Layered Earth Research Articles

First geodetic observations of a deep earthquake: The 2013 Sea of Okhotsk Mw 8.3, 611 km-deep, event

We analyze the first ever GPS observations of static surface deformation from a deep earthquake: the 24 May 2013 Mw 8.3 Sea of Okhotsk, 611 km-deep, event. Previous studies of deep earthquake sources relied on seismology and might have missed evidence for slow slip in the rupture. We observed coseismic static offsets on a GPS network of 20 stations over the Sea of Okhotsk region. The offsets were inverted for the best fitting double-couple source model assuming a layered spherical Earth. The seismic moment calculated from static offsets is only 7% larger than the seismological estimate from Global Centroid Moment Tensor (GCMT). Thus, GPS observations confirm shear faulting as the source model, with no significant slow-slip component. The relative locations of the U.S. Geological Survey hypocenter, GCMT centroid, and the fault from GPS indicate slip extending for tens of kilometers across most of the slab thickness.

Modeling deformation induced by seasonal variations of continental water in the Himalaya region: Sensitivity to Earth elastic structure

AbstractStrong seasonal variations of horizontal and vertical positions are observed on GPS time series from stations located in Nepal, India, and Tibet (China). We show that this geodetic deformation can be explained by seasonal variations of continental water storage driven by the monsoon. For this purpose, we use satellite data from the Gravity Recovery and Climate Experiment to determine the time evolution of surface loading. We compute the expected geodetic deformation assuming a perfectly elastic Earth model. We consider Green's functions, describing the surface deformation response to a point load, for an elastic homogeneous half‐space model and for a layered nonrotating spherical Earth model based on the Preliminary Reference Earth Model and a local seismic velocity model. The amplitude and phase of the seasonal variation of the vertical and horizontal geodetic positions can be jointly adjusted only with the layered Earth model, while an elastic half‐space model fails, emphasizing the importance of using a realistic Earth elastic structure to model surface displacements induced by surface loading. We demonstrate, based on a formal inversion, that the fit to the geodetic data can be improved by adjusting the layered Earth model. Therefore, the study also shows that the modeling of geodetic seasonal variations provides a way to probe the elastic structure of the Earth, even in the absence of direct measurements of surface load variations.

Seismic Impact of the Mw 9.0 Tohoku Earthquake in Eastern China

Abstract The 2011 Tohoku earthquake ( M w 9.0) in Japan caused significant coseismic displacement in much of eastern China, where seismic activity has been intense and damaging earthquakes numerous. Did the Tohoku earthquake change the stress field and seismic activity in eastern China? Here, we used a viscoelastic deformation model of layered spherical earth to calculate the coseismic and postseismic displacements and compared the results with Global Positioning System data for eastern China. Using the best‐fitting model, we then calculated the coseismic and postseismic Coulomb stress changes on major active faults in eastern China. Our results show that the Coulomb stress changes caused by the Tohoku earthquake on these faults are less than 0.002 MPa, so were unlikely to significantly alter the stress field or trigger earthquakes in eastern China. Online Material: Tables of coseismic displacement at 67 stations and time‐dependent displacements before and after the M 9.0 Tohoku earthquake.

Effect of Loop Geometry on TEM Response Over Layered Earth

A large horizontal loop located on the ground or carried by an aircraft are the most common sources of the transient electromagnetic method. Although topographical factors or airplane outlines make the loop of arbitrary shape, magnetic sources are generally represented as a magnetic dipole or a circular loop, which may bring about significant errors in the calculated response. In this paper, we present a method for calculating the response of a loop of arbitrary shape (for which the description can be obtained by different methods, including GPS localization) in air or on the surface of a stratified earth. The principle of reciprocity is firstly used to exchange the functions of the transmitting loop and the dipole receiver, then the response of a vertical or a horizontal magnetic dipole is calculated beforehand, and finally the line integral of the second kind is employed to get the transient response. Analytical analysis and comparisons depict that our work got very good results in many situations. Synthetic and field examples are given in the end to show the effect of loop geometry and how our method improves the precision of the EM response.

1-D DC Resistivity Modeling and Interpretation in Anisotropic Media Using Particle Swarm Optimization

We examine the one-dimensional direct current method in anisotropic earth formation. We derive an analytic expression of a simple, two-layered anisotropic earth model. Further, we also consider a horizontally layered anisotropic earth response with respect to the digital filter method, which yields a quasi-analytic solution over anisotropic media. These analytic and quasi-analytic solutions are useful tests for numerical codes. A two-dimensional finite difference earth model in anisotropic media is presented in order to generate a synthetic data set for a simple one-dimensional earth. Further, we propose a particle swarm optimization method for estimating the model parameters of a layered anisotropic earth model such as horizontal and vertical resistivities, and thickness. The particle swarm optimization is a naturally inspired meta-heuristic algorithm. The proposed method finds model parameters quite successfully based on synthetic and field data. However, adding 5 % Gaussian noise to the synthetic data increases the ambiguity of the value of the model parameters. For this reason, the results should be controlled by a number of statistical tests. In this study, we use probability density function within 95 % confidence interval, parameter variation of each iteration and frequency distribution of the model parameters to reduce the ambiguity. The result is promising and the proposed method can be used for evaluating one-dimensional direct current data in anisotropic media.

Magnetic transitions and the magnetocaloric effect in the Pr1−xYxMn2Ge2 system

Layered rare earth compounds in the RMn2X2 series (R = rare‐earth; X = Ge, Si) are of interest for potential cooling applications at lower temperatures as they enable the structural and magnetic behavior to be controlled via substitution of R, Mn, and X atoms on the 2a, 4d, and 4e sites respectively. We continue investigations of the Pr1−xYxMn2Ge2 magnetic phase diagram as functions of both composition and Mn–Mn spacing using X‐ray and neutron diffraction, magnetization and differential scanning calorimetry measurements. Pr1−xYxMn2Ge2 exhibits an extended region of re‐entrant ferromagnetism around x ∼ 0.5 with re‐entrant ferromagnetism at for Pr0.5Y0.5Mn2Ge2. The entropy values −ΔSM around the ferromagnetic transition temperatures from the layered antiferromagnetic AFl structure to the canted ferromagnetic structure Fmc (typically ) have been derived for Pr1−xYxMn2Ge2 with x = 0.0, 0.2, and 0.5 for ΔB = 0–5 T. The changes in magnetic states due to Y substitution for Pr are discussed in terms of chemical pressure, external pressure, and electronic effects.

Geoelectric Investigation of Some Parts of Ibrahim Badamasi Babangida University, Lapai, Nigeria

Aim: A geoelectric investigation of the subsurface layering of some locations in the Ibrahim Badamasi Babangida University, Lapai was carried out using the Schlumberger vertical electrical sounding (VES) method with a view to determining the depth of aquif er for groundwater development. Methodology: The G41 Geotron Resistivity Meter was used in obtainin g six (6) vertical electrical sounding data along dedicated profiles at different locations within the university, where there is concentration of developmental activities, while a global positioning system (GPS) enabled device was used in tracking the coo rdinates. The VES data so obtained were processed using the interpex 1xD sounding interpretation software. Results:The VES point near the Physics Block was observed to have the shallowest depth of aquifer at about 20m with the fractured basement of thick ness about 18m; while the aquifer at the VES point near the StudentsHostel which proved to be the best was about 38m thick at a depth of 46m. The results also show that the area is generally gently undulating and slopes from north to south with an averag e depth to aquifer formations of 36.75m. The lithology inferred from the interpretation showed a four layered earth model indicating the top lateritic layer, the silty/clayey layer, the weathered/fractured basement and the fresh basement which correlate d with the borehole logs in the vicinity

Layered rare earth hydroxides (LREHs): synthesis and structure characterization towards multifunctionality.

Layered rare earth hydroxides (LREHs) represent a new family of layered host compounds that integrate attractive physicochemical properties of rare earth elements with the wide tunability of guest anions. The compounds have attracted significant research attention, and potential applications have been found in various fields such as optics, catalysis, bio-medicine and so on. In this perspective, we describe our recent progress in the synthesis, structure characterization, and development of functionalities of the LREH compounds. A unique homogeneous alkalization method, in which RE ions are precipitated from a solution containing RE salt, concentrated target anions and hexamethylenetetramine, has been employed to effectively produce highly crystalline LREH samples. A range of anionic forms including chloride-, nitrate-, sulfate- and organodisulfonate-series, have been synthesized and structurally characterized. Two types of cationic rare earth hydroxide layers, {[RE2(OH)5(H2O)2](+)}∞ for the chloride- and nitrate-series and {[RE(OH)2(H2O)](+)}∞ for the sulfate- and organodisulfonate-series, are classified. Unique dehydration/rehydration behaviors or thermal phase evolution of the LREH compounds have been revealed and discussed in relation to the crystal structures. An outlook for potential applications of LREH compounds as anion exchangers, precursors to unique functional oxides, and optical phosphors is described.

3D MT Data Modelling using Multi-order Hexahedral Vector Finite Element Method, including Anisotropy and Complex Geometry

We will present the progress made on the development of a computational algorithm to model 3D Magnetotelluric data using Vector Finite Element Method (VFEM). The differential equations to be solved are the decoupled Helmholtz equations for the secondary electric field, or the secondary magnetic field, with a symmetric conductivity tensor. These equations are modified to include anisotropic earth and complex geometry (such as surface topography, and subsurface interfaces). The primary field is the solution of an air domain, homogeneous half-space or layered earth. This study will compare the application of two boundary conditions, the Generalize Perfect Matched Layers method (GPML) versus Dirichlet boundaries. Dirichlet boundary conditions are applied on the tangential fields, assuming that the boundaries lie far away from the inhomogeneous model. The GPML scheme defines an artificial boundary zone that absorbs the propagating and evanescent electromagnetic fields, to remove boundary effects (Fang, 1996). In this algorithm, high order edge elements are defined based on covariant projections for hexahedral elements (Crowley, et al., 1988). The vector basis functions are defined for the 12 edges (linear) element, 24 edges (quadratic) element, and 48 edges (cubic) element. By this definition, the vector basis will have zero divergence in the case of rectangular elements and relatively small divergence in the case of distorted elements. They are defined to study their numerical accuracy and speed, and to see if the divergence correction is automatically satisfied.

Regularized inverse problem of magnetometric resistivity response over a layered earth

A regularization technique can be applied to inverse problems arising in geoelectrical resistivity sounding. We propose here to use a regularized inversion scheme for interpretation of magnetometric resistivity data gathered from a horizontally stratied layered earth. An iterative scheme using the regularized conjugate gradient method is applied to estimate the conductivity parameters of the ground. The L-curve criterion is used to determine a suitable value of the regularization parameter. The nal inverted model obtained is qualitatively in good agreement with the real model from synthetic data. A comparison of inversion results obtained from our scheme, conventional conjugate gradient and Levenberg-Marquardt methods on a test data set clearly demonstrates an edge over the other two stated schemes as far as the robustness is concerned. The scheme described here has been successfully used for geophysical inversion of magnetometric resistivity data. Mathematics Subject Classication: 86A22, 86A25

Co-Seismic Effect of the 2011 Japan Earthquake on the Crustal Movement Observation Network of China

Great earthquakes introduce measurable co-seismic displacements over regions of hundreds and thousands of kilometers in width, which, if not accounted for, may significantly bias the long-term surface velocity field constrained by GPS observations performed during a period encompassing that event. Here, we first present an estimation of the far-field co-seismic off-sets associated with the 2011 Japan Mw 9.0 earthquake using GPS measurements from the Crustal Movement Observation Network of China (CMONOC) in North China. The uncertainties of co-seismic off-set, either at cGPS stations or at campaign sites, are better than 5 - 6 mm on average. We compare three methods to constrain the co-seismic off-sets at the campaign sites in northeastern China 1) interpolating cGPS coseismic offsets, 2) estimating in terms of sparsely sampled time-series, and 3) predicting by using a well-constrained slip model. We show that the interpolation of cGPS co-seismic off-sets onto the campaign sites yield the best co-seismic off-set solution for these sites. The source model gives a consistent prediction based on finite dislocation in a layered spherical Earth, which agrees with the best prediction with discrepancies of 2 - 10 mm for 32 campaign sites. Thus, the co-seismic off-set model prediction is still a reasonable choice if a good coverage cGPS network is not available for a very active region like the Tibetan Plateau in which numerous campaign GPS sites were displaced by the recent large earthquakes.

Tunable color generation of transparent composite films reinforced with luminescent nanofillers

A new composite film based on calcined layered rare earth hydroxides and poly(vinyl alcohol) shows a visible light transmittance comparable to that of the pure matrix, highly reinforced mechanical strength as well as tunable photoluminescence in rolled and folded shapes.

Groundwater Exploration in Alakuta-Awotan area of Ibadan, Southwestern Nigeria

Fast growing population of Alakuta-Awotan area of Ibadan has warranted the need to develop the water availability in the area. The area lies within the basement complex terrain of southwestern Nigeria typified by banded gneiss and minor intrusion of pegmatite and quartz vein. This study was carried out to characterize the aquifer units and anisotropic properties of fractures for evaluation of groundwater development in the area. Hydro-geologic investigation was carried out on forty-five wells to study the groundwater system and dynamics in shallow aquifers. This study was integrated with twenty-two Schlumberger vertical electrical soundings to further investigate the different subsurface geo-materials with the aim of delineating the thickness and continuity of the aquiferous zone. Six radial soundings were conducted to study the directional properties of the anisotropic rock and thus indicate the orientation of fractures and extent of the fracturing. Measured static water level and well head varied from 0.8 to 9.9 m and 182 m to 209 m respectively indicating that groundwater generally flow towards the eastern and southwestern parts from two main discharge sites in the northwestern part of the study area. Interpretation of the twenty-two Schlumberger vertical electrical soundings generally shows three layered earth structure notably top soil, saturated/sandy/lateritic clay and weathered/fractured/fresh basement which are mostly of the “H” curve type. Overburden isopach map revealed that the depth to the bedrock varies from 3 to 23 m. The main aquifer (weathered basement) is relatively shallow and most wells in the study area terminate in the second layer. The main water - bearing unit in the area of study is the weathered basement and the fractured basement which are within the second and third geoelectrical layers respectively. The weathered/fractured basement resistivity values vary from 62 Ohm-m and 9807 Ohm-m with thickness values ranging from 2.2 m to 36 m. Polygons produced from six radial sounding showed the predominant structural trends of fractures on the banded gneiss. The radial resistivity survey results show that there is significant anisotropy between 0 - 50 m depth generally striking NE-SW, NW-SE and N-S showing the major structural trend of basement fractures. Coefficient of Resistivity Anisotropy ranges between 1.03 and 1.38. Variation of apparent resistivity is strongest at the eastern and southern parts of the study area with coefficients of 1.33, 1.38, and 1.30. This relatively high coefficient of anisotropy implies higher- permeability anisotropy. The directionality of the trends could be responsible for the depressions in the weathered/ fractured basement. The regions with thick weathered/fractured basement/depressed zones are likely to be most promising sites for borehole drilling.

Three-dimensional inversion of airborne time-domain electromagnetic data with applications to a porphyry deposit

We inverted airborne time-domain electromagnetic (ATEM) data over a porphyry deposit in central British Columbia, Canada and recovered the 3D electrical conductivity structure. Full 3D inversion was required because of the circular geometry of the deposit. Typical analysis, which assumes a homogeneous or layered earth, produces conductive artifacts that are contrary to geologic expectations. A synthetic example showed that those misleading artifacts arise by assuming a 1D layered earth and that a 3D inversion can successfully solve the problem. Because of the computational challenges of solving the 3D inversion with many transmitters of airborne survey, we introduced a work flow that uses a multimesh strategy to handle the field data. In our inversion, a coarse mesh and a small number of soundings are first used to rapidly reconstruct a large-scale distribution of conductivity. The mesh is then refined and more soundings are incorporated to better resolve small-scale features. This strategy significantly speeds up the 3D inversion. The progressive refinement of the mesh also helps find the resolution limit of the data and an appropriate mesh for inversion, thus overcomputing on an unnecessarily fine mesh can be avoided. The final conductivity structure has features that emulate the expected geologic structure for a porphyry system and this substantiates the need and capability for working in 3D. However, the necessity for using 3D can depend upon the EM system used. A previous 1D interpretation of frequency-domain EM data at Mt. Milligan indicated a resistive stock. We reconciled this result with the present by computing the footprints of the frequency and time-domain surveys. The distribution of currents for the frequency-domain system was smaller than the length scale of the geologic target while the opposite was true for the time-domain data.

Numerical Modeling of Eccentered LWD Borehole Sensors in Dipping and Fully Anisotropic Earth Formations

Logging-while-drilling (LWD) borehole sensors are used to provide real-time resistivity data of adjacent earth formations for hydrocarbon exploration. This allows for a proactive adjustment of the dipping angle and azimuth direction of the drill and, hence, geosteering capabilities. The analysis of borehole eccentricity effects on LWD sensor response in full 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex></formula> 3 anisotropic earth formations is important for correct data interpretation in deviated or horizontal wells. In this paper, we present a cylindrical-grid finite-difference time-domain model to tackle this problem. The grid is aligned to the sensor axis to avoid staircasing error in the sensor geometry but, in general, misaligned to the (eccentered) borehole/formation interface. A locally conformal discretization is used to compute effective conductivity tensors of partially-filled grid cells at those interfaces, involving an isotropic medium (borehole) and a full 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\times$</tex></formula> 3 anisotropic medium in general (dipped earth formation). The numerical model is used to compute the response of eccentered LWD sensors in layered earth formations with anisotropic dipping beds.

Distortion decomposition of the magnetotelluric impedance tensors from a one-dimensional anisotropic Earth

SUMMARY The Earth is anisotropic at all scales, and models and interpretations of electrical anisotropy from observations are becoming more common. The magnetotelluric (MT) impedance tensor from an arbitrary 1-D anisotropic layered Earth admits an unusual form of six parameters (three complex impedances) with diagonal terms that are equal but of opposite sign. This particular form, somewhat unappreciated and neglected to date, can be exploited to deal with regional MT data that are distorted by galvanic effects of near-surface scatterers. Following prior distortion decomposition approaches, a method is proposed that statistically tests for its validity and solves for the determinable parts of the galvanic distortion and recovers the regional 1-D anisotropic responses. The method is demonstrated on synthetic data and real MT data from two different locations.

A thin-sheet model for global electromagnetic induction

We consider the frequency domain electromagnetic (EM) induction problem in a layered spherical Earth overlain by an inhomogeneous thin sheet of finite conductance, and derive an integral equation for the secondary electric fields resulting from surface conductivity variations. The Green′s function kernel of the integral equation is singular, and must in practice be regularized for a numerical treatment. We compare two approaches: truncation of the spherical harmonic representation of the Green′s function, and projection onto a discrete spatial grid, with elements of the resulting discrete system matrix evaluated as contour integrals. In either case rotational symmetry and the fast Fourier transform (FFT) may be exploited to reduce computational costs, and Krylov space methods may be used to solve the resulting discretized integral equation efficiently. We illustrate the methods for several source field geometries. The two regularization schemes result in solutions that agree well at large scales, but exhibit substantial differences as the nominal resolution is approached. The agreement improves with increased resolution. Although the level of solution errors for the two approaches is likely comparable in an L2 sense, the truncation approach results in Gibb′s oscillations near sharp conductivity contrasts. These may be reduced by tapering the truncated kernel, essentially equivalent to low-pass filtering the solution.

The Modeling of 2D Controlled Source Audio Magnetotelluric (Csamt) Responses Using Finite Element Method

This paper presents the modeling of 2D CSAMT responses generated by horizontal electric dipole using the separation of primary and secondary field technique. The primary field is calculated using 1D analytical solution for homogeneous earth and it is used to calculate the secondary electric field in the inhomogeneous Helmholtz Equation. Calculation of Helmholtz Equation is carried out using the finite element method. Validation of this modeling is conducted by comparison of numerical results with 1D analytical response for the case of homogeneous and layered earth. The comparison of CSAMT responses are also provided for 2D cases of vertical contact and anomalous conductive body with the 2D magnetotelluric (MT) responses. The results of this study are expected to provide better interpretation of the 2D CSAMT data.

The Excitation of Guided-waves by Underground Point Source: an Investigation with Theoretical Seismograms

Near-Source scattering of Rg into S appears to be the primary contributor to the low-frequency Lg. The authors further suggest that the prominent low-frequency spectral null in Lg is due to Rg from a compensated linear vector dipole (CLVD) source, and the low-frequency null in Rg excitation is due to a zero-crossing of the horizontal displacement eigenfunctions. In this study, the mechanism of the excitation of Lg from explosions in layered earth structures are analyzed with theoretical seismograms. Our result shows that the CLVD source generates prominent Lg waves,and the null in the Lg spectra showing remarkably good agreement with those expected from Rg due to a CLVD source. We conclude that the derivative of displacement eigenfunction also takes a key role in the excitation of the null, only zero-crossing of the horizantall displacement eigenfunction can not fully explain it.

Study on Layered Basin Earth Excavation Construction Technology

Through comprehensively analyzing of the present earth excavation methods and combined with Kangwang road tunnel engineering earth excavation, proposing the layered basin earth excavation construction technology of excavation. We put the layered basin earth excavation construction technology of excavation in the practice of Kangwang road tunnel earth excavation, applying "MIDAS-GTS" software to simulate and cost accounting, from the technical and economic sides to validate this new layered basin earth excavation is advanced. And finally puts forward the conditions for the layered basin earth excavation, this new layered basin earth excavation construction technique can be popularized.