An audiomagnetotelluric investigation of the Southern Upland Fault : novel instrumentation, field procedures and 3D modelling

The feasibility of the magnetic variation field for electromagnetic investigations in the audiomagnetotelluric frequency range is tested at two separate locations in southeastern Scotland in the vicinity of the Southern Upland Fault complex (Lammermuir Fault). The Lammermuir Fault represents a lateral conductivity contrast in that it separates conductive Carboniferous sediments from resistive Ordovician meta-sediments. For one of the experiments and for the first time, a new generation of instruments (S.P.A.M. MkIII) could be used for data acquisition. The instrument is operated as a network of geophysical sensors, it uses digital data communication between sites and provides on-line data quality control of all connected channels. For the experiment, we installed a 15-channel configuration of three synchronous AMT sites. All recorded data were re-processed off-line, with a newly developed S.P.A.M. MkIII-cornpatible data processing package. The software is based on robust statistics and an integrated database helps to organise all relevant information. Despite some problems with noise, the overall quality of the data is satisfying. Both locations reveal strong vertical magnetic field anomalies. The anomalies are resolved convincingly in frequency and space. Contrary to our original assumptions, the observed anomalies at neither location can he modelled using two-dimensional techniques. However, three-dimensional thin sheet models show that strong current channelling into a surface conductor can explain very large induction arrows. Full three-dimensional modelling of the data is necessary to fit both, real and imaginary parts of the observed induction arrows. The modelling results indicate that a SE/NW striking conductive feature at the surface is duplicated at greater depth and dips towards the south. S.P.A.M. MkIII is ideally suited to collect data of densely spaced sites for such electromagnetic mapping experiments or possibly three-dimensional data sets in future projects. Vertical magnetic field anomalies can be detected over the whole AMT frequency range and they are more robust against noise than the electric fields. A further development of the geomagnetic variation mapping method at high frequencies is therefore highly recommended.

[1]  J. Tukey,et al.  The Fitting of Power Series, Meaning Polynomials, Illustrated on Band-Spectroscopic Data , 1974 .

[2]  A. Junge.  The telluric field in northern Germany induced by tidal motion in the North Sea , 1988 .

[3]  K. Bahr,et al.  Electrical Anisotropy in the Lower Crust of British Columbia: an Interpretation of a Magnetotelluric Profile after Tensor Decomposition. , 1993 .

[4]  A. Schultz,et al.  Analysis of zonal field morphology and data quality for a global set of magnetic observatory daily mean values. , 1983 .

[5]  W. D. Parkinson.  Directions of Rapid Geomagnetic Fluctuations , 1959 .

[6]  R. Banks.  Geomagnetic Variations and the Electrical Conductivity of the Upper Mantle , 1969 .

[7]  Induction in a layered plane earth by uniform and non-uniform source fields , 1973 .

[8]  Ulrich Schmucker,et al.  Anomalies of geomagnetic variations in the Southwestern United States , 1970 .

[9]  T. D. Gamble.  magnetotellurics with a remote reference , 1979 .

[10]  C. M. Swift,et al.  The application of audio-frequency magnetotellurics (AMT) to mineral exploration , 1973 .

[11]  R. Groom.  The Effects of Inhomogeneities on Magnetotellurics. , 1988 .

[12]  G. Egbert.  Multivariate analysis of geomagnetic array data: 2. Random source models , 1989 .

[13]  F. X. Bostick,et al.  THE ESTIMATION OF MAGNETOTELLURIC IMPEDANCE TENSOR ELEMENTS FROM MEASURED DATA , 1971 .

[14]  G. Egbert,et al.  Robust estimation of geomagnetic transfer functions , 1986 .

[15]  F. W. Jones,et al.  The Perturbations of Alternating Geomagnetic Fields by Conductivity Anomalies , 1970 .

[16]  Karsten Bahr,et al.  Geological noise in magnetotelluric data: a classification of distortion types , 1991 .

[17]  J. Coggon.  Electromagnetic and electrical modeling by the finite element method , 1971 .

[19]  M. Chouteau,et al.  Channelling contribution to tipper vectors: a magnetic equivalent to electrical distortion , 1993 .

[20]  D. Gough,et al.  Electrical conductivity and tectonics of Scotland , 1977, Nature.

[21]  J. Larsen.  Transfer functions: smooth robust estimates by least-squares and remote reference methods , 1989 .

[22]  R. Parr.  Development of magnetotelluric processing and modelling procedures : application to Northern England , 1991 .

[23]  S. Constable.  Constraints on Mantle Electrical Conductivity from Field and Laboratory Measurements , 1993 .

[24]  A. Walden.  Estimating confidence intervals for the gain and phase of frequency response functions , 1986 .

[25]  M. Chouteau,et al.  High-frequency magnetotelluric investigation of crustal structure in north-central Abitibi, Quebec, Canada , 1995 .

[26]  C. Swift,et al.  A magnetotelluric investigation of an electrical conductivity anomaly in the southwestern United States , 1967 .

[27]  G. Pickup,et al.  A high resolution magnetotelluric survey of the Milos geothermal prospect , 1989 .

[28]  G. Vasseur,et al.  Bimodal electromagnetic induction in non-uniform thin sheets with an application to the northern Pyrenean induction anomaly , 1977 .

[29]  K. Bahr,et al.  Interpretation of the magnetotelluric impedance tensor; regional induction and local telluric distortion , 1988 .

[30]  Alan G. Jones,et al.  Magnetovariational and Magnetotelluric Investigations in S. Scotland , 1980 .

[31]  Philip E. Wannamaker,et al.  A stable finite element solution for two-dimensional magnetotelluric modelling , 1987 .

[32]  F. W. Jones.  Induction in laterally non-uniform conductors: Theory and numerical models , 1973 .

[33]  R. Fortey,et al.  Faunal evidence for oceanic separations in the Palaeozoic of Britain , 1982, Journal of the Geological Society.

[34]  J. T. Smith,et al.  Three-dimensional electromagnetic modeling using finite difference equations: The magnetotelluric example , 1994 .

[35]  Alan D. Chave,et al.  A comparison of techniques for magnetotelluric response function estimation , 1989 .

[36]  S. L. Fontes,et al.  Processing of noisy magnetotelluric data using digital filters and additional data selection criteria , 1988 .

[37]  M. Ingham,et al.  An electrical model of the crust and upper mantle in Scotland , 1980, Nature.

[38]  D. G. Watts,et al.  Spectral analysis and its applications , 1968 .

[39]  Alan G. Jones,et al.  Decomposition and Modelling of the BC87 Dataset , 1993 .

[40]  J. Larsen.  Low Frequency (0-1-6*0 cpd) Electromagnetic Study of Deep Mantle Electrical Conductivity Beneath the Hawaiian Islands , 1975 .

[41]  L. Cagniard.  Basic theory of the magneto-telluric method of geophysical prospecting , 1953 .

[42]  A. T. Price,et al.  Electromagnetic induction in non-uniform conductors, and the determination of the conductivity of the Earth from terrestrial magnetic variations , 1939 .

[43]  George V. Keller,et al.  Electrical Methods in Geophysical Prospecting , 1981 .

[44]  P. Sule.  Broadband magnetotelluric investigation in southeast Scotland , 1985 .

[45]  R. Parr,et al.  Magnetotelluric studies in and adjacent to the Northumberland Basin, Northern England , 1993 .

[46]  John Clarke,et al.  Error analysis for remote reference magnetotellurics , 1979 .

[47]  J. Floyd.  The derivation and definition of the ‘Southern Upland Fault’: a review of the Midland Valley – Southern Uplands terrane boundary , 1994, Scottish Journal of Geology.

[48]  John Clarke,et al.  Magnetotelluric data analysis; removal of bias , 1978 .

[49]  I. Reddy,et al.  MAGNETOTELLURIC EFFECT OF DIPPING ANISOTROPIES , 1971 .

[50]  D. Livelybrooks,et al.  The simulation of magnetic variation anomalies using single-station data , 1993 .

[51]  Alan D. Chave,et al.  On electric and magnetic galvanic distortion tensor decompositions , 1994 .

[52]  R. C. Bailey,et al.  Decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion , 1989 .

[53]  C. Dumitrescu,et al.  Subsurface structure of SE Scotland from broadband magnetotelluric measurements , 1993 .

[54]  Curiudu.  Induction in a thin sheet of variable conductance at the surface of a stratified earth - I.Two-dimensional theory , 1984 .

[56]  P. Weidelt.  The inverse problem of geomagnetic induction , 1973 .

[57]  B. Hobbs.  Terminology and symbols for use in studies of electromagnetic induction in the Earth , 1992 .

[58]  C. M. Swift,et al.  On determining electrical characteristics of the deep layers of the Earth's crust , 1986 .

[59]  H. Wiese.  Geomagnetische Tiefentellurik Teil II: Die Streichrichtung der untergrundstrukturen des elektrischen Widerstandes, erschlossen aus geomagnetischen Variationen , 1962 .

[60]  J. T. Weaver.  Mathematical methods for geo-electromagnetic induction , 1994 .

[61]  R. D. Hibbs,et al.  Electromagnetic induction in three dimensional structures for various source fields. , 1978 .

[62]  Geophysical images of the deep crust : the , 2022 .

[63]  E. Lagios.  Gravity and other geophysical studies relating to the crustal structure of South-East Scotland , 1980 .

[64]  J. Leggett,et al.  A model for the crustal evolution of southern Scotland , 1983 .

[65]  G. W. Hohmann.  Three-Dimensional Induced Polarization and Electromagnetic Modeling , 1975 .

[66]  B. Hobbs.  The Calculation of Geophysical Induction Effects Using Surface Integrals , 1971 .

[67]  D. Boerner,et al.  Electromagnetic modelling of buried line conductors using an integral equation , 1995 .

[68]  L. Cocks,et al.  Oceans, island arcs and olistostromes: the use of fossils in distinguishing sutures, terranes and environments around the Iapetus Ocean , 1986, Journal of the Geological Society.

[69]  The BC87 Dataset: Application of Hypothetical Event Analysis on Distorted GDS Response Functions and Some Thin Sheet Modelling Studies of the Deep Crustal Conductor. , 1997 .

[70]  D. Livelybrooks,et al.  Inversion of electromagnetic induction data for the Iapetus Suture Zone in the UK , 1993 .

[71]  Alan D. Chave,et al.  On the robust estimation of power spectra, coherences, and transfer functions , 1987 .

[72]  Mapping the geothermal anomaly on the island of Milos by magnetotellurics , 1989 .

[73]  Gary R. Olhoeft,et al.  Tables of Room Temperature Electrical Properties for Selected Rocks and Minerals with Dielectric Permittivity Statistics , 1979 .

[74]  J. T. Weaver,et al.  Three-dimensional induction in a non-uniform thin sheet at the surface of a uniformly conducting earth , 1979 .

[75]  Alan G. Jones,et al.  Geomagnetic induction studies in Southern Scotland , 1977 .