Curie Point Depths of Macedonia and Thrace, N. Greece

—?The aeromagnetic data of Macedonia and Thrace were used to produce Curie point estimates. The data were high pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by a DGRF. The depth to the centroid, z ₀, of the deepest distribution of the magnetic dipoles was obtained by computing a least-squares fit to the lowest-frequency segment of the azimuthally averaged log power spectrum. The average depth to the top of the deepest crustal block was computed as the depth to the top, z _t , of the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest magnetic dipoles, the inferred Curie point depth, was then calculated from z _b =2z _0???z _t . The Curie depth estimates for Macedonia and Thrace range between 11.2 and 17.3?km. These results are consistent with the depths inferred by extrapolating known geothermal gradient and heat-flow values.     

Curie Point Depth of the Island of Crete (Greece)

—The aeromagnetic data of the island of Crete were inverted to produce Curie point estimates. The data were high-pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by IGRF. The depth to the centroid, z ₀?, of the deepest distribution of the magnetic dipoles, was obtained by computing a least-squares fit to the lowest-fre quency segment of the azimuthally averaged log power spectrum. The depth to the top of the deepest crustal block was computed as the depth, z _t ?, to the centroid of the second deepest distribution, using the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest dipoles, the inferred Curie point depth, is then z _b = 2z ₀?z _t ?. The Curie depth estimates range between 24 and 28 km. This is in accordance with the depths inferred by extrapolating heat-flow values measured in boreholes.     

Interpretation of Magnetic Anomalies and Estimation of Depth of Magnetic Crust in Slovakia

The magnetic map of Slovakia used in the paper was compiled as part of a project titled Atlas of Geophysical maps and profiles in 2001. The residual magnetic data were analyzed to produce Curie point estimates. To remove distortion of magnetic anomalies caused by the Earth’s magnetic field, reduction to pole transformation was applied to the magnetic anomalies using the magnetization angle of the induced magnetization. Anomalies reduced to the pole tend to be better correlated with tectonic structures. We applied a 3-km upward continuation to the residually compiled magnetic anomalies in order to remove effects of topography. The depth of magnetic dipoles was calculated by an azimuthally averaged power spectrum method for the entire area. Such estimates can be indicative of temperatures in the crust, since magnetic minerals lose their spontaneous magnetization according to Curie temperature of the dominant magnetic minerals in the rocks. The computed Curie point depths in the Slovakia region vary between 15.2 km and 20.9 km. Heat flow higher than 100 mWm⁻² occurs at the central volcanics and eastern part of Slovakia, where the Curie point depths values are shallow. The correlation between Curie point depths, heat flow and crust depth was investigated for two E-W cross sections. Heat flow and Curie point depth values are correlated with each other however, these values could not be correlated with crust depth. The Curie point isotherm, which separates magnetic and non-magnetic parts of the crust, is represented in two cross sections.     

Curie point depth from spectral analysis of aeromagnetic data from Cerro Prieto geothermal area,Baja California,México

Using aeromagnetic data acquired in the area from the Cerro Prieto geothermal field, we estimated the depth to the Curie point isotherm, interpreted as the base of the magnetic sources, following statistical spectral-based techniques. According to our results the Curie point isotherm is located at a depths ranging from 14 to 17 km. Our result is somewhat deeper than that obtained previously based only in 2-D and 3-D forward modeling of previous low-quality data. However, our results are supported by independent information comprising geothermal gradients, seismicity distribution in the crust, and gravity determined crustal thickness. Our results imply a high thermal gradient (ranging between 33 and 38 °C/km) and high heat flow (of about 100 mW/m²) for the study area. The thermal regime for the area is inferred to be similar to that from the Salton trough.     

Magnetic Signatures and Curie Surface Trend Across an Arc–Continent Collision Zone: An Example from Central Philippines

Ground and aeromagnetic data are combined to characterize the onshore and offshore magnetic properties of the central Philippines, whose tectonic setting is complicated by opposing subduction zones, large-scale strike-slip faulting and arc–continent collision. The striking difference between the magnetic signatures of the islands with established continental affinity and those of the islands belonging to the island arc terrane is observed. Negative magnetic anomalies are registered over the continental terrane, while positive magnetic anomalies are observed over the Philippine Mobile Belt. Several linear features in the magnetic anomaly map coincide with the trace of the Philippine Fault and its splays. Power spectral analysis of the magnetic data reveals that the Curie depth across the central Philippines varies. The deepest point of the magnetic crust is beneath Mindoro Island at 32 km. The Curie surface shallows toward the east: the Curie surface is 21 km deep between the islands of Sibuyan and Masbate, and 18 km deep at the junction of Buruanga Peninsula and Panay Island. The shallowest Curie surface (18 km) coincides with the boundary of the arc–continent collision, signifying the obduction of mantle rocks over the continental basement. Comparison of the calculated Curie depth with recent crustal thickness models reveals the same eastwards thinning trend and range of depths. The coincidence of the magnetic boundary and the density boundary may support the existence of a compositional boundary that reflects the crust–mantle interface.     

Soil water content vertical profiles under natural conditions: matching of experiments and simulations by a conceptual model

The prediction of soil moisture content, θ, as a function of depth, z, and time, t, is of fundamental importance for applications in many hydrological processes. The main objective of this paper is to provide an approach to solve this problem at a local scale in soils with vegetation. The matching of soil moisture vertical profiles observed under natural conditions in grassy plots and their simulations by a conceptual model is presented. Experimental measurements were performed in a plot located in Central Italy, complete with hydrometeorological sensors specifically set up and equipped with a time domain reflectometry system providing the water content, θ_e(z, t). A conceptual model framework earlier proposed for two‐layered soil vertical profiles was modified and adopted for simulations. The changes concern the incorporation of evapotranspiration, the reduction of the original model for applications also to homogeneous soil vertical profiles, and a correction for the differences existing between assumed and observed initial moisture contents. In the model calibration, it was found that the effects of vegetation could be represented adequately by a fictitious soil vertical profile with a more permeable upper layer of saturated hydraulic conductivity, K_s, independent of time. Then, for the validation events, the model simulations in the stages of both infiltration and redistribution/evapotranspiration reproduced appropriately θ_e(z, t) with typical values of root mean square error in the range 0.0017–0.0657. Similar results were obtained by applying the modified two‐layered model for simulations of experimental data observed in three other plots located in Northern Italy and Germany. For all four vegetated sites, the two‐layer profile better matched the experimental data than the assumption of a homogeneous profile. Thus, the conceptual approach based on a two‐layered scheme for representing θ(z, t) in soils with vegetation appears to be appropriate for many hydrological applications. Copyright © 2013 John Wiley & Sons, Ltd.     

The α-effect in 2D fast dynamos

We look at the large-scale dynamo properties of spatially periodic, time dependent, helical 2D flows of the form u(x, t)?=?(? _y ?ψ?(x, y, t), ?? _x ?ψ?(x, y, t), ?ψ (x, y, t). These flows act as kinematic fast dynamos and are able to generate a mean magnetic field uniform and constant in the xy-plane but whose direction varies periodically along z with wavenumber k. Using Mean Field Electrodynamics, the generation mechanism can be understood in terms of a k-dependent α-effect, which depends on the magnetic Reynolds number, R _m . We calculate this effect for different motions and investigate how its limit as k?→?0 depends on R _m and on the properties of the flows such as their spatial structure or correlation time. This work generalises earlier studies based on 2D steady flows to motions with time dependence.     

SIGNAL ET BRUIT EN MAGNETOTELLURIQUE *

Conventional processes of extracting magnetotelluric signals from noisy records are reviewed: instrument noises and noises that are generated close to the detectors can be eliminated by the usual auto- and crosscorrelation processes. Identification of coherent noises, such as pulses due to field sources that are not uniform over at least 100 km in oil exploration or 1000 km in crustal studies, is much more tedious. The 5 components H_x, H_y, H_z, E_x, E_y, of the magnetotelluric field have been recorded in many areas in France at different periods of the year, (a) in non-uniform field sources in the vicinity of electric railways and of 50 cycle power lines, and (b) in areas of strong inhomogeneity at depth on the flanks of steep structures and near the sea shore. Means for detecting non-uniformity are reviewed. Measuring the vertical component of magnetic pulses is a good way of estimating field uniformity: if H vertical/H horizontal <10%, the uniform field assumption is valid, and the classical restitution formulas can be used; if H vertical/H horizontal > 10%, uniformity can not be assumed and there is some difficulty in deciding whether non-uniformity is due to the field source or to anisotropy or inhomogeneities at depth. Several ways to solve this difficulty are described. The reliability of calculation of actual resistivity at various depths is examined as a function of the precision of apparent resistivity measurements.     

Electromagnetic scattering by a perfectly conducting half-plane in a conductive half-space

FollowingDmitriev (1960) a rigorous theoretical solution for the problem of scattering by a perfectly conducting inclined half-plane buried in a uniform conductive half-space has been obtained for plane wave excitation. The resultant integral equation for the Laplace transform of scattering current in the half-plane is solved numerically by the method of successive approximation. The scattered fields at the surface of the half-space are found by integrating the half-space Green's function over the transform of the scattering current.The effects of depth of burial and inclination, of the half-plane on the scattered fields are studied in detail. An increase in the depth of burial leads to attenuation of the fields. Inclination introduces asymmetry in the field profiles beside affecting its magnitude. Depth of exploration is greater for quadrature component. An interpretation scheme based on a phasor diagram is presented for the VLF-EM method of exploration for rich vein deposits in a conductive terrain.List of symbols x, y, z Space co-ordinates - Half-space conductivity - ₀ Free-space permeability - Excitation frequency (angular) - T Time - h Depth of the half-plane - a Inclination of the half-plane - E _x x-Directed total electric field - E _x ^p x-Directed primary electric field - E _xo ^p x-Directed primary electric field atz=0 directly over the half-plane - H _y y-Component of total magnetic field - H _y ^p y-Component of primary magnetic field - H _y0 ^p y-Component of primary magnetic field atz=0 directly over the half-plane - H _z z-Component of total magnetic field - H _z ^p z-Component of primary magnetic field - J _x Surface density ofx-directed scattering current - G Green's function - k ₀,K Wave numbers - u,u ₀,u ₁,u ₂ Functions - Space co-ordinate - s Variable in transform domain - Variable of integration - Normalized scattering current - Laplace transform of - N Normalized - , ₀, ₁, ₂ Functions - t Variable of integration - Skin depth - H Total magnetic field - H ^p Primary magnetic field - H ₀ ^p Primary magnetic field atz=0 directly over the half-plane - M,Q,R,S,U,V Functions - N ₁,N ₂ Functions     

A METHOD OF WEATHERING CORRECTION *

In western Canada, the near surface weathered zone has variable velocity and is underlain by more uniform subweathering material. The uphole time is often used to compensate for the weathered zone, but there are limitations to its use. A method is proposed using the refraction delay time which is exactly proportional to the weathering correction time for several commonly assumed weathering velocity functions. The method does not require accurate determination of velocities or weathering depth nor even the depth of shot, yet is theoretically exact. A strip off time t_r is used to replace the weathered zone mathematically with high velocity material where t_r= zD/F, where D is the delay time and F the Blondeau factor. An algorithm using first break times is proposed that is amenable to computer use for determining D for each geophone point. F might be constant in some areas but is more likely a variable to be solved in an automatic static program. Thus, structure and residual statics can be handled at once.     

Rotating deep annulus convection: The steady high amplitude wave regime—Internal thermal fields

Abstract

In the high-amplitude wave regime, at each point in the axial plane, the temperature has a maximum, a minimum, and a time (or longitudinally) averaged value due to the propagation of the regular waves. The axial-plane profile of each of these three temperature fields was measured for 53 different combinations of ω, Δr _w T and n. A few sample profiles are displayed.

Correlations of several internal thermal parameters (especially Ro _g,t ) with the externally imposed parameters, are given along with the spatial variation of the thermal fields. Ro _g,t is always less than the value which would occur if the symmetrical regime existed, while the variation of Sz _g,t with the imposed parameters is about the same as for the symmetrical regime. The parametric variation of the wave amplitude is the only quantity dependent on n. Based on the parametric variation of the fields, the wave regime diagram can be subdivided into several different regions : a quasi-conductive, a conductive-convective zone, and a region at large rotation rates and temperature differences characterized by Ro _g,t = Ro _g,w ², Sz _o,l = Ro _g,w , and ΔT′ = (z′)^0.6.

The Ro _g, l fields are compared to existing theory and some of the experimental stability diagrams of Fultz et al. (1964) are reconstructed using the internal instead of the externally imposed Rossby number. The transition from three to four waves via a “minor wave” is illustrated.     

<Emphasis Type="Italic">Lg</Emphasis> Body-Wave Magnitude Scaling for the Continental Margin around Korea and Japan

Regional body-wave magnitude scalings are essential for quantification of small and moderate-size earthquakes that are observed only up to regional distances. Crustally-guided shear waves, Lg, develop stably at regional distances in continental crusts and are minimally influenced by the source radiation patterns. Lg body-wave magnitude scalings, m_b(Lg),m_b(Lg), are widely used for assessment of sizes of regional crustal events. The m_b(Lg)m_b(Lg) scaling has rarely been tested in continental margins where Lg waves are significantly attenuated due to abrupt lateral variation of crustal structures. We test the applicability of m_b(Lg)m_b(Lg) scaling to the eastern margin of the Eurasian plate around the Korean Peninsula and Japanese islands. Both third-peak and root-mean-square (rms) amplitudes of Lg vary significantly according to the crustal structures along raypaths, causing apparent underestimation of m_b(Lg).m_b(Lg). Implementation of raypath-dependent quality factors (Q) allows accurate estimation of m_b(Lg),m_b(Lg), retaining the transportability of m_b(Lg)m_b(Lg) in the continental margin around Korea and Japan. The calibration constants for an rms-amplitude-based m_b(Lg)m_b(Lg) scaling are not determined to vary by region in the continental margin due to complicated crustal structures. The calibration constants are determined to be distance-dependent. Both the third-peak-amplitude-based and rms-amplitude-based m_b(Lg)m_b(Lg) scalings yield accurate magnitude estimates when raypath-dependent quality factors are implemented.     

The Displacement and Strain Field of Three-dimensional Rheologic Model of Earthquake Preparation

Based on the three-dimensional elastic inclusion model proposed by Dobrovolskii, we developed a rheological inclusion model to study earthquake preparation processes. By using the Corresponding Principle in the theory of rheologic mechanics, we derived the analytic expressions of viscoelastic displacement U(r, t) , V(r, t) and W(r, t), normal strains ε_xx (r, t), ε_yy (r, t) and ε_zz (r, t) and the bulk strain θ (r, t) at an arbitrary point (x, y, z) in three directions of X axis, Y axis and Z axis produced by a three-dimensional inclusion in the semi-infinite rheologic medium defined by the standard linear rheologic model. Subsequent to the spatial-temporal variation of bulk strain being computed on the ground produced by such a spherical rheologic inclusion, interesting results are obtained, suggesting that the bulk strain produced by a hard inclusion change with time according to three stages (α, β, γ) with different characteristics, similar to that of geodetic deformation observations, but different with the results of a soft inclusion. These theoretical results can be used to explain the characteristics of spatial-temporal evolution, patterns, quadrant-distribution of earthquake precursors, the changeability, spontaneity and complexity of short-term and imminent-term precursors. It offers a theoretical base to build physical models for earthquake precursors and to predict the earthquakes.     

A statistical study of the interplanetary magnetic field control of sporadic E-layer occurrence in the southern polar cap ionosphere

The influence of the interplanetary magnetic field (IMF) on the occurrence of sporadic E (Es)-layers in the southern polar cap ionosphere has been investigated. We statistically analysed ionogram and Doppler velocity observations made using a HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E; 81°S magnetic latitude) during the two summer campaign intervals 1 January to 18 February, and 1 November to 31 December 1997. The ionogram and Doppler velocity measurements were used to determine the Es-occurrence and electric field vectors (assuming E×B/B² drift), respectively. Concurrent IMF data were obtained from measurements made on board the Wind spacecraft. First, the gross properties of the IMF dependence of Es-formation were obtained: the occurrence rate was higher for negative B_y and/or positive B_z, and lower for positive B_y and/or negative B_z. To reconcile these gross properties with the electric field theory of Es-layer formation, the detailed diurnal variation of both Es-occurrence and the ionospheric electric field were obtained for different orientations of the IMF. The main statistical results are that: (1) the B_y component mainly controls the occurrence of the midnight Es-layers through its influence on the corresponding South West electric field; and (2) the B_z component mainly controls the occurrence of the evening Es-layers. However, the change in the occurrence rate for evening Es-layers was not related to the strength of the associated North West and North East electric fields. The total occurrence of Es-layers depended more on B_y than on B_z, owing to the dominance of B_y-controlled midnight Es-layers in the occurrence distribution. Nevertheless, the dependence of Es-occurrence on B_z was important. We suggest that the increase in Es-occurrence for positive B_z might be explained by the intermittent production of lower F-region ionisation by polar showers and squalls, which also increase in frequency and intensity for positive B_z. The importance of metallic ion transport within the ionosphere is also considered.     

APPROXIMATE INVERSION OF AIRBORNE EM DATA FROM A MULTILAYERED GROUND1

A complex transfer function c (or generalized skin depth) can be derived from data for the secondary magnetic field measured by a dipole system with small coil spacing at height h above the ground. This function has a useful property: For a uniform or layered ground, the real part of c yields the‘ centroid depth’z* of the in-phase current system as a function of frequency. This parameter can be combined with the apparent resistivity ρ_a derived by conventional methods. The function ρ_a(z*), if known over a broad frequency range, yields a smoothed approximation of the true distribution ρ(z) without an initial model. The relations between ρ_a(z*) and ρ(z) are studied for a number of multilayer models. An example of the application of the ρ_a*) algorithm to data from a groundwater survey is given.     

Generation of superDreicer electric fields in the solar chromosphere

The electric field generation at the front of the current pulse, which originates in a coronal magnetic loop owing to the development of the Rayleigh–Taylor magnetic instability at loop footpoints, has been considered. During the τ_A ≈ l/V _A ≈ 5?25 s time (where l is the plasma plume height entering a magnetic loop as a result of the Rayleigh–Taylor instability), a disturbance related to the magnetic field tension B _?(r,t), “escapes” the instability region with the Alfvén velocity in this case. As a result, an electric current pulse I_z(z ? V _A t), at the front of which an induction magnetic field E _z, which is directed along the magnetic tube axis and can therefore accelerate particles, starts propagating along a magnetic loop with a characteristic scale of Δξ ≈ l. In the case of sufficiently large currents, when B _? ²/8π > p, an electric current pulse propagates nonlinearly, and a relatively large longitudinal electric field originates E _z ≈ 2I _z ³ V _A/c ⁴a²B_z ²l, which can be larger than the Dreicer field, depending on the electric current value.     

A superposed epoch analysis of geomagnetic storms

A superposed epoch analysis of geomagnetic storms has been undertaken. The storms are categorised via their intensity (as defined by the Dst index). Storms have also been classified here as either storm sudden commencements (SSCs) or storm gradual commencements (SGCs, that is all storms which did not begin with a sudden commencement). The prevailing solar wind conditions defined by the parameters solar wind speed (v_sw), density (_sw) and pressure (P_sw) and the total field and the components of the interplanetary magnetic field (IMF) during the storms in each category have been investigated by a superposed epoch analysis. The southward component of the IMF, appears to be the controlling parameter for the generation of small SGCs (-100 nT< minimum Dst\leq-50 nT for\geq4 h), but for SSCs of the same intensity solar wind pressure is dominant. However, for large SSCs (minimum Dst\leq-100 nT for \geq4 h) the solar wind speed is the controlling parameter. It is also demonstrated that for larger storms magnetic activity is not solely driven by the accumulation of substorm activity, but substantial energy is directly input via the dayside. Furthermore, there is evidence that SSCs are caused by the passage of a coronal mass ejection, whereas SGCs result from the passage of a high speed/ slow speed coronal stream interface. Storms are also grouped by the sign of B_z during the first hour epoch after the onset. The sign of B_z at t=+1 h is the dominant sign of the B_z for \sim24 h before the onset. The total energy released during storms for which B_z was initially positive is, however, of the same order as for storms where B_z was initially negative.     

A discussion of structural and thermal control of magnetic anomalies on the mid-Norwegian margin

We discuss the correlation between the depth extent of magnetic sources, the Curie temperature depth and crustal structures on the mid-Norwegian margin. Spectral methods can be used to estimate the depth extent of magnetic sources, even if the bottom is located in the lower crust, however, only with limited resolution. The bottom of the magnetic surfaces is often regarded to represent the depth to the Curie isotherm. However, comparison with a 3D model based on the interpretation of potential field and seismic reflection data and thermal modelling shows that the depth extent of the magnetic sources is merely controlled by the overall geometry of the crystalline crust and not the temperature distribution. The observed changes in the magnetic field between the inner and outer part of the mid-Norwegian margin appears not to reflect, as previously assumed, the depth to the Curie temperature but the geometry of the basement and lower crust. Our 3D model of the mid-Norwegian margin reveals a basement configuration that involves a basement with different petrophysical properties, which can be connected with lithological basement units of onshore Norway.