地球深部物质电学性质实验研究

地球深部物质电学性质的实验研究是了解地幔热结构和地幔动力学特征的重要手段之一,已越来越受到地球物理学家的重视。介绍了地球深部物质电学性质实验研究的基本原理、基本方法、电导的影响因素、实验研究的意义和今后的研究方向。     

Structure and evolution of the Bay of Pozzuoli (Italy) using marine seismic reflection data: implications for collapse of the Campi Flegrei caldera

Digital marine seismic reflection data acquired in 1973 in the Bay of Pozzuoli, and recently reprocessed, were used to study the volcanological evolution of the marine sector of Campi Flegrei Caldera during the last 37 ka. In order to gain more information, interpretation also involved estimation of the "pseudo-velocity" and the "pseudo-density" from the resistivity logs of two onshore deep exploration wells. The main results are: (1) discovery of ancient pre-18 ka and post-37 ka submarine and mainly effusive volcanic activity, along coeval emission centers located at the edges of Campi Flegrei Caldera; (2) confirmation that the caldera collapse in the marine sector of Campi Flegrei seems strongly controlled by regional NE–SW and NW–SE structural discontinuities; (3) the finding of at least two episodes of collapse in the bay; and (4) identification of a post-18 ka volcanic deflation phase that has caused about 150–200 m of subsidence in the central sector of the Bay of Pozzuoli in the last 18 ka.Editorial responsibilty: T. Druitt     

Identification of karst features using seismic P-wave tomography and resistivity anisotropy measurements

A football stadium with a capacity of a hundred thousand spectators is under construction over a karst terrain, 10 km west of the old town of Istanbul, Turkey. A large cavity of approximately 30 m³ was detected beneath the sports field through a number of boreholes so that a geophysical survey was required to further investigate a portion of the sports field. We utilized seismic refraction tomography and dc-electrical method with rotated Wenner array to delineate zones with solution voids and cavities. Total core recovery (TCR) was 5–15% from boreholes where zones with low velocities were identified through tomographic inversion, whereas TCR values were above 60% in zones with higher velocities. Both low velocity zones in the tomographic images and increasing resistivity anisotropy with depth appear to indicate that the cavity extends toward the west and south at a depth of approximately 8 m, although the southward and westward extension changes in character.     

A comparison of electrical and electromagnetic methods for the detection of hydraulic pathways in a fractured rock aquifer,Clare Valley,South Australia

Within fractured rock, the irregular and often unpredictable distribution and geometry of hydraulically conductive fractures produces large spatial variations in bore yield and groundwater quality. As fractures act as conduits for flow of both groundwater and electrical charge, methods which can efficiently detect the distribution of electrical pathways can be used to infer characteristics of significant hydrological parameters. This study compares the capabilities and limitations of electrical data obtained from direct current (DC) and electromagnetic (EM) surface azimuthal measurements, and from DC borehole-to-surface and cross-borehole measurements, for the interpretation of major hydrological structures in Clare Valley, South Australia. Electrical and EM surface methods are limited by poor depth sensitivity and the presence of conductive overburden, but provide useful tools for determining directional variations in resistivity at sites lacking bedrock exposure and boreholes. Application of borehole-to-surface methods yielded a better-resolved interpretation of sub-vertical fracture strike and was useful in identifying lateral variations in bedrock heterogeneity. Improved flexibility and sensitivity to measurements at depth permitted cross-borehole electrical tomography data to be used in reconstructing the spatial distribution of sub-horizontal, laterally extensive, electrically conductive zones. While the technique is restricted to small-scale sites with multiple boreholes, inferences can be made on fluid connections over a much larger regional scale. It is important to note, however, that while electrical methods provide valuable information about in-situ hydraulic pathways, they do not provide a complete hydraulic characterisation. Such a task requires integration of surface and borehole geophysics, geologic mapping, sampling and pumping tests of wells with packed-off intervals.

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Resumen La geometría, y con frecuencia impredecible, distribución irregular de fracturas hidráulicamente conductivas dentro de roca fracturada genera variaciones espaciales grandes en producción de pozos y calidad de agua subterránea. Debido a que las fracturas actúan como conductos de flujo de agua y carga eléctrica, los métodos que detectan eficientemente la distribución de trayectorias eléctricas pueden utilizarse para inferir las características de parámetros hidrológicos significativos. Este estudio compara las capacidades y limitaciones de datos eléctricos obtenidos de mediciones azimutales superficiales electromagnéticas (EM) y de corriente directa (DC), y de mediciones de DC realizadas en la superficie y pozos así como mediciones realizadas entre pozos para la interpretación de estructuras hidrológicas principales en el Valle Clare, sur de Australia. Los métodos superficiales eléctricos y EM están limitados por sensitividad de profundidad pobre y la presencia de cubierta conductiva, pero aportan herramientas útiles para determinar variaciones direccionales de resistividad en sitios que carecen de pozos y afloramientos rocosos. La aplicación de métodos superficiales y de pozos aportan una mejor interpretación del rumbo de fracturas sub-verticales y fue útil en identificar variaciones laterales en la heterogeneidad del macizo rocoso. El mejoramiento de flexibilidad y sensitividad en las mediciones profundas permitió que los datos de tomografía eléctrica de los pozos fuera utilizado en la reconstrucción de la distribución espacial de zonas eléctricas conductivas, sub-horizontales y lateralmente extensas. Aunque la técnica está restringida a sitios de pequeña escala con múltiples pozos, puede realizarse inferencias sobre relaciones entre fluidos en una escala regional mucho más grande. Sin embargo, es importante notar que aunque los métodos eléctricos aportan información valiosa acerca de las trayectorias hidráulicas in-situ, aún no proporcionan una caracterización hidráulica completa. Esta tarea requiere integrar geofísica superficial y de pozos, mapeo geológico, muestreo y pruebas de bombeo en pozos con intervalos sin empaque.

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Résumé Au sein des roches fracturées, lirrégularité et limprédictible distribution et géométrie des fractures par lesquelles sécoulent leau souterraine produit de larges variations spatiales entre les débits des forages et les paramètres de la qualité de leau. Comme les fractures conduisent et leau et les charges électriques, les méthodes qui peuvent de manière efficiente détecter la distribution des courants électriques peuvent être utilisées pour analyser les caractéristiques des principaux paramètres hydrologiques. Cette étude compare les capacités et les limites des données produites par Courant Direct (DC en Anglais) et par mesure Electromagnétique azimutale de surface (EM en Anglais), et par DC entre surface et forage et entre plusieurs forages, pour linterprétation de la structure hydrologique de la Vallée de Clare, Australie du Sud. Les méthodes électriques et électromagnétiques sont limitées par la faible sensitivité à la profondeur et la présence dune couverture conductrice, mais procure des outils utiles pour déterminer les variations directionnelles en terme de résistivité là où la roche nest pas affleurante et dans les puits. Lapplication de la méthode dinvestigation «forage à surface» apporte une meilleure interprétation des fractures sub-verticales et des hétérogénéités latérales. Les tomographies électriques entre forage bénéficient de la flexibilité et de la sensibilité des mesures en profondeur, et permettent de délimiter lextension latérale des hétérogénéités sub-horizontales de zones conductrices. Alors que la technique est restreinte à de petits sites comprenant de nombreux forages, il est possible de reconstituer les connections hydrauliques à des échelles régionales. Il est important de noter que les méthodes fournissent des informations intéressantes mais pas des caractérisations hydrauliques complètes. Pour cela les données pourraient être complétées par des études plus poussées intégrant les différentes prospections géophysique, les données des cartes géologiques, des échantillonnages et des essais de pompage à différents intervalles de profondeur.

     

景德镇-温州大地电磁剖面深部电性结构研究

华南大陆历经长期复杂的构造演化,其浅部复杂而多样的构造形态与深部壳幔结构及其动力学机制有着密切联系,但当前的深部探测资料有限,深部构造并不明确。为了研究华南的深部岩石圈结构状态,布设了多条宽频长周期大地电磁联合探测剖面。通过对景德镇-温州剖面的数据处理,获得了该剖面的岩石圈电性结构模型。分析表明:华夏地块与扬子地块东部的江南造山带以江绍断裂为界;华夏地块整体呈现高阻特征,可能具有深部岩石圈背景的上虞-政和-大浦断裂将其分为华夏褶皱带与东南沿海岩浆岩带;扬子地块东部的江南造山带整体电阻率较低,结构更破碎,受到深部改造明显,岩石圈底界约为110 km;剖面发现3处低阻带通道,可能与软流圈上涌相关;江绍断裂呈喇叭状向深部张开,华夏地块与扬子地块的裂解拼合可能与断裂深部的高导体活动有关。华夏地块与扬子地块的接触状态呈现为双向汇聚,华夏地块在上地幔挤入扬子地块,深部接触边界可能越过江绍断裂带。     

Determining the condition of groundwater in evaluating the need for corrective measures: a case for the national groundwater-monitoring network in South Korea

Groundwater data (water level, electrical conductivity (EC) and temperature) have been collected since 1995 from the national groundwater-monitoring stations in South Korea. Recently, substantial groundwater-level decline and deterioration of groundwater quality were reported at several stations. Relevant authorities undertook to investigate the groundwater hazards and to devise mitigation measures for selected monitoring stations. However, there were no criteria to determine the status of the groundwater level and EC, compared with historic data. A methodology is presented, which defines the groundwater condition. Using the suggested criteria, water level and EC data are classified into normal, watch and warning levels, leading to different administrative measures and technical investigations. The primary criterion compares the observed values for the evaluation year (e.g. 2003) and the historic values of normal years preceding the evaluation year (e.g. 1996–2002), using box plots. The secondary criterion uses variation trends of the values with the aid of parametric and non-parametric trend tests. Final decisions are dependent on scores assigned to each test. According to the criteria suggested, detailed investigations and mitigation measures for water level decline are required for seven stations and those for water quality are needed for 18 stations because they reside within the warning classification.     

Integrated geophysical mapping of the Ifewara transcurrent fault system, Nigeria

Integrated geophysical methods involving magnetic and dipole–dipole resistivity (DDR) were conducted across a prominent zone of weakness clearly observable in Landsat MSS and SLAR images in the Precambrian basement complex of southwestern Nigeria. Up till now, the location and existence of this megascopic structure have not been confirmed using geophysical methods. With the objective of delineating this weak zone and its structural attributes, three traverses were established at 500 m intervals across it, and geophysical measurements were made at 10 m intervals along these traverses. Qualitative interpretation of the magnetic data obtained shows a diagnostic signature of a near-vertical fault, trending along a NNE–SSW direction. Also, the quantitative interpretation of the data using the non-linear least-squares regression technique indicates that the width of the magnetic anomaly ranges from 90 to 150 m, its dip angle varies between 75° and 85°; the anomaly is concealed by a regolith of approximately 15 m thickness. Furthermore, a 2D resistivity inversion of the field resistivity data reveals a three-layer model, representing thin resistive topsoil underlain by weathered bedrock, resistive bedrock with a distinct low resistivity zone located within the bedrock. The most plausible explanation for this low resistivity zone is that it was formed by shearing activities during Late Precambrian times. Conclusively, the integrated approach employed in this research confirms the existence of the supposed Ifewara shear zone (ISZ).     

http://www.sciencedirect.com/science/article/pii/S1674987112000254

The applications of intelligent techniques have increased exponentially in recent days to study most of the non-linear parameters.In particular,the behavior of earth resembles the non-linearity applications.An efficient tool is needed for the interpretation of geophysical parameters to study the subsurface of the earth.Artificial Neural Networks(ANN) perform certain tasks if the structure of the network is modified accordingly for the purpose it has been used.The three most robust networks were taken and comparatively analyzed for their performance to choose the appropriate network.The single-layer feed-forward neural network with the back propagation algorithm is chosen as one of the well-suited networks after comparing the results.Initially,certain synthetic data sets of all three-layer curves have been taken for training the network,and the network is validated by the Held datasets collected from Tuticorin Coastal Region(78°7′30″E and 8°48′45″N),Tamil Nadu.India.The interpretation has been done successfully using the corresponding learning algorithm in the present study.With proper training of back propagation networks,it tends to give the resistivity and thickness of the subsurface layer model of the field resistivity data concerning the synthetic data trained earlier in the appropriate network.The network is trained with more Vertical Electrical Sounding(VES) data,and this trained network is demonstrated by the field data.Groundwater table depth also has been modeled.     

Global 1° × 1° thermal model TC1 for the continental lithosphere: Implications for lithosphere secular evolution

This paper reports a new 1° × 1° global thermal model for the continental lithosphere (TC1). Geotherms for continental terranes of different ages (> 3.6 Ga to present) constrained by reliable data on borehole heat flow measurements (Artemieva, I.M., Mooney, W.D. 2001. Thermal structure and evolution of Precambrian lithosphere: a global study. J. Geophys. Res 106, 16387–16414.), are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low-quality heat flow data (ca. 60% of the continents). These data are supplemented by cratonic geotherms based on electromagnetic and xenolith data; the latter indicate the existence of Archean cratons with two characteristic thicknesses, ca. 200 and > 250 km. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1° × 1° grid and combined with the statistical age relationship of continental geotherms (z = 0.04  t + 93.6, where z is lithospheric thermal thickness in km and t is age in Ma) formed the basis for a new global thermal model of the continental lithosphere (TC1). The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle, with the strongest lateral temperature variations (as large as 800 °C) in the shallow mantle. A map of the depth to a 550 °C isotherm (Curie isotherm for magnetite) in continental upper mantle is presented as a proxy to the thickness of the magnetic crust; the same map provides a rough estimate of elastic thickness of old (> 200 Ma) continental lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle.Statistical analysis of continental geotherms reveals that thick (> 250 km) lithosphere is restricted solely to young Archean terranes (3.0–2.6 Ga), while in old Archean cratons (3.6–3.0 Ga) lithospheric roots do not extend deeper than 200–220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei; it is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the growth rate of the lithosphere since the Archean, which does not reveal a peak in lithospheric volume at 2.7–2.6 Ga as expected from growth curves for juvenile crust.A pronounced peak in the rate of lithospheric growth (10–18 km³/year) at 2.1–1.7 Ga (as compared to 5–8 km³/year in the Archean) well correlates with a peak in the growth of juvenile crust and with a consequent global extraction of massif-type anorthosites. It is proposed that large-scale variations in lithospheric thickness at cratonic margins and at paleoterrane boundaries controlled anorogenic magmatism. In particular, mid-Proterozoic anorogenic magmatism at the cratonic margins was caused by edge-driven convection triggered by a fast growth of the lithospheric mantle at 2.1–1.7 Ga. Belts of anorogenic magmatism within cratonic interiors can be caused by a deflection of mantle heat by a locally thickened lithosphere at paleosutures and, thus, can be surface manifestations of exceptionally thick lithospheric roots. The present volume of continental lithosphere as estimated from the new global map of lithospheric thermal thickness is 27.8 (± 7.0) × 10⁹ km³ (excluding submerged terranes with continental crust); preserved continental crust comprises ca. 7.7 × 10⁹ km³. About 50% of the present continental lithosphere existed by 1.8 Ga.     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km². Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.