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1.
All explanations of the high-conductivity layers (HCL) found by magnetotellurics in the middle or lower crust incorporate a mixture of a low-conductivity rock matrix and a highly conductive phase, for example graphite or saline fluids. In most cases the bulk conductivity of the mixture does not depend on the conductivity of the rock matrix but rather (1) on the amount of high-conductivity material and, in particular, (2) on its geometry. The latter is quantitatively described by the parameter 'electrical connectivity'. Decomposition of the observed bulk conductivity of the mixture into these two parameters results in an ill-posed problem. Even if anisotropy occurs in the HCL, three output parameters (highly conductive phase fraction, connectivity with respect to the X direction, connectivity with respect to the Y direction) have to be estimated from the two bulk conductivities of the anisotropic HCL. The additional information required for solving this problem is provided if instead of single-site data the conductivities from many field sites are evaluated: a sample distribution of the conductivity can then be obtained. Ensembles of random networks are used to create theoretical distribution functions which match the empirical distribution functions to some extent. The use of random resistor networks is discussed in the context of other established techniques for the treatment of two-phase systems, such as percolation theory and the renormalization group approach. Models of embedded networks explain the discrepancy between 'small' anisotropy (2-3) on the laboratory scale and large anisotropy (10-100) found in electromagnetic field surveys encompassing volumes of several cubic kilometres. Strong anisotropy can indicate low electrical connectivity, and a possible explanation is that a network stays close to the percolation threshold.  相似文献   

2.
Summary. Fold belts form due to shortening of deep basins on oceaic and continental crust. Basins on the oceanic crust should be characterized by a pronounced seismic anisotropy in the mantle lithosphere. Deep basins on the continental crust may develop from the stretching or the destruction of the lower crust under asthenospheric upwelling. These processes can produce seismic anisotropy in both the crust and mantle lithosphere. The character of the anisotropy is different for different basin forming processes. Considerable anisotropy should also arise from compression of the crust and mantle in fold belts. The formation of fold belts produces the original seismic anisotropy in continental lithosphere.  相似文献   

3.
Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.  相似文献   

4.
Summary. The Oblique Seismic Experiment (OSE) has been proposed to increase the usefulness of the IPOD crustal borehole as a means of investigating layer 2 of oceanic crust. Specific objectives are: to determine the lateral extent of the structure intersected by the borehole, to analyse the role of cracks in the velocity structure of layer 2, to look for anisotropy which may be caused by large cracks with a preferred orientation and, finally, to measure attenuation in oceanic crust.
The first successful Oblique Seismic Experiment in oceanic crust was carried out in 1977 March in a hole 400 miles north of Puerto Rico. An adequate study of lateral velocity variations was impossible because the hole was not deep enough, the hole was inadequately logged, and the small scale basement topography was not known. In general both P - and S -wave velocity profiles suggest that the crack density decreases with depth in layer 2. Velocities at the bottom of layer 2 are the same as matrix velocities for basalt, implying that crack density may be negligible at this depth. No convincing evidence for anisotropy in either layer 2 or 3 is found from travel time analysis. The hole was not deep enough to measure attenuation from normal incidence shots and amplitudes were not consistent enough to obtain a measure of attenuation from long range shots.  相似文献   

5.
Summary. Integral equations are formulated to solve the problem of electromagnetic induction in the Earth in the presence of thin finitely conducting oceans. Unlike earlier models, the oceans are assumed to be in electrical contact with the mantle and leakage of oceanic electric current is therefore possible. A powerful iterative method is developed to solve the very large system of equations and is applied successfully to a model problem. A feature of the integral equations is that they lead to a good first approximation. When fed into the iterative scheme this enhances the rate of convergence. Variable mesh size also appears possible.  相似文献   

6.
Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent‐ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental‐crust domain is characterized by tilted fault blocks generally overlain by thick syn‐rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic‐crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic‐type features directly abut or lap on continental‐type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along‐strike variation in the tectonic structure of the continental margin. The NE‐most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental‐crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.  相似文献   

7.
Deep seismic reflection studies in Israel - an update   总被引：1，自引：0，他引：1  
Summary. The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.  相似文献   

8.
Summary. Deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust. However, a lack of relevant laboratory-based conductivity measurements makes interpretation of such soundings difficult. Laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0.4 GPa, temperatures up to 300°C, and with variable pore fluid pressure up to the confining pressure. Extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures, for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone, the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries. The lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and/or lower pore fluid pressures, perhaps due to hydration reactions. In areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure, i.e. low to high, with increasing depth seems more likely.  相似文献   

9.
Summary. We have analysed a thirty-six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north-east of Hawaii. The electromagnetic fields are dominated by tides which have an appreciable oceanic component, especially in the east electric and north magnetic components. The techniques of data analysis included singular value decomposition (SVD) to remove uncorrelated noise. There are three degrees of freedom in the data set for periods longer than five hours, indicating a correlation of the vertical magnetic field and the horizontal components, suggesting source field inhomogeneity. Tensor response functions were calculated using spectral band averaging with both SVD and least squares techniques and rotated to the principal direction. One diagonal component, determined mainly by the north electric and east magnetic fields, is not interpretable as a one-dimensional induction phenomenon. The other diagonal term of the response function indicates a rapid rise in conductivity to 0.05 mho m−1 near 160 km. No decrease in conductivity below this depth is resolvable. Polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km. Model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system.  相似文献   

10.
An algorithm for the numerical modelling of magnetotelluric fields in 2-D generally anisotropic block structures is presented. Electrical properties of the individual homogeneous blocks are described by an arbitrary symmetric and positive-definite conductivity tensor. The problem leads to a coupled system of partial differential equations for the strike-parallel components of the electromagnetic field. E x, and H x These equations are numerically approximated by the finite-difference (FD) method, making use of the integro-interpolation approach. As the magnetic component H x, is constant in the non-conductive air, only equations for the electric mode are approximated within the air layer. The system of linear difference equations, resulting from the FD approximation, can be arranged in such a way that its matrix is symmetric and band-limited, and can be solved, for not too large models, by Gaussian elimination. The algorithm is applied to model situations which demonstrate some non-trivial phenomena caused by electrical anisotropy. In particular, the effect of 2-D anisotropy on the relation between magnetotelluric impedances and induction arrows is studied in detail.  相似文献   

11.
Summary. The crustal structure beneath the Vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge. Relatively normal oceanic crust, but with an upwarped seismic Moho, was found under the transverse ridge. We suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it. A region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault, gradually decreasing in thickness as the fracture zone is approached. There is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley. Under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure. Seismic layer 3 is absent, and the seismic velocities are lower than normal. The absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault. The low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault. This same fracturing allows water to penetrate through the crust, and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic Moho below its original depth.  相似文献   

12.
Summary. The temperature field and rates of cooling and solidification of the oceanic crust and upper mantle at an ocean ridge have been calculated as a function of spreading rate. The thermal model of the accretion process incorporates latent heat release associated with solidification of the basalt. liquid forming the ocean crust and uses a heat supply boundary condition on the vertical ridge axis model boundary. It is assumed that while oceanic layer 2 cools rapidly by hydrothermal circulation, oceanic layer 3 cools predominantly by conduction. Basalt liquid injection into the upper part of oceanic layer 3 is shown to solidify instantaneously while that injected into lower crustal levels takes up to 0.4 Myr to solidify. Material solidifying instantaneously is interpreted as corresponding to the dolerite unit of the ocean crust while that taking a finite time to cool is interpreted as corresponding to the gabbroic unit. The rate of cooling of the crust is shown to be faster for slower spreading rates and consequently the thicknesses of the dolerite and gabbro units are predicted to thin and thicken respectively with increase in spreading rate. The width of the molten region, or magma chamber, within the crust at the ridge axis is shown to be approximately proportional to spreading rate with chamber half widths of 1.5 and 10.0 km for half spreading rate of 1.0 and 6.0 cm yr−1. Below a critical half spreading rate of about 0.65 cm yr−1 no molten region exists and the crust is entirely doleritic.  相似文献   

13.
A general correspondence principle is presented that relates any time-domain electromagnetic diffusion field to an electromagnetic wavefield in a 'corresponding' configuration. The principle applies to arbitrarily inhomogeneous and anisotropic media and arbitrary transmitters and receivers. For the correspondence between the two types of electromagnetic fields to hold, the electric conductivity in the diffusive case and the permittivity in the wavefield case should have the same spatial variation, while the permeability distributions in space in the two cases are to be identical. Essential steps in the derivation of the correspondence principle are the use of the time Laplace transformation of causal signals, taken at real, positive values of the transform parameter, the Schouten-Van der Pol theorem in the theory of the Laplace transformation, and the reliance upon Lerch's theorem of the uniqueness of the interrelation between causal field quantities and their time-Laplace-transform representations at real, positive values of the transform parameter. Correspondence is then established between the tensorial Green's functions in the two cases, where the Green's functions are the point-receiver responses (either electric or magnetic field) to point-transmitter excitations (either electric- or magnetic-current source).
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented.  相似文献   

14.
Anomalous seismic crustal structure of oceanic fracture zones   总被引：2，自引：0，他引：2  
Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment.  相似文献   

15.
Summary. Interpretation of the effects of natural electromagnetic induction is often in terms of models in which changes in resistivity at great distances from the point of measurement are not taken into account. In this paper we will try to show that this is not a valid approximation for very low-frequency telluric currents, flowing near the surface in the Earth's crust, when the material separating the crust from the mantle has a sufficiently high electrical resistance. In this case the shape and dimensions of the circuit, as well as the resistivity distribution along the whole length of the path followed by the currents, play a part in determining the induced electromagnetic field. A number of experimentally verifiable consequences follow from this.  相似文献   

16.
MT and reflection: an essential combination   总被引：3，自引：0，他引：3  
Summary. At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/tectonic interpretations of the seismic sections.
Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.  相似文献   

17.
Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.  相似文献   

18.
While the inversion of electromagnetic data to recover electrical conductivity has received much attention, the inversion of those data to recover magnetic susceptibility has not been fully studied. In this paper we invert frequency-domain electromagnetic (EM) data from a horizontal coplanar system to recover a 1-D distribution of magnetic susceptibility under the assumption that the electrical conductivity is known. The inversion is carried out by dividing the earth into layers of constant susceptibility and minimizing an objective function of the susceptibility subject to fitting the data. An adjoint Green's function solution is used in the calculation of sensitivities, and it is apparent that the sensitivity problem is driven by three sources. One of the sources is the scaled electric field in the layer of interest, and the other two, related to effective magnetic charges, are located at the upper and lower boundaries of the layer. These charges give rise to a frequency-independent term in the sensitivities. Because different frequencies penetrate to different depths in the earth, the EM data contain inherent information about the depth distribution of susceptibility. This contrasts with static field measurements, which can be reproduced by a surface layer of magnetization. We illustrate the effectiveness of the inversion algorithm on synthetic and field data and show also the importance of knowing the background conductivity. In practical circumstances, where there is no a priori information about conductivity distribution, a simultaneous inversion of EM data to recover both electrical conductivity and susceptibility will be required.  相似文献   

19.
Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s−1; an upper 0.8 s−1 layer overlies a 0.4 s−1 layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.  相似文献   

20.
The standard 1-D inversion approach for the interpretation of transient electromagnetic (TEM) data usually fails in the presence of near-surface conductivity anomalies. Since multidimensional inversion codes are not routinely available, the only alternative to discarding the data may be trial-and-error forward modelling. We interpret data from a long-offset transient electromagnetic (LOTEM) survey which was carried out in 1995 in the Odenwald area, using 2-D finite-difference modelling. We focus on a subsegment of the LOTEM profile, which was shot with two different electric dipole transmitters. A model is found which consistently explains the electric and magnetic field data at eight locations for both transmitters. First, we introduce a conductive dyke under the receiver spread to explain sign reversals in the magnetic field transients. A conductive slab under one of the transmitters is required to obtain a reasonable quantitative fit for that transmitter. Consideration of the electric field data then requires a modification of the layered earth background. Finally, we study the response of a crustal conductor, which was the original target of the survey. The data are sensitive to the conductor, and for the investigated subset of the data the fits are slightly better without the conductive layer.  相似文献   

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