Measurement of magnetic properties of steel sheets

Magnetic methods are used in detection of environmental, engineering and military objects fabricated of thin ferromagnetic sheets having volume susceptibilities higher than 100 SI units. Magnetic modelling of such objects would be advantageous, but it requires knowledge of the susceptibility and remanence values of sheet materials, which is scarce. We introduce a magnetometer method for the determination of susceptibility and remanence on thin steel samples. The area of the sample must be so large that its within-sheet magnetization remains below the saturation state. The measurements are made in normal office surroundings in the Earth's magnetic field with an ordinary fluxgate magnetometer. The square-shaped sheet samples measured in this work have an edge length of 17.5 cm and a thickness in the range 0.5–1.0 mm. During the measuring procedure the sample is placed in four positions on a subvertical measurement board. For each position, the magnetic field in the dip direction of the board plane is measured on the opposite sides of the sample. The secondary field values are averaged for each sample position in order to reduce the effect of sample inhomogeneities. With these data, the susceptibility and remanence of the sample in its edge directions are then determined, based on a model curve which is calculated numerically using thin-sheet integral equations. The susceptibilities measured for different steel types (cold rolled and hot-dip zinc-coated steel sheets) varied in the range 200–500 SI units, and the remanence varied in the range 1000–20 000 A/m. No systematic differences were observed between the magnetic properties of various steel types. The repeatability of the susceptibility measurements was good (variations < 5%) but the remanence could be changed by 50% between repeated determinations. The measured susceptibility range signifies that pieces of steel with a typical thickness of 0.5 mm remain below magnetic saturation when their edge dimension is larger than 5 cm. Therefore magnetic modelling of larger steel pieces must be made using the thin-sheet theory with known magnetic properties, whereas smaller saturated pieces can be alternatively modelled as an equipotential system.     

冀东油田钻井岩芯的磁学研究

论述了利用古地磁、磁组构综合研究进行钻井岩芯定向和确定沉积组构（沉积层面在现代地理坐标系下的）方向的方法．通过对冀东油田100块钻井岩芯实测结果的系统分析，指出该方法在岩芯磁组构方向确定和沉积剩磁成分能够分离出来的情况下，可用于地层产状未知和井孔近似直立情况下的钻井岩芯定向，并能给出沉积组构方向．对岩芯样品中的磁性矿物成分、成因进行了研究，分析了岩芯磁性特征与烃类富集的关系，指出磁参数分布特征，能为油气勘探开发、指示油气富集部位提供参考．     

同时考虑退磁和剩磁的有限体积法正演模拟

为分析同时考虑退磁和剩磁对磁测数据解释的影响,探讨了利用有限体积法求静磁场数值解的方法.从静磁场中的麦克斯韦方程出发,导出了有限体积法控制方程的离散表达式,对边界条件近似处理后求解方程组得到磁异常.通过与退磁改正计算结果对比,验证了方法的正确性,并分析得到忽略剩磁的相对误差与科尼斯布格比（Q）相关;利用有限体积法计算长方体模型在无地磁场情况下的磁异常和内部磁化强度,从数值模拟上说明剩磁也需要进行退磁改正,并表明退磁作用对剩磁的影响不仅与磁化率相关,而且与剩磁的方向和磁性体的形态相关;组合模型的计算结果对比表明,退磁作用对剩磁的影响还会因为临近强磁性体的作用发生改变.在青海灶火河西工区的应用说明,开展同时考虑退磁和剩磁的解释方法对准确识别强磁性岩体具有实用价值.     

非滞后剩磁各向异性

由于非滞后剩磁各向异性和等温剩磁各向异性的参数直接与岩石中的携磁矿物颗粒有关,它比传统的磁化率各向异性更明确地指示岩石组构,在构造地质研究中有较大应用前景．本文介绍了非滞后剩磁及其各向异性的测定方法,并以一个实例阐述了它在构造地质中的应用．     

Anisotropy of magnetic remanence: A brief review of mineralogical sources,physical origins,and geological applications,and comparison with susceptibility anisotropy

The magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low-field magnetic susceptibility (AMS). However, alternative methods based on remanent magnetization (measured in the absence of a magnetic field) rather than induced magnetization (measured in the applied field) have distinct advantages for certain geological applications. This is particularly true for; (1) adjunct studies in paleomagnetism, in order to assess the fidelity with which a natural remanence records the paleofield orientation; (2) studies of weakly magnetic or weakly deformed rocks, for which susceptibility anisotropy is very difficult to measure precisely; and (3) quantitative applications such as strain estimation. The fundamental differences between susceptibility and remanence (and their respective anisotropies) are due to several factors: (1) susceptibility arises from all of the minerals present in a sample, whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals; (2) ferromagnetic minerals are generally more anisotropic than para- and diamagnetic minerals; (3) for ferromagnetic minerals, remanence is inevitably more anisotropic than susceptibility; and (4) a number of common minerals, including single-domain magnetites, possess an inverse anisotropy of susceptibility, i.e., they tend to have minimum susceptibility parallel to the long axis of an individual particle; remanence is immune to this phenomenon. As a consequence of all these factors, remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample.Contribution number 9102 of the Institute for Rock Magnetism, University of Minnesota.     

Palaeomagnetism of silurian lavas of Somerset and Gloucestershire,England

Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y.     

Imprinting chemical remanent magnetization in claystones at 95 °C

Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer.In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m^− 1/day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m^− 1/day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m^− 1/day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.     

Magnetic study of the dolerite dyke near Amarpur,Dhanbad (Bihar)

Summary The dolerite dyke of post-lower Gondwana age reported earlier by the authors [5]²) has been taken for detailed magnetic study. Seven magnetic traverses have been taken across the dyke using Schmidt vertical force variometer. Negative anomalies varying from 1400 to 2200 gammas have been observed. Magnetic susceptibility of samples from the dyke has been determined on both cores and crushed material, using the Model MS-3 susceptibility bridge. The average susceptibility of 20 samples has been found to be 1930x10^–6 C.G.S. units. Direction and intensity of remanence on 10 oriented samples have been determined by an Astatic magnetometer. The average intensity of remanence has been found to be 3.13x10^–3 C.G.S. units and the average direction has a declination of 323° and inclination-68° (up dip). The Koenigsberger ratio varies from 1.6 to 5.6. The high negative magnetic anomalies have been explained in terms of remance-that more than 80% of the anomaly is due to remance and that negative anomalies are due to negative inclination of remanent direction of the rocks. The magnetic direction for the dyke gave the position for Dhanbad as 51° south latitude in Jurassic period. This is in conformity with the deductions made by other workers from the palaeomagnetic studies of the Deccan and Rajmahal traps. The virtual geomagnetic pole in Jurassic period as deduced from the palaeomagnetism of the dyke has a position of latitude 8 1/2°N and longitude 71°W, in the southern Carribean. On palaeomagnetic evidence this dyke has been correlated with Rajmahal traps of eastern Bihar.     

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.     

Magnetic studies of basalt fragments recovered by deep drilling in Iceland,and the “magnetic layer” concept

The magnetic susceptibility of 1300 samples of igneous rock drill cuttings obtained from eight deep drill holes in Iceland has been measured, in order to directly provide limits on the thickness of the layer which is the source of the magnetic anomalies over Iceland. The remanent magnetism of some of the material has also been studied, and the variation of magnetic susceptibility in 740 lava flows from eastern Iceland has been analysed as a function of depth of burial.All the results indicate no systematic change of susceptibility with depth up to 2.0 km. The Curie point of all deeply buried basalts in Iceland appears to be close to that of magnetite, so that the magnetic layer may be 5 km or more in thickness when susceptibility contrasts are considered; lateral contrasts in primary remanence may reach to 3 km depth. Derivation of a magnetic layer thickness in Iceland from analyses of magnetic anomalies, using methods which have been conventionally applied to marine magnetic anomalies could, on the other hand, yield much lower apparent thickness values (less than 1 km).We therefore argue that estimates of the magnetic layer thickness in oceanic regions should be based on considerations of magnetite Curie point isotherm behaviour, rather than on anomaly analysis.     

聂拉木地区高喜马拉雅岩石磁组构及其构造含义

利用岩石磁化率量值椭球体与岩石构造应变椭球体的共轴性,在缺乏岩性标志层的高喜马拉雅结晶基底（聂拉木地区）进行岩石磁组构研究,进而研究喜马拉雅造山带的变形期次和过程.磁组构特征表明：磁化率各向异性度P在樟木镇北1km处的片岩最高为172,在康山桥南1km处最低,为113;百分率各向异性度H在1182%～4507%之间;磁性线理L在102～109之间,磁性面理F在106～160之间,磁性面理F比磁性线理L发育;磁化率椭球体的形状因子T在019～076之间,磁化率椭球的扁率E在104～149之间,磁化率椭球体的形状为一压扁椭球;樟木镇北1km到肉切村,平均最小磁化率主轴方向D3为S－N（除聂拉木北1km的MA9外）,大多数样品最小磁化率主轴方向倾角I3＞51°,最高达726°,局部发育有拉长形磁化率量值椭球体.本文研究结果表明,聂拉木地区早期经历强烈的韧性变形,推测可能是一条巨大的右旋逆冲韧性变形带,晚期构造掀斜.     

Relationships between magnetic susceptibility and heavy metals in urban topsoils in the arid region of Isfahan, central Iran

Electromagnetic induction (EMI) method results are shown for vertical magnetic dipole (VMD) configuration by using the EM38 equipment. Performance in the location of metallic pipes and electrical cables is compared as a function of instrumental drift correction by linear and quadratic adjusting under controlled conditions. Metallic pipes and electrical cables are buried at the IAG/USP shallow geophysical test site in São Paulo City, Brazil. Results show that apparent electrical conductivity and magnetic susceptibility data were affected by ambient temperature variation. In order to obtain better contrast between background and metallic targets it was necessary to correct the drift. This correction was accomplished by using linear and quadratic relation between conductivity/susceptibility and temperature intending comparative studies. The correction of temperature drift by using a quadratic relation was effective, showing that all metallic targets were located as well deeper targets were also improved.     

中国松辽盆地白垩纪岩石磁化率、剩磁强度与古气候意义

本文对中国松辽盆地白垩系沙河子组、营城组、登娄库组、泉头组、青山口组、姚家组、嫩江组岩石进行了古地磁系统采样,采样地区主要在松辽盆地中部、北部的大庆油田主要勘探开发区及吉林东部营城煤矿煤田钻井。采样钻井分布在西起齐齐哈尔、白城,东至哈尔滨一带的广大区域内,共计22个钻孔。对其中900余块岩样进行了磁化率、剩磁强度测量(表1),并统计了白垩系各个岩段的平均磁化率(表2)。     

吉林陨石中铁镍合金的磁学性质

铁镍合金是陨石中重要的磁性物质,其中铁纹石、镍纹石和四方镍纹石是球粒陨石中的主要铁镍合金.然而,迄今针对陨石中铁镍合金的磁学性质研究仍非常缺乏.本文研究了吉林陨石中的铁纹石、四方镍纹石、以及陨硫铁的磁学特征.实验表明,镍含量为6%~7%的铁纹石是该陨石中最主要的铁镍合金物质,它具有低矫顽力和高的热稳定性,居里温度~750 ℃.镍含量为~48%的四方镍纹石具有高矫顽力和高的热稳定性,居里温度~565 ℃,它是剩磁的主要载体.陨硫铁在室温为反铁磁性,不具有载剩磁能力,在60 K左右存在一个低温转换,在氩气中加热较稳定而在空气中加热被氧化转化为磁铁矿.这些研究结果为鉴定球粒陨石中的磁性物质提供了依据.     

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling

I investigate large-scale deep crustal structures of the Nankai subduction zone and neighboring region using regional magnetic and gravity anomalies, heat flow measurements, and earthquake hypocenters. It is found that ages, dip angles, and geothermal states of the subducting slab have direct influences on mantle wedge serpentinization. The weakest serpentinization observed in the Nankai forearc region is associated with the youngest downgoing plate of the Shikoku Basin. Conspicuous gravity anomalies identified in the forearc region are coincidental spatially with magnetic anomalies after the reduction to the pole, a mathematical procedure that helps relocate magnetic sources and boundaries, and allows us to more easily interpret magnetic data. It is argued that these patches of magnetic and gravity anomalies are caused by the same sources of anomalous density and magnetization, and are linked directly to preexisting structures such as magnetic anomalies and their boundaries in the subducting oceanic crust. Since the gravity and magnetic anomaly patches are found to be closely related to interplate seismogenic behaviors in the Nankai subduction zone, I suggest that major magnetic boundaries in the Shikoku Basin are likely weak places for slab tears that trigger seismic segmentations along the subduction zone.     

一个变泥质岩包体样品的系统热磁试验及其矿物学意义

利用一件采自河北汉诺坝周坝地区变泥质岩包体样品,结合系统的低温和高温磁性测量结果,探讨了应用热磁实验鉴别样品中所含原生磁性矿物的多解性问题. 结果表明, 饱和等温剩余磁化强度（SIRM）在室温～250℃以及280℃～380℃的降低分别由高钛钛磁铁矿的剩磁解阻过程（一种物理过程）以及由磁赤铁矿转换成赤铁矿（一种矿物相变）引起.样品在500℃以后磁化率的升高则是由磁铁矿从钛磁赤铁矿中出溶所致.因此,κ T（即磁化率随温度变化）曲线中呈现约580℃居里点是由加热过程中次生的磁铁矿引起,而并非代表原始（即加热前）样品中的磁铁矿成分.     

Anisotropic inversion of refracted waves in vertical cable data in the presence of dip

Refracted arrivals are analysed to estimate the near‐surface anisotropy of marine sediments using a vertical‐cable (VC) configuration. In the presence of dip, the horizontal and vertical ray‐slownesses are obtained from the observed apparent slownesses in the up‐ and downdip directions using a sum or difference at each azimuth. The multiple azimuths generated by a VC geometry permit the ray‐slowness distribution of the marine sediments to be determined. An inversion procedure is developed to provide dip and anisotropy parameters for refractive layers from the measured refraction traveltimes in multilayered azimuthally isotropic and anisotropic media. Two sets of transversely isotropic models are used to analyse the azimuthal variations of apparent and ray slownesses. In the first set, we fix the anisotropic parameters of the models but vary the dip (0°, 5° and 10°) to test the effects of the presence of dip. In the second set, we vary the P‐wave anisotropy strength (5.2%, 10.3%, 15.8% and 22.0%) to examine the sensitivity and accuracy of ray‐slowness approximations which are independent of dip. We test this inversion procedure on synthetic P‐wave VC data calculated for six different models by a finite‐difference method. The results of applications to real VC data acquired from the North Sea are also presented.     

SOME NEW MAGNETIC TRANSFORMATIONS*

All magnetic transformations are governed by one simple differential relation between the observed and the transformed quantities. A magnetic map for any component, at any location, and for any given direction of magnetization can be converted into one for which any one, two, or all three parameters differ. Three new magnetic transformations are introduced: (i) reduction to equator, (ii) orthogonal reduction, and (iii) elimination of remanence. The first eliminates (or minimizes) the asymmetry and the lateral shift of the measured total field anomalies, exactly as in Baranov's reduction to pole. The second produces perfect asymmetry so that a symmetrical target lies vertically below the zero anomaly point, midway between the maximum and minimum. When remanence is a contributing factor, the direction of resultant magnetization must be known a priori in all cases, except for transformation of one component into another in the same area. Explicit working formulae are presented for reduction to equator and pole, and orthogonal reduction.     

Theoretical analysis of thermomagnetic properties, low-temperature hysteresis and domain structure of titanomagnetites

Magnetic hysteresis of coarse-grained titanomagnetites at room temperature is characterised by low coercive force, low relative remanence, and a high ratio of coercivity of remanence to coercive force. These properties are generally interpreted in terms of multidomain structure. At low temperatures, however, ulvöspinel-rich compositions exhibit hysteresis properties similar to those of single-domain assemblages, and on this basis Radhakrishnamurty and Deutsch have proposed an alternative interpretation of the domain structure of titanomagnetites having x 0.3 in terms of a mixture of single-domain and superparamagnetic particles. Low apparent Curie temperatures are attributed to the effects of thermal agitation above the blocking temperature.

We have examined theoretically the effects of thermal agitation on the low- and high-field thermomagnetic curves and find that observed Curie temperatures in general represent an intrinsic property of the magnetic mineral present, rather than reflecting thermal agitation. The high coercive force and relative remanence at low temperatures for titanomagnetites having x > 0.5 can be explained on the basis of the interaction of domain walls with crystal defects when the large increases in magnetocrystalline anisotropy and magnetostriction with decreasing temperature are taken into account. We discuss the evidence for the existence of domain walls in coarse-grained ulvöspinel-rich titanomagnetites and conclude that multidomain structure is well established.

It is also shown that fine titanomagnetite grains may have more than one blocking temperature. In any temperature interval for which superparamagnetic grains are present they will disproportionately influence susceptibility and low-field hysteresis.     

二维大地电磁各向异性参数对视电阻率的影响研究

大地电磁法是广泛应用于深部地质结构探测、油气和矿产资源勘查等领域的一种地球物理方法.电性各向异性对电磁观测数据有很大影响,但介质各向异性参数对不同模式视电阻率的影响还较少有较为系统的研究.本文基于Maxwell方程组,推导了二维大地电磁场在任意各向异性介质中电场和磁场相互耦合的变分方程,结合有限单元法及并行计算编写了二维大地电磁任意各向异性正演程序,采用三角形网格剖分.验证程序正确性后,以倾斜板状体作为模型来研究三个主轴电阻率及三个旋转欧拉角和四种模式的视电阻率之间的关系.结果表明,主轴各向异性时,xy模式视电阻率几乎只受x方向电阻率影响,yx模式视电阻率主要受y方向电阻率影响,但同时也受z方向电阻率一定影响;三个欧拉角中只有倾角不为零时,yx模式视电阻率受倾角大小的影响较大,xy模式视电阻率几乎不受倾角的影响;只有走向角不为零时,四种模式的视电阻率同时受x、y两个主轴电阻率和走向角的大小的影响.