High Conductivity Zone Research Articles

Detection of the deep crustal structure of the Qiangtang terrane using magnetotelluric imaging

To determine the deep electrical structure of the Qiangtang terrane, northern Tibetan Plateau, we reanalysed three broadband magnetotelluric (MT) profiles collected by China University of Geosciences (Wuhan) in 1993–1994 and derived corresponding 2-D resistivity models. In these models, a remarkable high-conductivity layer that was divided into northern and southern parts and that had formed an antiformal zone of high conductivity beneath the central Qiangtang terrane was visible. This high-conductivity layer corresponds well with the trace of the southward subduction of the mélange from the Jinsha River suture inferred from surface geology. According to our resistivity models, the crust of the northern Qiangtang terrane is mostly composed of mélange. The underplated mélange may be closely related to the formation of the high-conductivity layer and provide a reasonable explanation for the cause of special geophysical features in this area. In addition, the similarity between these resistivity models provides evidence that the Qiangtang anticlinorium extends eastward to at least 92°E, whereas the differences between them may offer an explanation for the gradual narrowing of the metamorphic belt from west to east in the central Qiangtang terrane.

Crustal structure of an intraplate thrust belt: The Iberian Chain revealed by wide-angle seismic, magnetotelluric soundings and gravity data

The Iberian Chain is a Cenozoic intraplate thrust belt located within the Iberian plate. Unlike other belts in the Iberia Peninsula, the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure. The Iberian Chain crust was investigated by means of a NE-SW refraction/wide-angle reflection seismic transect and two magnetotelluric profiles across the chain, oriented along the same direction. The seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile. The resulting velocity–depth model shows a moderate thickening of the crust toward the central part of the profile, where crustal thickness reaches values above 40km, thinning toward de SW Tajo and NE Ebro foreland basins. The crustal thickening is concentrated in the upper crust. The seismic results are in overall agreement with regional trends of Bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling. The magnetotelluric data consist of 39 sites grouped into two profiles, with periods ranging from 0.01s to 1000s. Dimensionality analyses show significant 3D effects in the resistivity structure and therefore we carried out a joint 3D inversion of the full impedance tensor and magnetic transfer functions. The Mesozoic and Cenozoic basins along the Chain are well characterized by shallow high conductive zones and low velocities. Elongated conductors reaching mid-crustal depths evidence the presence of major faults dominating the crustal structure. The results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the Iberian Chain. They are consistent with a Cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the Iberian Chain and the generation of its present day topography.

Deep-crustal magma reservoirs beneath the Nicaraguan volcanic arc, revealed by 2-D and semi 3-D inversion of magnetotelluric data

A long-period magnetotelluric (MT) experiment was conducted in early 2009 in western Nicaragua to study the electrical resistivity and thus fluid/melt distribution at the Central American continental margin where the Cocos plate subducts beneath the Caribbean plate. Strike analysis yields a preference direction perpendicular to the profile, with moderate deviation from two-dimensionality, however. Two-dimensional modeling maps the sediments of the Nicaraguan Depression and a high-conductivity zone in the mid-crust, slightly offset from the arc. Further conductors are modeled in the backarc. However, these features are probably artifacts when a 2-D program is applied to data which show moderate 3-D characteristics. 3-D inversion clarifies the situation, and the major remaining conductive structure is now quasi directly beneath the volcanic chain and interpreted as a deep-seated magma deposit. Conductivity in the backarc is also relatively high and may either be caused by still existing partial melts beneath the Paleocene to Miocene volcanic arcs or by related metallic deposits in the aureoles of hydrothermal alteration.

Ability of High-Resolution Resistivity Tomography to Detect Fault and Fracture Zones: Application to the Tournemire Experimental Platform, France

The Experimental Platform of Tournemire (Aveyron, France) developed by IRSN (French Institute for Radiological Protection and Nuclear Safety) is composed of a tunnel excavated in an argillite formation belonging to a limestone–argillite–limestone subhorizontal sedimentary sequence. Subvertical secondary fault zones were intercepted in argillite using drifts and boreholes in the tunnel excavated at a depth of about 250 m located under the Larzac Plateau. A 2D 2.5 km baseline large-scale electrical resistivity survey conducted in 2007 allowed detecting in the upper limestones several significantly low electrical resistivity subvertical zones (Gelis et al. Appl Geophys 167(11): 1405–1418, 2010). One of these discontinuities is consistent with the extension towards the surface of the secondary fault zones identified in the argillite formation from the tunnel. In an attempt to better characterize this zone, IRSN and MINES ParisTech conducted a high-resolution electrical resistivity survey located transversally to the fault and fracture zones. A 760-m-long profile was acquired using two array geometries and take-outs of 2, 4 and 8 m, requiring several roll-alongs. These data were first inverted independently for each take-out and then using all take-outs together for a given array geometry. Different inverted 2D electrical resistivity models display the same global features with high (higher than 5000 Ωm) to low (lower than 100 Ωm) electrical resistivity zones. These electrical resistivity models are finally compared with a geological cross-section based on independent data. The subvertical conductive zones are in agreement with the fault and fracture locations inferred from the geological cross-section. Moreover, the top of a more conductive zone, below a high electrical conductive zone and between two subvertical fault zones, is located in a more sandy and argillaceous layer. This conductive zone is interpreted as the presence of a more scattered fracture zone located at depth between two fault zones. This zone could be correlated with the fractured zones identified at 250-m depth in underground works. This study highlights the interest of multi-scale approaches to image complex heterogeneous near subsurface layers. Finally, this study shows that the electrical resistivity tomography is a useful and powerful tool to detect fault and fracture zones in upper limestones. Such a method is complementary to other geophysical and geological data.

Investigation of ultra-light weight proppant application in shale fracturing

Horizontal wells with long propped fractures are needed to maximize productivity for ultra-low permeability shale and tight reservoirs. Conventional slickwater fracturing with sand can produce long fractures, but only the near wellbore region is propped due to fast settling of sand in water. Ultra-light weight proppants (ULWPs) can be used to reduce proppant settling. A comprehensive investigation of ULWP performance in shale reservoirs has been conducted by utilizing fracture conductivity experiments, fracture modeling and reservoir simulation. Based on experimental fracture conductivity measurements, empirical conductivity correlations are developed for the three ULWPs as a function of proppant areal concentration at reservoir confining stress and temperature. A fracturing model is used to predict the proppant concentration distributions during fracturing operations from which conductivity distribution along fractures is estimated. The conductivity distribution is then put into a reservoir simulator to predict gas production from a fractured shale well. Simulation study shows that at the same injection concentration in slickwater, all three ULWPs create more propped area and longer propped distance than sand, but the fracture conductivity is significantly lower. ULWPs have strong productivity advantages over sand in 0.1μD gas reservoir by creating more propped areas. The productivity advantage of using ULWPs decreases with increasing reservoir permeability and decreasing production time. For 1year production and 0.1μD shale reservoirs, the acceptable maximum price is $10/lb for ULW-1, $6/lb for ULW-2, and $2.5/lb for ULW-3. The maximum price increases as production time increases. By adding small amount of ULWPs into sand, both the high conductivity zone at the bottom and low conductivity zone at the upper and deeper zones can be achieved, which benefits both short and long-term production.

Effects of High Salinity Wastewater Discharges on Unionid Mussels in the Allegheny River, Pennsylvania

AbstractWe examined the effect of high salinity wastewater (brine) from oil and natural gas drilling on freshwater mussels in the Allegheny River, Pennsylvania, during 2012. Mussel cages (N = 5 per site) were deployed at two sites upstream and four sites downstream of a brine treatment facility on the Allegheny River. Each cage contained 20 juvenile northern riffleshell mussels Epioblasma torulosa rangiana). Continuous specific conductance and temperature data were recorded by water quality probes deployed at each site. To measure the amount of mixing throughout the entire study area, specific conductance surveys were completed two times during low-flow conditions along transects from bank to bank that targeted upstream (reference) reaches, a municipal wastewater treatment plant discharge upstream of the brine-facility discharge, the brine facility, and downstream reaches. Specific conductance data indicated that high specific conductance water from the brine facility (4,000–12,000 µS/cm; mean 7,846) compared to the reference reach (103–188 µS/cm; mean 151) is carried along the left descending bank of the river and that dilution of the discharge via mixing does not occur until 0.5 mi (805 m) downstream. Juvenile northern riffleshell mussel survival was severely impaired within the high specific conductance zone (2 and 34% at and downstream of the brine facility, respectively) and at the municipal wastewater treatment plant (21%) compared to background (84%). We surveyed native mussels (family Unionidae) at 10 transects: 3 upstream, 3 within, and 4 downstream of the high specific conductance zone. Unionid mussel abundance and diversity were lower for all transects within and downstream of the high conductivity zone compared to upstream. The results of this study clearly demonstrate in situ toxicity to juvenile northern riffleshell mussels, a federally endangered species, and to the native unionid mussel assemblage located downstream of a brine discharge to the Allegheny River.

Application of Time Domain Electromagnetic (Tdem) Methods for Mapping of Salt Fresh Water Intrusions and Evaluate The Porosity in Carbyn's Cove, Wandoor and Khurumedhera Beaches in South Andaman

Time Domain Electromagnetic (TDEM) methods are successfully used to delineate the geometry of different hydrogeological targets to determine porosity of saline and freshwater zones. TDEM can be used to determine both fluid resistivities (ρw) and bulk resistivities (ρ) which are required to calculate the porosity by applying Archie’s equation. By studying saline water intrusion near coastal zone, where ρw and ρ can be determined very accurately by applying TDEM method. From the literature review, it is revealed that the TDEM method is the most suitable geophysical technique for seawater intrusion. TDEM technique is very sensitive to high conductive (low resistivity) zones hence this technique can be applied to study the saltwater intrusion into the fresh water aquifers. Therefore, the aim of the presence study is to find out the porosity of saline water zone for the EM data generated with TDEM technique in the areas of Carbyn’s Cove, Khurumedhera and Wandoor beach of South Andaman The TDEM images shows the range of interface between saline water and fresh water at depth of 4 – 10 m from the shoreline to distance of 20 m in Carbyns cove beach, 7 – 18 m from 10m distance in Wandoor beach and 11 – 17 m from 30 m in Khurumedhera beach, indicating the varying amount of mixing of sea-water throughout the aquifer. The porosity of saltwater and freshwater shows that the low porosity indicates high salinity corresponds to salty brackish water with porous/ sandy or saturated clay and high porosity indicates low salinity corresponds to intermediate quality fresh water zone with Sand/gravel or minor clay. It is reasonably presumed from present field study, a close correspondence between the high resistivity zones that encompass with lower total dissolved solids concentration (fresh water) and lower resistivity region was incorporated with the higher amount of total dissolved solids concentration. TDEM soundings with Central Loop Technique appear to be a suitable tool to demarcate the transition zones between fresh water and salt water.

Pressure and thermal regimes and systems in the sedimentary sequence of central and eastern Saudi Arabia

Reservoir pressure data in central and eastern Saudi Arabia reveals the presence of two normally pressured (NPR 1and NPR 2) and two overpressured (OPR 1 and OPR 2) regimes. The NPR 1 comprises the sequence from the Saq through the lower Jilh Formations, from outcrops to 15,000 ft below ground. The NPR 2 comprises the sequence from the upper Jilh through the Dammam Formations and extends from outcrops to 14,000 ft. The OPR 1 comprises the sequence from the Saq through the Khuff Formations at depths from 12,000 to 19,000 ft. Overpressures in the OPR 1 are a result of reduced porosity and permeability, and gas generation from the Qusaiba shale. The OPR 2 comprises the lower Jilh Formation at depths between 10,000 and 13,000 ft. Compaction and/or hydrocarbon charging from the Qusaiba shale could be the reason for overpressures in the OPR 2. The NPR 1, NPR 2, and OPR 1 are divided into pressure systems by stratigraphic seals, but these are locally breached by unconformities or faults forming local anomalies. Geothermal gradients illustrate shallow and deep thermal regimes (STR and DTR) separated by a high thermal conductivity zone (HTCZ). The STR extends from land surface to 8000 ft., with low and high gradients in the up-dip and down-dip areas, respectively. The DTR extends from 10,000 to 19,000 ft., with no differences in gradients between the up-dip and down-dip areas. The HTGZ lies between 8000 and 10,000 ft., with low gradients.

An audio-magnetotelluric investigation of the Otjiwarongo and Katima Mulilo regions, Namibia

As an additional opportunistic component to the southern African magnetotelluric experiment, natural-source audio-magnetotelluric (AMT) data were acquired during phase IV to investigate the local-scale electric conductivity subsurface structure in the Otjiwarongo and Katima Mulilo regions (Namibia) as an aid in locating the installation points for high-voltage direct current earth electrodes. The study showed that the shallow subsurface of areas containing one measurement site in the Otjiwarongo region and three sites in the Katima Mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes. Both of the AMT surveys are situated close to the edge of the orogenic Neo-Proterozoic Damara mobile belt (DMB). Previous studies all suggest the existence of a highly conductive midcrustal zone, which correlates well with the spatial location of the DMB. Two-dimensional inverse modeling of the Otjiwarongo AMT data confirms the existence of the high-conductive zone at midcrustal depths (10–15 km). The high conductivity of the DMB is explained by the presence of interconnected graphite in the marble units present. The Katima Mulilo inversion results are characterized by a conductive upper crustal layer that does not form part of the DMB conductive belt. It was deduced that at the uppermost subsurface (maximum [Formula: see text]), Kalahari sediments are responsible for the high conductivity observed, whereas at greater depth (up to 6 km), its cause remains enigmatic, albeit the hypothesis of ironstone or graphite being present and causing the observed conductive upper crust.

Electrical Conductance Map for the Kachchh Rift Basin: Constraint on Tectonic Evolution and Seismotectonic Implications

Geomagnetic field variations recorded by an array of magnetometers spread across the Kachchh Rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations. A non-uniform thin-sheet electrical conductance model is developed to account for the salient induction patterns. It indicates that the imaged conductivity anomalies can be related to the sediment-filled structural lows in between the fault bounded uplifts. It is suggested that sagging structural lows preserved the marine sediments deposited during the Mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with Late Eocene transgression. Depth integrated electrical conductance helped in mapping two depo-centres: along the ENE-WSW trending Banni half-Graben bounded by the Kachchh Main fault on the south and, second, along the Vinjan depression formed in response to the subsidence between the Vigodi fault and westward extension of the Katrol Hill fault together with the westward bending of the Median High. Presence of metamorphosed graphite schist clasts in shale dominated Mesozoic sequence and/or thin films of carbon resulting from the thermal influence of Deccan activity on Carbonate-rich formations can account for the high electrical conductivity anomalies seen in the depo-centres of thick Mesozoic and Tertiary sediments. Additionally two high conductivity zones are imaged encompassing a block defined by the 2001 Bhuj earthquake and its aftershocks. In agreement with gravity, magnetic and seismic velocity signatures, aqueous fluids released by recrystallizing magmatic bodies intruded in association with Deccan trap activity account for mapped high conductivity zones. High fluid pressure in such a fractured domain, surrounding the intruded magmatic plugs, perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 Bhuj earthquake.

Comparison of a karst groundwater model with and without discrete conduit flow

Karst aquifers exhibit a dual flow system characterized by interacting conduit and matrix domains. This study evaluated the coupled continuum pipe-flow framework for modeling karst groundwater flow in the Madison aquifer of western South Dakota (USA). Coupled conduit and matrix flow was simulated within a regional finite-difference model over a 10-year transient period. An existing equivalent porous medium (EPM) model was modified to include major conduit networks whose locations were constrained by dye-tracing data and environmental tracer analysis. Model calibration data included measured hydraulic heads at observation wells and estimates of discharge at four karst springs. Relative to the EPM model, the match to observation well hydraulic heads was substantially improved with the addition of conduits. The inclusion of conduit flow allowed for a simpler hydraulic conductivity distribution in the matrix continuum. Two of the high-conductivity zones in the EPM model, which were required to indirectly simulate the effects of conduits, were eliminated from the new model. This work demonstrates the utility of the coupled continuum pipe-flow method and illustrates how karst aquifer model parameterization is dependent on the physical processes that are simulated.

Integrated geophysical imaging of a concealed mineral deposit: A case study of the world-class Pebble porphyry deposit in southwestern Alaska

We combined aeromagnetic, induced polarization, magnetotelluric, and gravity surveys as well as drillhole geologic, alteration, magnetic susceptibility, and density data for exploration and characterization of the Cu-Au-Mo Pebble porphyry deposit. This undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks, presenting an exploration challenge. Individual geophysical methods primarily assist regional characterization. Positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit, likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration. The mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit, and mafic rocks also exhibit strong chargeability. Conductivity anomalies similarly reflect widespread sulfides as well as water-saturated glacial sediments. Mineralogical and magnetic susceptibility data indicate magnetite destruction primarily within the Cu-Au-Mo mineralized area. The magnetic field does not show a corresponding anomaly low but the analytic signal does in areas where the deposit is not covered by postmineralization igneous rocks. The analytic signal shows similar lows over sedimentary rocks outside of the mineralized area, however, and cannot uniquely distinguish the deposit. We find that the intersection of positive chargeability anomalies with analytic signal lows, indicating elevated sulfide concentrations but low magnetite at shallow depths, roughly delineates the deposit where it is covered only by glacial sediments. Neither chargeability highs nor analytic signal lows are present where the deposit is covered by several hundred meters of sedimentary and volcanic rocks, but a 3D resistivity model derived from magnetotelluric data shows a corresponding zone of higher conductivity. Gravity data highlight geologic features within the deposit, including shallow diorite sills that locally contain higher-grade mineralization. The results thus show ways in which an integrated survey approach might be used to distinguish zones of potentially economic mineralization.

An approach for the geophysical assessment of fissuring of estuary and river flood embankments: validation against two case studies in England and Scotland

This paper proposes a two-stage geophysical approach to map the vertical cracking and the structural integrity of flood embankments made up of clay geomaterials susceptible to fissuring. The first stage is based on a ‘coarse-resolution’ investigation using conventional electrical resistivity tomography (ERT) equipment to identify the fissured zones in the embankment. This step is complemented by an additional geophysical technique, electromagnetic, to verify the ERT measurements. The second stage is based on a ‘high-resolution’ investigation using a miniature ERT system previously developed at the laboratory scale for detailed mapping of the fissure patterns. The ‘coarse-resolution’ stage is the major focus of this paper and was validated against two case studies in England and Scotland. Longitudinal ERT survey provided a tomographic picture of the upper desiccated zones of the embankments and fissured areas in 2-D, validating the range of resistivity results obtained previously on a fissured clay model in the laboratory. A transversal embankment resistivity tomography was also completed to show the positions of fissured zones in detail in the field. The electromagnetic technique as a fast screening tool allowed cross checking the ERT results and was also efficient in detecting high and low conductivity zones, indicating areas of potential weakness during flash floods and heavy rain. The southern embankment in England showed more fluctuations in the conductivity and resistivity than the north embankment in Scotland, likely to be due to the differences in climate, vegetation and location characteristics between the two sites. Conclusions were also drawn on the potential weaknesses for both embankments and the effect of vegetation on conductivity measurements.

Magnetotelluric study of the Remiremont-Epinal-Rambervillers zone of migrating seismicity, Vosges (France)

AbstractThe magnetotelluric method has been used to image the deep electrical structure of the Remiremont-Epinal-Rambervillers region in the French Vosges Massif, which has presented a significant seismic activity in the past decades. Several earthquakes of moderate magnitude (up to 5.1) occurred in this area with a systematic migration along a nearly N-S direction. Inversion of the magnetotelluric data reveals zones of high electrical conductivity. A large conductive body presents a significative spatial correlation with the region that was most recently affected by earthquakes. This conductive body is interpreted as a consequence of the presence of a fluid filled basement fault network in proximity to the zone affected by the last seismic crisis, where fluid pressure diffusion takes place for several years after the main shock and participates in maintaining a microseismic activity.

A Non-Local Richards Equation to Model Unsaturated Flow in Highly Heterogeneous Media under Nonequilibrium Pressure Conditions

We study the impact of mass transfer between high and low conductivity zones on flow in unsaturated media. A dual continuum approach that assumes capillary-dominated flow in the slow continuum gives a system of coupled flow equations for the water saturations in the mobile and in the slow domain, whose coupling term is directly related to the evolution of the averaged water content in the slow domain. We show that linearization of the nonlinear diffusion equation that governs capillary flow in the slow continuum captures well the essential features of the time evolution of the averaged water content in the slow domain. This allows one to derive a non-local Richards equation for the water content in the mobile domain that is characterized by a memory kernel that encodes the local mass transfer dynamics as well as the geometry of the slow zones. Comparison of the model predictions to the results of numerical simulations of infiltration in a vertically layered medium shows that the non-local approach describes well nonequilibrium effects due to mass transfer between high and low conductivity zones.

Conductivity and scattering Q in GPR data: Example from the Ellenburger dolomite, central Texas

Total attenuation ([Formula: see text]) in ground-penetrating radar (GPR) data is a composite of intrinsic and scattering attenuations ([Formula: see text] and [Formula: see text]). For nonmagnetic materials, [Formula: see text] is a combination of the effects of real conductivity and dielectric relaxation. The attenuation for real conductivity [Formula: see text] in the GPR frequency band is a function of frequency while the dielectric relaxation is frequency-independent. These frequency behaviors allow separation of the attenuation types by attributing and fitting the [Formula: see text] decay shape with frequency to the conductivity, and by attributing the magnitude of [Formula: see text] to the sum of conductivity and dielectric relaxation attenuations at each frequency. Total attenuation is calculated from GPR data using spectral ratios, and [Formula: see text] is obtained by fitting a smooth lower bound to [Formula: see text]; the difference between [Formula: see text] and [Formula: see text] estimates the scattering contribution [Formula: see text]. Scatterer size spectra are evaluated using [Formula: see text] for 2D, and [Formula: see text] for 3D, propagation (where [Formula: see text] is wavenumber and [Formula: see text] is the scatterer size). We illustrate with 2D synthetic data and three field 2D crosshole profiles from an outcrop of an Ellenburger collapsed paleocave environment in central Texas. Between the three pairs of holes, we estimate the breccia sizes from the scattering spectra [Formula: see text]. To image the anisotropic electrical conductivity distributions, we use simultaneous iterative reconstruction tomography. There is a correlation between the low wavenumber features of the results of the current conductivity tomography and those in previous velocity tomography, and with surface data results that are predicted and calculated from GPR data attributes. Low- and high-conductivity zones tend to follow either the GPR facies distributions, lithological boundaries, or the larger of the fractures. Correlations are not visible where the breccias are finer because these tend to be more randomly oriented, and/or below the resolution of the GPR data.

Delineation of shallow resistivity structure around Malvan, Konkan region, Maharashtra by neural network inversion using vertical electrical sounding measurements

Modeling resistivity profiles, especially from hard rock areas, is of specific relevance for groundwater exploration. A method based on Bayesian neural network (BNN) theory using a Hybrid Monte Carlo (HMC) simulation scheme is applied to model and interpret direct current vertical electrical sounding measurements from 28 locations around the Malvan region, in the Sindhudurg district, southwest India. The modeling procedure revolves around optimizing the objective function using the HMC based sampling technique which is followed by updating each trajectory by integrating the Hamiltonian differential equations via a second order leapfrog discretization scheme. The inversion results suggest a high resistivity structure in the north-western part of the area, which correlates well with the presence of laterites. In the south-western part, a very high conductive zone is observed near the coast indicating an extensive influence of saltwater intrusion. Our results also show that the effect of intrusion of saline water diminishes from the south-western part to the north-eastern part of the region. Two dimensional modeling of four resistivity profiles shows that the groundwater flow is partly controlled by existing lineaments, fractures, and major joints. Groundwater occurs at a weathered/semi-weathered layer of laterite/clayey sand and the interface of overburden and crystalline basement. The presence of conduits is identified at a depth between 10 and 15 m along the Dhamapur–Kudal and Parule–Oros profiles, which seems to be potential zone for groundwater exploration. The NW–SE trending major lineaments and its criss-cross sections are indentified from the apparent and true resistivity surface map. The pseudo-section at different depths in the western part of the area, near Parule, shows extensive influence of saltwater intrusion and its impact reaching up to a depth of 50 m from the surface along the coastal area. Further, the deduced true electrical resistivity section against depth correlates well with available borehole lithology in the area. Present analyses suggest that HMC-based BNN method is robust for modeling resistivity data especially in hard rock terrains. These results are useful for interpreting fractures, major joints, and lineaments and crystalline basement rock and also for constraining the higher dimensional models.

Fe3+-rich augite and high electrical conductivity in the deep lithosphere

Research Article| February 01, 2012 Fe3+-rich augite and high electrical conductivity in the deep lithosphere Xiaozhi Yang; Xiaozhi Yang * Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany *E-mail: xiaozhi.yang@uni-bayreuth.de. Search for other works by this author on: GSW Google Scholar Catherine McCammon Catherine McCammon Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Xiaozhi Yang * Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany Catherine McCammon Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany *E-mail: xiaozhi.yang@uni-bayreuth.de. Publisher: Geological Society of America Received: 08 Aug 2011 Revision Received: 05 Sep 2011 Accepted: 13 Sep 2011 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (2): 131–134. https://doi.org/10.1130/G32725.1 Article history Received: 08 Aug 2011 Revision Received: 05 Sep 2011 Accepted: 13 Sep 2011 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Xiaozhi Yang, Catherine McCammon; Fe3+-rich augite and high electrical conductivity in the deep lithosphere. Geology 2012;; 40 (2): 131–134. doi: https://doi.org/10.1130/G32725.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Magnetotelluric soundings have detected high-conductivity zones in some regions of the deep lithosphere, and these conductive areas were usually interpreted to reflect the presence of hydrous phases, silicate/carbonatite melts, or water incorporated in olivine. Augite megacrysts are the most abundant single-crystal mantle samples entrained by volcanic basalts/kimberlites in a wide variety of tectonic environments, and are usually characterized by higher Fe (∼5%–12% in total FeO), Fe3+ (∼25%–45% in Fe3+/Fetotal), and H2O (∼200–1800 ppm) contents. Based on experimental work, we show that Fe3+- and H2O-rich augites have electrical conductivities that are several orders of magnitude higher than those of dry olivine. High conductivities in at least some regions of the lithospheric mantle can be explained by the presence of small amounts of augite (and other pyroxenite analogues, e.g., ∼10% to even less depending on composition, temperature, and models used for the estimation), which may ultimately indicate the infiltration of silicate melts and their subsequent crystallization and interaction with wall rocks in the deep lithosphere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geophysical Investigation For Groundwater Using Electrical Resistivity Method - A Case Study Of Annunciation Grammar School, Ikere Lga, Ekiti State, South-Western Nigeria

Geophysical survey involving electrical resistivity methods has been carried out at Annunciation grammar school, Ikere- Ekiti, southwestern Nigeria with the view to delineate the geo-electric characteristics of the basement complex and evaluate its groundwater potential in the area. A total of 5 vertical electrical sounding stations were established within the site along 5 traverses. The schlumberger configuration was used for the data acquisition. The half -currents electrode (���� 2 ) used range from 1 to 100m. The quantitative interpretation of the VES curves involved the use of partial curve matching and the 1-D computer iteration technique. The depth sounding interpretation results were used to generate geo-electric sections from which the aquifer was delineated. The geo-electric section drawn from the results of the interpretation reveal five subsurface layer which comprises of the topsoil, lateritic sand, partially weathered, weathered and fractured basement. The weathered and fractured layers constituted the aquiferous zone in all the stations. Hence, from this project work it is recommended that boreholes can be sited in high conductivity zones in VES 1,2,3 and 4 as they contain probable aquifers. The depth of any borehole should be located between 15m and 30m to take advantage of the basement fractures

Conductivity distribution beneath Lascar volcano (Northern Chile) and the Puna, inferred from magnetotelluric data

During two field campaigns in 2007 and 2010, long-period and broadband magnetotelluric measurements were conducted in the Central Andes of northern Chile and northwestern Argentina at a latitude of 23.7°S. The study area spans from the Precordillera over the active volcanic arc of the Western Cordillera until the western part of the Puna. A special focus was on Lascar, a subduction related stratovolcano with an historical activity characterized by fumarolic emissions and occasional sub-plinian eruptions, like the one occurred on 1993. The broadband instruments, covering a period range between T=0.005s and 1000s, were installed in the proximities of Lascar volcano, while the long-period devices (T=10s–10,000s) were installed on a profile slightly south of Lascar, crossing the Salar de Atacama basin, the volcanic arc and reaching the western border of the Puna.Remote reference and robust techniques were used for data processing. Induction vectors, phase tensor ellipses and strike direction of the conductivity distribution were calculated, showing some 3-D behavior for the Lascar sites at shallower depths, with induction vectors close to the edifice influenced partially by the topography. 3-D modeling and inversion revealed conductive anomalies beneath Lascar volcano, but also beneath the Puntas Negras volcanic chain. For the long-period transect, the behavior is closer to a 2-D case, with more stable strike direction coherent with induction vectors and phase tensor ellipses. The major result of 2-D inversion is a vast high-conductivity zone in the backarc crust, which seems to be the southern extension of a large highly conductive anomaly observed in prior studies beneath the Altiplano.