L. Szarka

Geophysical aspects of man-made electromagnetic noise in the earth—A review

AbstractThe paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows:- EM distortion effects of man-made constructions below and over the Earths surface defined as ‘passive-noise’,- field observation of EM disturbances due to ‘active’ man-made sources,- EM source mechanism of some important active sources from a geophysical point of view,- efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways,- application of man-made EM noise for geophysical prospecting. The paper is based on world-wide EM noise survey studies published mainly in geophysical journals.

Depth of Investigation and Vertical Resolution of Surface Geoelectric Arrays

Depth of investigation and vertical resolution values are determined and tabulated for 30 surface geoelectric arrays that have non-zero response, i.e., a depth of investigation characteristic (DIC) function due to a buried thin horizontal sheet. In accord with experience, results show a general reciprocal relationship between depth of investigation and vertical resolution. The most frequently used arrays in multi-electrode studies (i.e., Wenner-a, Wenner-b, Schlumberger, dipole-axial arrays and the pole-dipole array) offer reasonable compromises between depth of investigation and vertical resolution. Depth of investigation can be increased by using the pole-pole array; vertical resolution can be improved with, for example, the a10 or c10 arrays. Current focussing does not increase the depth of investigation for the horizontal thin-sheet model. The complete set of depth of investigation and vertical resolution values permits exact physical comparison of various geoelectric arrays and provides simple but useful rules for practical geoelectric applications, e.g., how to develop multi-electrode systems with higher vertical resolution, or how to select arrays to satisfy special exploration requirements.

Magnetotelluric and audiomagnetotelluric measurements in Finland

Abstract Joint Finnish—Hungarian MT (magnetotelluric) and AMT (audiomagnetotelluric) measurements were carried out in Finland in the framework of the international ELAS project. The conditions for MT measurements are favorable at these latitudes. Five MT and 150 AMT stations gave information on the electrical conductivity distribution in the area: AMT results guided the choice of MT sites with minimal near-surface distortion effects and helped the interpretation of the MT soundings; the MT measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near-surface dyke structures. This result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area. Any information about the upper mantle would require the use of Sq harmonics because of the crustal conductivity anomalies detected by the MT measurements.

Geoelectric mapping of near‐surface karstic fractures by using null arrays

The term “null array” is introduced for those electrode configurations where the measured potential difference is zero above a homogeneous half‐space when using a measuring dipole M0N0. Different types of null arrays (three‐electrode, Schlumberger, and dipole axial/equatorial null arrays) and their corresponding traditional arrays are studied. It was shown in a field study carried out in a karstified limestone area covered by thin sediments that it is possible to obtain geologically meaningful results with null‐array techniques. The main features of the null‐array data are as follows. (1) Null‐array data appear to be more spatially variable than the classical data. The spatial variability provides information about the presence of karstic fractures in the subsurface; (2) The null‐array anomalies caused by nearly vertical karstic fractures in the limestone basement do not decay with depth as quickly as the classical array anomalies. (3) The strike direction of the fractures is much less ambiguous than that...

Contribution of magnetic measurements onboard NetLander to Mars exploration

Abstract In the frame of the international cooperation for Mars exploration, a set of 4 NetLanders developed by an European consortium is expected to land on the planet during the forthcoming years. Among other instruments, the geophysical package of each lander will include a magnetometer. The different possible contributions of magnetic measurements onboard the NetLander stations are presented. Intrinsic planetary field and remanent magnetisation investigations by means of magnetometers onboard a network of landers are first considered, and the information that can be thus derived on the Martian core dynamo and surface rocks, soil, and dust is discussed. The contribution of permanent recording of the magnetic transient variations at a network of surface stations is then discussed. The transient variations of the magnetic field at the surface of a planet has a primary external source, the interaction between the environment of the planet and solar radiation, and a secondary source, the electric currents induced in the conductive planet. The continuous recording of the time variations of the magnetic field at the surface of Mars by means of three component magnetometers installed onboard NetLander stations will therefore allow study of both the internal structure of Mars and dynamics of its ionised environment. The expected characteristics of transient magnetic variations, and their relation with plasma flow and current in the Mars ionised environment are discussed. The use of the network magnetic data to probe the internal structure of Mars is also considered. The used techniques are presented, and the information that can be thus obtained on the Mars permafrost, lithosphere and mantle structure illustrated by numerical simulations. Finally, the specifications of the instrument allowing to achieve these objectives are discussed, and the instrument described.

Annual fluctuation in amplitudes of daily variations of electrical signals measured in the trunk of a standing tree.

Electrical potential differences between electrodes inserted in eight selected sites of the trunk of a Turkey oak (Quercus cerris) and the ground have been recorded continuously since November 1997. The mean amplitudes of daily electric variations were determined for time intervals when the meteorological and atmospheric electricity conditions were undisturbed. It was found that the mean daily amplitudes show a characteristic annual fluctuation. Both in 1998 and in 1999 two peaks were observed: the first maximum appears suddenly about the end of March, and the second one was seen in late June, July. Between November and March the mean amplitudes were smaller by about one order of magnitude.

Geoelectric imaging properties of traditional arrays and of the optimized Stummer configuration

In this paper a systematic, semi-empirical comparison is presented between two-dimensional geoelectric models and their inversion images, obtained by using five different electrical resistivity arrays and an optimized Stummer configuration. Eight different models (more or less in order of growing complexity) are studied and both noise-free and noisy data cases are considered. The results show that (1) the quality of the inversion images obtained with traditional arrays depends significantly on the model and on the noise level, (2) among the traditional arrays it is definitely the dipole-dipole array that provides inversion images mostly similar to the geoelectric models, (3) the inversion images obtained by using the optimized Stummer configuration are even more similar to the original geoelectric model than those obtained by the dipole-dipole array. It means that the optimized Stummer array is even better than the best traditional array, the dipole-dipole array, especially in the deepest part of the inversion images. We conclude that in a general field situation the Stummer configuration is good enough for not being forced to search specific configurations. As presented, optimization procedures, involving null arrays could even further improve the quality of the inversion images obtained by using the Stummer configuration.

Magnetotellurics (MT) in mountains-noise, topographic and crustal inhomogeneity effects

Abstract In a mountainous region the aim of magnetotelluric sounding is to detect deep fractures, indicated by electric conducting zones in the crust. Nevertheless, some factors, such as the EM noise of different character, the rugged topography, the electric inhomogeneities, are increased in a mountainous region, and to determine or overcome them special measuring and data processing techniques should be applied. This paper reviews these factors or effects illustrating them with examples from the Eastern Alps and the Hungarian Central Mountains.

Electromagnetic Induction Profile (PREPAN95) from the East European Platform (EEP) to the Pannonian Basin

Complex electromagnetic measurements along a profile crossing different great tectonic units in East-Central Europe (East European Platform, Paleozoic Europe and Carpatho-Pannonian region) were carried out by Hungarian, Polish, Slovak and Ukrainian institutes. Beside recent MT sounding curves, geomagnetic induction vectors, polar diagrams, apparent resistivity and phase pseudosections, a short description of earlier magnetotelluric (MT) and magnetovariational (MV) experiments is also given. For interpretation of these data 1D and 2D inversions were used after separation of the quasi E and B polarized curves. The Carpathian conductor clearly appears in the magnetovariational profile. MT soundings indicate that the mantle conductive basement — presumabely the asthenosphere — steeply deepens from the Neogene Pannonian Basin characterized by high heat flow towards the much colder EEP through the Paleozoic area.

Geomagnetic induction study using the NetLander network of magnetometers

Abstract The NetLander mission will provide a unique opportunity to probe the internal structure of Mars from continuous magnetic recordings at the surface of the planet. When the resistivity varies only with depth only, and the externally originating variations are homogeneous at the scale of the studied area, the resistivity distribution can be related to the ratio of the vertical magnetic field to the spatial gradients of the horizontal magnetic field. Assuming the field at the surface of Mars behaves as a superposition of plane waves, the spatial gradient determination is equivalent to wave vectors identification. We present a multi-point data analysis technique initially developed in the frame of Cluster, the future ESA multi-spacecraft mission. It allows to obtain optimum frequency–wave vector spectra estimation from simultaneous magnetic field recordings made at a subset of three NetLander stations. The analysis of synthetic data have made it possible to study the effects of experimental constraints for NetLander on the frequency–wave vector spectrum estimation.