On the reduction of time variations for geomagnetic repeat station measurements

For most previous geomagnetic surveys, the time variations of the observatory closest to the site of observation have been used for reduction. The accuracy of repeat station measurements depends upon the distance between the station and the observatory. Nowadays, the requirements for the accuracy of repeat station measurements are much higher, especially with respect to local secular variation anomalies. On the other hand, transportable variograph stations are available, by means of which time variations can be recorded closer to the station site. The reduction steps for this method are:

1. Reduction of measured values to the base-line of the nearby variograph station;
2. Computation of the nearest quiet night value using the baseline of the variograph;
3. Computation of the difference between this value and the corresponding one at the observatory.

The problems are:

- the stability of the base-line values of the variographs;

- the question as to what degree a quiet night value represents the normal value at that time (e.g. influence of aD _st field and/or other fields).

A repeat station survey of the Federal Republic of Germany was carried out in 1982. During the same time, eleven stations recorded the time variations in that area. We discuss the aforementioned aspects by way of an example taken from this survey.     

Reduction of geomagnetic measurements at sea in the vicinity of the geomagnetic equator

Summary On the dayside of the Earth, a huge current system, the so-called equatorial electrojet, flows along the magnetic dip equator in the ionosphere, mainly in easterly direction. At the surface of the Earth it generates strong variations of the geomagnetic field which severely impair marine geomagnetic measurements. Independent of its changing intensity, the position of the current system is remarkably constant.For marine geomagnetic measurements in the South China Sea and the Sulu Sea the latitude dependence of the variations was determined from the data themselves, using an appropriate functional relation. By a digital filter it was taken into consideration that the slowly changing parts are less latitude-dependent. Thus, with one recording station, it was possible to reduce the variations to only 15% of their original size for distances up to 500 km.Using the comprehensive data set of two research cruises into the Dangerous Grounds area of the South China Sea, it is shown which statements on the structure of the crust are possible with the corrected data.

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Die Reduktion erdmagnetischer Messungen auf See in der Nähe des magnetischen Äquators

Zusammenfassung Entlang des magnetischen Inklinations-Äquators fließt auf der Tagseite der Erde in der Ionosphäre ein riesiges Stromsystem, der sogenannte Äquatoriale Elektroje. Es erzeugt an der Erdoberfläche starke zeitliche Veränderungen des erdmagnetischen Feldes, die seemagnetische Vermessungen erheblich beeinträchtigen. Die Lage des Stromsystems ist auch bei wechselnden Stromstärken bemerkenswert konstant. Für seemagnetische Messungen im Südchinesischen Meer wurde die Breitenabhängigkeit der Variationen unter Benutzung eines geeigneten Funktionsansatzes aus den Daten selbst bestimmt. Durch eine Filterung wurde berücksichtigt, daß langsam veränderliche Anteile weniger stark breitenabhängig sind. Dadurch gelang es, die Variationen unter Benutzung einer einzigen Registrierstation in Entfernungen bis 500 km auf nur 15% zu reduzieren.An Hand des umfangreichen Meßdatensatzes zweier Forschungsfahrten in das Gebiet der Dangerous Grounds im Südchinesischen Meer wird gezeigt, welche geowissenschaftlichen Aussagen über den Aufbau der Erdkruste nach der Verbesserung möglich werden.

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Réduction des mesures de magnétisme à la mer effectuées au voisinage de l'équateur géomagnétique

Résumé Du côté jour de la terre, un flux plasmatique puissant appelé électrojet équatorial s'écoule le long de l'équateur magnétique (champ total et composante horizontale identiques) dans l'ionosphère, principalement en direction de l'Est. Ce flux engendre à la surface du globe de fortes variations du champ géomagnétique, qui ont pour effet de perturber sévèrement les mesures de magnétisme à la mer. La position de ce courant est d'une constance remarquable, et indépendante de ses variations d'intensité.Pour les mesures de magnétisme effectuées dans les mers de Chine Sud et de Sulu, les variations en fonction de la latitude ont été déterminées à partir des données elles-mêmes, au moyen de relations fonctionelles appropriées. L'emploi d'un filtre digital a permis de tenir compte du fait que les éléments variables lentement sont moins dépendants de la latitude. C'est ainsi qu'avec une station d'enregistrement, il a été possible de réduire les variations à 15% seulement de leur valeur originale jusqu'à des distances de 500 km.En s'appuyant sur la collection très complète de données recueillies au cours de deux campagnes de recherche effectuées dans la zone des Hauts-fonds dangereux, au Sud de la mer de Chine, on montre les hypothèses qu'il est possible de formuler quant à la structure de la croûte, à partir des données corrigées.

     

Some models of the geomagnetic field in western Europe from 1960 to 1980

Achache et al.'s averages over 10 west European observatories of the annual mean magnetic components X, Y, Z are studied from 1953 to 1979. A 1970 jerk is fitted to the data by least squares, and the fit is quantitatively compared with that of a polynomial having the same number of free parameters as the jerk (a quintic). A crude correction for the 11-y sunspot cycle is also attempted. The jerk and quintic are equally good fits to X, the jerk is clearly better for Y, and the quintic is modestly better for Z. The jerk amplitudes of X and Z but not Y depend heavily on whether a sunspot correction is made, and such a correction may be required to estimate the energy of the internal part of the jerk in the manner of Malin and Hodder. The sunspot correction to Y probably cannot be brought above the noise because it is small and highly correlated with both the jerk and the quintic. The sunspot corrections to X and Z are statistically significant but depend on whether the core signal is taken as a jerk or a quintic. Courtillot and Le Mouël's physical model for the jerk predicts that the jerk amplitudes of X, Y, Z will be proportional to the longitudinal derivatives of X, Y and Z. This prediction is roughly verified for the magnitudes but not the signs; it is just possible that the signs in X and Z are not statistically significant.     

地磁台站连续观测分钟值连接阈值选择

对流动地磁矢量观测数据进行日变通化时,需借助周边地磁台站长时段连续观测分钟值数据,以计算测点处观测时刻至通化日零时H、D和Z三分量变化量,而台站实际相对记录数据多存在相邻两日非正常台阶差值,需予以消除,以保证数据的连续性。选取满洲里、德都、蒙城、泰安、静海、隆尧、锡林浩特和长春8个固定地磁台站2020年5月相对记录数据,讨论H、D和Z分量的连接阈值。为筛选得到更多的台阶值,首选较小值进行连接尝试,分析台阶值在相邻两台站的同步性,并统计台阶值出现前后连续分钟值的正常波动幅度,讨论台阶值的合理性,进而对阈值进行调整,得到3个分量的合理阈值分别为1.0 nT、0.35 nT和0.4 nT。在实际应用中,由于不同台站的地磁观测数据质量存在差异,根据需要可适当提高阈值,尤其是Z分量阈值。     

Prediction of geomagnetic storms from solar wind data with the use of a neural network

An artificial feed-forward neural network with one hidden layer and error back-propagation learning is used to predict the geomagnetic activity index (D_st) one hour in advance. The B_z-component and _Bz, the density, and the velocity of the solar wind are used as input to the network. The network is trained on data covering a total of 8700 h, extracted from the 25-year period from 1963 to 1987, taken from the NSSDC data base. The performance of the network is examined with test data, not included in the training set, which covers 386 h and includes four different storms. Whilst the network predicts the initial and main phase well, the recovery phase is not modelled correctly, implying that a single hidden layer error back-propagation network is not enough, if the measured D_st is not available instantaneously. The performance of the network is independent of whether the raw parameters are used, or the electric field and square root of the dynamical pressure.     

泾阳台地磁资料的初步分析

应用加卸载、差值、斜率法及小波分析等方法,对陕西各地磁台的资料进行处理分析,结果显示：关中地区较大范围内的地磁场变化稳定;泾阳台、乾陵台的地磁场,在昆仑山口西8．1级地震前存在异常现象,且对省内近期发生的ML4．0以上地震有前兆反映。     

Spectrum of the geomagnetic field in central Europe

mam na n uu nmaaum n a mumuu ¶rt; n. ammu ama 27-¶rt;a auau uuauuu n amuaa 7 ¶rt;nu amu nu¶rt; 1960–68 . ¶rt;a m ¶rt; mum nma maa m nmaum n nu a amumu.     

拉萨台地磁资料的深部电性反演

本文分析了拉萨台长周期成分的感应尺度Ｃ值。周期范围从０．３－６７ｄ，超过两个量级。用Ｍｏｎｔｅ－Ｃａｒｌｏ法对所得的Ｃｒｅ（Ｔ）和Ｃｉｍ（Ｔ）结果进行反演计算，反演结果给出了深部电导率结构可能的多解范围和最佳拟合的电性模型。所谓最佳拟合是指目标函数值最小的一种拟合。     

用FFT消除地电观测对地磁数据的干扰

山丹地震台地磁观测资料受到地电观测的严重干扰,造成地磁观测资料应用精度下降。本研究采用Fourier变化对地电干扰进行剔除。结果认为:该方法对处理此类干扰具有良好的效果。     

Analysis of central western Europe deformation using GPS and seismic data

The kinematic field of central western Europe is characterized by relatively small movements (around 1–2 mm/year) and diffuse seismicity with earthquakes occurring mostly in the shallow crust (within 15 km), prevalently concentrated along the Alps and the European Cenozoic Rift System (ECRIS). In order to study and constrain the current crustal kinematic field we reconstructed the velocity and the strain field using permanent GPS stations, belonging to different networks (AGNES, EUREF, REGAL, RGP). The 2D strain rate tensor has been calculated using the method of least-squares collocation. Our results show that the area of maximum compression is located along the Alpine chain, where maximum values of 7 ± 2 nstrain/year are found, while maximum extension is measured between the Armorican Massif and the Massif Central, where values of 4 ± 2 nstrain/year are reached.The earthquakes with M > 3.0, have been used to estimate the seismic strain rates, while the style of the seismic deformation was reconstructed from the fault plane solutions (FPS) available from the literature. Relatively high values of seismic strain rates (around 10 nstrain/year) are measured along the Alpine Chain and the ECRIS. Results obtained by geodetic and seismic data are quite in agreement and reflect the different tectonic evolution of the geological features characterizing the area of study. The orientation of the compressional geodetic and seismic strain axes are NW-SE in most of the area of study, on account of the action of plate boundary forces. A rotation of the same axes to N-S direction along the eastern Alps, possibly related to the Adria convergence, is found.     

基于重复测线不符值的航空磁力矢量仪飞行试验数据精度评估

目前,航空磁力测量是快速获取陆地和近海区域高精度高分辨率地磁场信息的非常有效的技术手段.本文利用航空磁力矢量仪的15条野外飞行试验重复测线,完成了对航空磁力矢量仪的飞行测线数据的精度评估,结果表明,飞行测线上的地磁场总强度的测量精度,在200 m高度为3 nT,700 m高度为0.8 nT;三分量精度为4～9 nT.     

Subgrid variability of snow water equivalent at operational snow stations in the western USA

The spatial distribution of snow water equivalent (SWE) is a key variable in many regional‐scale land surface models. Currently, the assimilation of point‐scale snow sensor data into these models is commonly performed without consideration of the spatial representativeness of the point data with respect to the model grid‐scale SWE. To improve the understanding of the relationship between point‐scale snow measurements and surrounding areas, we characterized the spatial distribution of snow depth and SWE within 1‐, 4‐ and 16‐km² grids surrounding 15 snow stations (snowpack telemetry and California snow sensors) in California, Colorado, Wyoming, Idaho and Oregon during the 2008 and 2009 snow seasons. More than 30 000 field observations of snowpack properties were used with binary regression tree models to relate SWE at the sensor site to the surrounding area SWE to evaluate the sensor representativeness of larger‐scale conditions. Unlike previous research, we did not find consistent high biases in snow sensor depth values as biases over all sites ranged from 74% overestimates to 77% underestimates. Of the 53 assessments, 27 surveys indicated snow station biases of less than 10% of the surrounding mean observed snow depth. Depth biases were largely dictated by the physiographic relationship between the snow sensor locations and the mean characteristics of the surrounding grid, in particular, elevation, solar radiation index and vegetation density. These scaling relationships may improve snow sensor data assimilation; an example application is illustrated for the National Operational Hydrologic Remote Sensing Center National Snow Analysis SWE product. The snow sensor bias information indicated that the assimilation of point data into the National Operational Hydrologic Remote Sensing Center model was often unnecessary and reduced model accuracy. Copyright © 2012 John Wiley & Sons, Ltd.     

数字地磁台网数据应用进展

目前,国家地磁台网中心汇集了全台网数字化观测资料,并在全国地震行业网内实现了数据准实时共享,本文主要从技术管理、常规产出和探索研究3个方面,介绍国家地磁台网中心目前相对完善并提供Web服务的数据应用情况。技术管理方面的应用主要是为了监控台网数据质量,常规产出则是产出各种地磁报告,而探索研究方面的应用则是尝试利用各种方法研究地磁变化。     

Levelling aeromagnetic survey data without the need for tie‐lines

A new methodology that levels airborne magnetic data without orthogonal tie‐lines is presented in this study. The technique utilizes the low‐wavenumber content of the flight‐line data to construct a smooth representation of the regional field at a scale appropriate to the line lengths of the survey. Levelling errors are then calculated between the raw flight‐line data and the derived regional field through a least squares approach. Minimizing the magnitude of the error, with a first‐degree error function, results in significant improvements to the unlevelled data. The technique is tested and demonstrated using three recent airborne surveys.