Induction Arrows Research Articles

Regional conductance map of Andaman and Nicobar region

To image the electrical conductivity distribution, fluxgate magnetometers are operated at five sites in Andaman and Nicobar region. Transfer functions are estimated for the period range 8–128 min, from nighttime transient geomagnetic variations, using robust regression analysis. The observed induction arrows in Andaman Islands are found to point towards east despite deep sea located towards its west. This indicates that fore-arc basin (Andaman–Nicobar deep) is more conducting than the region of outer non-volcanic Island arc. Thin sheet model requires the conductance of 10,000–35,000 S (with increase conductivity towards the south) for explaining the observed induction pattern. The observed induction pattern at Andaman–Nicobar stations can be explained in terms of high conducting Cretaceous–Tertiary sediments filling the Andaman–Nicobar deep. High conductivity over Invisible bank has been attributed to the partial melts/volatile fluids derived from the subducting Indian plate that are intruding into the eastern margin of fore-arc basin through the West Andaman Fault (WAF). The induction pattern at Great Nicobar station (Campbell Bay) may be related to the highly conducting sediments filling the Mergui basin along with mafic intrusions. Also crustal transition occurs below the Mergui Terrace at the Malayan coast contributing to the enhanced conductivity anomaly.

Deep resistivity structure of the Trans-European Suture Zone in Central Poland

The deep resistivity structure was estimated along a 400-km profile in central Poland crossing the Malopolska Massif (MM), the Lysogory Unit (LU), the Trans-European Suture Zone (TESZ) and ending at the East European Craton (EEC). Magnetotelluric transfer functions, corresponding to 20 sites, were supplemented by magnetovariational responses obtained at the geomagnetic observatories situated at the same tectonic units. Such a combination made it possible to extend the initial period range, which is from fractions of a second to several hours, up to months in order to reliably cover crustal and upper-mantle depths. The geoelectrical structures, revealed using 2-D inversions, do not contradict the known features of the lithosphere structure determined using seismic and gravity data along the profile. The subsurface conductance, varying from approximately 10 Siemens at the inner part of the EEC to about 600 Siemens in the TESZ, is produced by sediments, the deep part of which contains conductive, highly mineralized water. The existence of two crustal conductive faults at the southwest and northeast of the TESZ were established mainly by the use of induction arrows. It was also revealed that rather high mantle conductivity beneath the MM, LU and TESZ at depths of about 150–200 km contrasts with the resistive upper mantle of the EEC. This can be interpreted as the decrease of asthenosphere conductance and/or as its submersion beneath the EEC. Generally, the results confirm the idea that the TESZ forms not only specific seismic boundaries but also causes peculiar conductivity anomalies in the crust and upper mantle.

Deep geoelectrical structure in and around the southern Korean Peninsula by GDS study

This paper presents lateral conductivity variations in the Earth's crust in and around the southern Korean Peninsula, which were mapped using magnetic field variations recorded at ten sites in Korea. GDS data can provide an efficient way to identify crustal units having different conductivities. And a three-dimensional (3-D) magnetotelluric (MT) modeling based on the induction arrows revealed a major conductivity anomaly in and around the Korean Peninsula. The comparison of observed and modeled induction arrows strongly suggests evidence of a relationship between main tectonic belts and the deep conductivity anomalies in this area. The overall pattern of induction arrows in this area appears to indicate a northwest-southeast direction, which is similar to that of the so-called ‘sea effect’, affected by the surrounding sea. However, the results of observations in the middle of the peninsula imply an anomalous pattern in the tectonic area near the Imjin River Belt. Induction arrows in the mid-southern area appear to be related to a thick sedimentary pile, the Ogcheon Belt, which forms another tectonic boundary in the Korean Peninsula. The overall pattern of arrows observed in the coastal area and islands was affected by the sea effect. The pattern, which shows southward arrows similar to those in the previous study of the southern coastline of the peninsula and Jeju Island near Kyushu, Japan, might indicate highly conductive anomalies (HCL) (Shimoizumi et al., 1997). However, the sea effect seems to be weak in the western coastal region, probably because the Yellow Sea is much shallower than the East Sea (Sea of Japan), and this phenomenon leads to the possibility of discerning an anomalous pattern beneath the Yellow Sea.

Electrical anisotropy of asthenosphere in a region of window to mantle underneath Eastern Indian Craton

We attempt to resolve electrical anisotropy in the mantle underneath the Eastern Indian Craton (EIC) (∼3.3 Gyr) with relatively short-period (<2000 s) magnetotelluric (MT) measurements. The phase-sensitive regional strike is frequency dependent. The average strike changes from N45°W for frequency >1 Hz to N25°W for frequency <0.1 Hz. This, along with negligible skew, suggests a two-dimensional (2-D) structure. A 2-D geoelectric model shows a rare example of highly resistive continental lower crust (CLC) of 1 S conductance instead of the generally observed highly conductive CLC. The presence of a resistive CLC provides unambiguous determination of the Moho. Two distinct phase-sensitive regional strikes for frequencies less than 0.1 Hz and >1 Hz indicate that the conductivity structure in the crust does not coincide with the mantle structure. This, along with the resistive CLC, provides a window to the mantle, helping in resolving the mantle structure. The phase data showing consistent phase splitting and negligible magnitude of induction arrow suggest an anisotropic mantle. The 1-D modeling shows that the anisotropic mantle is at a depth of about 175 km. The phase-sensitive strike at this depth deviates from the direction of the present-day Indian Plate (moving at a minimum speed of 3.7 ± 0.2 cm/year) by a minimum of 43°. This discrepancy could be due to either resistance to the mantle flow by the present-day plate motion or complex convection of mantle or the combined effect of these.

Geoelectric response of Archean lithosphere in the western Superior Province, central Canada

Magnetotelluric (MT) soundings were made at more than 230 sites in the western Superior Province, in central Canada, as part of the Lithoprobe Western Superior Transect investigations. The Superior Province forms the nucleus of the North American continent. Its western part contains a regional pattern of east–west trending 100–200 km wide granite–greenstone and metasedimentary belts or subprovinces. The objectives of the MT study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries, and to determine if there exists a signature in the upper mantle related to the formation of the craton. The exceptionally large MT database for the western Superior Province provides a three-dimensional sampling of the resistivity structure. In this paper, the primary MT responses from the data set, including geoelectric strike, apparent resistivity and phase, are used to define first-order geoelectrical structures in the region. Apparent resistivity responses indicate the ca 3.0 Ga North Caribou terrane contains extremely resistive crust. Crust to the south, accreted at ca. 2.7 Ga, is significantly more conductive, with resistivities similar to in the southeast Superior Province. The results suggest that the supracrustal component of the North Caribou terrane provides minimal enhancement of the crustal conductivity. At the southern margin of the former North Caribou superterrane, induction arrows define a conductivity anomaly that extends in an east–west direction for at least 600 km and causes a reversal of arrows in a region 100–200 km wide. This anomaly is interpreted to be caused by a conductive component of the 2.7 Ga metasedimentary rocks of the English River subprovince. The observations demonstrate the ability of electromagnetic methods to map some Neoarchean continental margins. In the southwest Superior Province, geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere. This result provides support to the interpretation that the lithospheric mantle of the western Superior Province has remained intact since the 2.7 Ga accretion of the subprovinces. The MT responses from the western Superior Province have thus defined a number of large-scale resistivity structures providing information on tectonic structure and history, and confirming the value of electromagnetic imaging of Archean lithosphere.

Electrical structure across a major ice‐covered fault belt in Northern Victoria Land (East Antarctica)

A Geomagnetic Depth Sounding profile was performed across the glaciated Rennick Graben and the adjacent fault‐bounded terranes of northern Victoria Land in East Antarctica. Induction arrows analysis and a 2D inversion model provide a unique deep electrical resistivity window beneath these fault zones. The electrical resistivity break across the Lanterman Fault is apparently restricted to the upper crust, suggesting that this strike‐slip fault may not represent a deep lithospheric suture. Further east, a westward‐dipping conductor is traced to a depth of 40 km beneath the Robertson Bay Terrane. It may image a remnant of the paleo‐Pacific oceanic plate, which subducted beneath the Bowers Terrane. Within the Wilson Terrane, the Rennick Graben is an upper‐crust resistive block. The Rennick Graben lacks a deep crustal or upper mantle conductor, in contrast to several continental rifts. However, similar resistive lower crust underlies some other major strike‐slip fault belts.

A high-resolution magnetotelluric survey of the Iapetus Suture Zone in southwest Scotland

SUMMARY Magnetic variation (MV) surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the Southern Uplands of Scotland from the North Sea to Galloway. A new magnetotelluric (MT) survey across the conductor in Galloway was designed to improve the resolution of structure in the depth range 1–15 km, and, at the same time, assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition, processing, analysis and modelling. A 40 km long profile was measured, doubled for part of its length at a separation of 1 km, with a typical site spacing of 2 km or less. It was supplemented by shorter parallel and orthogonal lines, giving a total of 40 sites. Galvanic distortion models involving regional 2-D structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites, confirming the picture from induction arrow maps that many sites were influenced by 3-D structures on scales of a few kilometres and upwards. Other indicators of the electrical strike (regional and local MV measurements, the spatial structure of the Groom–Bailey regional impedance phase and of the rotated off-diagonal phase), gave values more in agreement with the geological strike of N52°E, which was adopted as the most appropriate coordinate system into which to resolve the data. The MT impedances were accordingly rotated into directions N38°W (‘TM’) and N52°E (‘TE’), and inverted using the 2-D code of Rodi & Mackie. The investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters, and to different data subsets. The ‘TE’ mode data were insensitive to structural detail, and the best fit achieved was unacceptable, suggesting the influence of structure outside the plane of the section. The ‘TM’ mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies. Forward modelling showed that the model generated from the ‘TM’ mode also fitted the MV data adequately. The inversion resolved the conductive zone into distinct blocks, with edges matching faults mapped in the surface geology: the Leadhills, Fardingmullach and Orlock Bridge Faults, and the Moffat Valley lineament. In the resistivity image, the faults are vertical structures, disrupting other features to a depth of 15 km, which agrees with the view that, although they originated as thrusts, they were reactivated by strike-slip motion during oblique closure of the Iapetus Ocean. Within some of the blocks are highly conductive regions, no more than 4 km wide. The most likely explanation is graphite or other metallic mineralization, localized in shear zones.

Three-dimensional magnetotelluric modelling of the serra da cangalha impact crater, northeastern brazil

Three-dimensional (3-D) forward modeling results of a magnetotelluric (MT) survey carried out around the Serra da Cangalha impact crater located in northeastern region of Parnaiba basin Brazil are presented. The 13 km impact structure has been investigated because of its explicit 3-D shape and scientific interest. We recorded MT data in the period range between 0.001s and 1095s at 25 sites. The sites are aligned across the crater structure along three radial profiles (NE ? SW, NW ?SE and ENE ? WSW). The MT results at some sites are strongly affected by 3-D effects. A 3-D finite-difference forward modeling algorithm was applied in order to explain the observed data. The central part of the model covers an area of approximately 169 km2. The model of the study area, constructed using a 3-D mesh of 22 ´ 19 ´ 25 (29325 cells) was based on the a priori information of the geology of the region, and the results of the two-dimensional (2-D) inversion results. Eleven frequencies were used for the 3-D modelling from 0.01 to 1000 Hz. Induction vectors that show the conductivity distribution around the crater region were calculated for the 25 MT sites. The real induction arrows at short periods point towards the center of the crater and indicate the existence of a 3-D circular structure. The resistivity structure obtained suggests perturbations of the upper crust due to the meteorite impact; the base of the uplifted structure extending to 3.0 km depth was well delineated. The observed 3-D effects are explained as a shallow resistive ring that represents a local 3-D body embedded in a regional layered earth. The results confirm the importance of 3-D forward modeling in impact crater studies. The study provided insight into the post-impact electrical signature of the upper crust.

Crustal conductivity anomalies in central Sweden and southwestern Finland

Lateral conductivity variations in the Earth's crust in central Sweden and southwestern Finland were mapped using magnetic field variations recorded simultaneously by 29 Gough-Reitzel-type magnetometers in summer 1985. Three events of 3 or 4 h were digitized and analysed to obtain single-station induction arrows and interstation horizontal field transfer functions. The 1985 magnetometer array covered part of the BEAR (Baltic Electromagnetic Array Research) array operated in 1998 on the Fennoscandian Shield. Results from eight BEAR sites are used to control and to supplement the 1985 array results. The conductivity of the crust was found to increase in the western part of the array in western central Sweden. Crustal conductors were found both in the western part of the array and in the Baltic Sea area in the eastern part of the array. 3G-D modelling based on the induction arrows revealed a major conductivity anomaly in the middle and lower crust under the archipelago of Southwest Finland.

Integrated modeling of EM response functions from Peninsular India and Bay of Bengal

Existing sets of magnetovariational data from the large numbers of sites distributed across the peninsular India and those in the Bay of Bengal are reanalysed to obtain inter-site vertical and horizontal field transfer functions. Maps of induction arrows relocate the earlier reported conductive zones beneath the Palk-Strait and a regional scale anomaly in the offshore region, immediately southwest of the southern tip of Indian Peninsula, named South India Offshore Conductivity Anomaly (SIOCA). Period dependence of the induction arrows suggests that with increasing period SIOCA tends to control the induction pattern over the entire peninsula. Presentation of the horizontal transfer functions in the form of ellipses of anomalous currents helps to characterize the period and spatial behaviour of horizontal fields at seafloor sites. Integrated thin sheet modeling of the on-land and seafloor induction features suggest that the greater part of the anomalous behaviour of the horizontal fields at seafloor sites can be attributed to the shielding effects due to seawater. The weak anisotropic behaviour of the horizontal fields at selected sites can be explained in terms of the concentration of the induced currents in the sediment filled troughs on either side of the 85°E Ridge. Several lines of geophysical evidence favour the hypothesis that SIOCA, low velocity zone, low magnetization anomaly, all centered near the southern tip of the India, are the relics of the interaction of Marion Plume outburst with Indian lithosphere.

Feasibility study of the geoelectric structure of the Araguainha impact, Brazil

We have re-examined the modelling of magnetotelluric and GDS soundings carried out on the Araguainha Dome, Brazil, published a few years ago, in the light of a new modelling method and with particular stress on the induction of the vertical component of the magnetic field, which had been neglected in the early publications. By careful selection of the period range of the measured data, we obtain a set of radially symmetric induction arrows. This symmetry is taken as a first order assumption of the modelling scheme. The result is an improvement of the resolution of the radial distribution of depths to the crater basement. Due to the poor MT site coverage of the crater area, this work must be considered a feasibility study rather than a final modelling contribution.

Effects of anisotropy on the two-dimensional inversion procedure

Summary In this paper we show some of the effects that appear in magnetotelluric measurements over 2-D anisotropic structures, and propose a procedure to recover the anisotropy using 2-D inversion algorithms for isotropic models. First, we see how anisotropy affects the usual interpretation steps: dimensionality analysis and 2-D inversion. Two models containing general 2-D azimuthal anisotropic features were chosen to illustrate this approach: an anisotropic block and an anisotropic layer, both forming part of general 2-D models. In addition, a third model with dipping anisotropy was studied. For each model we examined the influence of various anisotropy strikes and resistivity contrasts on the dimensionality analysis and on the behaviour of the induction arrows. We found that, when the anisotropy ratio is higher than five, even if the strike is frequency-dependent it is possible to decide on a direction close to the direction of anisotropy. Then, if the data are rotated to this angle, a 2-D inversion reproduces the anisotropy reasonably well by means of macro-anisotropy. This strategy was tested on field data where anisotropy had been previously recognized.

Geomagnetic coast and other effects deduced from the new observatory at Visakhapatnam, India

Summary Coastal effects observed in the long-period Sq Solar quist day ranges and short-period changes over 30 min intervals during magnetic substorms (bays), as recorded at a new magnetic observatory at Visakhapatnam, on the east coast of India, during 1995 in the H, Z and D elements, are analysed and the preliminary results are discussed. The results are compared with those from Alibag and Hyderabad (located more or less at the same latitude) based on simultaneous 1995 records. Short-period induction vectors are computed and the arrows presented and discussed at the three locations. While the enhancement associated with close proximity to the coast in Sq (Z) at Alibag on the west coast compared to the central inland station of Hyderabad (enhancement of 17–37 per cent) is quite prominent in all three seasons of the year, at Visakhapatnam it is only marginally observed during the d-season. At bay frequencies, medium-sized induction arrows point towards the nearest deep-sea conductive body at both Alibag and Visakhapatnam, while it is towards the southeast with a considerably reduced magnitude at Hyderabad. These induction arrows and the effects on Sq (Z), Sq (H) and Sq (D) ranges are discussed in conjunction with the Parkinson arrows and Sq (Z), Sq (H) and Sq (D) ranges from Margao (west coast)–Dharwar–Bellary–Mahanandi–Chirala (east coast) at the latitude of the 16°N profile of an earlier array experiment, and the two are interpreted together.

Geomagnetic depth sounding in the Northern Apennines (Italy)

A Geomagnetic Depth Sounding (GDS) survey covering the Northern Apennines of Italy has been carried out in the period 1992–94. Induction arrows maps and hypothetical event Fourier maps were constructed to obtain an electromagnetic imaging of this area. Since the two dimensional (2-D) character comes out from those maps for periods greater than 32 minutes, a 2-D inverse modeling was carried out. The model responses show that a deep conductive layer (>5000 S) underlies the Apennine chain at about 20 km depth. The transition between the Adriatic and the Tyrrhenian domains is marked by a rather sharp vertical offset in this conducting layer. In the northwest sector of the studied area an anomalous high conductivity behavior is superimposed on the regional trend, which corresponds to the geothermal field of Larderello-Travale.

Magnetotelluric and geomagnetic depth soundings around the Torres Syncline Hinge, southeast Paraná Basin, Brazil

A MT/GDS survey has been carried out around the NW plunging Torres Syncline in the southeastern border of the Paraná Basin, in Brazil. Induction arrows show a NW‐trending conductive anomaly nearly coincident with the hinge of the syncline and ocean effects varying as a function of the distance from the coast. 2‐D inversions of MT data along two profiles parallel to the coast and across this anomaly have distinguished electrical anomalies in the upper crust. The most prominent of these anomalies is an electrical conduit dipping to the NW from a depth of 500 m at the coast to 1000 m at a distance of about 100 km further inland. The conductor, located in the crystalline basement, can be possibly ascribed to residual metasomatic fluids concentrated along the hinge of the downfolded Torres Syncline, as a fossil record of the Early Cretaceous volcanic events that affected the Paraná Basin and underlying lithosphere.

MAPPING OF HIGH ELECTRICAL CONDUCTIVITY ON THE TORRES SYNCLINE HINGE, SOUTHEASTERN PARANÁ BASIN

A preliminary interpretation of the electrical structures beneath the SE Paraná Basin is presented based on electromagnetic geophysical soundings carried out around the Torres Syncline. GDS induction arrows show a NW-trending conductive anomaly nearly coincident with the hinge of the syncline and strong "ocean effects" observed at longer periods in sites near the coastline. 1 -D inversions of MT data at sites located inside this anomaly have distinguished a large electrical conduit, dipping to the NW from a depth of 500 m at the coast to 1000 m at a distance of about 100 km further inland. The conductor, located in the crystalline basement, could be possibly ascribed to residual metassomatic fluids concentrated along a fractured hinge of the downfolded Torres Syncline, as a fossil record of past magmatic activities in the Early Cretaceous that affected the Paraná Basin and underlying lithosphere.

Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Magnetovariational (MV) data from 14 sites on a transect of the South Island of New Zealand are presented. The induction arrows clearly show the effects of induction in the surrounding oceans. Comparison with the responses of an analogue model of the effect of the oceans on induction in the New Zealand region indicates that the induction arrows also contain information on anomalous on-land conductivity structure. The use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two- and three-dimensional numerical models pertinent to the New Zealand situation. Difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal, in both real and quadrature arrows, across the South Island. Two-dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the Southern Alps of an anomalously conducting region previously identified by magnetotelluric (MT) sounding. Use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation.

Analogue model study of EM induction in elongated conductors—2D and 3D induction arrow responses

Magnetic induction arrow responses of elongated conductors embedded in a uniformly resistive host earth and underlain by a conductive substratum are studied with the aid of laboratory analogue model measurements. The dependence on period, the overburden depth, and the underlying conductive basement depth, are examined for three different conductor lengths that were chosen so as to include the transition in the responses from two dimensional (2D) to those of three dimensional (3D) structures. The characteristic period T c (the period at which the in-phase anomalous response of the elongated conductor model is maximum and the quadrature response is minimum) is found to depend on the conductor length, the overburden depth, and the conductive basement depth. A decrease in conductor length or conductive basement depth, each have the effect of decreasing both the response amplitude and the characteristic period, while increasing the overburden depth has the effect of increasing the characteristic period and decreasing the response amplitude. At short periods, say below the characteristic period, the responses appear to be roughly 2D, and to become progressively more 3D-like with increasing period above the characteristic period.

Induction arrows from offshore floating magnetometers using land reference data

SUMMARY Induction arrows are a traditional output of magnetovariational experiments, and represent transfer functions used quantitatively in the inversion of geomagnetic depth sounding data to give Earth electrical conductivity structure. In this paper, a technique is tested in which ‘total-¢eld’ variations are combined with horizontal-¢eld data recorded simultaneously at remote stations in order to derive induction arrows. The method is ¢rst demonstrated using total-¢eld data recorded on land by an aeromagnetic base station, and then applied to data obtained from magnetometers £oated oishore on the sea surface. The £oating magnetometers were deployed in two con¢gurations: one tethered to the sea£oor; the second free-£oating.

Seafloor electromagnetic induction studies in the Bay of Bengal

Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85° E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85° E Ridge along 14° N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across 85° E Ridge along 12° N latitude. Here we used the data from Annamalainagar Magnetic Obervatory (ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9° N latitude with land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding (VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 Ωm and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 Ωm) followed by a very low resistive (0.1–50 Ωm) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features such as deep-sea fans of Bay of Bengal, the resistive 85° E Ridge and the sea water column above the seafloor stations. VGS and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal.