Geomagnetic Transfer Functions Research Articles

Applications of dispersion relations to the geomagnetic transfer function

The geomagnetic transfer function is nowadays used to constrain the magnetotelluric inversion procedure given that this function contains complementary information to the impedance tensor. For the models usually employed by inversions, the real and imaginary parts of the geomagnetic transfer function are related by dispersion relations. The computation of the dispersion relations involves the Hilbert transform, and here we discuss different expressions to compute them. This computation was verified using synthetically generated geomagnetic transfer function from 2D and 3D models. The dispersion relations were applied on two cases: (a) to study the consistency between the real and imaginary parts of field recorded data, and (b) to develop a procedure to complete or extend the amount of measured data.

Evidence of electrical anisotropy in limestone formations using the RMT technique

Azimuthal resistivity surveys are often applied to complement hydrological information or to improve the location of observation boreholes in pump tests. Symmetric electrode configurations cannot distinguish anisotropy from lateral changes or dipping layers, but asymmetric arrays (e.g., the offset Wenner array) can. Tensor radiomagnetotellurics (RMT) is presented as an alternative method in studies of electrical anisotropy in the shallow subsurface. The electromagnetic and geomagnetic transfer functions provide information about the dimensionality of the data. These transfer functions can also be used to find the directions of anisotropy. Data with an anisotropic signature can be inverted for a one‐dimensional (1D) azimuthal anisotropy model. The method is faster than the azimuthal resistivity method. A 380‐m‐long profile of tensor RMT data (12.7–243 kHz) from limestones that overlie shale on the island of Gotland, Sweden, is used to show the merits of the method. The data have a clear anisotropic signature. The data are inverted for a three‐layer 1D model with azimuthal anisotropy using two different approaches: (1) a moving median filter of five neighboring stations and neglecting static shift parameters; and (2) treating each station separately and including static shifts of the electric field in the inversion. Both inversions show models having a marked anisotropy with anisotropy factors of 3.7 and 4.5, respectively, in the limestones. The second approach has a significantly better data fit. However, the first approach is preferred because the models are smoother from station to station.

The relative influence of different types of magnetotelluric data on joint inversions

Diverse magnetotelluric data (apparent resistivity, phase or geomagnetic transfer function) used during the inversion process provide different information on the model as a consequence of data error. The relative influence of these data constitutes a subject of interest on the inversion process. This influence can be evaluated from the error ratio between two types of data; thus, when phase and the logarithm of apparent resistivity are involved, the well-known ratio of one half is obtained. A new error ratio between the geomagnetic transfer function and the logarithm of apparent resistivity is presented. We deduced this ratio, which is bounded by one half of the amplitude of the geomagnetic transfer function. In order to verify this new ratio, we employed a technique based on the study of the RMS misfit, obtained after an intensive inversion computation whilst taking different error values for the different data. This technique was applied to synthetic and experimental data, and the results agree with the proposed value. This value should be taken into account for setting error floors when performing joint data inversion in order to obtain the same influence from the different data.

Static shift levelling using geomagnetic transfer functions

Galvanic distortion of magnetotelluric (MT) data is a common problem in the study of the Earth’s electrical properties. These distortions are local, they affect independently each MT site, and where restricted to distortion of the electric field are manifest as vertical shifts in the apparent resistivity curves (static shift). The removal of the static shift is necessary to avoid misinterpreting MT data. We present a method that allows us to partially retrieve the regional response of the TE-mode data in a 2D case. The method determines relative changes between distortion parameters along a profile, and is based in the Faraday’s law, and uses only magnetotelluric responses: measured impedance tensor and geomagnetic transfer function (tipper). The method is valid under the assumption that the variation of horizontal magnetic field can be neglected, and a test for checking this criterion has been developed. The mathematics involved in the procedure are straightforward, and can be stated as a linear regression. We present successful applications to both synthetic and real (COPROD data) datasets.

Magnetotelluric signature of the western Cantabrian Mountains

The magnetotelluric signature of the Alpine part of the Cantabrian Mountains was investigated in a N‐S magnetotelluric profile, from the Duero basin to the Cantabrian shoreline. A two‐dimensional conductivity model was obtained from the joint inversion of TM, TE modes and the geomagnetic transfer functions. A shallow conductor associated with the sediments of the Duero basin, and three north‐dipping conductors in the upper and middle crust were detected. Of the latter, one is interpreted as the sole detachment of the Cantabrian Mountains, whereas the others correlate with the Cabuérniga and San Vicente de la Barquera faults. This geometry suggests that the Cantabrian Mountains comprise uplifted Variscan basement. A comparison with a seismic reflection profile shows a good correlation between these conductors and seismic features. The lower crust is characterised by high conductivities.

Correction to “DC trains and Pc3s: Source effects in mid‐latitude geomagnetic transfer functions” by Gary D. Egbert, Markus Eisel, O. Sierra Boyd, and H. Frank Morrison

Geophysical Research LettersVolume 27, Issue 10 p. 1565-1565 Free Access Correction to “DC trains and Pc3s: Source effects in mid-latitude geomagnetic transfer functions” by Gary D. Egbert, Markus Eisel, O. Sierra Boyd, and H. Frank Morrison This article corrects the following: DC trains and Pc3s: Source effects in mid-latitude geomagnetic transfer functions Gary D. Egbert, Markus Eisel, O. Sierra Boyd, H. Frank Morrison, Volume 27Issue 1Geophysical Research Letters pages: 25-28 First Published online: January 1, 2000 Gary D. Egbert, Gary D. Egbert College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, ORSearch for more papers by this authorMarkus Eisel, Markus Eisel College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, ORSearch for more papers by this authorO. Sierra Boyd, O. Sierra Boyd Engineering Geoscience, University of California, Berkeley, CASearch for more papers by this authorH. Frank Morrison, H. Frank Morrison Engineering Geoscience, University of California, Berkeley, CASearch for more papers by this author Gary D. Egbert, Gary D. Egbert College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, ORSearch for more papers by this authorMarkus Eisel, Markus Eisel College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, ORSearch for more papers by this authorO. Sierra Boyd, O. Sierra Boyd Engineering Geoscience, University of California, Berkeley, CASearch for more papers by this authorH. Frank Morrison, H. Frank Morrison Engineering Geoscience, University of California, Berkeley, CASearch for more papers by this author First published: 15 May 2000 https://doi.org/10.1029/2000GL900008AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume27, Issue1015 May 2000Pages 1565-1565 RelatedInformation

Electrical anisotropy from electromagnetic array data: implications for the conduction mechanism and for distortion at long periods

The superposition of surface and middle crustal electrical conductivity anomalies is considered for the particular case of an electrically anisotropic middle crust. Two simple techniques which facilitate estimation of the two conductances of the anisotropic midcrustal layer are presented. Two tests for the electrical anisotropy hypothesis are introduced, evaluating the magnetotelluric (MT) phases and geomagnetic transfer functions from electromagnetic array data. If the midcrustal anisotropic structure is very inhomogeneous, then it will act on long period electric fields like a local scatterer, and it will impose static shifts on these long period electric fields. Static shift removal techniques that are adapted to this situation are introduced. Field data examples for two frequently occurring anomaly types are presented: (1) regular anisotropy: neighbouring sites have similar MT phases, conductances and regional strikes; (2) irregular anisotropy, which is just one feature of a more heterogeneous midcrustal layer. Then, neighbouring sites have smoothly varying MT phases, conductances and regional strikes, and regional skew occurs in the particular frequency band that belongs to the depth of the middle crust. The combined interpretation of both the conductance and the anisotropy of the conductance yields a quantitative argument against the fluid paradigm: anisotropy can indicate a reduced degree of interconnection of the conductive component. If a conductance of more than 1300 S is to be explained by brines which are only partly interconnected, then either a very high prosity or a very high salinity or an unreasonably large thickness of the conductive layer is required.

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.

Interpretation of the short period variations in geomagnetic anomalies of Shaanxi region

A temporary profile from Shiquan in the southern part of Shaanxi Province to Ningxian in Gansu Province for observation of geomagnetic short period variations was set up. The characteristics of geomagnetic short period variations in the profile region was analyzed through geomagnetic transfer functions at measurement sites (Fan, et al, 1998). According to the theory of multi-scale analysis of wavelet transform, the components affected by regional origin in the region is extracted in this paper. The components of regional origin is used as original data in the inversion study for investigation of 2-D conductivity structure corresponding to the regional origin. The inversion method (Zhdanov, Traynin, 1997) which is based on the minimum of the total energy of the residual field between observational electro-magnetic field and the calculated theoretical field of conductivity model and the conception of migrated residual field is introduced into our inversion of the data of magneto-variation sounding. The result of the inversion gives the underground conductivity structure in depth range of 100 km beneath the profile, and shows that a high conductivity area is located under Wugong and Qianling. The main characteristics of the inversion method is briefly discussed, that is, it can reflect the depth and the distribution of anomalous conductivity area comparatively well, and large number of parameters of inversion model is allowed to be used in the inversion.

A high electrical conductive zone at lower crustal depth beneath the Betic Chain (Spain)

Magnetotelluric data with periods of up to 2000 s were collected and the conductivity structure of the crust and upper mantle was imaged in the Central Betic chain. Interpretation of the impedance tensor components and the geomagnetic transfer functions was made in terms of a two-dimensional resistivity model along a NW–SE profile from the Guadalquivir foreland basin and across the boundary between the External and the Internal Betics. Elongated high conductivity zones in the upper and middle crust related to fluid circulation along both sediments and faults were detected. The conductors in the upper crust coincide with the areal distribution of the Cenozoic and Mesozoic sedimentary rocks from the Guadalquivir basin and the External Betics and the conductors in the middle crust with the major Miocene extensional detachments affecting the basement rocks of the Internal Betic zone. A high conductivity zone at lower crustal levels beneath the outer Internal Betics was also detected next to an area where earlier seismic reflection data had imaged a duplication of the reflective Moho. This conductor was interpreted as partial melting of a southeast dipping subducted Iberian lower crust. The geodynamic significance of this subduction depends mainly on whether the overthrusting lower crust belongs to the Alboran domain or to the Iberian plate. In the former case it would correspond to a major plate boundary and in the latter to an intraplate feature.

Great guam earthquake (M sz=8.1) and time changes in geomagnetic transfer functions

Great guam earthquake (M sz=8.1) and time changes in geomagnetic transfer functions

Local time variation of geomagnetic transfer functions

SUMMARY The local time (LT) variation of the geomagnetic transfer function at the 32-min period was examined for observatories distributed worldwide. Two distinct variation types (types 1 and 2) were found in the real part of A. Type 1 is conspicuous at lower latitudes and its seasonal variation is small, whereas type 2 is found at higher latitudes and has its maximum in summer. These two types of LT variation are seen globally and are conspicuous when solar activity is high. The amplitudes of both types of variation vary from 0.018 to 0.078, and are independent of the mean values at each observatory. These values are relatively small, but the amplitude of Chambon la Foret is larger than the mean value, which shows that Au changes its sign in local time. The amplitudes of type 1 and 2 variations decrease and increase with geomagnetic latitude, respectively. These features suggest that they are generated by some global external fields. The most probable cause is the Sq field, although the Dp field may contribute to type 1 variation. On the other hand, for the islands of the Pacific Ocean at low latitudes, such as Honolulu and Chichijima, the type 1 variation appears not in the real but in the imaginary part of A, which suggests that currents induced in the ocean also contribute to the local time variation.

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.

Estimation of geomagnetic transfer functions from nonuniform magnetic fields induced by the equatorial electrojet: A method to determine static shifts in magnetotelluric data

Local conductivity inhomogeneities in the near‐surface commonly shift magnetotelluric (MT) apparent resistivity curves in magnitude by a frequency independent factor (static shift). Proper interpretation of MT data requires detection and compensation of these static distortions. A variation of the Z/H method applied to single site recordings of magnetic variations of the equatorial electrojet (EEJ) produces static‐free geomagnetic transfer functions in the same period range as MT impedances. Numerical modeling demonstrates that a comparison of these undistorted geomagnetic transfer functions with distorted MT impedances at periods between 10 and 100 seconds determines the magnitude of static shifts. By using a priori information on the morphology of the EEJ field and removing regional two‐dimensional (2‐D) and three‐dimensional (3‐D) effects from the magnetic data, the single‐site Z/H method provides a reasonable estimate of the static shift in MT data from northeast Brazil. Critical factors in determining MT static shifts with this method are (1) good control on the morphology of the EEJ field and on the ratio of the regional uniform field to the nonuniform EEJ field at the magnetic equator, (2) a model reproducing the source field, (3) information on the distance between the MT site and the EEJ center, and (4) removal of regional 2‐D and 3‐D effects from the magnetic data.

On the problems concerned in “The subter-ranean electrical conductivity structure at the Chinese Great Wall Station on Antarctic (CG-WSA)” by Xiaoping Zeng, Yunfang Lin and Yuechen Zhao

By means of the Capon spectral analysis technique Zenget al. (1995) calculated the frequency response of the geomagnetic transfer functions for the Chinese Great Wall Station on Antarctic, and made a preliminary inference for the underground electrical conductivity structure. In the present note, the writer considers that there are some problems concerned in their paper. Maybe it is worthwhile discussing these problems as below.

Geomagnetic Transfer Functions in Japan Obtained by First Order Geomagnetic Survey.

Repeated measurements of the Earth's time varying magnetic field have been made across Japan by the Geographical Survey Institute. Such measurements are known as a first order geomagnetic survey. There are 105 geomagnetic stations located uniformly across the whole of Japan. Since 1987, triaxial fluxgate magnetometers have been used to obtain one-minute data at almost all stations. From the geomagnetic data, transfer functions were determined for each station. Traditional vertical field transfer functions of wide period range (from 4 min to 128 min) were obtained, making use of the interstation method. Additionally, we calculated transfer functions for the horizontal field. The distribution of the transfer functions can be used to form a reference map of geomagnetic induction in Japan. Moreover, the distribution of the horizontal transfer functions in Japan has been examined for the first time. After eliminating the effect of the sea water using thin-sheet models, large regional anomalies are found to remain. These anomalies are diagnostic of the resistivity structure and show patterns of current channeling in Japan.

Study on electric variations of media in epicentral area by geomagnetic transfer functions

Taking the graph of the apparent resistivityρ yx in Lanzhou area, its geomagnetic transfer function variations and the investigation of moderate-strong local earthquakes as example and from the viewpoint of seismomagnetic effect, the authors found that subterranean conductivity values might be applied to the division of Lanzhou area from the surface downward into four layers, i. e., the anomalous, the transitional, the non-anomalous and the anomalous layer. Only the transfer function modular |A| and |B|, magnetic azimuth α P with its variance Δα P , and the total varianceσ z in the anomalous layers showed obvious anomalies with time changes. Thus, it could be conjectured that the anomalous layer was closely associated with the anisotropic variations of conductivity of the subterranean media in the seismicity area. This phenomenon might provide a way to the further study of its mechanism. It is needed to study the inherent connotation deeply.

Electrical resistivity model of the crust and upper mantle from a magnetotelluric survey through the central Pyrenees

SUMMARY An MT profile which roughly coincides with the south side of the ECORS-Pyrenees deep reflection seismic profile was made from the North Pyrenean zone through the central Pyrenees and Ebro basin. The periods recorded ranged from 0.0128s to 2000 s. Analysis of MT data shows a 2-D conductive structure. The strike direction of the structure is E-W, parallel to the Pyrenean chain. Interpretation of the data includes geomagnetic transfer functions and is constrained by some relevant geometric crustal features observed in seismic data. Accordingly, a 2-D resistivity cross-section has been obtained. The most striking results are: (1) detection of subvertical conductors in the upper crust coinciding with major fault zones; (2) detection of a deep, high-conductivity zohe at the contact between the Iberian and the European plates: (3) the fact that the European plate is thicker and more resistive than the Iberian one. Interpretation of the deep high conductivities suggests the existence of partial melting in the Pyrenees related to the subduction of the Iberian plate beneath Europe.

Research on earthquake prediction from geomagnetic pulsation

This paper has presented a research on the method of using digitized data of geomagnetic pulsation observation to predict earthquakes and the research results. According to the theory of inductive magnetic effect, the observation of geomagnetic pulsation events can detect the preseismic conductivity and structure anomalies of subsurface media more effectively than the conventional geomagnetic observations, especially the short-impending anomalies before earthquakes. Our research results have also indicated that the geomagnetic pulsation transfer function has obvious short-impending anomalies and typical anomaly forms before earthquakes. Using the proposed method, satisfactory prediction results have been obtained.

A magnetovariational study in Central Italy: Standard techniques

magnetovariational study was performed in Central Italy, with an array of magnetometers located at Radicondoli (SI), Radda in Chianti (SI), Roccalbegna (GR), Città di Castello (PG) and Pennabilli (PS) from February to May 1992. Geomagnetic transfer functions in the frequency domain were calculated using the standard least squares tecnique. The induction arrows for the periods T= 32 min and T= 128 min, the Hypothetical Event (HE) maps and two IZ/HI pseudosections across the array, show the magnetovariational effect of the upper mantle anomaly in the Tuscan-Tyrrhenian area and the contrast between different crustal types.