Inter-station Transfer Functions Research Articles

Spatial gradients of geomagnetic temporal variations causing the instability of inter-station transfer functions

Spatial gradients in the primary geomagnetic fields directly contribute to both the amplitudes and phases of inter-station transfer functions (IS-TFs). This suggests that, for the analysis of subsurface resistivity structures, IS-TFs should be carefully treated by checking the establishment of the plane-wave assumption. Geomagnetic time-series data include various and complicated characteristics and accordingly, time–frequency domain analysis is suitable for the discussion of spatial gradients of time-varying geomagnetic fields. However, such evaluations are complicated by the huge amount of information contained in the spectrograms from several stations. Therefore, we propose a Multi-Channel Nonnegative Matrix Factorization (MC-NMF) method that can decompose raw spectrograms into several components, allowing the spatial gradient of each geomagnetic temporal variation to be identified. We confirm that such components actually affect the estimation of IS-TFs using data acquired at the Kakioka and Memambetsu magnetic observatories in Japan. We derive the year-to-year changes in IS-TFs from each set of paired stations among Kakioka, Kanoya, and Memambetsu observatories. Although the IS-TFs should exhibit opposite polarities (a negative correlation) when the input and output observatories are swapped; surprisingly, some of them have “identical” polarities. The application of MC-NMF shows that the analyzed geomagnetic data include several components that have various spatial gradients. Although IS-TFs sometimes fail to give the expected implication regarding the spatial gradients of geomagnetic temporal variations, MC-NMF can verify whether the IS-TFs exhibit any spatial gradients. Thus, the use of IS-TFs with MC-NMF can yield better implications regarding subsurface resistivity information.

On the relevance of source effects in geomagnetic pulsations for induction soundings

Abstract. This study is an attempt to close a gap between recent research on geomagnetic pulsations and their usage as source signals in electromagnetic induction soundings (i.e., magnetotellurics, geomagnetic depth sounding, and magnetovariational sounding). The plane-wave assumption as a precondition for the proper performance of these methods is partly violated by the local nature of field line resonances which cause a considerable portion of pulsations at mid latitudes. It is demonstrated that and explained why in spite of this, the application of remote reference stations in quasi-global distances for the suppression of local correlated-noise effects in induction arrows is possible in the geomagnetic pulsation range. The important role of upstream waves and of the magnetic equatorial region for such applications is emphasized. Furthermore, the principal difference between application of reference stations for local transfer functions (which result in sounding curves and induction arrows) and for inter-station transfer functions is considered. The preconditions for the latter are much stricter than for the former. Hence a failure to estimate an inter-station transfer function to be interpreted in terms of electromagnetic induction, e.g., because of field line resonances, does not necessarily prohibit use of the station pair for a remote reference estimation of the impedance tensor.

Synthesis of natural electric and magnetic Time‐series using Inter‐station transfer functions and time‐series from a Neighboring site (STIN): Applications for processing MT data

AbstractSynthesis of natural electric and magnetic Time series using Interstation transfer functions and time series from a Neighboring site (STIN) is a new approach for recovering natural electric and magnetic fields and reduce the influence of anthropogenic noise. The proposed approach modifies the windows of the local electric and magnetic time series that are affected by noise with time series resulted from modifying the spectra of the magnetic time series from a neighboring site with the interstation transfer functions between the local and neighboring sites. The STIN method was tested with artificially contaminated electric and magnetic time series. Comparison between STIN‐corrected time series and original noncontaminated time series shows high similarity, both in the time and frequency domains. Differences were quantified using the normalized root‐mean‐square error, the correlation coefficient, and the signal‐to‐noise ratio. The STIN method was also applied to two sites affected by unconstrained anthropogenic noise, thus demonstrating the ability and accuracy of STIN in synthesizing natural electric and magnetic fields and reducing the influence of anthropogenic noise. The synthesized time series provided by STIN show the method to be valuable for magnetotelluric (MT) geophysical applications, by increasing the reliability when constrains the MT impedance tensor and by reducing the scatter of data points when the time series are affected by noise, particularly for longer periods. As STIN is based on interstation transfer functions, the electric and magnetic time series can be treated independently, enabling computation of the MT impedance tensor even when the electric and magnetic time series of the local site were recorded at different times.

Horizontal magnetovariational responses in geothermal exploration: synthetic model studies

Horizontal magnetic inter-station transfer functions, which are less distorted by near surface inhomogeneities and more sensitive to the deep targets, have been incorporated with other magnetotelluric data sets to delineate the electrical conductivity distribution in geothermal zones. Geothermal settings composed of salty fluid saturated faults and fractures, and hydrothermal alterations show a wide variation of the bulk resistivity. This makes electromagnetic methods, particularly magnetotellurics which is able to detect the geoelectric structures of deep targets, ideal techniques for the exploration of geothermal regions. In this research, the horizontal magnetovariational data are combined with other magnetotelluric transfer functions and their capabilities for improving the electrical conductivity structure obtained from 2D inversion process are examined. To this end, several synthetic models-representative of the conductivity changes in geothermal fields or conceptual modelshave been utilized. The results from two-dimensional (2D) inversion confirm the effectiveness of horizontal magnetic tensor as a part of the transfer functions, especially for the detection of deep conductive features.

A new methodology to estimate magnetotelluric (MT) tensor relationships: Estimation of Local transfer-functIons by Combining Interstation Transfer-functions (ELICIT)

SUMMARY Anewmethodologytoestimatemagnetotelluric(MT)tensorrelationships,calledEstimationof Local transfer-functIons by Combining Interstation Transfer-functions (ELICIT), is proposed wherebytheMTtensorrelationshipsofthelocalsitearederivedusingonlyinterstationtransfer functions. The MT impedance tensor and the geomagnetic transfer function at the local site are characterised by combining interstation tensor relationships between electric and magnetic fields at the local site with the horizontal magnetic fields acquired at a neighbouring site. The main property of the proposed method is that the employed interstation transfer functions are independently constrained, without the need to acquire the electric and the magnetic fields at the local site simultaneously to recover the local MT tensor relationships. Due to this property, the ELICIT method offers new possibilities for MT data acquisition and processing, providing significant improvements when the magnetic time-series at the local site are affected by local noise or are truncated. Error analysis shows that, even when magnetic fields are truncated, the quality of the results obtained following the ELICIT method are similar to those we would obtainifthemagneticfieldshadnotbeentruncated.Anotherimportantpropertyisthatdifferent neighbouring sites can be used to recover the tensor relationships at the local site. Averaging of results obtained using different neighbouring sites can be performed to improve the statistics. For our example data, when the ELICIT method is used to improve the statistics, errors of the estimates for periods between 1000 and 20000 s periodicities are clearly reduced. All interstation transfer functions are calculated doing remote reference and the bootstrap method is used to compute the errors, when necessary. Long period magnetotelluric data acquired in the Pyrenees and in the Atlas Mountains in Morocco, and magnetic data provided by ∗ ∗

Pseudo‐remote reference processing of magnetotelluric data: a fast and efficient data acquisition scheme for local arrays

ABSTRACTThe basic physical properties of the magnetic source field, namely its homogeneity and spatial coherence, have been used for a variety of magnetotelluric processing techniques including remote reference processing. In the present work we propose a data acquisition and processing technique for a large number of stations distributed over a localized area ideally on a grid. For pseudo‐remote reference processing it is necessary to use the following station setup: five‐component MT data are only measured at some sites (base stations) while at the majority of sites (local stations) only the electric and vertical magnetic fields are recorded. The impedance tensor and vertical magnetic transfer functions at each local station are computed by assigning the magnetic fields of a base station to the local station as if they had been measured there. This approach can lead to biased or erroneous estimates of local transfer functions at stations in the vicinity of strong conductivity contrasts that can be corrected using the interstation transfer functions between the horizontal magnetic fields measured at the base station(s).We test this approach with a data set collected in the vicinity of the Groß Schönebeck geothermal test site. Magnetotelluric data were collected at 146 local and 5 base stations distributed over an approximately 5 km × 25 km wide grid with site spacing ranging from 500 m × 500 m to 1000 m × 1000 m in the frequency range 128–0.001Hz. The obtained pseudo‐remote reference transfer functions are generally smooth and consistent and conductivity models obtained from 2D inversion of the data are in agreement with previous conductivity models from the study area.

Coast effect distortion of marine magnetotelluric data: Insights from a pilot study offshore northeastern Japan

We report on strong coast effect distortions observed for broadband marine magnetotelluric (MT) data collected on the forearc offshore northeastern Japan. Eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple-station algorithm, yielding good MT responses and inter-station transfer functions at periods of 7–10,000 s. Transverse electric (TE) mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s, while the transverse magnetic (TM) mode responses are galvanically depressed below the TE responses. An analysis of inter-station transfer functions confirms that the apparent resistivity cusps are a magnetic field, rather than electric field, phenomenon, consisting of an amplitude minimum and rapid phase change around a characteristic frequency. Poynting vectors for a TE coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below, after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin. We show that the characteristic frequency and position of the TE mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor, the depth of the ocean, and the distance from the coastline. By including coastlines and bathymetry in 2D inversion, we recover the seafloor conductivity structure along the forearc, demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere.

Transfer function approach to signal discrimination of ULF geomagnetic data

In order to study earthquake-related ULF geomagnetic field changes, it is important to discriminate the noises such as magnetic pulsations originated from solar–terrestrial interactions and artificial noises from DC-driven trains and factories. For this aim, the interstation transfer functions and wavelet transform method have been proposed and applied to the data obtained at the ULF electromagnetic sensor array at the Boso Peninsula, Japan. It is concluded that this interstation transfer function approach has a capacity for signal discrimination; that is, it shows effectiveness to eliminate noises originated from the external ionospheric sources and its secondary effects.

Processing And Interpretation Of Electromagnetic Induction Array Data

The simultaneous nature of array data can be exploitedin electromagnetic induction studiesfor three general purposes.First, one or more reference sites can be usedto reduce bias, improve signal-to-noise ratios, and provide bettercontrol over source complications and coherent noisein estimates of MT impedances and otherEM transfer functions (TFs).Although a single good reference site can dramatically improveTF estimates, improvements due to multiple sites are often rather modest, because local noise is the limiting factor.Secondly, arrays allow for estimation of inter-station transfer functions, and maps of anomalous horizontal field variations. Relatively straightforward modifications to inversion codes wouldallow quantitative interpretation of these additionalconstraints on resistivity variations.Finally, with arrays it is possible to estimate the response of theEarth to a richer spectrum of external source excitations. In particular,the natural extension to the usual uniform source assumption implicit inthe MT method allows for three curl-free magnetic gradient sources.Quantitative interpretation of the response of a three-dimensionalEarth to these sources could provide additionalconstraints on large scale variations in crustal and uppermantle resistivity, and might help to overcome problemsdue to aliasing of near surface distortion in widely spaced MT data.

Short-term precursor of the geomagnetic horizontal field transfer functions

In this paper, using the variometer data of Changli station and Baijiatuan station from 1972 to 1997, the inter-station transfer functions of horizontal field and the single-station transfer functions of vertical field have been calculated. With analyzing the whole changing process of the transfer functions, we have discovered that only the horizontal field transfer functions arose obvious shor-term precursors before Tangshan earthquake. This fact may provide a new method for short-term prediction of earthquake.

DC trains and Pc3s: Source effects in mid‐latitude geomagnetic transfer functions

Magnetotelluric (MT) data from two sites 150 and 300 km southeast of San Francisco, California (geomagnetic dipole latitude: 43 degrees, L approximately 1.9) show that the usual MT assumption of spatially uniform external magnetic fields is violated to a significant degree in the period range 10–30 s. Inter‐station transfer functions exhibit large systematic temporal variations which are consistent with a combination of two distinct sources: electromagnetic noise due to the San Francisco Bay Area Rapid Transit (BART) DC electric railway, and Pc3 geomagnetic pulsations. There is a suggestion in the data that some of the Pc activity may actually be excited by BART.

Robust multiple-station magnetotelluric data processing

SUMMARY Although modern magnetotelluric ( MT) data are highly multivariate (multiple components, recorded at multiple stations), commonly used processing methods are based on univariate statistical procedures. Here we develop a practical robust processing scheme which is based on multivariate statistical methods. With this approach we use data from all channels to improve signal-to-noise ratios, and to diagnose possible biases due to coherent noise. To illustrate our approach we use data from two- and three-station wide-band MT arrays from an area south of San Francisco, California, where contamination of the MT signal by spatially coherent cultural electromagnetic noise is severe at some periods. To deal with such coherent noise we adopt a two-stage procedure. In the first stage we focus on reducing the effects of incoherent noise, and testing for the presence of coherent noise. To this end we have developed a robust multivariate errors-in-variables ( RMEV) estimator, which estimates background noise levels, cleans up outliers in all channels, and determines the ‘coherence dimension’ of the array data. In the absence of coherent noise, the coherence dimension of the data will be two (corresponding to two polarizations of the plane-wave MT source fields). In this case the RMEV estimator provides direct estimates of MT impedances and inter-station transfer functions. We show, with synthetic and real data examples, that in some cases these estimates can be significantly better than those obtained with more standard robust remote reference estimators. When MT data is severely contaminated by coherent noise (as for our example arrays for periods of 4-50s) the coherence dimension of the data will exceed two. The RMEV estimate thus provides a clear warning of coherent noise contamination. Although there appears to be no completely general automatic way to deal with this circumstance, useful results can be obtained from severely contaminated data in some cases. We show in particular how the RMEV estimator can be adapted to separate the MT signal from coherent noise for two special cases: when at least one site is unaffected by coherent noise, and when coherent noise sources are intermittent. We give examples of significant improvements in MT impedance estimates obtained with the RMEV estimate for each of these cases.

The anomalous reactions of the geomagnetic horizontal field transfer functions before Tangshan earthquake

We have collected the data of variometers in period from 1972 through 1984 at Changli and Baijiatuan stations. The results calculated from 19 groups of data show: 1) There are no significant anomalies of A, B at Baijiatuan and that of B at Changli, but there are evident anomalous changes for A u, Av at Changli station. This result basically agrees to the a, b calculated using the method of measuring magnetograms, which demonstrates that the anomalies limited in the aftershock area and its neighbouring regions. 2) There are some changes of the interstation transfer functions of C u, Cv, Fu, Fv before Tangshan earthquake, and the changes of the image parts C v, Fv are more obvious, which is in agreement with the research results for Hualian earthquake. However, according to the results of Carlisle M=5.0 earthquake by Beamish (1982), the real parts F u, Cu change more significantly.

The simulation of magnetic variation anomalies using single-station data

Hypothetical event maps of the vertical field derived from single-station magnetic variation transfer functions are biased by the anomalous horizontal fields. An iterative scheme, proposed earlier, is used to estimate and correct for the anomalous fields. The scheme here is applied to single-station transfer functions from 90 sites in southern Scotland and northern England. It converges to a stable solution in four iterations, and the results compare favourably with those predicted in the southern part of the area using measured inter-station transfer functions. The resultant hypothetical event maps of the anomalous horizontal fields show that the Northumberland Trough and Southern Uplands conductivity anomalies are both strikingly linear features which correlate more closely than previously supposed with other geological and geophysical observations.

Single- and inter-station transfer functions for non-synchronous geomagnetic arrays-II. the results of the field data analysis

The theory of methods of computing single- and inter-station transfer functions in both the spectral and time domains was developed in paper[1]. Both approaches are applied to the variation data recorded at field stations along two non-simultaneous profiles traversing the eastern margin of the Bohemian Massif, where a zone of anomalous induction seems to mark an important geological boundary of formations with different histories of development. The results of both analyses are found to coincide within reasonable bounds of 20–30% in the principal induction characteristics.

Single- and inter-station transfer functions for non-synchronous geomagnetic arrays — I. Theory of methods used in data analysis

The theory of the multivariate coherence analysis(spectral domain approach) is developed for calculating single- and inter-station transfer functions and corresponding vector induction characteristics from time variations of the geomagnetic field components. An alternative approach of calculating similar induction characteristics using a time domain algorithm is shown.

The interpretation of the Northumberland Trough geomagnetic variation anomaly using two-dimensional current models

Summary. The electromagnetic fields at the surface are non-linear functionals of the electrical conductivity of the Earth. However, the anomalous magnetic fields which are created by lateral variations in the conductivity are linear functionals of the internal current density. The inverse problem analysed in this paper is that of using the spatial structure of the vertical field, defined along a profile perpendicular to the strike of a 2-D structure, to infer the lateral and vertical distribution of current parallel to the strike. In the Northumberland Trough survey, magnetometers at some sites are operated simultaneously with a reference instrument at Durham, and interstation vertical field transfer functions were fitted to the data. They are subsequently used to predict the vertical field at each site which is associated with a specific horizontal field at the reference. Other magnetometers were run separately, and only single-station transfer functions could be computed. These yield predictions of the vertical field which are biased by anomalous horizontal fields. When the spatial structure of the anomaly is known, the horizontal component of the field can be predicted from the vertical by using the constraint that the field derives from a potential which satisfies the Laplace equation. The corrections that are required to convert from singlestation to inter-station transfer functions can be estimated from the maps of the horizontal field. This approach is used to generate vertical field values at sites spaced at 10 km intervals along a 100 km profile traversing the Alston Block and Northumberland Trough in northern England. The map of the horizontal component anomalies relates more simply to the structure than the vertical component map. The surface magnetic fields do not define a unique distribution of current density withm the Earth. The non-uniqueness is removed by fixing the direction and bounding the magnitude of the current density. The inversion of the set of constraints is solved by linear programming methods. The current density in the Carboniferous sedimentary rocks filling the Northumberland Trough is calculated by combining estimates of their conductivity based on Archie’s Law with measurements of the electric field at a magneto

The use of equivalent current systems in the interpretation of Geomagnetic Deep Sounding data

Many geomagnetic variation anomalies are probably caused by the channelling, through small-scale bodies, of electric currents induced in much larger conductors elsewhere. Consequently, the direct interpretation of anomalous magnetic fields by modelling the electromagnetic response of conductive structures may give misleading results. It is suggested that, rather than attempting to proceed directly from the electromagnetic fields to conductivity models, we should instead take the intermediate step of determining the distribution of anomalous current flow. Maps of the anomalous fields over a conductive structure can be generated from inter-station transfer functions. If it is assumed that the internal currents are concentrated in a thin sheet at a specified depth, the equivalent current system in the sheet can be computed directly from the vertical magnetic field. The most straightforward method of performing this calculation is to compute the Fast Fourier Transform of the magnetic field data, and then to apply a wavenumber filter. The presence of any vertical currents invalidates the thin sheet model. However, if the spatial distribution of a horizontal component of the anomalous magnetic field is also known, the presence of any vertical currents can be detected directly, and their position determined. The value of the methods is illustrated by applying them to the interpretation of a Geomagnetic Deep Sounding survey of the Kenya rift valley.

The mapping of induced currents around the Kenya Rift: a comparison of techniques

Summary. Single-station and inter-station transfer functions are derived from data recorded by a magnetometer array in and around the Kenya Rift Valley. Different methods of presentation of the transfer function estimates are compared in terms of their ability to define lateral variations in electrical conductivity. When, as in East Africa, the conductivity structure is complex and three-dimensional, by far the most useful method of presentation is to use the transfer functions to simulate the anomalous internal fields associated with regional current flow at a particular azimuth. The use of inter-station rather than single-station transfer functions is almost essential where the amplitudes of anomalous horizontal fields are of the same size or greater than the normal fields. Horizontal field transfer functions prove to be just as useful as those usually calculated for the vertical component, and provide strong additional constraints on the internal current system. Maps of simulated vertical and horizontal fields of internal origin indicate the importance of current channelling around the Rift Valley. A conductor just to the east of Nairobi apparently funnels regionally induced currents into two conductors associated with the Rift and dome; one at shallow depths beneath the Rift floor, and a deeper body to the east.