GDS Data Research Articles

3-D anisotropic modelling of geomagnetic depth sounding based on unstructured edge-based finite-element method

SUMMARY Geomagnetic depth sounding (GDS) is a geophysical electromagnetic (EM) method that studies the deep structure and composition of the Earth by using long-period EM signals from geomagnetic observatories and satellites. In this paper, a 3-D anisotropic GDS modelling algorithm is developed. The curl–curl equation is discretized using the edge-based finite-element method on unstructured tetrahedral grids. In order to solve the computationally demanding problem of EM modelling on a global scale, the complex linear system is first separated into the equivalent real linear systems and then the real system is iteratively solved by the flexible generalized minimum residual method with a block diagonal pre-conditioner. This will greatly reduce the condition number of the linear system and thus speed up the solution process. We verify the accuracy of the proposed algorithm by comparing our results with the existing methods. After that, we design a subduction zone model to simulate the EM responses under isotropic and anisotropic environments, respectively. The numerical results show the high efficiency of the proposed algorithm and the response differences between isotropic and anisotropic models. This research can provide theoretical and technical support for the high-accuracy and efficient inversion of GDS data for the geo-dynamic study.

Deep Resistivity Structure of Basalt‐Covered Central Part of Paraná Basin, Brazil, From Joint 3‐D MT and GDS Data Imaging

AbstractThe deep structure beneath the basalt‐covered sediments of Paraná basin is not well understood. Magnetotelluric (MT) broadband (1,000 to 0.001 Hz) data recorded at 296 selected stations along three new survey lines were first inverted in three dimensions to reveal the local scale resistivity structure of the central part of the basin where there are three deep exploration wells with resistivity logs for model validation. The 3‐D MT inversion model correctly identified the resistive (>100 Ωm) flood basalts and postvolcanic cover layer, the underlying electrically conductive (~10 Ωm) sediments, and the highly resistive (>1,000 Ωm) crystalline basement at the well locations and provides a reliable average crustal resistivity structure consisting of ~20–24 km thick highly resistive upper crust and ~20 km thick conductive lower crust above a resistive mantle. Subsequent basin‐scale joint inversion of highest quality MT data sets (261 stations) and geomagnetic depth sounding array data imaged two prominent subparallel belts of N‐S and NNE‐SSW trending steep conductors in the deep crust and upper mantle, one located directly beneath the present‐day axis of thickest sedimentary deposits in the basin and another near the coastal location where dolerite dyke swarms intrude the outcropping Precambrian basement. Other roughly orthogonal bands (E‐W to NW‐SE) of linear crustal conductors radiate from these two main belts. The resulting networks of steep conductors at upper to midcrustal level seem to mark the fossil plumbing system of Paraná magmatic province and are interpreted as preexisting shear zones exploited by the voluminous Early Cretaceous magmatism in the region.

Residual layer thickness in nanoimprint: Experiments and coarse-grain simulation

A refined coarse-grain method for simultaneous calculation of the resist viscous flow in nanoimprint lithography and the stamp/substrate deformation is presented. The method has been realized in an effective simulation tool for the verification of stamp structures on standard personal computers, by using the GDS data of the stamp design. From the presented comparison of calculated and experimental results, it can be concluded that the coarse-grain modeling allows predicting the residual layer thickness with accuracy better than 10%.

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

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

Socioeconomic and geographical influences on primary dental care preferences in a population of young children

To compare the socioeconomic profiles of children registered in the GDS, with those using the CDS services and unregistered children. Secondly to examine the effects of socioeconomic status on travelling to access primary dental care, and finally to map out the location of unregistered children in relation to primary dental care services. The study was carried out in 1998 in Ellesmere Port in the North West of England. The study population was all children younger than 6 years who used primary dental care services in, or were residents of, Ellesmere Port. The study population was categorized into those registered with a GDS dentist, those using CDS services and those unregistered by matching GDS and CDS data to the HA population register. Socioeconomic status was measured using the Super Profiles geodemographic classification. The relationship between service preferences and travelling to access primary dental care with socioeconomic status were compared using cross-tabulations and chi square tests. There was a significant socioeconomic trend evident in the use of dental services. Two thirds of those using CDS services came from the most deprived area types. Of those who were unregistered half lived in the most deprived area types compared with one third of those registered with the GDS. Those who travelled into Ellesmere Port to access primary dental care were significantly more likely to live in an affluent area. Unregistered patients were homogeneously spread across the town. The ability to match GDS and CDS data to the HA population register is essential to understand how dental services are used by the local population. Children from deprived areas are more likely to use the CDS and a service local to their homes, therefore primary dental care services for deprived communities have to be provided locally.

A study of primary dental care provision in young children with special reference to deprived communities

Objective To compare the socioeconomic profiles of children registered in the GDS, with those using the CDS services and unregistered children. Secondly to examine the effects of socioeconomic status on travelling to access primary dental care, and finally to map out the location of unregistered children in relation to primary dental care services. Setting The study was carried out in 1998 in Ellesmere Port in the North West of England. Subjects and materials The study population was all children younger than 6 years who used primary dental care services in, or were residents of, Ellesmere Port. The study population was categorised into those registered with a GDS dentist, those using CDS services and those unregistered by matching GDS and CDS data to the HA population register. Socioeconomic status was measured using the Super Profiles geodemographic classification. The relationship between service preferences and travelling to access primary dental care with socioeconomic status were compared using cross-tabulations and chi square tests. Results There was a significant socioeconomic trend evident in the use of dental services. Two thirds of those using CDS services came from the most deprived area types. Of those who were unregistered half lived in the most deprived area types compared with one third of those registered with the GDS. Those who travelled into Ellesmere Port to access primary dental care were significantly more likely to live in an affluent area. Unregistered patients were homogeneously spread across the town. Conclusions The ability to match GDS and CDS data to the HA population register is essential to understand how dental services are used by the local population. Children from deprived areas are more likely to use the CDS and a service local to their homes, therefore primary dental care services for deprived communities have to be provided locally.

Magnetotellurics and seismotectonics in the analysis of active domains: An essential combination?

The geoelectric structural trends (GST) derived from the spatial analysis (rotation/decomposition) of MT and GDS data in the vicinity of active deformation zones, exhibit remarkable correlation with seismotectonic trends and nodal planes of earthquake mechanism solutions, in practically all the cases we have examined in Greece and abroad. In the upper crust of an active domain, the GST may be explained in terms of conductors formed by water infiltrating through fault planes and related discontinuities, aligned microcracks and anastomosing shear zones. If this interpretation is correct, the GST should be good indicators of the corresponding trends and location of active faulting. In this context, the spatial analysis of geoelectromagnetic data is important in constraining the geometry of active faulting and hence, the modes of deformation. Two characteristic examples are presented and discussed from the island of Milos, Greece, located in the Hellenic Volcanic Arc and experiencing extensional tectonics and the island of Terceira, the Azores, simultaneously experiencing normal and strike-slip faulting.

Inversion of GDS data of northwest Himalaya using EM2INV

This paper reviews the validity of earlier models obtained after quantitative interpretation of GDS data and presents a fresh model using the inversion scheme EM2INV. The 2-D inversion of data is more objective than the earlier interpretation performed by using trial and error method. The inversion results indicate that the present model differs from the earlier ones. The reason could be that available GDS data are sufficient only for deriving the horizontal variation of subsurface resistivity. In order to study the vertical resistivity variation additional MT sounding data would be required. It would therefore be desirable to carry out MT survey in the specified area. A more comprehensive/appropriate model could be derived from joint inversion of GDS and MT data.

The Present State of Art of Long Period Magnetotellurics in the Western Part of the Bohemian Massif

Research into the complex geological structure of the Middle European Variscides in the western part of the Bohemian Massif has largely intensified in connection with the KTB deep borehole project in the Oberpfalz, Germany. First results of long period magnetotelluric studies along an about 100 km long profile in SW Bohemia, close to the KTB drilling site, are presented here. They represent a partial step within a broader project of constructing a 3-D geoelectrical model of the western margin of the Bohemian Massif and resolving the relations of anomalous geoelectrical features to the geological structures of the region. The data roughly fit a composite model of a 2-D regional structure, striking E-W, overlaid by a layer of highly distorting near-surface inhomogeneities. The distorting layer displays large anisotropy with preferred conductivity in NW-SE to NNW-SSE direction. Attempt is made to estimate the absolute static shift of the MT curves. Several variants of 2-D models for the deep regional structure are discussed, including an anisotropic model which can qualitatively explain the discrepancy of the principal directions indicated by the MT and GDS data. The relation of the results obtained along the particular profile to the regional geoelectrical background is studied.

The BC87 Dataset: Application of Hypothetical Event Analysis on Distorted GDS Response Functions and Some Thin Sheet Modelling Studies of the Deep Crustal Conductor.

Galvanic distortion of the regional MT responses was inferred from the analyses of the BC87 dataset presented at the MT-DIW1 in 1992. Since perturbation currents also produce an anomalous magnetic field, the GDS response functions were suspected to be distorted in the long period range. In order to reveal the correct regional strike direction, hypothetical event analysis was applied to the data from all sites. With this technique, common regional information, such as strike and impedance phase, can be recovered from distorted response functions. Although the BC87 GDS data are noisy, and the distortion is more of an inductive than of a purely galvanic nature, the resulting regional strike direction of 60° agrees well with the azimuth suggested from previous MT decomposition methods. Thin sheet modelling studies of the geometry of the deep crustal conductor (L-shaped or oblique) at 1000 s imply that along the BC87 profile induction arrows are generally produced by current concentrations around a resistive region associated with the Nelson Batholith. Comparison of the model induction arrows with those determined from the BC87 data indicates that the model featuring the oblique, NE striking conductor (SABC) may be favoured.

The geoelectrical methods in geophysical exploration : M.S. Zhdanov & G.V. Keller, (Elsevier; Methods in Geochemistry and Geophysics, 31), ISBN (hardback) 0 444 89678 3, price Dfl 375.00 (US$214.25), 1994, 873 pp, index

Part 1 Electrical methods in geophysics: in the beginning the mind set and the future. Part 2 General concepts of electromagnetic field behaviour: Maxwell's equations diffusing electromagnetic fields static electromagnetic fields quasi-stationary electromagnetic fields. Part 3 Properties of rocks and minerals: properties and units properties in a parametric sense properties in an existential sense geoelectrical mesostructures and megastructures electrical properties of the atmosphere and the ocean. Part 4 Electromagnetic environment of planet Earth: Earth currents of external origin industrial noise atmospheric electric sources mechanically generated electric fields extraneous sources of electric fields electromagnetic fields generated by the oceans. Part 5 Direct current and induced polarization methods: point and dipole sources on a uniform Earth electric fields in one-dimensional Earth structures fields of a point source for two- and three-dimensional structures vertical electric sounding and apparent resistivity induced polarization magnetic field measurements. Part 6 Natural-field electromagnetic methods: the Tikhonov-Cagniard approach linearity of relationships between components of the magnetotelluric field electromagnetic fields in a horizontally homogenous medius electromagnetic fields in horizontally inhomogeneous media magnetotelluric and magnetovariational survey methods data reduction interpretation of MT data interpretation of MVP and GDS data. Part 7 Controlled source electromagnetic methods: principles of controlled source electromagnetic methods electromagnetic sounding electromagnetic profiling. Part 8 Modelling and simulation: exact analytical solutions for special cases thin sheet models the integral equation method finite difference modelling finite element method physical and analog modelling other approaches to modelling. Part 9 Insensitivity and ambiguity: insensitivity for a laterally uniform Earth ambiguity for a laterally uniform Earth. Part 10 Practical aspects of data acquisition: sources detection of electric and magnetic fields sampling filtering survey design. Part 11 Interpretation: rules, intuition and pragmatic interpretation pseudo-inversion of soundings by transformation inversion continuation of electromagnetic fields electromagnetic migration comprehensive three-dimensional interpretation. Part 12 Other platforms, other methodologies: airborne electromagnetic (AEM) methods marine electrical surveys bore hole assisted methods spontaneous polarization (SP) method ground penetrating radar (GPR) the piezoelectric method (PEP). Part 13 A baker's dozen of case histories: minerals exploration - Que River Deposit, Tasmania, Australia physical volcanology - Oshima volcano, Japan groundwater exploration - Apodi Valley, Rio Grande do Norte, Brazil. (Part Contents).

Screening, detection and management of depression in elderly primary care attenders. I: The acceptability and performance of the 15 item Geriatric Depression Scale (GDS15) and the development of short versions.

One-hundred and ninety-eight elderly subjects attending their general practitioners (GPs) were asked to complete the 15 item Geriatric Depression Scale (GDS15). Analysable results were obtained from 194 (98%). Of these, 67 (34%) scored above the GDS15 cut-off (4/5) for significant depressive symptomatology. 87.6% found the questionnaire to be acceptable and only 3.6% found it very difficult or very stressful. The GDS15 had a high level of internal consistency (Cronbach's alpha = 0.80). All the individual items of the GDS15 associated significantly (P < 0.01) with total score and 'caseness'. A single question "do you feel that your life is empty?" identified 84% of 'cases'. In an attempt to devise short scales to screen elderly primary care patients for depression, the data were subjected to logistic regression analysis. Ten (GDS10), four (GDS4) and on (GDS1) item versions were generated. Agreement between these short scales and the GDS15 in the original sample was 95, 91 and 79% respectively. Cronbach's alpha was 0.72 for the GDS10 and 0.55 for the GDS4. The short scales were then validated in an independent sample of 120 patients in whom both GDS data and the results of a detailed psychiatric interview (the Geriatric Mental Status Schedule, GMS) were available. The sensitivity and specificity of the GDS10 against GMS caseness were 87 and 77% (cut-off 3/4); those of the GDS4 were 89 and 65% (cut-off 0/1) and 61 and 81% (cut-off 1/2). Sensitivity and specificity for the GDS1 were 59 and 75%. It is concluded that these short scales may be useful in helping GPs and practice staff to identify elderly patients with significant depressive symptoms.