中国地区地磁场水平梯度的计算与分析

根据2000年中国地区156个地磁台站和复测点的实测数据以及国境外的6个地磁台站实测数据,加上国境外36个IGRF点,分别利用Taylor多项式模型和曲面Spline模型,建立了中国地区地磁场总强度(F)、磁偏角(D)和磁倾角(I)的地磁模型,通过对两种模型中各分量的φ(纬度)和λ(经度)进行微分,计算得到F、D、I的水平梯度值,并绘制相应分量沿南北方向和东西方向的水平梯度图,将其与2000年IGRF10的水平梯度图作比较,最后对水平梯度的空间分布以及三种梯度图进行了分析比较.结果表明:F和I的水平梯度主要随纬度变化,F随纬度增加,变幅约为7.0～6.0 nT/min,在中部地区(25°～35°N,90°～120°E)强度达到最大;I随纬度的增加,变幅约为2'/min～1'/min;D的水平梯度随纬度和经度而变化,随着经度和纬度的增加,变幅分别约为0.4'/min～-0.3'/min和-0.1'/min～-0.4'/min.     

地球非偶极磁场对虚地磁极计算的影响

古地磁学使用的虚地磁极（VGP）是在地心偶极磁场假设下计算的,由于地球非偶极磁场的存在,VGP一般不同于真地磁极（RGP）.为了定量检验非偶极磁场对VGP的影响,本文利用国际参考地磁场模型IGRF 1900～2000,在全球5°×5°的“虚拟测点”网格上计算了VGP和RGP的位置,并求出两种磁极的经纬度偏差和二者的角距离.结果表明,南极地区VGP与GP的角距离最大,可达26°,南大西洋和欧亚大陆北部最大达到24°和18°,其余地区一般小于15°.VGP对RGP的偏差与地磁场分布有关：在非偶极磁场较弱的地区（如太平洋半球）,纬度偏差一般不大（≤10°）,但是在主要地磁异常区（如南大西洋和南极地区）,VGP对RGP的纬度偏差可达25°.VGP对RGP的经度偏差要比纬度偏差大得多,例如在欧亚大陆北部地区,经度偏差分布在-180°到180°的大范围内.     

Transitional field clusters from uppermost Oligocene volcanic rocks in the central Walker Lane, western Nevada

Three sections of the Candelaria Hills volcanic sequence, west-central Nevada, appear to have recorded parts of two transitional field records or reversal excursions. Paleomagnetic data and / laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25.7 Ma and about 23.8 Ma. Fifteen sites yield northeast declination, moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination, moderate negative inclination directions. Both populations of site mean directions, representing a total of 12 independent eruptive units, are highly discordant to a time-averaged late Tertiary field direction, and neither can be explained by a geologically reasonable magnitude of vertical axis rotation. Virtual paleomagnetic poles (VGPs), estimated from the directional data, lie at low to intermediate latitudes; 29 of the 31 flows at intermediate latitudes (<60°), and 11 at very low latitudes (<30°). Two well-grouped VGP clusters are defined by these data with each cluster roughly corresponding to one of the age groups. Stratigraphically corrected VGPs from most of the 23.8 Ma group roughly cluster at intermediate to low latitudes at about 150°E longitude. The cluster at about 150°E corresponds to VGP clusters that have been interpreted to reflect a long lasting near-dipole configuration during several field reversals. The second stratigraphically corrected cluster lies at intermediate to low latitudes at about 80°E longitude and, notably, is defined by pyroclastic flows of the 25.7 and 23.8 Ma age groups. The VGP data at about 80°E do not fall into any previously identified preferred longitudinal band, however, they are consistent with data from some sedimentary records of reversal excursions in western North America. We recognize that the VGPs returned to a preferred location in both age populations, which we interpret as a preferred directional position, thus reflecting a potentially stable non-dipole component during a complete reversal or a reversal excursion. The observation that the VGPs maintained a preferred location during separate high amplitude events supports the hypothesis that preferred VPG clusters and thus persistent non-dipole field components can factor into the behavior of the geomagnetic field during full reversals or reversal excursions.     

地磁场长期变化的自洽正态统计模型

从地磁场时间变化尺度可分的理论考虑和观测事实出发，建立地磁场Reynolds分解，强调地磁场时均部分和长期变化部分均包含所有的多极子分量；重新解释Constabe和Paker关于规一化Gduss系数的统计结果，从而将地磁场长期变化自洽正态模型自洽化．在这一结构最简单的统计模型中，地磁角度要素和总强度的统计分布均为非正态．地磁场方向及其等价表示虚偶极磁矩方向服从广义Fisher分布，可以解释地磁场方向和虚磁偶极矩方向的椭圆分布长轴大致正交的观测事实．模型的VGP角散布不能解释古地磁观测，意味着时均场中显著存在非轴向偶极子分量．解析结果加强了Egbert关于VGP路径优势经度为不均匀采样所致的推论．     

Analysis of magnetic inclination in borecores

Summary In palaeomagnetism it is usually assumed that the distribution of directions of natural remanent magnetization at a site is Fisherian. This assumption is used here for an analysis of dispersion on a sphere when only inclination (and not declination) of NRM directions is known. By this method, in spite of the lack of declination data, the mean inclination and precision of the parent Fisherian population are estimated, together with the probable errors in these estimates. The method is tested against known Fisherian populations and is then used for dealing with data from borecores, where the way-up of cores and dip of the hole are known, but the azimuth of cores is not. Uses of the information derived from this analysis include estimations of age of NRM, support for palaeomagnetic data from surface studies, and comparison of dispersion of the palaeomagnetic field with that of the present geomagnetic field.     

电离层对流和极光区电集流的地磁链观测

本文采用31个高纬地磁台站资料考察1997年5月15日一次中等磁暴期间极光区电集流和电离层对流的空间分布和时间变化;其中20站处于纬度60°N～80°N之间的西半球,而另11站是偶极磁经度约为120°E的欧洲IMAGE地磁站链.对此纬度链和经度链上各站1 min精度地磁资料的综合分析结果表明,极光区电集流中心的相对强度及其纬度位置是随世界时和地方时区不断变化的.电集流中心所处位置的变化可能是其中心的南北移动造成的,也可能是中心带与磁纬圈间的相互倾斜所致.另一方面,电离层对流形态和晨昏对流圈的经向跨度及其两端的位置是基本不变的.有关结论得到同期的非相干散射雷达EISCAT观测的证实和补充.     

Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.     

A paleomagnetic and rock-magnetic study of a neogene lava flow sequence in La Gomera (Canary Islands,Spain)

We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (T_C = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (T_C = 120 to 200°C) and an intermediate Curie-temperature phase (T_C = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α₉₅ = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A₉₅ = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter S_B = 5.9 with an upper confidence limit S_up = 8.0 and a lower confidence limit S_low = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.     

Paleointensity in the Quaternary West Eifel volcanic field, Germany: preliminary results

Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.     

Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

时空Kriging算法在区域地磁场插值中的应用及改进

本文以区域地磁场为应用背景,研究了时间域信息对地磁场插值效果的影响.用时空Kriging算法进行插值时,在时空Kriging变差函数的拟合过程中,将角度信息引入向量距离,定义了新的向量距离形式,提出了改进的时空Kriging算法,并对权重的选取进行讨论.对经度87.2°E—126.6°E,纬度19.0°N—49.6°N范围内的32个台站时均值地磁场数据进行插值实验.在添加了时间信息之后,插值结果在兼顾经度方向和纬度方向变化的同时,边值问题也得以缓解,明显优于传统方法.交叉验证结果表明,随着时间域信息和向量角度的加入,Kriging算法的性能得到改进,地磁场的插值精度依次提高.对比实验中,改进Kriging算法的精度最高、性能最稳定,MAE集中在1nT左右,MSE集中在[0,5]的范围内.     

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from¹⁴C analyses on the Lomond sediment and comparison with other¹⁴C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations.The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced.The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.     

Plio-pleistocene paleomagnetic record from the Michoacán-Guanajuato Monogenetic Volcanic Field (Western Mexico)

The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated by ⁴⁰Ar/³⁹Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian statistical parameters are k = 28, α₉₅ = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position P_lat = 83.9°, P_long = 321.6°, K = 34, A₉₅ = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study of the scatter of the virtual geomagnetic poles giving S_F = 15.9 with S_U =21.0 and S_L = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35 Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1. Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine sediments at about 590 ka.     

Variations of angular elements of the geomagnetic field in Europe during the last 24 centuries

The analysis of variations in angular elements of the geomagnetic field during the period since 350 B.C. to the present day according to the findings from the study of thermal magnetization of baked archaeological samples from England, France, and East Europe showed that the key feature in the behavior of the geomagnetic inclination in all three regions is a millennial variation. The trend in the behavior of the inclination of the geomagnetic field can be regarded as a manifestation of a variation with a characteristic time scale of several thousand years. Despite the general likeness of variations in inclination and declination of the ancient geomagnetic field, they also exhibit a noticeable dissimilarity. The paths of the virtual geomagnetic pole reconstructed from the variations of angular elements of the geomagnetic field in East Europe indicate that the geomagnetic polar motion is quasi-cyclic. The duration of the first cycle was about 1000 years, while the second cycle has not been completed due to the change of the motion to the opposite direction in the middle of the XVII century.     

Analysis of geomagnetic secular variations 10 000 to 30 000 years bp,Lac du Bouchet,France

Cores of the bottom sediments from Lac du Bouchet, in the Haute Loire, France (44.9° N, 3.8° E) have been taken using Mackereth type pneumatic piston corers. The sediments have been dated by palynological control back to 15 000 years bp (years before present), and by radiocarbon age determinations using both conventional and accelerator methods. The depth/time transform provisionally used here dates the sequence from 10 000 to 30 000 years bp. The stacked records of demagnetised remanent magnetisation from eight cores have been examined. Fourier and maximum entropy method spectral analyses show good agreement. Inclination and declination spectra are different in that the dominant inclination periods tend to fall in the negative part of the complex spectrum, while the dominant declination periods tend to fall in the positive part. The VGP path traced out by the remanent magnetisation vector shows predominantly clockwise looping, consistent with essentially westward drifting geomagnetic sources. The detailed form of the VGP path is affected by the degree of smoothing, but clockwise looping always predominates averaging ∼ 67% over the whole time interval investigated. A band of frequencies between ∼ 0.9 and ∼ 0.3 cycles per thousand years is associated with the strongest bias towards clockwise rotation (∼ 80% to 90%) while a band between ∼ 0.3 and 0.1 cycles per thousand years is associated with only ∼ 50% clockwise rotation. Individual core records show some negative inclinations at horizons where inclination minima occur suggesting that ‘excursions’ should be regarded as extreme values of secular variation rather than aborted polarity reversals of the main field.     

Statistical Analysis of Palaeomagnetic Data from the Last Four Centuries: Evidence of Systematic Inclination Shallowing in Lava Flow Records

The main objective of this work is to compare directional (declination and inclination) volcanic and archaeomagnetic data for the last four centuries (~1600–1990) with the historical geomagnetic predictions given by the GUFM1 model which spans from 1590 to 1990. The results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model. However, when comparing the volcanic data with the model predictions, marked inclination shallowing is observed. This systematically lower inclination has already been observed in local palaeomagnetic studies (Italy, Mexico and Hawaii) for the 20th century, by comparing recent lava flows with the International Reference Geomagnetic Field (IGRF) model. Here, we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3° lower than the historical geomagnetic field model predictions.     

磁极性转换与地磁漂移

地磁极性转换期间地球磁场变化特征是地磁场最基本最重要的特征之一,是现代地磁学的研究热点之一。本文简要地论述了地磁场署期期间虚地磁极和地球磁场强度变化特征的研究现状。     

Inclination shallowing and preferred transitional VGP paths

Transitional VGP paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90° away from their site longitudes, the latter also being clustered. VGP paths obtained from lava flow sequences, though much fewer, appear not to show these biases, suggesting a rock-magnetic influence on VGP paths recorded in sediments. Inclination shallowing of detrital magnetic remanence, enhanced under low transitional field strengths, is the most likely candidate.

We illustrate the effects of inclination shallowing by applying a simple shallowing model (tan I_R = f tan I_A, where I_A is the inclination of the magnetic remanence and I_A is the inclination of the ambient field) with field variation to hypothetical data sets. Shallowing-induced clustering increases as f decreases and becomes extreme as f approaches 0.1.

We have used the model to ‘de-shallow’ the available set of transitional VGP sediment records for various values of f. The probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases. When f drops to 0.13 there is a 50% chance of getting at least as much grouping as observed. To decide if inclination shallowing is a dominant factor in the clustering, we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions. Field and laboratory redeposition data are not yet adequate to resolve this question.     

Paleomagnetic and sedimentological studies at Lake Tahoe,California-Nevada

Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of¹⁴C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.     

海洋磁测日变校正的纬度改正方法研究

目前,在远海开阔性海域磁测中,尚难以有效设立日变站,致使日变改正仍存在着困难.本文利用地磁台站实测资料对地磁日变的纬度分布特征进行研究,发现日变值随纬度变化具有由Sq电流中心向南北两侧递增的规律,且二者之间的关系可用二次函数近似描述。以此规律为基础我们建立了纬度改正方法,该方法以经度链为基站,利用基站数据得到日变值与磁纬度的拟合函数以进行纬度改正,再调整时差作为经度校正即得到计算站的日变值.实测数据计算结果表明,相较于加权平均法,此方法在远距离(经度差达40°)仍能保持较高的校正精度(4 nT),因而能更好地适用于远海磁测.