Deep Conductivity Structure Research Articles

Stripping induced polarization effects from airborne electromagnetics to improve 3D conductivity inversion of a narrow palaeovalley

The electrical conductivity distribution within wide palaeochannels is usually well-mapped from airborne electromagnetic data using stitched 1D algorithms. Such stitched 1D solutions are, however, inappropriate for narrow valleys. An alternative option is to consider 2D or 3D models to allow for finite lateral extent of conductors. In airborne electromagnetic data within the Musgrave block near the well-studied Valen conductor, strong induced polarization (IP) and superparamagnetic (SPM) effects make physical property and structure estimation even more uncertain for deep channel clays, particularly those whose channel widths are comparable to their depth of burial. We developed a recursive data fitting algorithm based on dispersive thin sheet responses. The separate IP and SPM components of the fit provide near-surface chargeability and SPM distributions, and the associated electromagnetic (EM) fit provides stripped data with monotonic decays compatible with a simple nondispersive conductivity model. The validity of this stripped data prediction was tested through a comparison of 1D conductivity-depth imaging and 3D inversion applied to the original data and the stripped data. Due to the forked geometry of the deep conductivity structure in the region we investigated, we successfully used 3D rather than 2D inversion to predict the conductivity distribution related to the EM data. We recovered from the stripped data a continuous conductivity structure consistent with a branching, clay-filled palaeovalley under cover.

AMT survey in the Outokumpu ore belt, eastern Finland

ABSTRACTThe Outokumpu belt in eastern Finland is known for numerous polymetallic (Cu–Co–Zn–Ni–Ag–Au) sulphide ore deposits and it is a potential area for new discoveries. Audiomagnetotelluric (AMT) data have recently been collected to image the subsurface conductivity structure of the belt. The data were acquired along five profiles aiming to explore deep conductors, study their geometry and investigate the deep conductivity structure of the belt.The acquired AMT data were mostly adequate for 2D inversion, as the survey area is characterised by thin, laterally extensive conductors indicated by airborne electromagnetic data, deep drillings and regional strike analysis of the impedance tensor data. 2D smooth inversion was carried out jointly for transverse electric and transverse magnetic data. The results show southeast-dipping and sub-horizontal continuous conductors southeast of the Outokumpu old mine, suggesting the presence of prospective Outokumpu assemblage rocks. One ∼ 1 km deep sub-horizontal conductor is verified by a deep drill-hole located ∼ 8 km southeast of the mine. The results demonstrate a similar southeast-dipping structure in the Vuonos mine area, which is located ∼ 7 km northeast of the Outokumpu old mine. An eastwards-dipping conductor detected in the Miihkali serpentinite area indicates deep exploration potential in this area. Besides the smooth 2D inversion, a sharp-boundary 2D inversion was performed and the resulting model was assessed with the existing geological interpretation of the Miihkali area. In the area of the Sotkuma gneiss inlier, conductors are absent from the uppermost ∼ 6 km. This feature may represent an uplifted fault block rather than a thin thrust sheet of Archaean basement rocks.

A Comparison of Peak Electric Fields and GICs in the Pacific Northwest Using 1‐D and 3‐D Conductivity

AbstractGeomagnetically induced currents (GICs) are a result of the changing magnetic fields during a geomagnetic disturbance interacting with the deep conductivity structures of the Earth. When assessing GIC hazard, it is a common practice to use layer‐cake or one‐dimensional conductivity models to approximate deep Earth conductivity. In this paper, we calculate the electric field and estimate GICs induced in the long lines of a realistic system model of the Pacific Northwest, using the traditional 1‐D models, as well as 3‐D models represented by Earthscope's Electromagnetic transfer functions. The results show that the peak electric field during a given event has considerable variation across the analysis region in the Pacific Northwest, but the 1‐D physiographic approximations may accurately represent the average response of an area, although corrections are needed. Rotations caused by real deep Earth conductivity structures greatly affect the direction of the induced electric field. This effect may be just as, or more, important than peak intensity when estimating GICs induced in long bulk power system lines.

Seismic activity and deep conductivity structure of the Eastern Carpathians

We present results of a study of the seismicity and the geoelectric structure of the Eastern Carpathians. After the evaluation of the seismicity, new methods of processing and analyzing seismic data are developed, which allow constructing an averaged horizontal-layered velocity model of the crust in the Carpathian region of Ukraine, tracing the seismic active faults and localizing the seismic events both in horizontal and in vertical direction with a better precision. For the study of the conductivity structure beneath the Eastern Carpathians, the collected magnetovariation and magnetotelluric data are used. The depth of electrical conductivity anomalies are estimated and the resulting quasi-3D model of the conductivity structure beneath the Carpathians is compared with the seismicity in the depth range of 10 ± 2.5 km. The comparison suggests possible geological mechanisms: the seismic events occur mainly in resistive solid rock domains which surround aseismic high-conductivity zones, consisting of at least partially melted material. Aqueous fluids, or a joint effect of several mechanisms, may also play an active role in this distribution.

New government geophysics from the Onshore Energy Security Program

SUMMARY As part of the recently completed Onshore Energy Security Program, Geoscience Australia has delivered new types of pre-competitive data to as well as the more familiar government pre-competitive data types such as aeromagnetics. These new data provide industry with additional information for assessing the geological prospectivity of a region and making targeting decisions. The visionary AWAGS continental airborne magnetic and radiometric survey enabled the first ever national merging of all radiometric surveys to produce the Radiometric Map of Australia. The rigorously calibrated data supports quantitative normalisation of uranium anomalies against rock-type, and hence identification of zones of uranium enrichment, at either a prospect or regional scale. Geoscience Australia also undertook three large AEM surveys (>50,000km2) to map regional scale geological structures important for uranium deposition. These surveys were effective in imaging depth of regolith, geological unconformities, shale units and fault zones. Often these features are not apparent in regional magnetic or gravity data. MT data are now acquired along all deep crustal seismic transects to image the deep conductivity structure. These data are highly complementary to the structural information interpreted from the seismic sections. MT can also detect conductivity features in the non-reflective crust below the Moho.

Deep conductivity structure beneath the northern Brasília belt, central Brazil: Evidence for a Neoproterozoic arc-continent collision

The Brasília belt, in central Brazil, experienced a broad range of tectonic events, including several periods of compressional and extensional deformations during the Proterozoic. Magnetotelluric data were collected at 27 sites along a profile crossing the northern part of the belt. Strike analysis and distortion decomposition indicated that most of the data are two-dimensional (2D) and fit a regional geoelectric strike angle of N20E, parallel to the tectonic grain. 2D conductivity model is constrained by available gravity and seismic refraction data and reveals the cumulative effects of the multiple tectonomagmatic episodes that affected the area in the past on current crustal and upper mantle structure. The results show that the resistive lithospheric root of the São Francisco cratonic plate is restricted to the easternmost part of the profile and does not extend westward beneath the Brasília belt. The external zone of the fold belt, composed of slightly metamorphosed rocks at the surface, presents crustal conductors that can be tentatively interpreted as regions with fossil fluid flows at mid-crust depth beneath a gold mineralization zone and as a very shallow mineralized zone within the metamorphic rocks. The external zone is separated to the west from the Goiás massif by a strong conductivity contrast, indicative of a suture zone along the Rio Maranhão fault system. Crustal conductivity structure under the Niquelândia complex is distinct from what is observed in other parts of the Goiás massif, in agreement with the proposal of the complex being an allochthonous block. In spite of the large tectonothermal events that affected the Goiás magmatic arc there are no significant conductive anomalies at crustal depths in its eastern side. The most prominent feature detected in the geoelectric model is a huge high conductivity anomaly below the entire Goiás massif in the central part of the belt, mainly concentrated at the uppermost mantle but also affecting mid to lower crustal depths. It can be interpreted as associated with thermal events either related to Mesoproterozoic and/or Neoproterozoic continental rifting processes or to delamination and detachment processes at the end of the evolution of the belt, in the Neoproterozoic. Effects related to oceanic lithosphere subduction during the collision and accretion of the Goiás massif and Goiás magmatic arc to the western passive margin of the São Francisco cratonic plate in the Neoproterozoic may alternatively explain the anomaly.

Electrical and electromagnetic investigations for HVDC ground electrode sites in India

High Voltage Direct Current (HVDC) power transmission systems require setting up of specially designed ground electrodes at terminal ends of the transmission line to close the circuit with an earth return path. The design parameters of these electrodes need the information about the electrical conductivity structure within a radius and depth of several km of the site in order to ensure that the injected current penetrates deep enough into the earth. Further, detailed conductivity structure, up to 100–200 m depth, of the electrode site covering an area of less than a sq. km is also needed to ensure safe limits for the step and touch potential at the site. Electrical and electromagnetic methods are very useful tools for this purpose. However, artificial source variants such as deep direct current (DC) resistivity sounding pose logistic problems for target depths greater than a couple of kilometers and in inaccessible areas. We have employed magnetotelluric (MT) and electrical resistivity tomography (ERT) tools to investigate the detailed deep and shallow electrical conductivity structure, respectively, of several potential sites in India for a ±800 kV, 6000 MW HVDC multi-terminal system. Investigations of a site in Assam revealed the presence of highly resistive crustal rocks at the depth of about 1.3 km beneath a thick pile of conductive sediments, rendering the site unsuitable even though the shallow conductivity was favorable. At another site identified after analyzing available geological and geophysical data, we inferred the presence of conductive structure up to at least 4.0 km depth. ERT investigation at this site revealed a favorable conductive structure except for the presence of a 20-m-thick near-surface resistive layer. This information has been useful for the electrode design. We suggest that a strategy suitably combining MT and ERT is useful in the selection of ground electrode sites.

Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan

Abstract The Network-Magnetotelluric (NMT) method is well-suited for investigating deep and large-scale conductivity structure; however, application of the method is strongly dependent on the availability of telecommunication facilities (specifically, metallic transmission cables). To overcome the problem posed by the progressive replacement of metallic transmission cables with fiber cables, we developed a modified NMT (modified NMT) method consisting of purpose-built electrodes, making use of local metallic telecommunication lines, without a transmission cable. We first applied this modified NMT method over the Kii Peninsula, southwestern Japan, undertaking two-dimensional conductivity modeling along a transect across the central part of the peninsula. The model is characterized by a large (∼20 km wide and depths of 10–60 km) and highly conductive (< 10 Ω m) zone in the central part of the peninsula between the Conrad discontinuity and the upper surface of the Philippine Sea slab. This zone contains the hypocenters of many deep low-frequency tremors but regular earthquakes are rare. The zone also corresponds to a high-V p /V s area. The presence of fluid in the zone plays a key role in the absence of regular earthquakes, occurrence of deep low-frequency tremors, and elevated V p /V s values, as well as enhancing conductivity.

Electromagnetic images of the deep structure of the Trans‐European Suture Zone beneath Polish Pomerania

A large‐scale international electromagnetic experiment has been carried out in northwest Poland and northeast Germany. The main goal was to study the deep conductivity structure across the Trans‐European Suture Zone, which is the most prominent tectonic structure of Phanerozoic age in Europe. Electromagnetic measurements were carried out mainly along seismic profiles P2, LT‐7, and LT‐2 crossing the suture zone and running in the northeastern direction. Strike and dimensionality analyses indicate that a geo‐electrical strike of N60°W common to both profiles LT‐7 and P2 can be estimated. This strike direction was used to project and rotate all transfer functions and both profiles were subjected to 2D inversion using three different approaches. The results show the presence of highly conductive Cenozoic‐Mesozoic sedimentary cover reaching depths up to 3 km. A significant conductivity anomaly beneath the central part of the TESZ, called the Central Polish Anticlinorium, has been well resolved at mid‐crustal depths. The upper mantle of the Precambrian East European Craton is more resistive than, adjacent to the West, the younger Paleozoic Platform.

EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations

There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research, the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals. Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended. At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary. A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale, near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities. The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions.

Updating the map of Earth's Surface conductance

Studying the Earth's deep conductivity structures, important for developing our understanding of the dynamics of the Earth, is complicated due to effects of the shallow conductive structures on the electromagnetic (EM) responses for periods larger than hours. The results of the deep EM soundings can be heavily distorted by the surface shell conductance, which varies from fractions of Siemens (S) inland to up to tens thousand of Siemens in the oceans. Thus, separating the effects caused by those variations and by deep conductivity structures is an important step during interpretation of the data.This article reports on efforts to overcome these difficulties by providing high‐resolution, global maps of the Earth's surface shell conductivity structure, from which deep conductivity can be interpolated. Using finescale regional surface schemes of conductance for the shallow structures (S‐maps) overlain and compiled into broader spatial maps, scientists will b e able to use data products from these efforts to accomplish research goals of the currently running USArray (http://www.emscope.org) and for the planned Euro‐Array (http://www.euroarrayorg), projects that aim in part to regionally map the conductivity structures at upper and middle mantle depths by using magnetotelluric (MT) and magnetovariation (MV) methods.

海底拡大系下の非対称電気伝導度構造

A comparative study of deep mantle structures beneath spreading centres revealed asymmetry of lithosphere and mantle. Multidisciplinary marine expeditions were conducted at two survey sites : one around a super-fast spreading ridge in the East Pacific Rise, and the other around a slow and stable ridge segment of the continental breakup region, the Gulf of Aden. At the former site, a Mantle ELectromagnetic and Tomography experiment (MELT) was conducted with a large number of seismometers and electromagnetometers deployed at the seafloor to image the shape and the geometry of partial melt bodies below the fast spreading centre. At the latter, a seafloor magnetotelluric array was deployed to obtain a twodimensional electrical conductivity structure beneath the continental breakup region, and the structure was compared with precise swath mapping and sea surface gravimetry. At both sites, the deep electrical conductivity structures showed remarkable asymmetry with respect to the ridge crest. Partial melt bodies that correspond to the seafloor-producing factory at the surface could not be observed right below the ridge crest, but they significantly shifted by several tens of kilometers from the ridge crest. The asymmetry also appeared in bathymetry, gravity and seismic structures. This implies that the mid-ocean ridges are created at the contacts of different oceanic mantles rather than right above the symmetric partial melt bodies.

Investigation of the electrical conductivity beneath China using geomagnetic spatial gradient method

The data of the year 1992 from 25 geomagnetic observatories affiliated to the geomagnetic network of State Seismological Bureau of China were processed using the principle of geomagnetic spatial gradient method. Through finding out the polynomial form of optimum fitting, comparatively good C values for four harmonic components of diurnal variations were obtained. By using the inverse method of non-linear underdetermined problem, the electrical conductivity structures under the observatories were investgated. It is shown that there are differences of the C values and conductivity structures in the deep underground under the south-western part and northern parts and other parts of China. We studied the possibility of improving the gradient method for investigation of the deep underground conductivity structure, and it is indicated that the gradient method is hopeful in the investigation of earth’s deep conductivity structure and the applied studies concerned.

Deep Conductivity Structure of the North Anatolian Fault Zone and the Istanbul and Sakarya Zones Along the Gölpazari-Akcaova Profile, Northwest Anatolia

Magnetotelluric (MT) measurements at six locations along a 90-km profile across the area between Golpazari and Akcaova have been modeled in two dimensions to increase understanding of the deep conductivity structure of the western part of the North Anatolian fault zone (NAFZ) and the Istanbul and Sakarya zones. It is well known from surface geology that the branch of the NAFZ that passes through the Pamukova Valley with an E-W strike separates the region into two sub-areas, containing contrasting sets of geological features. These two areas also exhibit significant differences in terms of their deep conductivity structure. Electrical resistivity is quite low (10 ohmm) south of the fault at an approximate depth of 26 km, compared to the area north of the fault zone. This low-resistivity zone may provide an indication of partial melting at this depth. In the northern part of the profile beneath Ucgaziler (DUC) and Akcaova (DAK), a five-layered conductivity sequence obtained by magnetotelluric modeling and t...

Deep electrical conductivity structures of the Appalachian Orogen in the southeastern U.S.

Long period magnetotelluric data across the southeastern Appalachians image deep crustal and upper mantle relics of ancient orogenic events. Inversions of the responses show: (1) Beneath the Appalachian mountains there is a sub‐horizontal conductor at 15–20 km depth which dips to the southeast at the surface trace of the Brevard fault. (2) At the location of the Central Piedmont suture, there is a crustal conductor which dips towards the southeast, interpreted as a structure related to the Acadian suture. (3) Upper mantle conductors were found at 80 km depth northwest of the Blue Ridge and at 140 km depth southeast of the Eastern Piedmont. Between these, there is a northwest‐dipping resistive gap, possibly representing the remnant structure of the Alleghanian collision.

Geothermal exploration of kos island, Greece: Magnetotelluric and microseismicity studies

This paper reports the results of magnetotelluric (MT) and microseismicity studies, conducted as part of a multi-disciplinary project to explore the geothermal potential of the island of Kos, Greece. The MT survey, comprising 18 soundings, was carried out in the bandwidth 128 Hz–40 s, in order to determine the deep conductivity structure in the geothermally prospective western part of the island. Rigorous dimensionality analysis has indicated that the geoelectric structure could adequately be approximated with 1-D interpretation tools. Two significant and seemingly communicating conductive zones of potential geothermal interest were found within the first 2 km. The first is extensive and shallow, detected at depths of 400–600 m; the second is deeper (1000–1300 m), but of considerably smaller lateral dimensions. A very deep relative conductor ( <25 Ω m ) was also detected at depths of 7–10 km, which is thought to comprise part of an old magma chamber with brine-saturated rocks. The microseismicity studies revealed the partial or total attenuation of shear waves in many microearthquake records. The analysis of these observations determined the vertical and lateral extent of that attenuation zone, the greatest part of which is located underneath the marine area between western Kos and Nissyros island to the south, extending approximately from near the surface to about 1.5 km depth. The nature of this zone is discussed in terms of fluid concentration due to the geothermal system of the area.

Geomagnetic variation anomaly and deep electric conductivity structure in the area around Tangshan

Geomagnetic variation anomaly and deep electric conductivity structure in the area around Tangshan

A magnetovariational study in the SW alpine arc: a short report

Magnetovariational studies are useful for the investigation of deep conductivity structures within the Earth. Equipment for geomagnetic time variations research, consisting of four sets of fluxgate magnetometers with microloggers for acquisition, storage and transfer of data, has been installed in northwestern Italy. Geomagnetic variation events along two profiles, covering the area roughly corresponding to the subduction and obduction system of the Ivrea body, have been recorded during Summers 1986 and 1987. This area is characterized by structural anomalies of the lithosphere and asthenosphere, at the contact between the European and African plates. After an analysis in time and frequency domains, results partially confirm some previous findings on the electrical conductivity structure in the inner part of the SW Alpine arc.

Induction of currents in long submarine cables by natural phenomena

The influence of induced currents on long submarine cables has been observed for nearly a century. The currents originate from time variations in the magnetic field of external origin (magnetospheric, atmospheric tides) and earth and ocean tides. Steady earth currents of internal origin have also been proposed as a possible source. The induced currents will depend upon the length, depth, and geomagnetic location of the cables. Present understanding of the phenomena is reviewed. Also discussed are potential scientific problems which might be addressed with data from such long cables and possible limitations on the information obtained. Such problems include studies of (1) the deep conductivity structure of the earth, (2) telluric currents flowing in mid‐oceanic ridges, (3) the spatial extent of external source fields, and (4) a possible role of electromagnetic eddy currents in fluctuations of the earth's spin rate.

Magnetic Induction Fields (2-30 cpd) on Hawaii Island and their Implications Regarding Electrical Conductivity in the Oceanic Mantle

*Summary. Geomagnetic time-variations observed at several sites on the island of Hawaii are analysed for the effects of island bathymetry as well as for the inductive response of the deeper mantle. The data are generally consistent with the deep conductivity profile derived using lower frequency, electromagnetic data from the Island of Oahu. Hawaii data fit better if that model is modified to give the upper 200 km of the mantle a lower conductivity of 0.02 S/m compared to 0.1 S/m for Oahu. The data are represented by a complex, frequencydependent function of location, Tu, relating the vertical variation Z to a component U of the horizontal variation (T, = Z/U). The direction of U is nearly frequency independent at each site but is different for each site. Below a frequency of about 30 cycles per day, the functions, T,, at any two sites are found to be related by a real constant. This suggests that the deeper conductivity structure is the same beneath each site. This result is consistent with quasi-static induction in a non-uniformly conducting thin sheet above a stratified conductivity structure. The response of such a model can be written as Tu =Aq, where 4 is a quasi-uniform, complex, frequency-response function characterizing the effect of the deep conductivity and A is a spatially dependent parameter parameterizing the effect of variable conductivity in the thin sheet. The parameter A may be estimated by fitting observational estimates of T, to models of deep conductivity structure.