Broadband Magnetotelluric Research Articles

Electrical conductivity structure of north-west Fennoscandia from three-dimensional inversion of magnetotelluric data

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project “Magnetotellurics in the Scandes” (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 simultaneous long period and 220 broad-band MT sites were occupied in the summers of 2011 to 2013.The 3-D inversion of the MaSca data was obtained using the ModEM 3-D code. The full impedance and tipper data were used for the inversion. The rocks of Archaean and Proterozoic basement towards east and the Caledonian nappes towards west are modelled as resistive structures. In the central and southern parts, the whole crust is resistive and reflects the Trans-Scandinavian Igneous Belt granitoids. The middle to lower crust of the Svecofennian province is conductive. An uppermost crustal conductor is revealed in the Skellefteå Ore District. The south end of the Kittilä Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. A map of the crustal conductance for the north-west Fennoscandian Shield is presented.

Magnetotelluric array data analysis from north-west Fennoscandia

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project “Magnetotellurics in the Scandes” (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 synchronous long period and 220 broad-band MT sites was occupied in the summers of 2011 to 2013. We estimated MT transfer functions in the period range from 0.003 to 105s.The Q-function multi-site multi-frequency analysis and the phase tensor were used to estimate strike and dimensionality of MT data. Dimensionality and strike analyses indicate generally 2-D behaviour of the data with 3-D effects at some sites and period bands. In this paper we present 2-D inversion of the data, 3-D inversion models are shown in the parallel paper. We choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Seven crustal-scale and four lithospheric-scale 2-D models are presented.The resistive regions are images of the Archaean and Proterozoic basement in the east and thin Caledonian nappes in the west. The middle and lower crust of the Svecofennian province is conductive. The southern end of the Kittilä Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. The thickest lithosphere is in the Palaeoproterozioc Svecofennian Domain, not in the Archaean. The thickness of the lithosphere is around 200km in the north and 300km in the south-west.

Geoelectrical baseline model of the subsurface of the Hontomín site (Spain) for CO2 geological storage in a deep saline aquifer: A 3D magnetotelluric characterisation

The magnetotelluric (MT) method was used to characterise the underground research laboratory (URL) for CO2 storage in a deep saline aquifer at Hontomín (Spain). A total grid of 109 closely-spaced broadband MT sites was acquired in the study area covering an areal extent of 3×5km2. Different three-dimensional (3D) inversion codes were employed to invert the MT data in the period range of 0.001–10s (frequency range 1000–0.1Hz), with all of them giving similar results. The final preferred 3D model validates a previously published two-dimensional (2D) MT study and is supported by a variety of multidisciplinary data (e.g., well log, 3D seismic and hydrogeochemistry data). The 3D model constitutes the baseline electrical resistivity model of the site that will be used for the future time-lapse electromagnetic (EM) monitoring experiments of the URL. The 3D resistivity distribution shows the dome-like structure of the saline aquifer and images fracture regions, thus identifying the most likely leakage pathways and consequently, the monitoring requirements of the Hontomín site.

Magnetotelluric imaging of upper crustal partial melt at Tendaho graben in Afar, Ethiopia

We report on a recent magnetotelluric (MT) survey across the Manda Hararo magmatic segment (MHMS) within the Tendaho graben in the Afar Depression in northeastern Ethiopia. Twenty-two broadband MT sites with ∼1 km station spacing were deployed along a profile with the recorded data covering a period range from 0.003 s to 1000 s. A two-dimensional (2-D) resistivity model reveals an upper crustal fracture zone (fault) and partial melt with resistivity of 1–10Ωm at a depth of >1 km. The partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the Afar Stratoid Series basalts. We estimate a melt fraction of about 13% based on geochemical and borehole data, and bulk resistivity from the 2-D MT inversion model. The interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat. Within the MHMS and Tendaho graben, a magma intrusion is a plausible explanation for the upper crustal conductor. The inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also makes the Tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy.

Magnetotelluric imaging of the Ohaaki geothermal system, New Zealand: Implications for locating basement permeability

Closely-spaced broadband MT (magnetotelluric) data measured on a profile through the Ohaaki geothermal system show that the basement greywacke directly beneath the geothermal field is resistive. There is no evidence in these MT data for the existence of an intrusive heat source directly beneath Ohaaki. However, these data confirm that a dipping conductive zone links the near surface low resistivity anomaly associated with the geothermal field to a deep region of low resistivity that is offset to the northwest. The resistivity of the dipping conductor is consistent with a zone of fracturing within the basement meta-sediments, and is interpreted to show the location of upwelling high-temperature fluids that supply the Ohaaki geothermal field from a deep magmatic source.

First geoelectrical image of the subsurface of the Hontomín site (Spain) for CO2 geological storage: A magnetotelluric 2D characterization

Magnetotelluric (MT) data provided a two-dimensional (2-D) resistivity model of the subsurface of the Technology Demonstration Plant (TDP) located in Hontomín (Burgos, Spain) for geological storage of CO2 in a deep saline aquifer. A first north–south profile of 22 broadband MT soundings was acquired to study the viability of the method to characterize the TDP subsurface. Stations were deployed at approximately 200-m intervals and the data were recorded in the period range of 0.001–100s. The dimensionality analysis proved the validity of the 2-D assumption, and therefore a joint 2-D inversion of the TE and TM modes was undertaken constraining the modelling process according to a vintage resistivity log acquired for oil exploration. The 2-D model provides the first geoelectrical image of the structure and links the different lithologies to electrical properties. Results locate the main reservoir and seal units in the period range of 0.1–1s. Thereby it is proved the validity of the method to supply a high resolution baseline model able to fully image the geoelectrical structure of the Hontomín site in the pre-injection state.

Magnetotelluric investigation complement multi-disciplinary geophysical data

As part of the Australian Government?s Onshore Energy Security Program, Geoscience Australia has acquired magnetotelluric data along 12 deep crustal seismic reflection transects in Australia. The magnetotelluric projects, which total more than 640 stations over 3700 km in distance, have been undertaken by Geoscience Australia in collaboration with state and territory geoscience agencies. Broadband and long period MT data have been acquired for investigating geological structures from the shallow surface to upper mantle. These data, along with the deep seismic reflection, magnetic, gravity and geological data form the basis for multi-disciplinary investigations of crustal architecture, and energy and mineral potential, providing precompetitive information to industry and researchers.

Crustal-scale electrical conductivity anomaly beneath inflating Lazufre volcanic complex, Central Andes

Large-scale surface deformation was observed at Lazufre volcanic center in the Central Andes of Northern Chile/Northwestern Argentina by means of Interferometric Synthetic Aperture Radar (InSAR). Uplift started there after 1998 and increased dramatically in the following years up to a rate of 3 cm/a. Lazufre is now one of the largest deforming volcano systems on Earth, but the cause for uplift – likely influx of magmatic material into the crust – is still poorly understood.In the beginning of 2010 a magnetotelluric survey was conducted to delineate the electrical conductivity distribution in the area. Several long-period magnetotelluric (LMT) sites and two broadband magnetotelluric (BBMT) sites were set up on an EW trending profile crossing the volcanic center; furthermore some LMT sites were arranged circularly around Lazufre complex and adjacent Lastarria volcano. Data were processed using an algorithm for robust and remote reference transfer function estimation. Electrical strike directions were estimated and induction arrows were derived. Although electrical strike is rather ambiguous, in a first step a 2-D resistivity model was calculated. The most prominent feature of this model is a well conducting structure rising from the upper mantle to the shallow crust beneath the center of elevation. This can be interpreted as partial melts ascending from the asthenospheric wedge and feeding a potential magma reservoir beneath Lazufre volcanic center. An improved model is finally achieved by 3-D inversion, supporting this feature. We assume that these rising melts are the source of the observed uplift at Lazufre complex.

Anomalous electrical structure in the northwestern Paraná Basin, Brazil, observed with broadband magnetotellurics

Abstract The deep structure of the northern part of the Parana Basin has been investigated through the analysis of a SW–NE magnetotelluric (MT) profile composed of 24 broadband stations with periods from 0.001 to ∼1000 s that are regularly spaced ∼10 km apart. The SW portion of the profile crosses a negative Bouguer anomaly with amplitudes of ∼20 mGal. Our analyses show that there are two lithospheric domains with different gravity and electric conductivity properties. In the regularized two-dimensional MT inversion model the mid-to-lower crust in the SW part of the profile is almost two orders in magnitude more conductive compared to the NE portion. This difference in conductivity extends to the uppermost mantle depths, suggesting the existence of a major lithospheric discontinuity in the north-central Parana Basin, locally coincident with the position of the Apore River. We interpret this discontinuity as a juxtaposition of terranes with distinct natures and ages, possibly as result of tectonic accretion during the formation of Western Gondwana.

Geophysical Images of the North Anatolian Fault Zone in the Erzincan Basin, Eastern Turkey, and their Tectonic Implications

The collision between the Arabian and Eurasian plates in eastern Turkey causes the Anatolian block to move westward. The North Anatolian Fault (NAF) is a major strike-slip fault that forms the northern boundary of the Anatolian block, and the Erzincan Basin is the largest sedimentary basin on the NAF. In the last century, two large earthquakes have ruptured the NAF within the Erzincan Basin and caused major damage (M s = 8.0 in 1939 and M s = 6.8 in 1992). The seismic hazard in Erzincan from future earthquakes on the NAF is significant because the unconsolidated sedimentary basin can amplify the ground motion during an earthquake. The amount of amplification depends on the thickness and geometry of the basin. Geophysical constraints can be used to image basin depth and predict the amount of seismic amplification. In this study, the basin geometry and fault zone structure were investigated using broadband magnetotelluric (MT) data collected on two profiles crossing the Erzincan Basin. A total of 24 broadband MT stations were acquired with 1–2 km spacing in 2005. Inversion of the MT data with 1D, 2D and 3D algorithms showed that the maximum thickness of the unconsolidated sediments is ~3 km in the Erzincan Basin. The MT resistivity models show that the northern flanks of the basin have a steeper dip than the southern flanks, and the basin deepens towards the east where it has a depth of 3.5 km. The MT models also show that the structure of the NAF may vary from east to west along the Erzincan Basin.

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.

A new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake (Mw 7.6) hypocentre zone and its tectonic implications

SUMMARY We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2-D inversion of broad-band MT data of two profiles of lengths 32 km (AA′) and 52 km (BB′) revealed a thick (∼3 km) highly conductive (1–4 Ω-m) surface layer of fluvio-marine Mesozoic–Cenozoic sediments. The models delineate the hypocentre zone located at ∼20–25 km depth that manifests the high resistivity–conductivity transition zone. The accumulation of compressive stresses post-rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F1 got initiated along the high resistivity–conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20–30 km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40 km depth, intersected by the F1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thick-skinned deformation.

New geoelectrical characterisation of a continental collision zone in the West-Central Pyrenees: Constraints from long period and broadband magnetotellurics

Continental collision dominates the development of a large number of mountain ranges on Earth. Although the geological evolution of these regions is highly studied, lithospheric-scale physical processes are less well characterised and their interpretations frequently differ. A new magnetotelluric profile crossing the West-Central Pyrenees with broadband and long period magnetotelluric data constrains the geoelectrical features of this continental collision zone confining the geological and physical processes that take place at a lithospheric scale. Three geoelectrical structures associated with partial melting and its melt migration are imaged for the first time in the West-Central Pyrenees suggesting an Iberian subducted lower crust reaching the depth of 70km. This result, supported by an analysis of the thermal evolution of the Iberian subducted lower crust, reinforces the hypothesis of partial melting in this region. From the long period magnetotelluric data, the asthenosphere has been imaged as a geoelectrical structure with electrical resistivity values between 20Ωm and 70Ωm. The lithosphere–asthenosphere boundary close to the collision zone has been constrained between 90km and 110km depth below the Iberian plate and between 120km and 160km depth below the transition zone between plates and the European plate, in agreement with the previous geophysical data. Additionally, other geoelectrical anomalies imaged have been associated with an unexpected major Variscan boundary located below the Ebro basin, Silurian sediments and free fluids in the Gavarnie thrust, and with a thick sequence of Jurassic and Early Cretaceous sediments situated below the North Pyrenean Thrust Sheet.

Magnetotelluric imaging of upper‐crustal convection plumes beneath the Taupo Volcanic Zone, New Zealand

Broadband MT (magnetotelluric) data were recorded that form an array of measurements at the south‐eastern margin of the TVZ (Taupo Volcanic Zone), in the central North Island of New Zealand. These array data are used to investigate mechanisms by which the TVZ's extraordinarily high heat flux is transported to the surface. Taken together with seismological data, these MT data show compelling evidence that support a model of hydrothermal convection within the brittle (upper ∼6–7 km) part of the crust. Both 2‐D and 3‐D inversion models of these MT data show vertical low‐resistivity zones that connect surface geothermal fields to an inferred magmatic heat source that lies below the brittle‐ductile transition.

Two-dimensional resistivity imaging in the Kızıldere geothermal field by MT and DC methods

Magnetotelluric (MT) and Schlumberger (DC) surveys were conducted to identify conductive zones inside metamorphic complex in the easternmost part of B. Menderes Graben. In this study, using these surveying data, creation of some two-dimensional images of the shallow and deeper parts of the area was aimed. Broadband (200–0.002 Hz) MT and Schlumberger data were acquired along four profiles crossing the B. Menderes Graben with a total of 18 MT and 19 DC stations. These data were modeled using two-dimensional inverse techniques. The main findings are: (1) presence of wide conductive regions with very low resistivity (< 3 Ω m) are imaged at depth of ~ 100 m, extending to maximum ~ 2000 m depth, and these conductive regions signify potential geothermal resource in the area, (2) presence of deep conductive regions (< 75 Ω m) at a depth from ~ 15 km to 35 km, which reflects the shallow asthenosphere, could be correlated with the presence of high enthalpy geothermal system and its heat source.

Magnetotelluric deep soundings, gravity and geoid in the south São Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south São Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean–Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Ωm) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Ωm) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m 3) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (~ 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO 2, H 2O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the São Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Paraná and Espinhaço (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (~ 3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the São Francisco–Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous.

Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: Constraints from broadband magnetotellurics

A 1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the “thermal” thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mW m − 2 ), Western Kimberley Block 190 km (44 mW m − 2 ), Rehoboth Terrane 180 km (45 mW m − 2 ) and Ghanzi-Chobe/Damara Belt 160 km (48 mW m − 2 ). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident. Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P– T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40–42 mW m − 2 ), very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites. The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and the subsequent lithospheric heating/thinning event required to account for its present-day lithospheric structure, are not well constrained. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138–167 km. It appears most likely that the depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is explained by refertilisation of the lower lithospheric mantle.

MT measurements in the western part of the Paleoproterozoic Skellefte Ore District, Northern Sweden: A contribution to an integrated geophysical study

A 2D conductivity model of the Kristineberg area in the Skellefte Ore District, Northern Sweden, has been derived from new magnetotelluric measurements. This complements an intensive geophysical and geological study of the area, including reflection seismics, gravity and aeromagnetic data modeling as well as geological field observations. In a pilot study, 20 broadband MT stations were installed in May 2007 along a 20 km long north–south profile. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction of N75°E is not consistent over the whole profile. The new conductivity model of the upper crust agrees well with the results from the seismic studies. Interpreting both independent data sets confirms the major features from the previous model, such as the thickness of the Revsund granites in the south, the existence of a structural basement with metasedimentary origin, and gives new insight into the nature of the volcanic rocks and their possible mineral content.

The Jeju dataset: Three-dimensional interpretation of MT data from mid-mountain area of Jeju Island, Korea

Magnetotelluric (MT) surveys have been performed along five lines in the Jeju volcanic island located at South Sea of Korea. The purposes of the MT surveys are to see if there still remains thermal regime and if there exist deeply extended fractures or an aquifer system beneath the mid-mountain region of Mt. Halla in Jeju. By performing Audio-frequency MT (AMT) surveys together with MT surveys at each site and by operating far-remote reference site of approximately 480 km apart, we could get a broadband (10 − 4 –10 4 Hz) MT dataset of totally 108 sites with very good quality. 3D inversion of MT data from 88 measurement sites gives a reasonable estimation of the subsurface structures beneath the island. An induction vector analysis and 3D inversion results commonly indicate the existence of a conductive anomaly extended down to a few-kilometer depth beneath the central part of the island, which can be related to ancient volcanic activities. Because the island is surrounded by electrically conductive sea and has steep topographic variation, measured MT data in terms of impedance is seriously biased by distortion of electric fields and is suffered from the static shifts. The Jeju MT dataset is valuable for the research purpose in the sense that the dataset contains so many problems, some of which can frequently be observed in every field data, and that the geology of Jeju Island is relatively simple.

Hydrogeologic assessment of the Amchitka Island nuclear test site (Alaska) with magnetotellurics

Amchitka Island, in Alaska, was used for underground nuclear testing from 1965 to 1971. Since the test program concluded, there have been concerns about the possible release of radionuclides into the marine environment of the Aleutian Islands. The hydrogeology of islands such as Amchitka is characterized by a layer of freshwater overlying a saltwater layer, with the salinity increasing across a transition zone (TZ). Hydrogeologic modeling can provide an estimate of the timing and amount of radionuclide release from the explosions beneath Amchitka Island. This modeling is inconclusive because of a lack of information regarding subsurface structure. To address this problem, magnetotelluric (MT) data were collected on Amchitka Island in 2004. Broadband MT data were recorded on profiles passing through three explosion sites to give information about subsurface porosity and salinity. A 2D MT inversion produced models of sub-surface electrical resistivity and showed a pattern of increasing, decreasing, and increasing resistivity with depth at each test site. The depth at which resistivity begins to decrease defines the top of the TZ. The deeper increase in resistivity approximates the base of the TZ. The depths of the top and bottom of the TZ were determined as follows: Cannikin 900–2500 m; Long Shot 600–1700 m; Milrow 900–1700 m. Uncertainties were estimated for these depths. Effective porosities were also estimated and ranged from 10%–20% at the surface to 1%–3% at 3-km depth. These porosities are higher than those assumed in several hydrogeologic models, and give longer transit times from the explosion to the marine environment. Subject to the limits of the analysis, it appears that each of the cavities resulting from underground nuclear explosions is located in the TZ from fresh to saltwater. This implies shorter transit times to the marine environment than if the detonations had been located in the saltwater layer.