Geophysical transitions across the northwest Atlantic magnetic quiet-zone border

Deep and surface magnetic measurements, gravity and subbottom seismic profiling data have been gathered across a part of the northwest Atlantic smooth-rough magnetic border. These data indicate that the transition involves only a change in the magnetic field, without associated gravity or subbottom topographic signature. Model studies suggest that a change in basement magnetization from 0.010 emu/cm³ to 0.005 emu/cm³ could account for the magnetic field change across the boundary from the rough to the smooth sectors. Various theories previously proposed to explain this magnetic boundary are discussed with respect to these newer data; reduction of basement magnetization by weathering of the proto-Atlantic Ocean floor as described by Drake et al. (1968) is the preferred explanation.     

Preparation of Magnetic Hydroxyapatite and Their Use as Recyclable Adsorbent for Phenol in Wastewater

Magnetic hydroxyapatite (HAP), which combined superparamagnetic Fe₃O₄ nanoparticles and HAP, composite materials were prepared by ultrasound method in this paper. It has also been found that these materials have the ability to adsorb phenol in wastewater. The magnetic materials were investigated by scanning electron microscope, X‐ray diffraction (XRD), Fourier transform infrared spectroscopy, thermal gravimetric analysis, vibrating sample magnetometer, and N₂ adsorption in order to elucidate the morphology, structure, and other properties. When the prepared magnetic materials were calcined at 200°C, the prepared Fe₃O₄ was oxidized to Fe₂O₃, possessing loose‐shaped holes with a high specific area of 325.2 m²/g, a magnetization intensity of 12.5 emu/g, and the N₂ adsorption isotherm belongs to porous adsorption type I. Moreover, the magnetic HAP can adsorb 90% phenol in wastewater. This means that it is an excellent recyclable phenol sorbent for sewage treatment. Experiments confirmed that the Freundlich adsorption isotherms model applies to lower phenol concentrations (0–50 mg/L), while for high phenol concentrations (50–500 mg/L) the Langmuir adsorption isotherms model fits. The magnetic sorbents have the capacity to regenerate after reaching adsorption saturation using ethanol as eluant and external magnetic field as separation unit. The efficiency of adsorption was reduced only by 10% over a six time use period.     

Lunar magnetic field palaeointensities determined by an anhysteretic remanent magnetization method

Anhysteretic remanent magnetization (ARM) is used as a means of estimating lunar magnetic field palaeointensities from several Apollo 11 and 16 samples. A value of 1.4 Oe was obtained by this method for an Apollo 16 sample of age 3.9 AE and this value is in close agreement with the value obtained by the conventional Thellier method (1.2 Oe) carried out on the same sample. A further sample which was of age 4.0 AE and which had been reheated at 3.84 AE also showed evidence of a primary magnetization acquired in a field of this magnitude. Determinations on two younger samples (about 3.6 AE) gave palaeofields of about one quarter of this value (0.33 and 0.38 Oe respectively). These estimates of field strengths are considerably higher than previously reported.     

Magnetic aftereffect in systems of single-domain interacting particles and magnetic viscosity of rocks

Using the model of single-domain interacting particles, certain magnetic aftereffects are examined. Under the thermal-activation mechanism of magnetization approximation, dependences of viscous remanent magnetization on time, temperature, and magnetic field are obtained that agree with experimental observations. We evaluate the dependence of the time of formation of viscous magnetization on the heating temperature of a sample is carried out. Viscous magnetization, caused by diffusion in magnetic particles, is calculated. A model of viscous magnetization in a system of heterogeneous interacting particles is constructed. The coupling of the degree of chemical heterogeneity of particles and the rate of viscous magnetization is established.     

A preliminary magnetic study of soil samples from west-central Minnesota

Various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west-central Minnesota. There is a marked change in magnetic properties as a function of depth in the core. X-ray analysis and Curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil. The intensity of anhysteretic remanent magnetization (ARM) decreases sharply as the depth increases. In contrast, the stability of ARM was found to be higher for the subsoil. The surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization (VRM). The VRM acquisition coefficient (S_a) of the subsoil (S_{a= 3.18 × 10^?6emu g^?1, 3.18 × 10^?6A m² kg^?1}) was about ten times weaker than that of the top soil sample (S_a = 3.868 × 10^{?7emu g?1, 3.868 × 10?7A m2 kg?1}). The magnetic domain state indicator, the ratio of coercivity of remanence to coercive force, H_cr/H_c, was 1.5 and 3.85 for the top soil and subsoil, respectively. It appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size. We conclude that the magnetic grains in surface soil samples were more single-domain (SD) like whereas the magnetite grains in the subsoil samples were more likely in pseudo-single-domain (PSD) or small multidomain (MD) range. The observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs.     

A Robust Nonlinear Inversion for the Interpretation of Magnetic Anomalies Caused by Faults,Thin Dikes and Spheres Like Structure Using Stochastic Algorithms

A geophysical interpretative method is proposed to depth, amplitude coefficient (effective magnetization intensity), and index parameter (effective magnetization inclination) determination of a buried structure from magnetic field data anomaly due to a fault, a thin dike or a sphere-like structure. The method is based on the nonlinearly constrained mathematical modelling and also on the stochastic optimization approaches. The proposed interpretative method was first tested on a theoretical synthetic model with different random errors, where a very close agreement was obtained between the assumed and the evaluated parameters. The validity of this method was also tested on practical field data taken from United States, Australia, India and Brazil, where available magnetic data existed and were previously analyzed by different interpretative methods. The agreement between the results obtained by our developed method and those obtained by the other geophysical methods is good.     

The use of a portable fluxgate magnetometer for the estimation in the field of the direction and stability of remanent magnetization of rock specimens

Summary A portable magnetometer of the fluxgate type is described, which is capable of measuring in the field the direction and intensity of remanent magnetization of rock samples of 2 cm diameter with an accuracy of the order of 6° for intensities of 10^–3 emu/cc or more. The measurement and calculation can be carried out in less than five minutes, and a precision of the order of 2° can be obtained if more readings are taken so that ten minutes are required for a complete measurement. The chief limitation to the precision of measurement was found to be inhomogeneity of magnetization of the specimens. The instrument has been used to estimate stability of natural remanent magnetization where A.C. cleaning apparatus is not available, and the method of doing so is briefly described.Now with English Electric Computers.     

Determination of the ancient geomagnetic field elements from thermal remanent magnetization corrected for magnetic anisotropy

A method is proposed for correcting the magnetic anisotropy of baked magnetic material from archaeological objects and rocks intended for the study of its magnetization and for gaining data on the angular elements of the ancient geomagnetic field. The application of this correction decreases the spread of individual determinations obtained in studying the sample magnetization from an object and increases the determination accuracy of the average value of the ancient geomagnetic field elements. This, in turn, makes it possible to increase the accuracy of its dating.     

Bildungsbedingungen von Zinkferriten im Hinblick auf die Entfernung von Schwermetallen aus Abwässern durch Magnetseparation

Conditions of the Formation of Zinc-bearing Ferrites in Regard of Heavy Metal Removal from Wastewater by Magnetic Separation Magnetic separation techniques can be applied for heavy metal removal from wastewater if it is reached to link together the heavy metals with a substance which is sufficiently influenced by a magnetic field. Such a substance is the ferrimagnetic magnetite which can be prepared under special conditions – by oxidation of ferrous hydroxide – in wastewater. In this paper, zinc was chosen as an example for technically relevant, diamagnetic heavy metals. The investigations deal with the influence of zinc onto the structure and the magnetic properties of the reaction products produced by oxidation of zink-bearing ferrous hydroxide in aqueous solutions at room temperature. The oxidation was caused by synthetic air passing through the suspension. During the reaction, the parameters pH value, redox potential, and concentration of dissolved oxygen were observed continuously. The plots of these parameters versus time yield typical courses which can be used as measurement for the progress of the reaction. The results show that magnet ite can only be prepared if the molar ferrous concentration at the beginning is four times higher than the concentration of the dissolved oxygen. Furthermore, the oxidation rate must be slow, a condition which could be achieved by mass transfer controlled kinetics. The products of the oxidation of zinc-bearing ferrous hydroxide show a colour between black and brown. They consist of magnetite, zinc-bearing ferrite and amorphous iron hydroxide. It can be observed that with increasing initial zinc concentrations, increasing amounts of zinc-bearing ferrite but also of amorphous iron hydroxide are produced. Therefore, it seems that the impact of zinc on the reaction is in such a way that more amorphous compounds instead of the thermodynamically stabile ferrite are formed; The magnetic properties show also a dependence on the initial zinc concentration: The saturation magnetization decreases with an increase of the zinc concentration. The reaction product which results from the experiment with an initial mole ratio of Zn:Fe = 1:1 points out only paramagnetic behaviour.     

Fair function minimization for interpretation of magnetic anomalies due to thin dikes, spheres and faults

A new method is proposed to interpret magnetic anomalies due to a thin dike, a sphere, and a fault like structure, where depth, horizontal location, effective magnetization intensity and effective magnetization inclination of a buried structure are simultaneously obtained. The proposed method is based on Fair function minimization and also on stochastic optimization modeling. This new technique was firstly tested on a theoretical synthetic data randomly generated by a chosen statistical distribution from a known model with different random noises components. This mathematical simulation shows a very close agreement between the assumed and the estimated parameters. The applicability and validity of this method are thereafter applied to magnetic anomaly data taken from United States, Australia, India, and Brazil. The agreement between the results obtained by the new method and those obtained by other interpretative methods is good and comparable. Moreover, the depth obtained by such a method is found to be in high accordance with that obtained from drilling information.     

A New Best-Estimate Methodology for Determining Magnetic Parameters Related to Field Anomalies Produced by Buried Thin Dikes and Horizontal Cylinder-like Structures

A new best estimate methodology is proposed and oriented towards the determination of parameters related to a magnetic field anomaly produced by a simple geometric-shaped model or body such as a thin dike and horizontal cylinder. This approach is mainly based on solving a system of algebraic linear equations for estimating the three model parameters, e.g., the depth to the top (center) of the body (z), the index parameter or the effective magnetization angle (θ) and the amplitude coefficient or the effective magnetization intensity (k). The utility and validity of this method is demonstrated by analyzing two synthetic magnetic anomalies, using simulated data generated from a known model with different random errors components and a known statistical distribution. This approach was also examined and applied to two real field magnetic anomalies from the United States and Brazil. The agreement between the results obtained by the proposed method and those obtained by other interpretation methods is good and comparable. Moreover, the depth obtained by such an approach is found to be in high accordance with that obtained from drilling information. The advantages of such a proposed method over other existing interpretative techniques are clarified, where it can be generalized to be automatically applicable for interpreting other geological structures described by mathematical formulations.     

一种地磁（△Z）化极方法

地磁异常ΔZ受到岩石斜磁化方向畸变。应用函表示ΔZ后,能将磁异常ΔZ化到磁极而消除了这一畸变。由此得出垂直磁化磁异常三分量ΔZ_⊥、ΔX_⊥、ΔY_⊥,它们的导数以及向上和向下延拓公式。这一方法将有助于地磁资料的进一步解释。     

磁化强度矢量反演新方法及其在卡拉塔格铜多金属矿床找矿中的应用

磁异常的反演是地球物理勘探的重要手段,三维磁化率反演是磁异常定量解释中的一种重要方法.由于剩磁的存在使得磁化方向与地磁场方向产生偏差,从而影响了磁异常反演与解释的精度.本文基于磁异常模量反演和磁化强度矢量反演方法得到了一种新的磁化强度矢量反演方法.与以往的磁化强度矢量反演方法相比,该方法以磁异常模量反演得到的磁化率模型为约束,采用Lp范数正则化方法求解,提高了磁化强度矢量反演的精度和效率.本文通过模拟试验的反演计算,验证了这种磁化强度矢量反演方法的有效性.最后,将本文方法应用于新疆东天山卡拉塔格地区航磁数据的解释,获得了地下空间不同磁性差异的磁性体的空间分布特征,为进一步分析研究区隐伏矿床提供了重要信息.     

关于用有限元法作磁法勘探正演计算的理论问题

本文提出用有限元法解磁法勘探的正问题,不再需要引入均匀磁化假定。 传统的正演计算方法是在均匀磁化的假定下建立的,它不能适应解释复杂磁异常的需要。近几年来,国外学者提出了用迭代方法作非均匀磁化条件下的正演计算,对形状比较简单的磁性体取得了较好的效果。但随着磁性体的形状变得比较复杂,计算所需时间很快增加,精度和稳定性也明显降低。以求能量函数极小的变分原理为依据,用有限元法作非均匀磁化条件下的正演计算,得到的磁位势函数满足磁性体内外的偏微分方程和全部界面条件（文中对此作了证明）。因此,这种方法可以研究各种复杂因素的影响,宜于计算非均匀磁化磁性体的有效磁化强度和磁异常。     

Combining paleointensity methods: A dual-valued determination on lunar sample 10017,135

A single-heating procedure is presented which makes possible the determination of two partially independent values of paleofield intensity for a given sample, one serving as a check to the other. The approach combines data required for Shaw-type and “ARM-method” determinations and in so doing furnishes a value of the ratio of TRM to ARM acquisition efficiency (f′) corrected for any physicochemical alteration to the magnetic carriers which may have occurred during laboratory heating.

Applicability of the Shaw-method to Fe-bearing samples is favorably demonstrated through simulated paleointensity determinations conducted on synthetic samples containing multi-domain grains. Moreover, coercivity spectra corresponding to anhysteretic remanent magnetization (ARM) are found to be considerably more sensitive to thermally induced alteration when compared with those corresponding to thermoremanent magnetization (TRM).

The combined Shaw-ARM procedure was successfully applied to lunar basalt sample 10017,135 rendering a paleointensity of 0.82 ± 0.11 Oe. The Thellier-Thellier method, however, was not able to provide a meaningful determination on the neighboring chip (number 136). These apparently conflicting findings may be explained by one or more of the following possible interpretations: (1) multiple step-wise heatings cause considerably more damage to the carriers of remanence than does a single-heating procedure; (2) the rock possesses extreme variability in magnetic properties from one sub-sample to the other; (3) the natural remanent magnetization in this lunar basalt is not a simple TRM.     

强剩磁强退磁条件下的二维井中磁测反演

强剩磁、强退磁改变了总磁化强度的大小和方向,给磁测资料解释带来困难.为此,本文利用二维井中磁测数据反演磁化强度矢量的二维分布.首先利用井中磁测的磁异常模量反演磁化强度大小的分布.然后,在已知磁化强度大小分布的前提下,拟合磁场分量,反演磁化强度方向的分布.其中,磁化强度大小和方向均用共轭梯度法求解,并通过预优矩阵改善磁化强度大小的反演效果.理论模拟说明,该方法能准确获得磁化强度矢量分布.磁化强度矢量反演结果包括感磁、剩磁及退磁的影响,这为研究强剩磁、高磁化率矿床提供了一种有效方法.     

Magnetism of natural pyrrhotite,haematite and ilmenite

Summary The paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization J_n, the volume susceptibility , the specific magnetization of saturation _so, the Curie temperature , the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization J_n, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the -transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.     

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

An equivalent layer magnetization model obtained from inversion of long-wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. The magnetization maximum nearly coincides with a gravity high elongated north-south and extending into Tennessee. Previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. Taken together, the geophysical data suggest a large, localized mass of intracrustal rock which is both dense and very magnetic. A simple magnetization/density model is given which accounts for the gravity and long-wavelength aeromagnetic anomalies due to the body. We interpret it as a mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. The inferred mean magnetization (4 A/m) of the body is large, but not inconsistent with values reported by others for deep crustal bodies associated with long-wavelength magnetic anomalies. Such magnetization levels can be achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.     

Ancient magnetic field determinations on selected chondritic meteorites

Paleofield intensity determinations involving a comparison of the stable natural remanence (NRM) component with a laboratory thermoremanence (TRM) were carried out on nine chondrites selected in Brecher and Fuhrman (1979a, this issue, hereafter called Paper I), as well as on two manifestly unsuitable controls. To judge their reliability: (1) heat-alteration was monitored by comparing saturation coercivity spectra before and after heating; and (2) the NRM and TRM intensity and stability were compared to those of residual magnetization following zero-field cooling (TRM₀) from above the Curie point of kamacite (Ni---Fe). The latter criterion separates the role of an external magnetic field (of 0.43 Oe) at cooling from intrinsic contributions to magnetic grain alignments, due to accretionary, metamorphic or shock-oriented petrofabrics.

In some chondrites (e.g., Brownfield, H3B; Holyoke, H4C; Farley, H5A), a surprisingly large (10% NRM) and stable TRM₀ proved so similar to NRM and TRM, that sizeable spurious “paleofields” — comparable to paleointensities obtained — were derived by the standard method for zero-field cooling. In other chondrites, with negligible TRM₀ (1% of NRM) and irregular AF demagnetization curves, more reliable paleofield strengths in the range 0.01–0.09 Oe were obtained (e.g., Cavour, H6C). These seem representative of magnetic fields at the end of metamorphism intervals (10⁷ years after accretion) and/or at post-shock cooling. Thus, field strengths obtained from ordinary chondrites are typically weaker (by factors of 10–100) than those reliably determined from carbonaceous chondrites and ureilites, suggesting temporal decay of nebular magnetic fields, from the end of accretion until the end of metamorphism and early catastrophic-collisional stages.     

Remanent Magnetization in Portland-Cement-Based Materials

The purpose of this research is to demonstrate the possibility of applying one geophysical method to studying untraditional systems as is the case with Portland-cement-based materials. The research demonstrates how conventional paleomagnetic methodology can be employed in studying the mode of magnetic recording in present-day industrial materials. Portland-cement admixtures such as fly ashes and furnace slags should be discriminated, because those particles interact in soils and sediments in nature. Moreover, a better undertanding of magnetic remanent acquisition in model materials can serve to improve the interpretation of magnetic remanent acquisition in natural rocks formed a long time ago. The magnetic constituents of Portland-cement paste and mortar acquire a magnetic remanence due to their alignment with the earth's magnetic field at the casting place. This magnetization can be measured using ordinary paleomagnetic techniques. The alignment of the individual magnetic particles accounts for the intensity of the magnetic remanence, which can be increased by adding water and by vibration before setting and hardening. Blast furnace slag admixtures also add to the enhancement of the intensity of remanence. The magnetization of Portland-cement-based materials shows a near linear relationship with the water /cement (w/c) ratios employed in the experimental work; the w/c ratios range between 0.2 – 0.6 in pastes and 0.3 – 0.6 in mortar. Stable remanent magnetization was obtained during the first seven days of setting and hardening, a period necessary for magnetic particles to become locked parallel to the earth's magnetic field. The stability of magnetic remanence predicts the usefulness of the methodology in studying the properties of Portland cement and particularly in the control of iron-bearing admixtures.