大地电磁全信息资料三维共轭梯度反演研究(英文)

在对张量阻抗数据、倾子数据和共轭梯度算法深入分析的基础上,我们实现了大地电磁全信息资料三维共轭梯度反演算法。基于全信息资料的三维共轭梯度反演研究,探讨了同时利用五个电磁场分量整理得到的大地电磁资料进行三维反演定量解释的方法以及全信息数据在三维反演中的作用。理论模型合成数据的反演结果表明,在三维反演中使用张量阻抗和倾子数据结合的全信息数据的反演结果优于只使用张量阻抗数据(或只使用倾子数据)的反演结果,提高了反演结果的分辨率和可信度。合成数据的反演算例也验证了所实现的大地电磁全信息资料三维共轭梯度反演算法的正确性和稳定性。     

Airborne VLF Measurements and Variations of Ground Conductivity: A Tutorial

Airborne very low frequency (VLF) data are routinely collected by national agencies and commercial companies together with other passive geophysical measurements of the static magnetic field and radiometric data. The purpose of this paper is to demonstrate that both standard three-component VLF and tensor VLF (TVLF) data contain a lot of useful quantitative and qualitative information about the electrical conductivity distribution in the upper few hundred meters of the crystalline basement. We first give a new derivation of the fundamental transfer functions (the tipper) used in the TVLF technique. We then show that the tipper can be estimated from simultaneous measurements of the wave magnetic fields from at least two transmitters with somewhat different frequencies, and present a simple model by which the maximum error introduced by the difference in frequencies can be found. Single transmitter scalar VLF maps emphasise those conductive structures that have dominant strikes in the direction of the transmitter. Multiple transmitter transfer functions are dependent only upon the underlying conductivity structure. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic magnetotelluric (MT) soundings. In such cases three-component VLF measurements can be modelled easily upon appropriate rotation of the co-ordinate system to “strike” co-ordinates. Single frequency transfer functions (tippers) have real and imaginary parts that carry information on not only lateral contrasts in conductivity, as usually stated in text books, but, taken together, they provide a robust tool for determining the background conductivity level away from distinct conductors, and they can also be used to discriminate between deep and shallow conductors. Based upon simulations using multi-frequency data, it can be concluded that such a new development would dramatically increase the resolving power of airborne VLF measurements.     

Annual variations in the electromagnetic field of the earth and electrical conductivity of the geological environment

Synchronous annual variations in the geoelectric and geomagnetic field are studied on the basis of long-term electromagnetic monitoring. It is shown that the annual geoelectric variations have intraterrestrial origin and are not related to the annual geomagnetic variations. Temporal variations in the magnetotelluric impedance and magnetic tipper, which characterize the electrical conductivity of the geological environment, are analyzed. It is established that annual variations in the magnetotelluric impedance mainly describe the variations in the electrical conductivity of surface crustal layers and are less sensitive to the deep electrical conductivity of the Earth. The annual variations in the imaginary magnetic tipper at the periods of 1000–3000 s probably reflect the changes in conductivity of a deep transversal low-resistive zone (the fault). It is suggested that annual variations in the geoelectrical and geomagnetic fields, as well as in the electrical conductivity of the geological environment, arise as a response to the changes in the geodynamical processes caused by the revolution of the Earth around the Sun.

     

3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit,British Columbia,Canada

Z‐axis tipper electromagnetic and broadband magnetotelluric data were used to determine three‐dimensional electrical resistivity models of the Morrison porphyry Cu–Au–Mo deposit in British Columbia. Z‐axis tipper electromagnetic data are collected with a helicopter, thus allowing rapid surveys with uniform spatial sampling. Ground‐based magnetotelluric surveys can achieve a greater exploration depth than Z‐axis tipper electromagnetic surveys, but data collection is slower and can be limited by difficult terrain. The airborne Z‐axis tipper electromagnetic tipper data and the ground magnetotelluric tipper data show good agreement at the Morrison deposit despite differences in the data collection method, spatial sampling, and collection date. Resistivity models derived from individual inversions of the Z‐axis tipper electromagnetic tipper data and magnetotelluric impedance data contain some similar features, but the Z‐axis tipper electromagnetic model appears to lack resolution below a depth of 1 km, and the magnetotelluric model suffers from non‐uniform and relatively sparse spatial sampling. The joint Z‐axis tipper electromagnetic inversion solves these issues by combining the dense spatial sampling of the airborne Z‐axis tipper electromagnetic technique and the deeper penetration of the lower frequency magnetotelluric data. The resulting joint resistivity model correlates well with the known geology and distribution of alteration at the Morrison deposit. Higher resistivity is associated with the potassic alteration zone and volcanic country rocks, whereas areas of lower resistivity agree with known faults and sedimentary units. The pyrite halo and ≥0.3% Cu zone have the moderate resistivity that is expected of disseminated sulphides. The joint Z‐axis tipper electromagnetic inversion provides an improved resistivity model by enhancing the lateral and depth resolution of resistivity features compared with the individual Z‐axis tipper electromagnetic and magnetotelluric inversions. This case study shows that a joint Z‐axis tipper electromagnetic–magnetotelluric approach effectively images the interpreted mineralised zone at the Morrison deposit and could be beneficial in exploration for disseminated sulphides at other porphyry deposits.     

Methods and results of monitoring of the natural telluric field in the Baikal rift zone

Variations in horizontal and vertical components of the telluric field are studied within the range of periods from a few tens of seconds to diurnal and longer variations. Variations caused by an external ionospheric source are used for studying the geoelectric heterogeneity of the medium. For this purpose, an electric tipper is introduced by analogy with the geomagnetic field and its frequency response is examined. The magnetotelluric impedance is studied using data of remote geomagnetic observatories. According to the data of monitoring of the electric tipper and magnetotelluric impedance, anomalous effects are revealed that can be related to earthquakes with K ≥ 12. A method is developed for the extraction of electric field anomalies produced by intraterrestrial electric sources.     

Processing of magnetotelluric data

The processing of magnetotelluric data involves concepts from electromagnetic theory, time series analysis and linear systems theory for reducing natural electric and magnetic field variations recorded at the earth's surface to forms suitable for studying the electrical properties of the earth's interior.The electromagnetic field relations lead to either a scalar transfer impedance which couples an electric component to an orthogonal magnetic component at the surface of a plane-layered earth, or a tensor transfer impedance which couples each electric component to both magnetic components in the vicinity of a lateral inhomogeneity.A number of time series spectral analysis methods can be used for estimating the complex spectral coefficients of the various field quantities. These in turn are used for estimating the nature of the transfer function or tensor impedance. For two dimensional situations, the tensor impedance can be rotated to determine the principal directions of the electrical structure.In general for real data, estimates of the apparent resistivity are more stable when calculated from the tensor elements rather than from simple orthogonal field ratios (Cagniard estimates), even when the fields are measured in the principal coordinates.     

Carriers of magnetism and vertical inhomogeneities in ultramafic-mafic rocks of the Kondyor Massif (Aldan shield)

Dunite samples from a borehole drilled in the platiniferous concentrically-zoned Kondyor Massif are studied by electron spin resonance (ESR). The spectrum profiles, relative intensities I, and volume magnetic susceptibilities κ are analyzed. These values experience strong irregular variations, sometimes by an order of magnitude, in the upper and medium parts of the column, at depths from 100 to 400 m; and the variations decay at greater depths. The magnetic properties of the samples are determined by iron (II) ions in the olivine lattice and by iron (III) ions in the magnetite and pyrrhotite microphases and in the products of breakdown of the solid solution: chromiferous magnetite, chromoferrite, etc. The I and κ values are directly related: κ_max = 27.8 × 10⁻³ SI units, κ_min=2.63 × 10⁻³ SI units, and κ_mean = 12.7 × 10⁻³ SI units. The maximum κ values are found in the zones with elevated contents of magnetite and pyrrhotite particles, and the minimum ones, in zones with few medium and small clusters with Fe³⁺ ions. The uneven distributions of solid solutions and magnetic phases over depths are suggested to be related to the disturbances in the conditions of crystallization.     

Vertical and horizontal components of the lake Baikal electrotelluric field and their relation to the electric conductivity

Synchronous observations of the electrotelluric field were performed on the lake ice and the coast. The ice-based vertical/horizontal field ratio at periods of a few seconds to a few minutes ranges from a few hundredths to a few thousandths, which is comparable to the noise level. The results of these observations favor a plane wave model. At the coastal station, the vertical and horizontal fields are similar, which is related to geoelectric heterogeneity of the medium. An electric tipper is used to control the behavior of heterogeneity in the spectral-time domain. Anomalous changes associated with earthquakes of K ≥ 12 are revealed from data of the tipper monitoring at epicentral distances of up to 300 km. A longitudinal MTS curve free from the effect of local geoelectric heterogeneities is obtained from ice-based data. Deep conducting layers are recognized as minimums in this curve.     

On small-scale near-surface distortion in controlled source tensor electromagnetics1

Based on a Born approximation of a thin sheet integral equation, it is shown that small-scale surficial heterogeneity significantly distorts the electromagnetic field excited by electric dipoles only when either the source or the receiver are located on the heterogeneities. When a surface heterogeneity is beneath the source, the associated distortion of the electromagnetic field is manifest as a change in the effective electric dipole moment. Hence the magnetotelluric transfer functions and impedance relations remain undistorted in this case. When a surface heterogeneity is beneath the receiver, the electric field is severely distorted, but the magnetic field is only slightly distorted. The impedance tensor is therefore strongly distorted, but the tipper vector is almost unaltered. Since the controlled source tipper is a function of 1D earth conductivity, it is proposed that tipper data should be used in the first stage of 1D interpretation. For a 1D earth, the tipper vector must always point towards the source and, in the near-field limit, should have unit length. These two necessary conditions must be met by the measured tipper before it is interpreted one dimensionally.     

Nonlinear plasma wave processes observed in the polar cusp

We present observations of electric and magnetic field variations from proton (about few Hz) to electron cyclotron frequencies (about few kHz) obtained by STAFF instrument on Cluster satellites during two cusp crossings, at ∼6 R _E altitude, in September 2002. The cusp was identified by the presence of intensive fluxes of counter streaming electrons with low energies and broadband wave activity which is typical for this region. Special attention is given for the interval of measurements when the waveform of the magnetic field fluctuations was taken in this region by CLUSTER satellites. The wave has been processed using the wavelet and bispectral analysis. Results showing the cascade of turbulence and wave-wave interactions are presented in this paper. A three wave process can be responsible for the broadening of the wave spectra in the polar cusp.     

A new magnetic index based on the external part of vertical geomagnetic variations

Variations of geomagnetic components X, Y, and Z recorded in 19 Intermagnet European observatories in 2004 were analysed. The original data from all observatories were preliminarily processed. In the first step, periods longer than three hours were filtered out. In the second step, variations of vertical geomagnetic component Z were separated into external and internal parts. We introduced a non-dimensional index η defined as the square root of a ratio of the energy of the external part of the vertical component to that of the horizontal components. Maps of the surface distribution of a new magnetic index η for the area of Europe at selected time periods were created, and their time changes are presented. The time changes of η for selected observatories are also shown. Moreover, we discuss a very interesting phenomenon we discovered, that has never been described in geophysical literature. Namely, in the recordings of all the observatories we noticed the presence of very regular variations, observed almost exclusively in the vertical component Z, which is quite unusual. These regular variations occur in the form of sinusoidal “wave packets”. The amplitudes of these variations do not depend on the geomagnetic latitude and appear in the records of all the observatories we analyzed. They occur in quiet days, which suggests that their source is in the ionosphere.     

Inversion of waveform from the data of six moderate strong earthquakes in Chinese mainland to seismic moment tensors and source mechanism

Based on Generalized Seismic Ray Theory (Helmberger, 1968), a new quickly linear inversion method from the data of seismic waveform to seismic moment tensor and source mechanism for domestic earthquake is studied in this paper. Six moderately strong earthquakes which occurred in Chinese mainland in the past few years are studied. The seismic source parameters of these earthquakes, seismic moment tensors, scalar seismic moments, fault plane solutions and source time functionsetc, are obtained. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 261–268, 1993.     

An airborne tensor VLF system. From concept to realization1

Radio signals from very low frequency (VLF) transmitters distributed world-wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.     

Compressional wave events in the dawn plasma sheet observed by Interball-1

Compressional waves with periods greater than 2 min (about 10–30 min) at low geomagnetic latitudes, namely compressional Pc5 waves, are studied. The data set obtained with magnetometer MIF-M and plasma analyzer instrument CORALL on board the Interball-1 are analyzed. Measurements performed in October 1995 and October 1996 in the dawn plasma sheet at −30 R_E ≤ X_GSM and |Z_GSM| ≤ 10 R_E are considered. Anti-phase variations of magnetic field and ion plasma pressures are analyzed by searching for morphological similarities in the two time series. It is found that longitudinal and transverse magnetic field variations with respect to the background magnetic field are of the same order of magnitude. Plasma velocities are processed for each time period of the local dissimilarity in the pressure time series. VeloCity disturbances occur mainly transversely to the local field line. The data reveal the rotation of the veloCity vector. Because of the field line curvature, there is no fixed position of the rotational plane in the space. These vortices are localized in the regions of anti-phase variations of the magnetic field and plasma pressures, and the vortical flows are associated with the compressional Pc5 wave process. A theoretical model is proposed to explain the main features of the nonlinear wave processes. Our main goal is to study coupling of drift Alfven wave and magnetosonic wave in a warm inhomogeneous plasma. A vortex is the partial solution of the set of the equations when the compression is neglected. A compression effect gives rise to a nonlinear soliton-like solution.     

Anomalies in the electric field and electric conductivity of the Earth’s crust in relation to the Kultuk Earthquake on Lake Baikal

We analyze the variations in the horizontal and vertical components of the Earth??s electromagnetic field measured at nine points onshore along the southern part of Lake Baikal. It is found that variations with a period of a few tens of seconds have decayed 20 minutes before the earthquake. This is a regional effect. It is observed in the variations of the horizontal magnetic field. The intensity of the effect in the electric field is site-specific, which is due to the nonuniform distribution of electric conductivity in a medium, as revealed by the deep magnetotelluric sounding. The electric field also experiences a coseismic variation associated with the seismic wave. This effect is stronger pronounced in the vertical electric field. The bay-like anomalies that precede and accompany the Kultuk earthquake are revealed in the electric field of intraterrestrial origin and in the behavior of the electric tipper. These anomalies are concurrent with the rise of the ground water level in the borehole. The probable nature of the revealed anomalies is discussed.     

大地电磁测深数据处理——4元素和6元素阻抗张量元素的比较研究(英文)

阻抗张量元素的计算是在大地电磁测深数据处理的重要一步。按照常规,阻抗张量被定义为以Zxx,Zxy,Zyx,和Zyy为元素的2×2矩阵。在本次研究中,6个元素的阻抗张量的计算使用了一个含有Zxx,Zxy,Zyx,Zyy,Zxz和Zyz分量的2x3矩阵。对上述两类阻抗张量元素的属性进行了分析。利用由印度古吉拉特邦卡奇沉积盆地采集的5个分量大地电磁数据测试了文中的方法。从视电阻率和相位的计算中我们是观察到在大部分的频带范围内4个元素阻抗和6个元素阻抗Zxy和Zyx两类元素区别不大。然而,较长周期时间的数据,如超过100秒,观察到视电阻率的增加和相位的减少。我们还注意到,倾子幅度在大部分时间几乎是零,但较长周期(超过100秒),逐渐呈增加的趋势。卡奇沉积盆地的地电断面表明在较长的周期内浅层近水平层和深层异常高电导性的不均质层都可能是引起大的Hz分量的原因。这表明,磁场垂直分量Hz对在大的2D/3D结构区域内的电场参数估计发挥的重要作用。     

Analysing two‐dimensional effects in central loop transient electromagnetic sounding data using a semi‐synthetic tipper approach

We present a simple and feasible approach to analyse and identify two‐dimensional effects in central loop transient electromagnetic sounding data and the correspondingly derived quasi two‐dimensional conductivity models. The proposed strategy is particularly useful in minimising interpretation errors. It is based on the calculation of a semi‐synthetic transient electromagnetic tipper at each sounding and for each observational transient time point. The semi‐synthetic transient electromagnetic tipper is derived from the measured vertical component of the induced voltage and the synthetically calculated horizontal component. The approach is computationally inexpensive and involves one two‐dimensional forward calculation of an obtained quasi two‐dimensional conductivity section. Based on a synthetic example, we demonstrate that the transient electromagnetic tipper approach is applicable in identifying which transient data points and which corresponding zones in a derived quasi two‐dimensional subsurface model are affected by two‐dimensional inhomogeneities. The one‐dimensional inversion of such data leads to false models. An application of the semi‐synthetic transient electromagnetic tipper to field data from the Azraq basin in Jordan reveals that, in total, eight of 80 investigated soundings are affected by two‐dimensional structures although the field data can be fitted optimally using one‐dimensional inversion techniques. The largest semi‐synthetic tipper response occurs in a 300 m‐wide region around a strong lateral resistivity contrast. The approach is useful for analysing structural features in derived quasi two‐dimensional sections and for qualitatively investigating how these features affect the transient response. To avoid misinterpretation, these identified zones corresponding to large tipper values are excluded from the interpretation of a quasi two‐dimensional conductivity model. Based on the semi‐synthetic study, we also demonstrate that a quantitative interpretation of the horizontal voltage response (e.g. by inversion) is usually not feasible as it requires the exact sensor position to be known. Although a tipper derived purely from field data is useful as a qualitative tool for identifying two‐dimensional distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi‐synthetic transient electromagnetic tipper approach is particularly feasible as an a posteriori approach if no horizontal components are recorded or if the sensor setup in the field is not sufficiently accurate.     

REMOTE-REFERENCE MAGNETOTELLURICS: EQUIPMENT AND PROCEDURES*

During the past 3 years, major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation. The major factor in this improvement has been the introduction of the remote-reference technique, although the use of ultrasensitive magnetometers and minicomputers for in-field data processing has also been important. After a review of the remote-reference technique, this paper describes the equipment and procedures used for remote-reference magnetotellurics by the authors. Magnetometers using d.c. Superconducting Quantum Interference Devices typically have a sensitivity of 10^?14 T Hz^?1/2, a dynamic range of 10⁷ in a 1 Hz bandwidth, and a slewing rate of 3 × 10^?5.T s^?1 at 10 kHz. The electric field measurements use conventional Cu-CuSO⁴ electrodes. The remote magnetic reference signals are transmitted to the base station using FM analog telemetry. The data are collected and processed by a minicomputer based on an LSI-11 microprocessor; the essential results—for example, the apparent resistivities and the tipper components, with their probable errors—are available in the field. Practical details are given of the handling of superconducting devices, low temperature cryostats and liquid helium in the field. Various spurious noise sources are mentioned, and techniques for minimizing their effects are described.     

Airborne VLF measurements and mapping of ground conductivity in Sweden

Airborne VLF data are routinely collected by The Geological Survey of Sweden (SGU) as part of its bedrock mapping programme. In this paper we demonstrate that the novel Tensor VLF technique developed at Uppsala University and SGU can provide useful qualitative and quantitative information about the electrical conductivity distribution in the upper few hundred meters. Single transmitter scalar VLF maps emphasize those conductive structures that have dominant strikes in the direction of the transmitter. The tensor tipper (essentially the vertical magnetic field from currents along the strike direction) calculated from multiple transmitters is dependent only upon the underlying conductivity structure. Transformation of the tipper into the peaker (the horizontal divergence) has proven to enhance the lateral resolution while the transformation to the apparent resistivity can be used to discriminate different rock types. Two case histories from the application of VLF data are presented in this study. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic MT soundings. Direct inversion of the real and imaginary parts of the tipper provides more quantitative information about the subsurface resistivity distribution.     

Response of the high-latitude polar ionosphere to changes in the orientation of the Poynting vector near the Earth orbit relative to the geomagnetic moment orientation

The effect of the mutual orientation of the Poynting vector P of the electromagnetic energy density in the solar wind and the vector M of the Earth’s magnetic moment (taking into account its orbital and diurnal motions) on the geomagnetic activity has been examined for the first time using the measurements of the solar wind parameters on the Earth orbit in 1963–2005. The component P _m of the vector P along the vector M is shown to have a pronounced annual variation with the extrema in November and May and a diurnal variation with the extrema at ∼6 and 18 UT. The phases of the variations are shown to be determined only by the geometric parameters and are independent of the sign of the sector structure of the interplanetary magnetic field. The experimental data on the planetary and high-latitude geomagnetic activity, which is a response to changes in the orientation of P relative to M, are presented. The power of the sources of the electromagnetic energy of the solar wind during strong geomagnetic disturbances is also estimated.