A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

Yan, Ping; Juanatey, Maria A. Garcia; Kalscheuer, Thomas; Juhlin, Christopher; Hedin, Peter; Savvaidis, Alexandros; Lorenz, Henning; Kück, Jochem

doi:10.1093/gji/ggw457

Cited by 18 publications

(30 citation statements)

References 51 publications

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“…Note that an alter- Depth (km) 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 Distance along CDP line (km) 6 shown at a vertical exaggeration of 2 : 1. The black line marks the depth to the highly conductive layer from MT data as mapped by Yan et al (2016). An excellent correspondence exists between the base of the uppermost seismically transparent zone and the mapped conductor.…”

Section: General Characteristics Of the Cosc Composite Seismic Profilmentioning

confidence: 91%

“…This alternative will be discussed later in the paper. The new magnetotelluric (MT) survey along the profile (Yan et al, 2016) provides evidence of a gently undulating surface of the prominent uppermost conductive layer, shown in Fig. 7, this being located at ca.…”

Section: General Characteristics Of the Cosc Composite Seismic Profilmentioning

confidence: 94%

“…Complementary to the seismic profiling, a magnetotelluric (MT) survey was conducted along the entire seismic profile in 2013 (Yan et al, 2016). Although this also suffered from the need for a diversion and, thus, follows the highly crooked seismic profile, it provides clear constraints on the depth to the top of the highly conductive alum shales.…”

Section: Juhlin Et Al: Seismic Imaging In the Eastern Scandinaviamentioning

confidence: 99%

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Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

et al. 2016

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Abstract. The Collisional Orogeny in the ScandinavianCaledonides (COSC) project, a contribution to the International Continental Scientific Drilling Program (ICDP), aims to provide a deeper understanding of mountain belt dynamics. Scientific investigations include a range of topics, from subduction-related tectonics to the present-day hydrological cycle. COSC investigations and drilling activities are focused in central Scandinavia, where rocks from the middle to lower crust of the orogen are exposed near the SwedishNorwegian border. Here, rock units of particular interest occur in the Seve Nappe Complex (SNC) of the so-called Middle Allochthon and include granulite facies migmatites (locally with evidence of ultra-high pressures) and amphibolite facies gneisses and mafic rocks. This complex overlies greenschist facies metasedimentary rocks of the doleriteintruded Särv Nappes and underlying, lower grade Jämt-landian Nappes (Lower Allochthon). Reflection seismic profiles have been an important component in the activities to image the subsurface structure in the area. Subhorizontal reflections in the upper 1-2 km are underlain and interlayered with strong west-to northwest-dipping reflections, suggesting significant east-vergent thrusting. Two 2.5 km deep fully cored boreholes are a major component of the project, which will improve our understanding of the subsurface structure and tectonic history of the area. Borehole COSC-1 (IGSN: http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in the summer of 2014, targeted the subduction-related Seve Nappe Complex and the contact with the underlying allochthon. The COSC-2 borehole will be located further east and will investigate the lower grade, mainly Cambro-Silurian rocks of the Lower Allochthon, the Jämtlandian décollement, and penetrate into the crystalline basement rocks to identify the source of some of the northwest-dipping reflections.A series of high-resolution seismic profiles have been acquired along a composite ca. 55 km long profile to help locate the COSC drill holes. We present here the results from this COSC-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014. These seismic data, along with shallow drill holes in the Caledonian thrust front and previously acquired seismic, magnetotelluric, and magnetic data, are used to identify two potential drill sites for the COSC-2 borehole.

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Section: General Characteristics Of the Cosc Composite Seismic Profilmentioning

confidence: 91%

Section: General Characteristics Of the Cosc Composite Seismic Profilmentioning

confidence: 94%

Section: Juhlin Et Al: Seismic Imaging In the Eastern Scandinaviamentioning

confidence: 99%

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Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

et al. 2016

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“…A second borehole, COSC‐2, is currently being planned and supposed to drill through the main décollement between the allochthons and the underlying autochthonous Precambrian basement. Many geophysical data were acquired in connection with the COSC project, including a high‐resolution reflection seismic profile (CSP; Figure ) [ Hedin et al ., ; Juhlin et al ., ] and 59 MT stations [ Yan et al ., ] along the CSP.…”

Section: Application To the Cosc Datamentioning

confidence: 99%

Two‐dimensional magnetotelluric inversion using reflection seismic data as constraints and application in the COSC project

Yan

Kalscheuer

Hedin

et al. 2017

Geophysical Research Letters

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We present a novel 2‐D magnetotelluric (MT) inversion scheme, in which the local weights of the regularizing smoothness constraints are based on the envelope attribute of a reflection seismic image. The weights resemble those of a previously published seismic modification of the minimum gradient support method. We measure the directional gradients of the seismic envelope to modify the horizontal and vertical smoothness constraints separately. Successful application of the inversion to MT field data of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project using the envelope attribute of the COSC reflection seismic profile helped to reduce the uncertainty of the interpretation of the main décollement by demonstrating that the associated alum shales may be much thinner than suggested by a previous inversion model. Thus, the new model supports the proposed location of a future borehole COSC‐2 which is hoped to penetrate the main décollement and the underlying Precambrian basement.

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“…Further improvements in models of mineral deposits can be obtained by including resistivity borehole logs as local prior information (e.g., Yan et al 2017a), by transferring structural boundaries observed in reflection seismic images or seismic velocity models to 2D or 3D inversion models of MT data (e.g., Le et al 2016b;Yan et al 2017b;Moorkamp 2017) or by performing joint inversions of MT, seismic and potential field data using petrophysical or structural coupling (e.g., Moorkamp et al 2011;Gallardo and Meju 2011;Haber and Gazit 2013;Takam Takougang et al 2015;Moorkamp 2017). Strictly speaking, well logs only represent the resistivity structure up to a few metres off the borehole and may be affected by borehole fluids and the invaded zone around the borehole.…”

Section: Resultsmentioning

confidence: 99%

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

Kalscheuer

Juhojuntti

Vaittinen³

2017

Surv Geophys

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A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of $ 300-1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and elect...

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A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

Cited by 18 publications

References 51 publications

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

Two‐dimensional magnetotelluric inversion using reflection seismic data as constraints and application in the COSC project

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

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A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

Cited by 18 publications

References 51 publications

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

Two‐dimensional magnetotelluric inversion using reflection seismic data as constraints and application in the COSC project

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

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Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden

Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden