Geomagnetic Response Functions Research Articles

The Pannon LitH2Oscope magnetotelluric array in the Pannonian Basin

AbstractThe Pannonian Basin is one of the best natural laboratories in the world to study the lithospheric response to continental extension and subsequent tectonic inversion. Here we address the topic of lithospheric structure by a combined geochemical and magnetotelluric analysis, which has been carried out in the framework of the Pannon LitH2Oscope project. The main objective was to detect the resistivity distribution over the entire lithosphere by magnetotelluric measurements, considering the lithological resistivity properties and relate the results to the structure and evolution of the Pannonian Basin. The Pannon LitH2Oscope MT array was used to estimate the depth of the Lithosphere-Asthenosphere Boundary (LAB), considering the legacy MT data and compared to previous estimates for the region. Using the MT and geomagnetic response functions, major structural zones of the Pannonian basin, such as the Mid-Hungarian Shear Zone or fault systems like the Makó Trough and the Békés Basin, were also imaged. In addition, we used the apparent resistivity soundings to compare 1D resistivity models computed from geochemistry and obtained from field MT measurements. This comparison provided new constrains for the composition, fluid and melt content variations at the local lithosphere-asthenosphere boundary. The Pannon LitH2Oscope MT dataset and the results presented in this paper provide input for more complex 3D inversions and further investigations of the lithospheric structure in the Carpathian-Pannonian region.

Electrical conductivity anomalies study

Anomalous currents in a well-conducting body arise due to local electromagnetic induction inside the anomalous body, as well as due to conductive redistribution (and concentration) of currents induced in the host medium on a large territory equal to the external source size. The local induction generates the so-called anomaly of geomagnetic variations of the magnetic or inductive type. Its characteristic property is that the secondary anomalous field cannot be greater than the primary normal field of geomagnetic variations. However, in some places on the earth’s surface, the normalized anomalous fields are greater than 1. Analytical solution of the EM induction problem for circular cylinder yields the physical explanation of two types of anomalous geomagnetic fields. The first term (proportional to the normal electric field E0) describes the conductive anomaly type, the second term (proportional to the normal magnetic field B0) describes the inductive anomaly type. The conductive type usually is much greater than the inductive one. The normalized anomalous field of the conductive type is not limited to 1 or any other value. It is proportional to two functions: V(T) — the non-decreasing function of the period T (0≤V≤1, V=1 corresponds to DC) which describes the degree of filling of the conductor by anomalous currents (result of the skin effect inside the anomaly) and the normal impe­dance of inclosing cross-section — the decreasing function of the period. Product of these functions has a maximum at some period T0. The position T0 is closely related to the total lengthwise conductance G[S×m] of the anomalous body, that is, the scale of the anomaly. On the period T0, the anomalous fields and the induction vector become real C=Cu and the imaginary induction vector Cv passes through zero changing sign. Thus, the spectral properties of the geomagnetic response functions were studied for two-dimensional anomalies with a generalization to three-dimensional conductors with varying cross-section. 18 crustal electrical conductivity anomalies were considered and their integral lengthwise conductance G was obtained. At all anomalies, the G values turn out to be in a relatively narrow range G=(1—8)·108 S∙m; this has geophysical significance.

Variations of Geomagnetic Response Functions before the 2011 Tohoku Earthquake

Induction vectors have been extensively calculated using data from 19 Japanese observatories for a dozen years preceding the huge 2011 Tohoku earthquake (EQ). At 6 observatories anomalous variations of induction vectors were separated in the years of 2008-2010 that can be identified as middle-term precursors. These observatories are located not at the shortest distance from the EQ epicenter, that is in agreement with the widely known phenomenon of spatial selectivity of EQ precursors. The analysis of horizontal tensors reveals a conductivity anomaly under the central part of the Boso peninsula (at 30 km from Tokyo) with a WNW-ESE strike coinciding both with the Sagami trough strike and the strike of well conducting 3 km thick sediments. A joint analysis of geoelectric and tectonic data leads to a preliminary conclusion that the Boso conductivity anomaly connects two large scale conductors: Pacific sea water and a deep magma reservoir beneath a volcanic belt. Between two so different conductors an unstable transition zone can be expected which should be sensitive to changes of stress. Applying our original processing including two steps analysis and elimination of annual and monthly periods, a short-term two-month-long precursor of bay-like form was successfully separated at the observatory of Kanozan, KNZ (over the Boso anomaly) despite its strong noise. All the results were obtained with advanced multi-windows multi-rr (remote reference) robust programs with a coherency control. Dependence of the results of induction vector calculation on geomagnetic activity was carefully studied, and this dependence is relatively strong when the magnetotelluric field and noise have approximately the same magnitude. But even in this case we could identify the precursor field.

Variations of geomagnetic response functions in Japan

Peculiarities of measurements have been described for response functions (RF), historical review of RF application in Japan has been given together with general condition of earthquake forecast (EQ). Detailed analysis of vast material of processing obtained under conditions of strong disturbances has been fulfilled. Anomalies of response functions have been distinguished, which occurred during several years before catastrophic EQ Tohoku, which might be most probably recognized as medium-term precursors of that EQ. Anomaly of electric conductivity has been revealed under the central part of Boso peninsula (Tokyo suburb) in the area of complicated joint of three lithosphere plates.

Variation of the components of the induction vector and the horizontal tensor before the Tohoku earthquake March 11, 2011 according to the data of Japanese geomagnetic observatories

This work is a continuation of a work [Babak et al., 2013]. The sources of variable magnetic field observed at the surface of the Earth are presented in the introductory part. The most reliable cases of lithospheric emission registered before strong earthquakes are described for five seismic regions of the World. The changes of electrical conductivity of the Earth's crust as a precursor of earthquakes are discussed. The review shows that geomagnetic response functions (induction vector and horizontal tensor) can be used as an earthquake precursor, which carry information both on lithospheric emission and changes in the electrical conductivity. Analysis of 18 Japanese geomagnetic observatories data (6 of them are away from the Tohoku earthquake epicenter less than 400 km) shows the presence of intense noise for periods less than 500 s. Application of noiseproof algorithms for calculation of response function components only partially reduced their influence. A large amount of processing was carried out and database of response functions for 10-20 years for 18 Japanese observatories was created. During the processing variety of record deficiencies were found, the cause of which is not always clear. Detailed processing of one-second data for 10 weeks before the Tohoku earthquake reveals anomalies of response functions temporal behavior: 7-9, 19-21, 33-36 and perhaps ~ 60 days before the event. Moments of amplitude and phase anomalies vary slightly for different periods of variations, for different components of the response functions, for different stations. This behavior is caused, on one hand, by the influence of geomagnetic activity and the contribution of noise, on the other hand, it can be the manifestation of precursors and co-seismic events, and this behavior may reflect a migration of lithospheric emission and/or changes in the electrical conductivity of the Earth's lithosphere. Decoding of this information can provide valuable information for understanding the physics of earthquake preparation and precursors formation.

Approximations for the 2-D coast effect on marine magnetotelluric data

Marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest. However, the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted. For a 2-D coastal model, this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast. A maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions. Based on a modelling study, we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity. Based on a simple analytical approach, we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest. While the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor.

Three-dimensional geomagnetic response functions for global and semi-global scale induction problems

SUMMARY Features of the geomagnetic response functions due to a 1000 km scale isolated conductivity anomaly embedded in a radially symmetric Earth with a surface heterogeneous layer were studied by three-dimensional (3-D) global forward modelling in order to understand the fundamental characteristics of the response functions for period of about 1–100 d and to examine the validity of 1-D like treatment for some of the responses. The geomagnetic deep sounding (GDS) response function has dipolar sensitivity to the lateral electrical conductivity contrast, while the D response (ratio of the eastward to northward component of geomagnetic field variation) shows the 3-D effect as a quadrupole-like distribution having peaks outside of heterogeneity. Although GDS and D responses were used in global induction studies in the past, we examined if the horizontal transfer function (HTF), the ratios of the geomagnetic north components at a station to those at other reference station, has enough sensitivity to the mantle heterogeneity and if it is worth to be included in global conductivity soundings. Modelling result shows that the spatial distribution of anomalous HTF is somewhat similar to the projection of heterogeneity on the surface so that the information different from those due to the GDS and D responses can be obtained by the HTF. The signatures of the conductivity heterogeneity in the GDS, D and HTF are detectable if the conductivity of the 1000-kmscale heterogeneous block is more than twice as conductive or five times as resistive as the surrounding medium. Both cases have an induction number of about 0.8. The anomalous parts of the response functions due to multiple heterogeneous bodies can be represented very well by the sum of those due to each isolated heterogeneity, if an induction number of each anomalous scattering body is less than about 2. Although the linearity breaks down when the induction number is larger, the difference between them is not significant compared with typical observational error of corresponding response functions. This implies that the sensitivity to an isolated block can be applied to discuss the resolution of multiple blocks, such as conductivity distribution having checkerboard pattern, and vise versa, in 3-D inversions. The distribution of 3-D GDS response implies that 1-D like treatment of the GDS response, such as compiling single station 1-D conductivity profiles to discuss regional differences of conductivity and correcting the static shift of magnetotelluric responses with converted GDS response, may cause serious problem if lateral heterogeneity is not negligible. This shows the necessity of modelling and analyses, which are compatible with 3-D structure.

A joint interpretation of electromagnetic and seismic tomography models suggests the mantle transition zone below Europe is dry

The three-dimensional (3D) electrical conductivity distribution in the mantle transition zone (MTZ) below Europe is resolved by directly inverting geomagnetic response functions from 12 selected European observatories. Comparison of the conductivity image with the results of global seismic tomography reveals a good correlation between low-conductivity and high-velocity anomalies at depths of 400–700 km beneath the Mediterranean subduction zone in central Europe. Interpretation of the 3D conductivity model combined with seismic tomography indicates that the European MTZ is drier than the MTZ beneath the western Pacific subduction zone, even though the subducted slab material is stagnating similarly in both regions.

Lateral variations of the mid-mantle conductance beneath Europe

Europe is a region with the largest density of geomagnetic observatories and several authors have used these data to estimate local geomagnetic response functions for various period ranges, typically of the width of 1.5 to 2.5 decades. By collecting the local response functions from 35 European observatories, and by their precise selection and subsequent combination, the independent regional geomagnetic induction data set could be extended to a period range of 4.5 decades. The initial local responses that were estimated by two magnetovariation methods, with two different external source fields employed, have been further supplemented by continental and global 11-year data, providing thus a data set extending over a period range from the harmonics of the daily variations up to 11 years. The combined responses have been inverted individually for each observatory by two techniques, by an Occam procedure and a stochastic 1D inversion for spherically symmetric Earth. The integrated mantle conductance has revealed rather regular lateral changes that have been used to design a mantle conductance image down to a depth of about 770 km. The presented conductance image can be correlated with major European tectonic units like the Baltic Shield and the Trans-European Suture Zone. To examine possible distortions to the inferred mantle conductance models due to large-scale near-surface heterogeneities, specifically those caused by the oceans, seas and large sedimentary basins, a spherical forward modeling was carried out for a radially symmetric conductor coated by an inhomogeneous thin shell with the variable surface conductance. The model responses for the 35 observatory positions were inverted in the same way as previously the experimental data. The results for 28 observatories have shown that the depth down to a pre-defined conductance level could be retrieved with a high accuracy of a few percents, but for seven southernmost observatories the recovery error increased up to 9%. With these seven observatories removed from the analysis, the effect of the seas and oceans on the upper and mid-mantle conductance estimates beneath Europe can be considered negligible.

Correlation of electrical conductivity and structural damage at a major strike‐slip fault in northern Chile

Large‐scale strike‐slip fault zones are often imaged as electrically conductive structures in the brittle crust. However, the relationship of conductivity and internal architecture of the fault zone remains largely unclear. This paper presents results of a study designed to compare the record of structural deformation across a fault zone with its electrical conductivity image. Two high‐resolution magnetotelluric profiles trend perpendicularly across the West fault, a branch of the Precordilleran fault system in northern Chile. The magnetotelluric and geomagnetic response functions in the frequency range from 1000 Hz to 0.1 Hz clearly image a fault zone conductor (FZC) about 350 m wide and 1500 m deep, trending along the surface trace of the fault. The position of the FZC and its geometric properties (width, dip) correlate with a region of intense fluid alteration and the orientation of fault‐related damage elements (minor faults, fractures). According to estimates of fluid salinity and rock porosity, the conductivity anomaly results from meteoric water penetrating into a permeable zone. This suggests that the increase in electrical conductivity is causally related to a mesh of minor faults and fractures, acting as a pathway for fluids. The FZC reflects the central and most fractured part of the damage zone. In view of similar published magnetotelluric studies, we infer a dependency of a fault's state of activity and the characteristics of the FZC such as its conductance, width, and depth extent. Ongoing deformation is the controlling factor for maintaining a permeable and electrically conductive fracture network.

The electrical conductivity and thermal profile of the Earth's mid‐mantle

Electrical conductivity in the Earth's mantle is sensitive to temperature and chemical environment. Recent laboratory measurements of electrical conductivity are combined with candidate mantle geotherms to produce synthetic electrical conductivity profiles. These profiles are used to forward model the Earth's geomagnetic response function C, results of which are compared with the observed globally averaged response function at periods of 3.5 days to 4 months. Candidate lower mantle geotherms, representing whole‐mantle and layered convection end‐members, are compared using published electrical conductivity measurements on alumina‐bearing and alumina‐free perovskite in the conductivity models. Comparison of the predicted response functions with the observed geomagnetic response of the Earth shows that a) if lower mantle alumina is incorporated into perovskite, then the lower mantle must be cool, and b) if the alumina is not incorporated in perovskite then the results are only consistent with a hot lower mantle. In addition, the maximum alumina content of lower mantle MgSiO3 perovskite is constrained at 4%.

New equipment and processing for magnetotelluric remote reference observations

Robust estimates of magnetotelluric and geomagnetic response functions are determined using the coherency and expected uniformity of the magnetic source field as quality criteria. The method is applied on data sets of three simultaneously recording sites. For the data acquisition we used a new generation of geophysical equipment (S.P.A.M. MkIII), which comprises novel concepts of parallel computing and networked, digital data transmission. The data-processing results show that the amount of noise on the horizontal components of the magnetic field varies considerably in time, between sites and over the frequency range. The removal of such contaminated data beforehand is essential for most data-processing schemes, as the magnetic channels are usually assumed to be free of noise. The standard remote reference method is aimed at reducing bias in response function estimates. However, this does not necessarily improve their precision as our results clearly show. With our method, on the other hand, we can filter out source field irregularities, thereby providing suitable working conditions for the robust algorithm, and eventually obtain considerably improved results. Contrary to previous concepts, we suggest rejecting as much data as feasible in order to concentrate on the remaining parts of high-quality observations.

On the minimization of correlated residuals

SUMMARY A variety of methods for solving overdetermined linear systems of equations are considered. The Gaussian method of least squares assumes a noise-free matrix of coefficients. The maximum-likelihood method assumes the same amount of noise in each column of the matrix and the resulting vector. Both methods represent special cases of the general problem of deriving unbiased solutions in the presence of noise. Here, the minimization of correlated residuals is introduced. A set of boundary conditions is required to adjust the statistical properties of the solution. This method minimizes the second term of the autocovariance function of the residuals and is independent of the noise. The operation of the method is outlined analytically using an univariate scatter problem and applied numerically to synthetic data of a linear regression problem. To demonstrate the applicability of the method in practical situations, geomagnetic response functions are derived from observational data using the Z : H method.

Magnetospheric and ionospheric signals in magnetic observatory monthly means: Electrical conductivity of the deep mantle

First differences of magnetic observatory monthly means for 1963–1982 were analyzed using techniques of spherical harmonic analysis and power spectral analysis. The external source signal is shown to be primarily zonal in geomagnetic coordinates. Prominent peaks are present in the power spectrum at frequencies of 1.0 cycle/yr and 2.0 cycles/yr. The annual signal is largest on the degree 2 external zonal spherical harmonic, while the semiannual signal is largest on the degree 1 and degree 3 external zonal spherical harmonics. The presence of the semiannual signal on odd‐degree spherical harmonics and of the annual signal on even‐degree spherical harmonics was predicted from symmetry considerations and the annual cycle of solar inclination. These signals are all modulated by the sunspot frequency and its harmonics. The degree 1 term is believed to be due mainly to magnetopause and ring currents while the degree 2 and degree 3 terms are believed to be due mainly to ionospheric currents. The degree 1 external zonal harmonic has a continuous spectrum in addition to the semiannual spectral peak. A corresponding degree 1 internal term is due to electromagnetic induction. The degree 1 continuous spectrum is useful for study of the electrical conductivity of the deep mantle. A global geomagnetic response function consistent with a mantle conductivity of about 10 S/m at the core‐mantle boundary has been derived.

Constraints on Mantle Electrical Conductivity from Field and Laboratory Measurements.

A global geomagnetic response function, sensitive to the average radial electrical conductivity structure of Earth’s mantle to depths of at least 1800 km, is obtained by averaging published, single-site response functions estimated at periods between 10 5 –1 0 7 seconds from magnetic observatory records. Although the error bars on the global response function are mostly smaller than 5%, Parker’s D + algorithm demonstrates compatibility with a one-dimensional model, both in terms of magnitude and distribution of data residuals. Smooth models in the sense of minimum first and second derivatives of log(conductivity) with log(depth) show conductivities increasing from 0.01 S/m 200 km deep to 2 S/m at a depth of 2000 km. Geotherms inferred from these conductivities using a laboratory model for the temperature dependence of dry subsolidus olivine yield temperatures of 1750 ◦ Ca ta depth of 410 km; hotter than the 1400 ◦ C for this depth inferred from published values for the equilibrium boundary of the olivine α → α+ β transition. Inclusion of a sharp jump in conductivity at the 660 km seismic discontinuity lowers the electrogeotherm to 1600 ◦ Ca t410 km, while an explicit penalty on the conductivity at this depth demonstrates that a temperature of 1400 ◦ is compatible with the global response function if 1000 S of additional conductance is included above 200 km. The electrical conductivity below the jump at 660 km is 1 S/m increasing to 2 S/m at 2000 km, in excellent agreement with recent diamond anvil measurements of lower mantle materials. Extension of the global response to higher frequencies is possible using data from magnetic satellites. One such study is shown to be in general agreement with the averaged response. The response of Earth to time variations in the externally generated geomagnetic field can be interpreted in terms of electrical conductivity with depth. The estimation of global geomagnetic response functions and their interpretation in terms of mantle electrical conductivity structure dates from the end of the last century, but the modern practice is perhaps typified by Banks’ (1969) response function and its inversion by Parker (1970). Since then interest has turned away from the estimation of global, radial electrical conductivity distributions. Roberts (1984) and Schultz and Larsen (1987), for example, attempted to extend the radial model by making single-site estimates of response functions at numerous geomagnetic observatories and examining them for inter-site variations. Both these papers concluded that lateral variations in conductivity exist, although Roberts decided that the oceans were chiefly responsible for variations in the response functions while Schultz and Larsen invoked lateral heterogeneity in the mantle. While surface (ocean) and mantle heterogeneities undoubtedly exert their influence on the data, this paper again addresses radial electrical conductivity structure in Earth’s mantle. This reexamination of a classical problem is motivated by several things. The data sets cited above have not been analyzed for radially symmetric global structure. Recent development of more versatile inversion schemes has provided new tools with which to study the data. Improvements in laboratory measurements relating electrical conductivity to temperature in mantle materials

The electrical conductivity of the upper mantle as estimated from satellite magnetic field data

The electrical conductivity of the upper mantle is estimated from low‐latitude magnetic field variations (magnetic storms) caused by large fluctuations in the equatorial ring current. The data base is derived from magnetic field magnitude data measured by satellites OGO 2, 4, and 6, which offer better global coverage than land‐based observatories. The procedures of analysis consist of 1) separation of the disturbance field into internal and external parts relative to the surface of the earth, 2) estimation of an electromagnetic response function Q(ω) which relates the internally generated magnetic field variations to the external variations due to the ring current, and 3) interpretation of the estimated response function using theoretical response functions for assumed conductivity profiles. Special consideration is given to possible oceanic and ionospheric effects. Best estimates of the geomagnetic response function Q(ω) for 0.2 to 2.0 cpd indicate an upper mantle conductivity of the order of 10−2 S/m. This conductivity is reasonable, especially when laboratory measurements of conductivity versus temperature for olivine are considered.

Observations of the electrical asthenosphere beneath Scandinavia

The recent recognition that long period (i.e., of the order of hours) electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments. Experiments carried out on the ocean bottom have met with considerable success in delineating the “electrical asthenosphere”, i.e., a local maximum in electrical conductivity (minimum in electrical resistivity) in the upper mantle. In this paper, observations of the time-varying magnetic field recorded in three regions of Scandinavia, northern Sweden (Kiruna—KIR), northern Finland/northeastern Norway (Kevo—KEV) and southern Finland (Sauvamaki—SAU), are analysed in order to obtain estimates of the inductive response function, C( ω), for each region. The estimated response functions are compared with one from the centre of the East European Platform (EEP), and it is shown that the induced eddy currents, at periods of the order of 10 3–10 4 s, in the three regions flow much closer to the surface than under the platform centre. Specifically, at a period of ~3000 s, these currents are flowing at depths of the order of: KEV—120 km; KIR—180 km; SAU—210 km; EEP—280 km; implying that the transition to a conducting zone, of σ ∼-0.2 S/m, occurs at around these depths. One-dimensional inversion of C ̂ KEV and C ̂ KIR shows that there must exist a good conducting zone, of σ = 0.1–1.0 S/m, under each of the two regions, of 40 km minimum thickness, at depths of: KEV 105–115 km; KIR 160–185 km. This is to be contrasted with EEP, where the ρ- d profile displays a monotonically decreasing resistivity with depth, reaching σ~0.1 S/m at > 300 km. Finally, a possible temperature range for the asthenosphere, consistent with the deduced conducvitity, is discussed. It is shown that, at present, there is insufficient knowledge of the conditions (water content, melt fraction, etc.) likely to prevail in the asthenosphere to narrow down the probable range of 900°–1500°C.

Diurnal characteristics of transfer functions at pulsation periods

Within a particular induction region the geomagnetic response function possesses a dependence on the wavenumber of the inducing field. To investigate possible relationships between response function and wave­ number, the local time dependence of the magnetic variation fields used to determine the response function can be retained. Such an analysis has been carried out at five UK sites using single-station transfer function analysis in the period range from 2000 to 40 s. To allow a level of significance to be established the local time estimates are compared with the time-averaged determinations. At periods greater than 500 s there is no significant local time variation of the transfer function estimates. At periods less than 500 s, corresponding to mid-latitude pulsation fields, significant local time variations are observed. At four of the five sites studied, the diurnal character in the transfer function estimate consists of a maximum response on the dayside and a corresponding nightside minimum. The behaviour is qualitatively related to a dayside/nightside asymmetry in the source region of the mid­ latitude pulsation field.