MT Survey Research Articles

New MT surveys and 3D resistivity imaging beneath the Ngozi-Rungwe volcanoes at the triple rift junction of the East African Rift System in SW Tanzania: Support for integrated interpretations of geothermal conceptual models

The geothermal system related to the Ngozi and Rungwe volcanoes, SW Tanzania, lies at the intersection of the west and east branches of the East African Rift System and has been investigated by many geoscientists for decades. Here we present a 3D electrical resistivity model based on 190 magnetotelluric resistivity soundings that have been integrated with geochemical and geological results to support the development of the geothermal resource conceptual model presented here. The model includes two separate reservoirs, a larger system located beneath the Rungwe volcano and a smaller chloride water reservoir located under the Ngozi caldera, which contains a neutral chloride hot spring with geothermometry >230°C. An extensive conductive clay cap with variable thickness extends along the 30 km long NW-SE trending Ngozi-Rungwe Fault Zone from the Kiejo area SE of the Rungwe volcano to the Ngozi caldera. The absence of geothermal surface manifestations directly over the inferred Rungwe upflow zone is consistent with effective sealing of the proposed underlying geothermal reservoir by the clay cap. The scarcity of thermal manifestations on the up-dip margins of the low-resistivity clay cap can be explained by coincidence of the base of the clay cap with impermeable Precambrian formations and by structural boundaries. This interpretation implies that the area with the highest geothermal resource potential is the Rungwe volcano where proposed drilling sites might intersect the proposed high-temperature reservoir.

3D structural model and slip-dilation tendency analysis of the Chiweta Zone: Geothermal system implications and fault reactivation potential

A 3D structural model and slip-dilation tendency analysis are used to assess the reactivation potential of shear and dilatational structures to investigate structurally controlled geothermal systems in the Chiweta Zone. The Chiweta Zone is in the northern part of the Malawi Rifted Zone, where an elevated heat flow anomaly and one of the hottest hot springs occur. Structurally controlled geothermal systems are poorly understood in Malawi and the entire Western Branch of the East African Rift System. The objectives of this study were to (1) construct a three-dimensional structural geometry of the faults identified with remote sensing and aeromagnetic data, (2) perform a slip-dilation tendency analysis to assess the fault reactivation potential under a stress state condition at depths between ∼700 and 1000 m.b.g.l. of the estimated reservoir, (3) identify, based on the results, which faults are critically stressed and serve as fluid conduits or barriers of the geothermal system, and (4) combine geological, geochemical, and geophysical data with our results to improve the conceptual model of the Chiweta Geothermal System (CGS). Our findings indicate that ∼ NW- and ∼NNE-striking faults southwest- and northwest dipping, respectively, show segments with the highest dilation values of Td ∼0.6 to Td ∼0.8 and probably behave as active fluid pathways. This is supported by the Chiweta Fault, a NW-striking and southwest-dipping fault that controls the only thermal manifestations in the Chiweta Zone. NW- and NE-striking faults dipping to the northeast and southeast, respectively, show lower dilation values of Td ∼0.1 to Td ∼0.5 and probably act as fluid barriers. These results allowed us to define, for the first time, possible structural boundaries of the Chiweta geothermal reservoir and to improve its geothermal conceptual model. The results of this study may help identify favorable areas to target the first exploratory wells and where further subsurface studies (e.g., MT surveys and seismic reflection) should be conducted in the Chiweta Zone. Finally, our methodology is expected to be applied to other geothermal prospects in the Western Branch of the East African Rift System and help advance geothermal energy production in this region.

Crustal Electrical Structure and Deep Metallogenic Potential in Northern Wuyi Area (South China), based on Magnetotelluric Data

AbstractThe northern Wuyi area, which is located in the northern Wuyi metallogenic belt, has superior mineralization conditions. The Pingxiang–Guangfeng–Jiangshan–Shaoxing fault (PSF) extends across the whole region regardless of whether or how the PSF relates to the near‐surface mineralization. We carried out an MT survey in the region and obtained a reliable 2D model of the crustal electrical structure to a depth of 30 km. In the resistivity model, we inferred that a continuous high conductivity belt that ranges from the shallow to deep crust is a part of the PSF. Then, we estimated the fluid content and pressure gradient to identify the deep sources of fluid as well as its pattern of motion pattern. Finally, we proposed a model for the deep metallogenic migration processes that combines geological data, fluid content data, pressure gradient data, and the subsurface resistivity model. The model analysis showed that the Jiangnan orogenic belt and the Cathaysia block formed the PSF during the process of com. The deep fluid migrated upward through the PSF to the shallow crust. Therefore, we believe that the PSF is an ore‐forming fluid migration channel and that it laid the material basis for large‐scale mineralization in the shallow crust.

Forward modeling of magnetotellurics using Comsol Multiphysics

Magnetotellurics is an electromagnetic geophysical method that has been widely used to study structures in Earth's subsurface. Numerical modeling of magnetotellurics is important for survey design, inversion, geological interpretation and many other aspects of geophysical studies. For example, modeling a subsurface conductive body in terms of its conductivity, geometry and dipping angle would yield substantial information on the phase response and sensitivity in an MT survey. While there are many different modeling techniques, the finite element method is most commonly used. In this effort, we present magnetotelluric models of layered Earth, uplift structures, auroral electrojets, and geomagnetically induced currents in power-line skywires using the commercial finite-element package Comsol Multiphysics. The AC/DC module in Comsol can be used to solve Maxwell's equations in the quasi-static limit for modeling the magnetotelluric response. One of the advantages of Comsol modeling is its Graphical User Interface (GUI), which allows users to solve complex single or multi-physics problems in a meshed domain. The use of Comsol also reduces the need for sophisticated computer coding when solving partial differential equations such as Maxwell's equations. In the effort presented here, we first discuss model validation for layered Earth geometries. We then present two examples of magnetotellurics modeling in impact crater and geomagnetically induced current studies. Numerical results were compared with analytical solutions or benchmark results whenever possible.

Lithospheric structure of the western Borborema Province from receiver functions and surface-wave dispersion: Implications for basin inversion

The Proterozoic Borborema province of NE Brazil was under extensional stresses during the Mesozoic, causing stretching and thinning of the lithosphere. During Cenozoic times, compression associated with the development of the South Atlantic and collision along the Andean front dominated throughout the Province. One of the main consequences of such compression might have been the formation of the Chapada de Araripe, an inverted rift basin located in the Western Province. We undertook a passive seismic experiment along a NS profile crossing the Borborema province and a short EW profile across the Chapada de Araripe, with the goal of providing constraints on the origin of the Chapada de Araripe. A variety of mechanisms can contribute to basin uplift, so understinding basin inversion is in turn important for understanding the tectonic evolution and geodynamics of NE Brazil. Through joint inversion of receiver functions and surface-wave dispersion, we show that the crust is typically 34–38 km thick under the western Province; in contrast, the crust is 43–46 km thick under the northern craton. No apparent crustal thinning is observed under the Araripe Basin, due to the presence of a relatively thick (10–14 km) high-velocity (4.1–4.3 km/s) lowermost crust. Our velocity models also image a drop in S-velocity (from 4.6 km/s to 4.3 km/s) at 135–145 km depth under the southern half of the profile, which we interpret as a mid-lithospheric discontinuity, and at 120–140 km depth under the Araripe Basin, likely representing the lithosphere-asthenosphere boundary. Our findings confirm the presence of thin lithosphere under the Araripe Basin - originally postulated by a collocated MT survey - that might have facilitated tectonic inversion of this basin through regional stress concentration and thermal buoyancy. These findings also suggest additional buoyancy contributed by a mafic lowermost crust right under the basin.

Geological Survey of Queensland: Newly released airborne data and MT survey in the North West Mineral Province

Geological Survey of Queensland: Newly released airborne data and MT survey in the North West Mineral Province

Detecting the Fingers of God: Optimising magnetotelluric survey design for mineral exploration

SummaryMagnetotellurics is a versatile exploration technique that can be used for imaging across scale lengths of tens to hundreds of metres, down to tens to hundreds of kilometres. Only in recent years has the potential of MT as an exploration tool been fully realised across varying depth scales. The optimum design of an MT survey for imaging the full mineral system has not yet been studied in detail, leaving questions such as, could the famous ‘Fingers of God’ conductivity anomalies that point to Olympic Dam, Wirrda Well and Vulcan deposits be resolved with larger site spacing or with a narrower period range? What about with an array instead of a transect? These questions are addressed using synthetic studies that take known conductivity anomalies that lead to mineral deposits, and trialling different survey layouts, period ranges and cover thickness to determine the ideal survey layout to make sure these features are not missed, whilst keeping survey acquisition expenditure and time to a minimum.

Monitoring geothermal reservoir developments with the Controlled-Source Electro-Magnetic method — A calibration study on the Reykjanes geothermal field

Surface geophysical monitoring techniques are important tools for geothermal reservoir management as they provide unique information on the reservoir development away from boreholes. For magmatic environments, electromagnetic (EM) methods are attractive monitoring tools as they allow to characterize the reservoir and hence potentially monitor changes related to fluid injection/production. Indeed, the electrical resistivity of reservoir rocks is highly dependent on the volume, chemistry and phase of the in-situ geothermal brine (e.g. liquid, vapor, supercritical).Passive EM techniques (e.g. magnetotellurics or MT) are traditionally used for geothermal exploration and a few recent studies have demonstrated its potential for monitoring reservoir development. One of the main challenges is though the presence of cultural noise and/or variability of the Earth magnetic field that can obfuscate the EM signals of interest. We have investigated the benefits and drawbacks of active EM surveying (Controlled-Source EM or CSEM) to tackle this challenge, first with a synthetic study and subsequently with an actual time-lapse survey acquired in 2016 and 2017 over the Reykjanes geothermal field in Iceland before (baseline) and after (monitor) the thermal stimulation of the supercritical RN-15/IDDP-2 geothermal well.The synthetic study showed that for geothermal fields having a resistivity structure similar to the Reykjanes field (i.e. a conductive caprock overlying a more resistive higher temperature reservoir), CSEM and MT measurements can both detect resistivity changes within the deep resistive reservoir, provided that measurement errors are small. Variations in many survey parameters (e.g. errors in receiver position/orientation, differences in recording devices, variations of near surface conditions, external noise) can create significant time-lapse CSEM measurement errors. Our actual time-lapse survey showed that when similar CSEM equipment is used during the baseline and monitor surveys and systematically d-GPS positioned, the remaining key parameter controlling the survey repeatability is the level of external noise. Since the influence of external noise on CSEM data can be artificially reduced (e.g. by increasing the transmitter dipolar moment), it offers the possibility to adapt the survey design to increase the chance of detecting the time-lapse signals of interest. On the contrary, little control is possible on the MT signal to noise ratio and hence repeatability.The time-lapse EM survey acquired over the Reykjanes geothermal reservoir showed indeed that a high CSEM survey repeatability can be achieved with electric field measurements (within a few percent) but that time-lapse MT survey is a challenging task because of the high level of cultural noise in this industrialized environment. To assess the quality of our CSEM dataset, we inverted the data and confronted the resulting resistivity model with the resistivity logged in the RN-15/IDDP-2 well. We obtained a good match up to 2-3km depth, i.e. enough to image the caprock and the liquid-dominated reservoir but not deep enough to image the reservoir in supercritical conditions. To obtain such an image, we had to jointly invert legacy MT data with our CSEM data. On the monitoring aspects, the analysis of changes in electric fields did not allow to identify any CSEM signal related to the thermal stimulation of the RN-15/IDDP-2 well. One possible explanation is the weakness of the time-lapse CSEM signal compared the achieved CSEM survey repeatability as a result of a limited resistivity change over a limited volume within the reservoir. Future reservoir developments in the supercritical reservoir (e.g. hydraulic stimulation, long-term fluid circulation) will most likely generate stronger resistivity variations over a larger volume than during the thermal stimulation of the well. This calibration study provides the basic information for deciding when and how an EM monitor survey must be performed for the monitoring of the Reykjanes geothermal reservoir but also for the definition of the monitoring strategy of similar high-enthalpy geothermal reservoirs.

3D MODEL OF THE DEEP STRUCTURE OF THE SVECOFENNIAN ACCRETIONARY OROGEN: A GEODYNAMIC INTERPRETATION

The results of this study are reported in two related successive publications. The article “3D model of the deep structure of the Svecofennian accretionary orogen based on data from CDP seismic reflection method, MT sounding and density modeling” (M. V. Mints, E. Yu. Sokolova, LADOGA Working Group) presented a 3D model of the deep structure of the Late Paleoproterozoic Svecofennian accretionary orogen. The model is based on harmonized data of geological mapping, FIRE-2-2a-1 and FIRE-3-3a seismic reflection profiling, using sections of the 3D crustal density model, results of magnetotelluric surveys along the Vyborg-Suojarvi profile in the Northern Ladoga area and materials from MT surveys previously carried out by Finnish specialists in Southern Finland. In this paper these results were used as the baseline for the construction of the evolutionary model and determination of the position of the Svecofennian orogen in the structure and history of the Paleoproterozoic Lauro-Russian intracontinental orogen. This Paleoproterozoic orogeny swept the predominant part of Lauroscandia, a paleocontinent that united the North American and the East European cratons. Interpretation of the structural features and evolution of the Lauro-Russian orogen leads to the conclusion that its onset and evolution had a causative relation to the development of the Paleoproterozoic superplume which, in turn, initiated plate tectonic events. The evolution of the Lauro-Russian orogen included three intensive development periods (~2.5; 2.2–2.1 and 1.95–1.87 Ga). In the axial zone of the intracontinental orogen there appeared the Svecofennian Pre-Labrador Ocean, and the accretionary Svecofennian and Pre-Labrador orogens formed along its continental margins after its closure.

3D MODEL OF THE DEEP STRUCTURE OF THE SVECOFENNIAN ACCRETIONARY OROGEN BASED ON DATA FROM CDP SEISMIC REFLECTION METHOD, MT SOUNDING AND DENSITY MODELING

A 3D model of the deep structure of the Late Paleoproterozoic Svecofennian accretionary orogen is presented. The model is based on coordinated data of geological mapping, seismic reflection surveys along the profiles FIRE-2-2a-1 and FIRE-3-3a, incorporates and utilizes cross-sections of the 3D density crustal model, the results of magnetotelluric surveys along the Vyborg-Suojarvi profile in the Northern Ladoga area and materials from MT surveys previously carried out in Southern Finland. The main structural features of the Svecofennian accretionary orogen were described relying on the 3D model. The accretionary complex is characterized by dipping tectonic layering: tectonic sheets some 15 km thick, made up of volcanic-sedimentary associations with electrically conductive graphite-bearing sediments and of granitoid rocks, plunge successively eastwards in a monotonous manner. At the lower crustal level the tectonic sheets of the accretionary complex become fuzzy, appearing in seismic reflection images as a homogenous acoustically semi-transparent medium where only fragments of the margins of some sheets can be discerned. The crust-mantle boundary is diffuse: the crust to mantle interface is determined by the gradual vanishing of the fuzzy boundaries of tectonic sheets and a gradational transition from the acoustically homogenized and semi-transparent lower crust to the transparent mantle. The ‘blurring’ of the images of rock contacts, which had originally produced acoustic impedance contrasts, is caused by melting and partial fusion. The analysis of the 3D model of the deep structure has brought about some new conclusions about the principal features and sources of the evolution of the Svecofennian accretionary orogen. A model of the onset and evolution of Svecofennian orogeny, which is considered as one of the components of the supercontinent Lauroscandia, is described in the paper by M. V. Mints “3D model of the deep structure of the Svecofennian accretionary orogen: a geodynamic interpretation”, which constitutes the second part of this study.

Metamorphic geothermal reservoir characterisation—A case study in the Jinlun geothermal area, Taitung, Taiwan

There are more than one hundred hot spring outcrops around the island of Taiwan, which implies that Taiwan is rich in geothermal resources; however, no commercial geothermal power plants have been established since 1993, the year of the abandonment of a 3 MW geothermal plant in Chingshui. Situated at the oblique collision zone between Eurasia and Philippine Sea plates, most of Taiwan’s hot springs are located in metamorphic terrain rather than volcanic areas. Previous study showed that geothermal resource is also rich in Jinlun area, whereas structural complexity and low permeability pose difficulties to geothermal exploitation. This paper aims to describe a geothermal conceptual model to facilitate the exploitation of the Jinlun geothermal field by integrating results from gravity, temperature surveys, surface geology, and information from recently conducted magnetotellurics survey and a new borehole. Results from the MT survey indicate a deep resistive structure down to 5 km depth. A zone of low resistivity (<5 ohm-m), which coincides with elevated temperatures observed in the borehole, could be attributed to thrust faulting. Based on 14C dating data, sources of geothermal brine could be connate water or meteoric water that travelled for a long distance. The main reservoir could be located at the fractured regions instead of porous strata.

DGPS AND MAGNETOTELLURIC CONSTRAINTS ON THE CONTEMPORARY TECTONICS OF THE SANTORINI VOLCANIC COMPLEX, GREECE

The study of vertical and horizontal crustal movements at the Santorini Volcanic Complex (SVC), as deduced by Differential GPS measurements revealed that an intricate pattern of five distinct domains with different horizontal kinematics: The West SVC (Akrotiri peninsula and Therassia) with very significant NNW-ward motion, North Thera with rather significant NW-ward motion, East Thera (Monolithos), with significant SE-ward motion, South Thera with significant NW-ward motion and, finally, central Thera with small westward motion. An apparently dextral, NNW-SSE oblique-to-strike-slip fault emerges as a prominent tectonic structure, separating the West SVC from the rest of the complex; this is the “Santorini Fault Zone”. Additional insight is afforded by the results of MT and GDS surveys: a significant NNW-SSE conductive zone was detected, which is collocated with the purported NNW-SSE fault zone indicated by DGPS analysis and may be explained as an epiphenomenal conductivity anomaly. The observed deformation pattern enables the drafting of a qualitative model of contemporary tectonics, which is also presented and discussed. The model is plausible but certainly incomplete and pending verification with numerical modelling and additional observations

Geophysical Responses from Mineral System Components in the Deep Crust and Upper Mantle

The concept of a mineral deposit forming via a mineral system that operates across areas of perhaps 1000s of squares kilometres and to mantle depths has important implications for greenfields mineral exploration. Geographically widespread datasets and deep penetrating geophysical methods are required to map key mineral system elements such as fluid/metal source zones and migration paths. Developed primarily for academic studies of the deep crust, there are several established geophysical techniques that can potentially be used to identify elements of mineral systems in the deep crust and upper mantle. Although the seismic reflection method produces the highest quality images, it is prohibitively expensive and the recommended approach is a combination of MT surveys and receiver function recordings with CCP stacking. Mineral system elements that can be detected in this fashion include major structures and geological boundaries which are potential controls on fluid flow and also areas of crust and mantle that have been altered by one or both of fluid creation and migration.

VLF -MT Survey around Nakadake crater at Aso Volcano

Shallow resistivity structure around Nakadake craters has been revealed by extensive VLF- MT survey from August to October 2006. This structure comprises a varied resistivity value at the upper and bottom layer of the western flank of craters; a conductive upper layer at the fourth Crater; and a resistive upper layer at southern part of craters. The conductive value of second layer at a line in the western flank and southern part of craters at several ten meter depth, interpreted as water saturated, and high resistivity at second line of western flank of craters as sheet of lava and at fourth crater is interpreted as lava rock. At western flank of craters, we found the resistivity value and the depth of second layer decrease in depth with decreasing of distance to craters line.

A comparison of survey methods to evaluate macrophyte index of biotic integrity performance in Minnesota lakes

Aquatic macrophytes shape trophic web dynamics, provide food and refuge for macroinvertebrates and fish, and increase nutrient retention, sediment stabilization, and water clarity. Macrophytes are well-suited as indicators of ecological health because they are immobile, relatively easy to sample and identify, and respond to anthropogenic disturbance on an ecological time scale. Aquatic plant monitoring programs can provide valuable information to water resource managers, especially in conjunction with macrophyte-based indices of biotic integrity (IBI). However, there are several current sampling designs and the precision of IBI scores has not been evaluated across different surveys. We evaluated the performance of the Minnesota macrophyte-based IBI for two survey designs; a point intercept (PI) survey and a belt transect (BT) survey. PI surveys are time intensive, especially on large lakes, whereas BT are less time intensive and have been used historically in Minnesota. Our objectives were to compare the PI surveys with BT surveys on the same lakes, and to modify the BT survey (MT survey) to improve information obtained from BT surveys. BT surveys consistently overestimated IBI scores compared to the PI method (t=6.268, df=60, p<0.001). Overall IBI scores calculated from MT surveys differed significantly from PI scores, but on average, MT surveys predicted scores only 3% lower than PI scores. Implementation of the Minnesota macrophyte-based IBI through the adoption of the MT survey approach would improve sampling efficiency and enable widespread documentation of the effects of landscape change, shifts in hydrologic regimes, and other anthropogenic activities on the integrity of lacustrine systems.

Two-dimensional resistivity structure of Unzen Volcano revealed by AMT and MT surveys

AMT and MT surveys were conducted to investigate at high resolution the spatial resistivity structure of Unzen volcano, with consideration given to understanding its regional dimensionality. Our phase tensor analysis supports the conclusion that the resistivity structure is two-dimensional, with the strike in the E-W direction. Two-dimensional inversions suggest that Unzen volcano is likely to comprise 4 layers: a high resistivity surface (greater than 1000 Ω m), an intermediate second layer (20 to several hundreds of Ωm), a low resistivity third layer (less than 20 Ω m), and a relatively high resistivity basement. We assume the upper-most high resistivity layer consists of undersaturated lava and pyroclastic flow deposits. The second and third layers are likely to be water-saturated and form an aquifer that seems to correlate well with the emergence of groundwater discharge at the surface. In deeper areas beneath the summit, a region with a resistivity of 20–80 Ω m is surrounded by areas of extremely low resistivity (less than 3 Ω m); this structural features in Unzen volcano was first identified in this study, but is typical of the resistivity structure observed in active volcanoes. Interpreting the results of well logs and geodetic studies of Unzen volcano in light of the findings of the present study and the resistivity structure of other active volcanoes, we suggest that Unzen volcano possesses a hydrothermal system of high-temperature fluids beneath its edifice; this hydrothermal system may play a non-negligible role in controlling heat and mass transfer in the magmatic system of Unzen volcano.

Preparation for drilling well IDDP-2 at Reykjanes

Preparation has begun for drilling the second deep IDDP well into the saline Reykjanes high-temperature field in SW-Iceland. The site selection for the IDDP-2 drillhole is under review and the prime candidate is essentially the same as the 1st priority site suggested for the Reykjanes field in 2003. More recent drillhole data and new MT surveys have amplified the justification for selecting that site. Deep drilling to 4–5km depth is an important part of the HS Orka exploration strategy for enhanced power production, either by direct use of high energy steam, or by attempting to enhance the field performance by re-injecting geothermal fluid deep into very hot rocks. Pending on several decisions and development in Iceland, outside the control of the IDDP energy consortium, the IDDP-2 well might possibly be drilled to 4–5km depth as early as 2014.

Full 3D Acquisition and Modelling with the Quantec 3D System - The Hidden Hill Deposit Case Study

This abstract will detail Quantec?s 3D system, a proprietary 3D DC/IP and MT acquisition developed by Quantec Geoscience Ltd., Canada by means of a case study over the Hidden Hill Deposit in central Nevada, USA. An integrated data acquisition, modelling and interpretation of the DC, IP, and MT surveys over a complex geological setting embedding a known epithermal gold and silver-bearing deposit are presented. The survey and interpretation are based on a true 3D acquisition and inversion of the geophysical data. The results show a very close correlation between the geophysical resistivity and chargeability signatures and the location of the known mineralization. The extremely large data volume collected in an omni-directional current source proved to be essential for a detailed and reliable imaging of the subsurface.

Electrical anisotropy in the Main Central Thrust Zone of the Sikkim Himalaya: Inference from anomalous MT phase

The Sikkim Himalaya in the eastern sector of the Himalayan collision belt shows some distinct features, such as the occurrence of the Main Central Thrust as a several-km-wide zone (MCTZ) of intense ductile deformation and its deviation from the general east–west strike of the Himalayan thrusts, suggesting that the tectonics of this region could be different from the regional Himalayan tectonics. We have carried out a MT survey across MCTZ to understand the tectonics of the region. The broadband MT data of the site at the northern boundary of the zone reveals anomalous phase exceeding 90° for frequencies smaller than 1Hz whereas other sites further north of this zone show normal behavior. Such anomalous phase behavior has been explained earlier in terms of anisotropy in the medium and special configuration of local conductors. We have analyzed the observed anomalous phase behavior in terms of anisotropy based on geological consideration that alternating bands of graphitic schists are present within high-grade crystalline gneisses at the northern boundary of MCTZ. Among various models analyzed, a block model of the crustal structure with strong anisotropy contrast between the block and the underlying anisotropic layer provides good fit to the observed data. We, thus, infer that the presence of a localized strongly anisotropic block at the northern boundary of MCTZ could be a viable explanation for the observed anomalous phase behavior.

MT and reflection seismics in northwestern Skellefte Ore District, Sweden

A seismic reflection and MT survey was carried out along a 27-km long transect in northwestern Skellefte District, as part of a bigger 3D modeling project. The main motivation for the data acquisition is to elucidate the geologic relationship between the known mineralizations in the Adak mining camp to the north and in the well studied Kristineberg area south of the transect. The seismic reflection data were acquired with a VIBSIST system, and show reflectivity down to 3 s. Apart from the conventional processing for crystalline environments, the seismic data was also subject to an azimuthal binning procedure and cross-dip analysis, allowing the orientation of planar reflectors in 3D. Regarding the MT data, it is primarily of good quality along the 17 installed sites. The inversion of the determinant of the impedance tensor yielded a stable 2D resistivity model, dominated by resistors corresponding to the postorogenic intrusions along the transect. Adding the location of the analyzed seismic reflectors in the MT inversion rendered an integrated model that facilitated a preliminary joint interpretation of the data sets. Overall, the results are in good agreement with surface observations and reveal a crude configuration of the geologic units below the transect. The most prominent outcomes are the lateral and depth extent of the large postorogenic intrusions in the area reaching to 5- or 6-km depth, the dimensions of the nearly vertical Brännäs gabbro extending to 6-km depth, and the presence of enhanced conductivities along the transect at about 10 km depth. The latter is probably related to the deep conductor previously identified in the district.