a stable magnetization constant in magnitude and direction from specimen to specimen, and a second component which varies randomly in direction. This second component may be introduced during the collection, the measurement, or the partial demagnetization of the specimen. It may also be present as a fundamental property of natural remanent magnetization, as most processes by which rocks become magnetized do not impart perfectly uniform magnetization to an entire rock formation. Whatever its origin, a randomly oriented component of magnetization is invariably present and produces an angular dispersion in measurements of the remanent magnetization of rocks. I 345

Piezomagnetic Effect at the Time of Local Earthquakes

THE magnetostrictive effects, or piezomagnetic properties, of rock under stress have been widely discussed and experimentally observed in the laboratory1–6. The results of these investigations indicate that a change in the sub-surface stress should manifest itself as a change in the susceptibility and remanent magnetization of rock and therefore in the local geomagnetic field. Wilson1 described laboratory observations of these effects with the suggestion that they be used to observe actual deformation of the Earth's surface. Direct observation of these effects under conditions such as those involving active fault stress, however, have never been reported.

Direction of polarization determined from magnetic anomalies

In traditional methods of interpretation of the results of magnetic surveys the effects due to permanent magnetization are neglected. Recent geomagnetic research on the remanent magnetization of rocks has shown this neglect to be unjustified. Moreover, techniques now being employed provide better measurements of magnetic-field variations than have ordinarily been available in the past. With most of the newer methods components of the field are measured in directions other than those generally treated in the earlier methods of interpretation. In order to take advantage of the new developments, equations for the magnetic field over a point dipole, a horizontal line of dipoles, a thin, dipping sheet, a thick, dipping sheet, and a sloping step are derived in the case where both the directions of measurement and polarization are arbitrary. It is found that these directions combine with other properties of the bodies to form parameters from which various features of the magnetic anomalies over the bodies can be determined. In terms of these combined parameters it is possible to obtain the higher derivatives of the expressions for the fields over these bodies and to develop methods of determining the unknown parameters of the bodies, including the direction of polarization, when magnetic profiles over them are given. Further, it is shown that the fields over four of the bodies treated can be obtained by successive differentiation of a single function. This fact is used in drawing charts for computing values of the fields and their derivatives at points along profiles over any of these bodies. Graphs are given, showing the position of special points such as peaks and inflection points on the profiles for any direction of polarization and measurement. Methods of calculation of the unknown parameters of the line of dipoles and the thin, dipping sheet when their anomalies are given, are outlined, followed by examples of calculation in the case of theoretically calculated sample profiles.

Magnetic Cleaning of Rocks in Palaeomagnetic Research

Summary It has been observed that the remanent magnetism of rocks is often the resultant of two magnetizations: firstly a magnetization with low coercive force, mostly directed by the local present Earth's field; and secondly a magnetization with high coercive force which shows the direction of the field from the period of the rock formation. It is shown that it is possible to remove the disturbing magnetization of low coercive force while saving a good deal of the fossil magnetization with the aid of partial demagnetization. This was carried out by a combination of alternating fields and heating. Generally a demagnetization with alternating fields up to 300 oersted (effective) at a temperature of 150 °C appeared to be sufficient to clean the rocks.

Remanent Magnetism of Rocks and the History of the Geomagnetic Field

Remanent Magnetism of Rocks and the History of the Geomagnetic Field