Anomalous Vertical Field Research Articles

On guided versus deflected fields in controlled-source electromagnetics

Abstract The detection of electrically resistive targets in applied geophysics is of interest to the hydrocarbon, mining and geotechnical industries. Elongated thin resistive bodies have been extensively studied in the context of offshore hydrocarbon exploration. Such targets guide electromagnetic fields in a process which superficially resembles seismic refraction. On the other hand, compact resistive bodies deflect current in a process which has more similarities to diffraction and scattering. The response of a real geological structure is a non-trivial combination of these elements—guiding along the target and deflection around its edges. In this note the electromagnetic responses of two end-member models are compared: a resistive layer, which guides the electromagnetic signal, and a resistive cylinder, which deflects the fields. Results show that the response of a finite resistive target tends to saturate at a much lower resistivity than a resistive layer, under identical survey configurations. Furthermore, while the guided electromagnetic fields generated by a buried resistive layer contain both anomalous horizontal and vertical components, the process of electromagnetic deflection from a buried resistive cylinder creates mainly anomalous vertical fields. Finally, the transmitter orientation with respect to the position of a finite body is an important survey parameter: when the distance to the target is much less than the host skin depth, a transmitter pointing towards the resistive cylinder will produce a stronger signal than a transmitter oriented azimuthally with respect to the cylinder surface. The opposite situation is observed when the distance to the target is greater than the host skin depth.

Overview of Geomagnetic Deep Soundings (GDS) as applied in the Parnaíba Basin, North-Northeast Brazil

Geomagnetic Deep Sounding (GDS) is an electromagnetic method of geophysics, which is capable of imaging the Earth’s interior in terms of electrical conductivity using natural geomagnetic transient variations. The method is particularly suited to map geological structures marked by large lateral conductivity contrasts. An overview of the methodology is presented for a magnetometer array study undertaken within and around the intracratonic Parnaiba Basin, north-northeast Brazil. The article describes the sequential steps of data processing, the results of numerical modeling, and the related geological/tectonic implications of the inferred conductivity distribution. In the initial stages of data processing, an advanced robust regression technique is applied to derive transfer functions used to diagnose the lateral conductivity distribution in the study region. The presentation of the transfer functions in the form of induction arrows helps to identify regions of enhanced conductivity. Contour plots and pseudo-sections of the anomalous vertical fields, estimated from the hypothetical event analysis on transfer functions, are essential to characterize the orientation and dimensionality of the electrical conductive structures of the region. The analysis of various frequency and polarization parameters indicates that an anomalous behavior of the magnetovariational field, with periods longer than one hour, is determined by currents induced in the seawater, and perhaps in the raised asthenosphere beneath the oceanic region. Yet, anomalous signatures observed at periods shorter than one hour are primarily controlled by concentrated currents in two inland conductive structures. They are the Parnaiba Basin Conductivity Anomaly (PBCA), that follows the trend of the Transbrasiliano Lineament in the eastern part of the Parnaiba Basin, and the LINK anomaly, that extends from the central part of the basin to the Marajo Graben. A non-uniform thin sheet modeling has mapped the lateral extent and estimated the depth-integrated conductances of PBCA and LINK to be of the order of 2000 S and 1000 S, respectively. The overall pattern of the inferred conductivity distribution helps to visualize the LINK anomaly as relics of a probable sedimentary channel connecting the Parnaiba Basin and Marajo Graben, and which could have acted as a gateway for sea transgressions during early stages of the basin evolution. The combined inversion and forward modeling of the GDS response functions provides the PBCA structural cross-section as an ensemble of a graben-like structure in the basement and a highly conducting block confined to the deeper central part of the basin, with an embedded resistive body in the middle. The existence of a broad conducting block confined to the central part of the basin is also consistent with magnetotelluric data, and the graben-like structure in the basement is corroborated by aeromagnetic data. The origin of graben-like structures in the basement could be possibly related to an extensional tectonism, whereas the resistive body is tentatively interpreted as a diabase dike or a recrystallized magmatic body intruded during a Cretaceous magmatic event. Carbon bearing sediments are suggested as an alternative to hydrated siliciclastic sediments to account for the high conductivity of the central block. A hydrothermal event associated with the Cretaceous magmatic activity may be the likely process to produce carbon through the pyrolysis of hydrocarbon-saturated Paleozoic sediments.

A thin-sheet model of electromagnetic induction in northern England and southern Scotland

Electric currents induced in the seas surrounding the British Isles influence the electromagnetic fields observed on land. Observational data suggest that , at certain periods, anomalous currents concentrate in a thin­ sheet comprising the shallow seas and onshore sedimentary sequences. The block and basin structure of northern England and southern Scotland provides a physical basis for the implementation of a thin-sheet approximation in quantitative electromagnetic modelling studies of the region. The model comprises a non-uniform thin-sheet at the surface of a layered half. space. The solution to this problem , as formulated by Vasseur & Weidelt, has been applied to a regional thin-sheet model of northern England and southern Scotland. A regional observational data set, comprising the anomalous vertical field at a period of 750s, is used to constrain possible models. Geological and geophysical observations are used to provide a realistic model for the major sedimentary basin of the region and the results obtained are compared with observations. The results suggest that such a basin is capable of establishing a substantial onshore perturbation in the anomalous vertical field, of similar geometry to that observed. Although the present study is necessarily limited in its depth resolution , it is suggested that other models presented go some way to defining the lateral extent of the conductive configuration for the region.

Local and regional induction in the British Isles

Electric currents induced in the shallow seas and deep ocean around the British Isles have a profound effect upon the electromagnetic fields observed at stations on the land. The configuration of anomalous currents changes with frequency, and causes corresponding changes in the geomagnetic transfer functions. Magnetic variations have been recorded at a dense network of sites in southern Scotland and northern England. Single­ station transfer functions have been used to generate hypothetical event maps of the anomalous vertical field, and hence to infer the configuration of the anomalous internal currents. At periods exceeding 2000 s, the vertical field is dominated by the effects of electric currents to the west, presumably in the Atlantic Ocean . In the period range 400-2000 s, anomalous currents are concentrated in a thin sheet comprising the shallow seas, the t hick sequences of post-Caledonian sedimentary rock which underlie them, and the extensions of the sedimentary basins into the land. The response of the individual basin is determined not only by its local conductivity structure, but also by the extent of its connection to the shallow seas,i.e. its regional importance within the conductive sheet. At periods less than 200 s on the other hand, the anomalous fields at inland sites are principally determined by the local geological structure . These results confirm conclusions reached from theoretical studies of electromagnetic induction in a heterogeneous surface layer (Park, Orange & Madden); that electromagnetic response data measured in a 3-D environment such as the Northumberland Basin must be interpreted using 3-D models . If one or 2-D models are used, the data must be corrected on the basis of regional measurements.

A magnetovariational study of a geothermal anomaly

Summary. During 1980 September and October an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the Rocky Mountains foothills of Alberta. The purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly. Examples of magnetograms and Fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients. A detailed transfer function analysis of the data is presented. Both single-station transfer functions and a solution of the induction matrix are calculated. The latter involves the representation of the normal fields by least-squares fitted planes and a justification of the validity of this method is given. Contour plots of the response of the anomalous vertical field to horizontal hypothetical events, and pseudosections of IZa/HnI for traverses across the array, are shown. These indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity. By treating the observed values of the in-phase part of IZa/Ha I along a traverse across the anomaly as due to vertical dipoles, it is shown that the top of the conductive body probably lies at 10-20km depth. The pseudosections suggest that the body extends at least to lower crustal depths. The correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust.

Geomagnetic depth sounding and magnetotelluric results from a seismically active region northeast of Quebec City

Observations of geomagnetic and telluric field variations have been made at four stations near La Malbaie, Quebec, in a seismically active region northeast of Quebec City. The data were analysed by transfer function and impedance tensor methods to study the nature of geomagnetic induction in the region and its structural implications. Transfer functions and telluric fields were influenced by several factors including the nearby St. Lawrence River, lateral variations in conductivity caused by geological complexities, and conduction of electric currents probably originating through induction in the ocean.It is demonstrated that it is possible to distinguish between induction by the vertical geomagnetic field and induction by the horizontal components. Anomalous vertical fields caused by vertical field induction are observed in the area and the electric currents associated with them have a flow pattern which is similar to that of currents induced by the horizontal field. Further, the Z-induced currents changed impedance tensor elements by as much as 20% though the effect was usually smaller than this. Nonetheless, the magnetotelluric data confirmed the presence of a conductivity anomaly in the Laurentides Park region of Quebec.

Electrical Conductivity Structure by Geomagnetic Induction at the Continental Margin of Atlantic Canada

Summary Geomagnetic variations measured at 10 stations in Atlantic Canada show significant laterally inhomogeneous induction. Transfer functions giving the systematic dip angle and direction of the variation field lines have been computed for periods from 20s to 120min and the results numerically modelled. High electrical conductivity exists starting at a depth of 15 km or less under the continental shelf off Nova Scotia and Newfoundland and perhaps under the Bay of Fundy. The apparent coast effect with a maximum at 30-min period results from the contrast between the highly conducting shelf structure and the more resistive inland rocks. The most likely explanation of the high conductivity is that there is highly saline interstitial water in the lower part of a 10km sedimentary section associated with evaporite, salt layers, or that part of the crust is hydrated in this area. Strong electric currents flow in the various arms of the Gulf of St Lawrence for short period (10 s to 10 min) inducing fields. Numerical models show that they can be explained by local induction in the shallow sea water. The details of the current flow indicate that significant conductive channelling must occur. One station on the north shore of the St Lawrence River has large long period (30 min) anomalous vertical fields. They probably result from a contrast in deep conductivity between the Appalachian and Canadian Shield geological provinces.