scholarly journals TEM Response of a Large Loop Source over the Multilayer Earth Models

2018 ◽  
Vol 2018 ◽  
pp. 1-7 ◽  
Author(s):  
A. K. Tiwari ◽  
S. P. Maurya ◽  
N. P. Singh
Keyword(s):  
Time Derivative ◽  
Earth Model ◽  
Computational Technique ◽  
Fourier Cosine ◽  
Layered Earth ◽  
Large Loop ◽  
Earth Models ◽  
Central Loop ◽  
And Inversion ◽  
Decay Form

The general expression of TEM response of large loop source over the layered earth models is not available in the literature for arbitrary source-receiver positions, except for the case of central loop and coincident loop configurations over the homogeneous earth model. In the present study, an attempt is made to present the TEM response of a large loop source over the layered earth model for arbitrary receiver positions. The frequency domain responses of large loop source over the layer earth model for arbitrary receiver positions are converted into the impulse (time derivative of magnetic field) TEM response using Fourier cosine or sine transform. These impulse TEM responses in turn are converted into voltage responses for arbitrary receiver positions, namely, central loop, arbitrary in-loop, and offset-loop TEM responses over the layered earth models. For checking the accuracy of the method, results are compared with the results obtained using analytical expression over a homogeneous earth model. The complete matching of both of the results suggests that the present computational technique is capable of computing TEM response of large loop source over the homogeneous earth model with high accuracy. Thereafter, the technique is applied for computation of TEM response of a large loop source over the layered earth (2-layer, 3-layer, and 4-layer) models for the central loop, in-loop, and offset-loop configurations and the results are presented in voltage decay form. The results depict their characteristic variations. These results would be useful for modeling and inversion of large loop TEM data over the layer earth models for all the possible configurations resulting from a large loop source.

Response of a layered earth to the Crone pulse electromagnetic system

Geophysics ◽  
1982 ◽  
Vol 47 (1) ◽  
pp. 63-70 ◽  
Author(s):  
S. K. Verma ◽  
S. S. Rai
Keyword(s):  
Half Space ◽  
Single Channel ◽  
Time Derivative ◽  
Measuring System ◽  
Difference Approximation ◽  
Earth Model ◽  
Theoretical Formulation ◽  
Electromagnetic System ◽  
Layered Earth ◽  
Derivative Formula

The response of a layered earth to the Crone pulse electromagnetic (PEM) system, which measures the decay of the time derivative [Formula: see text] has been computed. The transient vertical magnetic field component [Formula: see text] due to a vertical magnetic dipole is obtained by applying a Fourier series summation approach and using digital linear filters to compute the response at individual frequencies. Oscillations in [Formula: see text] due to Gibb’s phenomena are suppressed with Lanczos weights, and the derivative [Formula: see text] is computed numerically by using a linear difference approximation over five points. Decay curves for various half‐spaces are found to cross each other at different values of time. Thus, a single channel response cannot be used to estimate the half‐space resistivity uniquely. This can be achieved, however, by making use of responses at two different channels. Conductivity‐aperture diagrams for half‐space models are plotted for both [Formula: see text] and [Formula: see text]. For [Formula: see text], all the channel amplitudes show well‐defined peaks in the range of 0.3 to 5 ω-m, whereas for [Formula: see text] these peaks lie in the range of 0.7 to 20 Ω-m. This supports the finding by earlier workers that for higher conductivities a total field measuring system responds better than a derivative measuring system. A theoretical formulation is presented to compute the Crone PEM response of an n‐layered earth model. After checking the accuracy of the program, some results are presented for models with different numbers of layers. However, detailed investigation is reported for two‐layered earth only. It is found that overburden parameters can be determined by utilizing responses at two different channels. Nomograms to determine these parameters are presented. There nomograms reveal that for the resistive basement case the best discrimination can be done for the overburden resistivities in the range of 3 to 10 Ω-m and for thicknesses in the range of 0.5 L to 1.5 L (L being the coil separation). For the conductive basement situation the corresponding values are 3 to 15 Ω-m and 0.25 L to 1 L.

scholarly journals Viscous Effects in the Solid Earth Response to Modern Antarctic Ice Mass Flux: Implications for Geodetic Studies of WAIS Stability in a Warming World

2020 ◽  
Vol 33 (2) ◽  
pp. 443-459 ◽  
Author(s):  
E. Powell ◽  
N. Gomez ◽  
C. Hay ◽  
K. Latychev ◽  
J. X. Mitrovica
Keyword(s):  
Mass Flux ◽  
Crustal Deformation ◽  
Earth Model ◽  
Elastic Model ◽  
West Antarctica ◽  
Viscous Effects ◽  
Quarter Century ◽  
Viscosity Models ◽  
Earth Models ◽  
Warming World

AbstractThe West Antarctic Ice Sheet (WAIS) overlies a thin, variable-thickness lithosphere and a shallow upper-mantle region of laterally varying and, in some regions, very low (~1018 Pa s) viscosity. We explore the extent to which viscous effects may affect predictions of present-day geoid and crustal deformation rates resulting from Antarctic ice mass flux over the last quarter century and project these calculations into the next half century, using viscoelastic Earth models of varying complexity. Peak deformation rates at the end of a 25-yr simulation predicted with an elastic model underestimate analogous predictions that are based on a 3D viscoelastic Earth model (with minimum viscosity below West Antarctica of 1018 Pa s) by ~15 and ~3 mm yr−1 in the vertical and horizontal directions, respectively, at sites overlying low-viscosity mantle and close to high rates of ice mass flux. The discrepancy in uplift rate can be reduced by adopting 1D Earth models tuned to the regional average viscosity profile beneath West Antarctica. In the case of horizontal crustal rates, adopting 1D regional viscosity models is no more accurate in recovering predictions that are based on 3D viscosity models than calculations that assume a purely elastic Earth. The magnitude and relative contribution of viscous relaxation to crustal deformation rates will likely increase significantly in the next several decades, and the adoption of 3D viscoelastic Earth models in analyses of geodetic datasets [e.g., Global Navigation Satellite System (GNSS); Gravity Recovery and Climate Experiment (GRACE)] will be required to accurately estimate the magnitude of Antarctic modern ice mass flux in the progressively warming world.

Validity of Resolving the 785 km Discontinuity in the Lower Mantle with P′P′ Precursors?

2020 ◽  
Vol 91 (6) ◽  
pp. 3278-3285
Author(s):  
Baolong Zhang ◽  
Xiangfang Zeng ◽  
Jun Xie ◽  
Vernon F. Cormier
Keyword(s):  
Lower Mantle ◽  
Forward Modeling ◽  
Earth Model ◽  
Frequency Content ◽  
Waveform Data ◽  
Mantle Discontinuities ◽  
The Earth ◽  
Earth Models ◽  
Beamforming Technique ◽  
Ray Paths

Abstract P ′ P ′ precursors have been used to detect discontinuities in the lower mantle of the Earth, but some seismic phases propagating along asymmetric ray paths or scattered waves could be misinterpreted as reflections from mantle discontinuities. By forward modeling in standard 1D Earth models, we demonstrate that the frequency content, slowness, and decay with distance of precursors about 180 s before P′P′ arrival are consistent with those of the PKPPdiff phase (or PdiffPKP) at epicentral distances around 78° rather than a reflection from a lower mantle interface. Furthermore, a beamforming technique applied to waveform data recorded at the USArray demonstrates that PKPPdiff can be commonly observed from numerous earthquakes. Hence, a reference 1D Earth model without lower mantle discontinuities can explain many of the observed P′P′ precursors signals if they are interpreted as PKPPdiff, instead of P′785P′. However, this study does not exclude the possibility of 785 km interface beneath the Africa. If this interface indeed exists, P′P′ precursors at distances around 78° would better not be used for its detection to avoid interference from PKPPdiff. Indeed, it could be detected with P′P′ precursors at epicentral distances less than 76° or with other seismic phases such as backscattered PKP·PKP waves.

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