Necessity of Terrain Correction in Magnetotelluric Data Recorded from Garhwal Himalayan Region, India

The magnetotelluric (MT) method is one of the useful geophysical techniques to investigate deep crustal structures. However, in hilly terrains, e.g., the Garhwal Himalayan region, due to the highly undulating topography, MT responses are distorted. Such responses, if not corrected, may lead to the incorrect interpretation of geoelectric structures. In the present paper, we implemented terrain corrections in MT data recorded from the Garhwal Himalayan Corridor (GHC). We used AP3DMT, a 3D MT data modeling and inversion code written in the MATLAB environment. Terrain corrections in the MT impedance responses for 39 sites along the Roorkee–Gangotri profile in the period range of 0.01 s to 1000 s were first estimated using a synthetic model by recording the topography and locations of MT sites. Based on this study, we established the general character of the terrain and established where terrain corrections were necessary. The distortion introduced by topography was computed for each site using homogenous and heterogeneous models with actual topographic variations. Period-dependent, galvanic and inductive distortions were observed at different sites. We further applied terrain corrections to the real data recorded from the GHC. The corrected data were inverted, and the inverted model was compared with the corresponding inverted model obtained with uncorrected data. The modification in electrical resistivity features in the model obtained from the terrain-corrected response suggests the necessity of terrain correction in MT data recorded from the Himalayan region.

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A new variable‐magnetization terrain correction method for aeromagnetic data

Geophysics ◽

10.1190/1.1442244 ◽

1987 ◽

Vol 52 (1) ◽

pp. 94-107 ◽

Cited By ~ 28

Author(s):

V. J. S. Grauch

Keyword(s):

Magnetic Field ◽

Real Data ◽

Theoretical Models ◽

Correction Method ◽

Terrain Correction ◽

Aeromagnetic Data ◽

San Juan Mountains ◽

Terrain Effects ◽

Terrain Corrections ◽

Lake City

Terrain effects in aeromagnetic data are produced by rugged, magnetic topography. These effects mimic the shape of topography and can often be so large that they obscure anomalies of interest. Thus it is desirable to remove terrain effects from aeromagnetic data in order to isolate the anomalies to be investigated. However, removal of aeromagnetic terrain effects has been a longstanding problem. Previously developed methods have succeeded only in certain, specific geologic situations. I present a new aeromagnetic terrain‐correction method that is superior to the previously developed methods for the general case. This method takes into account the highly variable magnetic properties of rocks and can remove terrain effects whether the sources of interest are shallow or deep. The new method is based on the assumption that magnetic sources of interest are often geometrically unrelated to terrain. It finds the magnetization that gives a magnetic‐field residual with minimum correlation to terrain effects for a window of data within a grid of magnetic‐field values. By repeating the calculation for windows covering the entire grid, a grid of variable‐magnetization values is produced which is combined with topography to calculate a magnetic‐terrain correction. The variable‐magnetizaton method was extensively tested using theoretical models (where the answer is known) and using real data from the Lake City caldera area in the San Juan Mountains of southern Colorado. The tests demonstrated the method’s effectiveness in removing terrain effects from aeromagnetic data. Valid terrain corrections were not obtained where anomalies of interest correlated with terrain effects. However, these places are readily recognizable and easily corrected by editing some of the magnetization values.

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Bayesian inversion of magnetotelluric data considering dimensionality discrepancies

Geophysical Journal International ◽

10.1093/gji/ggaa391 ◽

2020 ◽

Vol 223 (3) ◽

pp. 1565-1583

Author(s):

Hoël Seillé ◽

Gerhard Visser

Keyword(s):

Learning Algorithm ◽

Synthetic Data ◽

Real Data ◽

Error Model ◽

Bayesian Inversion ◽

Magnetotelluric Data ◽

Data Set ◽

Likelihood Functions ◽

Training Images ◽

Phase Tensor

SUMMARY Bayesian inversion of magnetotelluric (MT) data is a powerful but computationally expensive approach to estimate the subsurface electrical conductivity distribution and associated uncertainty. Approximating the Earth subsurface with 1-D physics considerably speeds-up calculation of the forward problem, making the Bayesian approach tractable, but can lead to biased results when the assumption is violated. We propose a methodology to quantitatively compensate for the bias caused by the 1-D Earth assumption within a 1-D trans-dimensional Markov chain Monte Carlo sampler. Our approach determines site-specific likelihood functions which are calculated using a dimensionality discrepancy error model derived by a machine learning algorithm trained on a set of synthetic 3-D conductivity training images. This is achieved by exploiting known geometrical dimensional properties of the MT phase tensor. A complex synthetic model which mimics a sedimentary basin environment is used to illustrate the ability of our workflow to reliably estimate uncertainty in the inversion results, even in presence of strong 2-D and 3-D effects. Using this dimensionality discrepancy error model we demonstrate that on this synthetic data set the use of our workflow performs better in 80 per cent of the cases compared to the existing practice of using constant errors. Finally, our workflow is benchmarked against real data acquired in Queensland, Australia, and shows its ability to detect the depth to basement accurately.

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Frequency‐domain acoustic‐wave modeling and inversion of crosshole data: Part II—Inversion method, synthetic experiments and real‐data results

Geophysics ◽

10.1190/1.1443818 ◽

1995 ◽

Vol 60 (3) ◽

pp. 796-809 ◽

Cited By ~ 110

Author(s):

Zhong‐Min Song ◽

Paul R. Williamson ◽

R. Gerhard Pratt

Keyword(s):

Frequency Domain ◽

Real Data ◽

Forward Modeling ◽

Two Dimensions ◽

Inversion Method ◽

Inversion Algorithm ◽

Data Set ◽

Wave Inversion ◽

Full Wave ◽

And Inversion

In full‐wave inversion of seismic data in complex media it is desirable to use finite differences or finite elements for the forward modeling, but such methods are still prohibitively expensive when implemented in 3-D. Full‐wave 2-D inversion schemes are of limited utility even in 2-D media because they do not model 3-D dynamics correctly. Many seismic experiments effectively assume that the geology varies in two dimensions only but generate 3-D (point source) wavefields; that is, they are “two‐and‐one‐half‐dimensional” (2.5-D), and this configuration can be exploited to model 3-D propagation efficiently in such media. We propose a frequency domain full‐wave inversion algorithm which uses a 2.5-D finite difference forward modeling method. The calculated seismogram can be compared directly with real data, which allows the inversion to be iterated. We use a descents‐related method to minimize a least‐squares measure of the wavefield mismatch at the receivers. The acute nonlinearity caused by phase‐wrapping, which corresponds to time‐domain cycle‐skipping, is avoided by the strategy of either starting the inversion using a low frequency component of the data or constructing a starting model using traveltime tomography. The inversion proceeds by stages at successively higher frequencies across the observed bandwidth. The frequency domain is particularly efficient for crosshole configurations and also allows easy incorporation of attenuation, via complex velocities, in both forward modeling and inversion. This also requires the introduction of complex source amplitudes into the inversion as additional unknowns. Synthetic studies show that the iterative scheme enables us to achieve the theoretical maximum resolution for the velocity reconstruction and that strongly attenuative zones can be recovered with reasonable accuracy. Preliminary results from the application of the method to a real data set are also encouraging.

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TERRAIN CORRECTIONS FOR BOREHOLE GRAVIMETRY

Geophysics ◽

10.1190/1.1439936 ◽

1968 ◽

Vol 33 (2) ◽

pp. 361-362 ◽

Cited By ~ 5

Author(s):

J. R. Hearst

Keyword(s):

First Principles ◽

Surface Density ◽

Gravitational Constant ◽

Terrain Correction ◽

Correction Formula ◽

Free Air ◽

Terrain Corrections

The measurement of in‐situ density by borehole gravimetry has now become a commonly accepted, if not commonly used, practice (McCulloh, 1965, 1967; Howell et al., 1966; Hammer 1950). The expression for density as a function of gravity difference at two depths is given in a general form by McCulloh (1967) as [Formula: see text] [Formula: see text] where ρ is the density, F the free air gradient, [Formula: see text] the measured gravity difference between two depths, [Formula: see text] a correction for the effect of sub surface density differences (according to McCulloh, generally negligible), [Formula: see text] the terrain correction, [Formula: see text] a borehole correction, and k the gravitational constant. This equation can be obtained from first principles using Gauss’ law.

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Improvement of the conic prism model for terrain correction in rugged topography

Geophysics ◽

10.1190/1.1441243 ◽

1981 ◽

Vol 46 (7) ◽

pp. 1054-1056 ◽

Cited By ~ 13

Author(s):

Raymond J. Olivier ◽

Réjean G. Simard

Keyword(s):

Gravity Anomalies ◽

Terrain Correction ◽

Field Calculation ◽

Bouguer Gravity ◽

New Model ◽

Gravity Station ◽

Rugged Terrain ◽

Rugged Topography ◽

Terrain Corrections ◽

Prism Model

Terrain corrections for Bouguer gravity anomalies are generally obtained from topographic models represented by flat‐topped compartments of circular zones, utilizing the so‐called Hayford‐Bowie (1912), or Hammer’s (1939) method. Some authors have introduced improved relief models for taking uniform slope into consideration (Sandberg, 1958; Kane, 1962; Takin and Talwani, 1966; Campbell, 1980). We present a new model that increases the accuracy of the calculation of terrain correction close to the gravity station in rugged terrain, especially when conventional templates with few zones are used in field calculation.

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EXTENDED TERRAIN CORRECTION TABLES FOR GRAVITY REDUCTIONS

Geophysics ◽

10.1190/1.1440266 ◽

1972 ◽

Vol 37 (2) ◽

pp. 377-379

Author(s):

Jesse K. Douglas ◽

Sidney R. Prahl

Keyword(s):

Computer Program ◽

Rocky Mountain ◽

Terrain Correction ◽

Western Montana ◽

Inclined Plane ◽

Plane Model ◽

Mountain Area ◽

Terrain Corrections

This note extends the gravity terrain corrections for elevation differences beyond the tables originally published by Hammer (1939). Experience in the Rocky Mountain area has demonstrated to us the need for such an extension. The frustration encountered by the authors led to a computer program to calculate the terrain correction tables presented in this article. The mountain topography in western Montana is typical of an area not sufficiently regular to allow use of the less tedious inclined‐plane model presented by Sandberg (1958). The inclined‐plane and the cylinder models are designed for calculating the effects of local terrain and do not include a correctional factor for earth curvature. Large regional surveys require the Hayford‐ Bowie terrain correction zones. However, local surveys can be easily incorporated into these larger studies by Hammer to Hayford‐Bowie transition tables (Sandberg, 1959),

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Integrated seismic and electromagnetic model building applied to improve subbasalt depth imaging in the Faroe-Shetland Basin

Geophysics ◽

10.1190/geo2015-0137.1 ◽

2016 ◽

Vol 81 (1) ◽

pp. E57-E68 ◽

Cited By ~ 4

Author(s):

Martin Panzner ◽

Jan Petter Morten ◽

Wiktor Waldemar Weibull ◽

Børge Arntsen

Keyword(s):

Seismic Data ◽

Model Building ◽

Velocity Structure ◽

Seismic Velocity ◽

Real Data ◽

Magnetotelluric Data ◽

Electromagnetic Data ◽

Data Inversion ◽

Seismic Image ◽

Imaging Results

Subbasalt imaging has gained significant interest in the last two decades, driven by the urge to better understand the geologic structures beneath volcanic layers, which can be up to several kilometers thick. This understanding is crucial for the development and risking of hydrocarbon play models in these areas. However, imaging based on the reflection seismic data alone suffers from severe amplitude transmission losses and interbed multiples in the volcanic sequence, as well as from poor definition of the subbasalt velocity structure. We have considered a sequential imaging workflow, in which the resistivity model from joint controlled-source electromagnetic and magnetotelluric data inversion was used to update the velocity model and to improve the structural definition in the migrated seismic image. The quantitative link between resistivity and velocity was derived from well data. The workflow used standard procedures for seismic velocity analysis, electromagnetic data inversion, and well analysis, and thereby allowed detail control and input based on additional geophysical knowledge and experience in each domain. Using real data sets from the Faroe-Shetland Basin, we can demonstrate that the integration of seismic and electromagnetic data significantly improved the imaging of geologic structures covered by up to several-kilometer-thick extended volcanic sequences. The improved results might alter the interpretation compared with the imaging results from seismic data alone.

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Inner zone terrain correction calculation using interpolated heights

Contributions to Geophysics and Geodesy ◽

10.1515/congeo-2015-0021 ◽

2015 ◽

Vol 45 (3) ◽

pp. 219-235 ◽

Cited By ~ 3

Author(s):

Pavol Zahorec

Keyword(s):

Gravity Data ◽

Real Data ◽

Test Area ◽

Gravity Measurements ◽

Topography Model ◽

Terrain Corrections ◽

Elevation Model ◽

Regional Gravity ◽

Synthetic Models ◽

Simple Statistical Analysis

Abstract The discrepancy between real heights of gravity points and the elevation model has a significant impact on the terrain corrections calculation especially within the inner zone. The concept of interpolated heights of calculation points used instead of measured ones within the specified inner zone can considerably decrease the resulting errors. The choice of appropriate radius of the inner zone for use of interpolated heights is analysed on synthetic topography model as well as real data. The tests with synthetic models showed the appropriate radius of this zone is proportional to the deformation wavelength of the model. Simple statistical analysis of a particular elevation model can give an estimate of the appropriate radius for the calculation using interpolated heights. A concept with interpolated heights in the zone 0–250 m is used in actual practice in Slovakia. The analysis of regional gravity data from the Tatry Mountains test area indicates the searched radius should be about 100 m. Detailed gravity measurements from different areas showed the searched radius does not play so important role but the use of interpolated heights instead of measured ones is still relevant. The more reasonable method instead of using interpolated heights is also presented when calculating the topographic effect.

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Schumann resonance frequency variations observed in magnetotelluric data recorded from Garhwal Himalayan region India

Annales Geophysicae ◽

10.5194/angeo-27-3497-2009 ◽

2009 ◽

Vol 27 (9) ◽

pp. 3497-3507 ◽

Cited By ~ 8

Author(s):

R. Chand ◽

M. Israil ◽

J. Rai

Keyword(s):

Magnetic Field ◽

Time Series ◽

Resonance Frequency ◽

Himalayan Region ◽

Series Data ◽

Frequency Variation ◽

Schumann Resonance ◽

Magnetotelluric Data ◽

Electric And Magnetic Field ◽

Frequency Variations

Abstract. Schumann resonance (SR) frequency variation has been studied using Magnetotelluric (MT) data recorded in one of the world's toughest and generally inaccessible Himalayan terrain for the first time in the author's knowledge. The magnetotelluric data, in the form of orthogonal time varying electric and magnetic field components (Ex, Ey, Bx and By), recorded during 10 March–23 May 2006, in the Himalayan region, India, at elevations between 1228–2747 m above mean sea level (amsl), were used to study the SR frequency variation. Electromagnetic field components, in the form of time series, were recorded at 64 Hz sampling frequency at a site located away from the cultural noise. Spectral analysis of time series data, at a frequency resolution of 0.03 Hz, has been performed using Fast Fourier Transform (FFT) algorithm. Spectral stabilization in three Schumann resonance modes is achieved by averaging the power spectral magnitude of 32 data segments, each with 2048 sample data. Amplitude variation in the Schumann resonance frequency associated with day-night, sunrise and terminator effect was observed. Average diurnal variation in the first three Schumann resonance frequencies associated with magnetic field components is presented. The maximum frequency variation of about 0.3, 0.4 and 0.7 Hz was observed in the first, second and third mode, respectively. The frequency variations observed in electric and magnetic field components also show phase shift and varying attenuation. The SR frequency variation has been used to define the ionospheric electron density variation in the Himalayan region, India.

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Two-dimensional determinant inversion of marine magnetotelluric data and a field example from the Gulf of California, Mexico

Geophysics ◽

10.1190/geo2019-0735.1 ◽

2021 ◽

Vol 86 (1) ◽

pp. E37-E57

Author(s):

Shunguo Wang ◽

Steven Constable ◽

Valeria Reyes-Ortega ◽

Hormoz Jahandari ◽

Colin Farquharson ◽

...

Keyword(s):

Gulf Of California ◽

Subduction Zones ◽

Field Data ◽

Real Data ◽

Transverse Magnetic ◽

Two Dimensional ◽

Magnetotelluric Data ◽

Earth Structures ◽

3D Effects ◽

2D Data

Two-dimensional marine magnetotelluric (MT) observations are useful for offshore geologic studies, such as natural resource exploration, fault mapping, fluid estimation at subduction zones, and the delineation of the lithosphere-asthenosphere boundary beneath the seafloor. Earth structures are often assumed to be two dimensional, which allows MT data to be decomposed into a transverse electric (TE) mode and a transverse magnetic (TM) mode. The 2D assumption can effectively reduce acquisition and computational costs. However, offline 3D effects and other problems such as lack/failure of the compass on instruments are often encountered, making it difficult to decompose data into the TE and TM modes. In these cases, 2D inversion may be misleading or may not provide an acceptable misfit to the marine MT observations. Thus, we have developed a 2D determinant inversion to the marine MT method to mitigate these difficulties, implemented in the MARE2DEM code, and we tested its utility using synthetic examples and a field example. In the synthetic examples, the determinant inversion demonstrates an ability to overcome 3D effects caused by 3D anomalies and bathymetry. With confidence from the synthetic tests, we interpreted real data acquired in the Gulf of California, Mexico, where not only is the bathymetry 3D in nature, but the external compasses failed to record the orientation. The field data can not only be fit to a reasonable misfit with a determinant inversion, but the resolved conductive zones also have a good correlation with known faults. A comparison between the resistivity model from the field data and a seismic reflection section shows that a previously interpreted fault, the Wagner Fault, should be shifted 5 km toward the southwest and made slightly steeper. Thus, the implementation of the determinant inversion may provide a new approach for using problematic 2D data.

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