LIMITING PARAMETERS IN THE MAGNETIC INTERPRETATION OF A GEOLOGIC STRUCTURE

Geophysics ◽  
1962 ◽  
Vol 27 (6) ◽  
pp. 807-814 ◽  
Author(s):  
Gordon E. Andreasen ◽  
Isidore Zietz
Keyword(s):  
Goodness Of Fit ◽  
Drill Hole ◽  
Magnetic Interpretation ◽  
Basement Rocks ◽  
Randolph County ◽  
Susceptibility Contrast ◽  
Magnetic Profile ◽  
Limiting Parameters ◽  
Dip Angle ◽  
Depth Of Burial

A prominent aeromagnetic anomaly in Randolph County, Indiana, suggests the existence of a dikelike structure within the Precambrian basement rocks. Because of the ambiguity inherent in magnetic interpretation, a unique solution for the parameters involved (depth of burial, geometric configuration, and magnetic susceptibility of the mass producing the anomaly) is impossible. However, if one of the parameters is known it is sometimes possible to indicate a range of plausible values for each of the remaining parameters. The depth to the surface of the Precambrian rocks in Randolph County is known from drill‐hole data to be about 3,000 ft. As the depth of burial is known, limits need be set only on the thickness, angle of dip, and susceptibility contrast of the assumed dike. These limits are determined by a graphical method. Theoretical anomalies over the postulated dike are computed for different dike thicknesses, angles of dip, and susceptibility contrasts. The computed profiles are then fitted to an observed magnetic profile flown at right angles to the trend of the feature. Numerical values for the “goodness of fit” are calculated by using the statistical method of sums of squares. Two plots, one of the index of goodness of fit and the other of susceptibility as functions of dip angle and dike thickness, are made to show graphically the interdependence of the variables and the plausible range of each. The contoured plots of the goodness of fit and the susceptibility contrast show that the range of best fit is between dip angles of 30 and 60 degrees and dike thickness of 1,000 to over 4,000 ft. The contoured plot of the susceptibility contrast, translated into percentage of magnetite, shows this parameter to be independent of the dip angle, varying only with thickness.

Magnetic curve fit for a thin dike: Calculator program (TI 59)

Geophysics ◽  
1980 ◽  
Vol 45 (3) ◽  
pp. 447-455 ◽  
Author(s):  
Edwin J. Ballantyne
Keyword(s):  
Massive Sulfide ◽  
Sulfide Deposits ◽  
Curve Fit ◽  
Magnetic Curve ◽  
Original Field ◽  
Susceptibility Contrast ◽  
Magnetic Profile ◽  
Depth Of Burial ◽  
Minimum Points ◽  
Interpretation Problem

In the exploration for massive sulfide deposits, many of the sources of magnetic anomalies are long tabular dike‐like bodies which are thin in relation to the depth of burial. That is, the width of the dike is less than the depth of burial. A program is presented to aid in the inverse solution of this interpretation problem. Demagnetization effects are ignored, and the program will not function for symmetrical anomalies. Given field‐observed amplitudes and coordinates of maximum and minimum points on a total‐ or vertical‐field magnetic profile, this program fits the corresponding theoretical profile for a thin dipping magnetic dike. Coordinate and depth of the top of dike, dip of dike, and width‐susceptibility contrast of the dike are calculated. A theoretical anomaly profile may also be calculated for comparison with the original field curve. Successive iterations may be used to refine estimates of the calculated parameters.

Paleomagnetism and rock magnetism of East and West Clearwater Lake impact structures

2019 ◽  
Vol 56 (9) ◽  
pp. 983-993
Author(s):  
Jérôme Gattacceca ◽  
William Zylberman ◽  
Adam B. Coulter ◽  
François Demory ◽  
Yoann Quesnel ◽  
...  
Keyword(s):  
Rock Magnetism ◽  
Drill Hole ◽  
Hydrothermal Activity ◽  
Radiometric Dating ◽  
Impact Structure ◽  
The West ◽  
Impact Melt ◽  
Basement Rocks ◽  
East And West ◽  
The Impact

The East and West Cleawater Lake impact structures (Wiyâshâkimî Lake, Québec), ∼26 and 32 km in diameter, respectively, have been proposed to represent an impact doublet. We investigated their paleomagnetism to contribute to this debate. The paleomagnetic directions of the impact melt rocks and impact melt-bearing breccias from the West Clearwater structure are compatible with the radiometric age of 280–290 Ma previously determined for this structure and indicate that the impact occurred during a reverse polarity interval of the geomagnetic field. A similar remagnetization direction is found in the basement within 10 km of the structure center, whereas basement farther away from the center has escaped remagnetization by the impact. Samples for the East Clearwater structure come from two holes drilled in 1963 and 1964. Unfortunately, the drill hole through the melt rocks is tilted by 30° from the vertical with an unknown azimuth. The paleomagnetic inclination of these melt rocks cannot be constrained to better than between −28° and +32°. This is, however, distinct from the inclination of the melt rocks of the West Clearwater Lake impact structure (−27.8° ± 3.7°), suggesting that the two structures do not represent an impact doublet, in agreement with recent radiometric dating. The basement rocks and the melt rocks within 10 km of the center of the West Clearwater Lake impact structure show a magnetic signature of titanohematite that crystallized during postimpact hydrothermal activity under oxidizing conditions. This is not observed in the basement or the melt rocks from the East Clearwater Lake impact structure.

THE DIRECT APPROACH TO MAGNETIC INTERPRETATION AND ITS PRACTICAL APPLICATION

Geophysics ◽  
1949 ◽  
Vol 14 (3) ◽  
pp. 290-320 ◽  
Author(s):  
Leo J. Peters
Keyword(s):  
Field Data ◽  
Potential Problem ◽  
Scalar Potential ◽  
Direct Calculation ◽  
Magnetic Data ◽  
Practical Application ◽  
Magnetic Field Data ◽  
Magnetic Interpretation ◽  
Basement Rocks ◽  
Derivatives Of

This paper discusses the solution of the inverse potential problem and its practical application in the interpretation of field data which have a scalar potential distribution. The discussion will be in terms of the interpretation of magnetic data. Among the topics discussed are: the direct calculation of basement relief, the derivation of the potential and the horizontal components of the field from the vertical intensity, the continuation of the field upward, the continuation of the field downward towards its source, the calculation of derivatives of the vertical intensity with special attention to the second and fourth, and the estimation of depths to igneous basement rocks. The uses of these tools and the information of practical value which can be obtained by their use are discussed and illustrated. Methods of rapidly making calculations using magnetic field data are given.

scholarly journals Modeling of Self Potential (SP) Anomalies over a Polarized Rod with Finite Depth Extents

2019 ◽  
pp. 51-56
Author(s):  
T. S. Fagbemigun ◽  
M. O. Olorunfemi ◽  
S. A. Wahab
Keyword(s):  
Potential Distribution ◽  
Synthetic Data ◽  
Field Measurements ◽  
Finite Depth ◽  
Distribution Data ◽  
Geophysical Field ◽  
Dip Angle ◽  
Self Potential ◽  
Depth Of Burial ◽  
Sp Anomaly

Modeling is a powerful tool used by Geoscientists to provide pre-knowledge about the expectations of any geophysical field measurements. This study generates Self Potential (SP) anomalies over a typical dike-like structure to observe the influence of depth of burial and dip on SP anomalies. A computer program was developed from the potential distribution equation of an inclined polarized rod with a limited depth extent using Visual Basic (VB) programming language to produce synthetic data for potential distribution. The potential distribution data were used to generate theoretical SP anomaly curves for a polarized rod for varying depth of burial and dip. Twenty SP anomaly curves were generated with different dip values and depth of burial and from these curves, superimposed curves were also generated. The anomalies were analyzed for the effect of depth of burial and attitude or dip. The SP anomaly curves generated show that an increase in depth of burial causes a reduction in the peak negative amplitude of SP anomaly curves. For inclined polarized rod at relatively shallow depth (<2.0 m), the peak negative amplitude remains virtually the same with a positive shoulder over the down dip side of the target. Also as the dip angle decreases from 90o for a fixed depth of burial, the anomaly curves become asymmetrical. At 0o, the distance between the peak negative and peak positive amplitude of the anomaly curve is equal to the linear extent of the rod. Therefore, this study shows that the depth of burial inversely influences the amplitude of self-potential (SP) anomalies while the dip angle affects the form or symmetry of anomaly curves.

scholarly journals Magnetic anomaly interpretation for a 2D fault-like geologic structures utilizing the global particle swarm method

2021 ◽  
Author(s):  
Khalid S. ESSA ◽  
Yves Géraud ◽  
Alan B. Reid
Keyword(s):  
Magnetic Anomaly ◽  
Goodness Of Fit ◽  
Moving Average ◽  
Particle Swarm ◽  
Theoretical Models ◽  
Shear Zones ◽  
Real Field ◽  
Fault Parameters ◽  
Particle Swarm Method ◽  
Dip Angle

Abstract We establish a method to elucidate the magnetic anomaly due to 2D fault structures, with an evaluation of first moving average residual anomalies utilizing filters of increasing window lengths. After that, the buried fault parameters are estimated using the global particle swarm method. The goodness of fit among the observed and the calculated models is expressed as the root mean squared (RMS) error. The importance of studying and delineating the fault parameters, which include the amplitude factor, the depth to the upper edge, the depth to the lower edge, the fault dip angle, and the position of the origin of the fault, is: (i) solving many problem-related engineering and environmental applications, (ii) describing the accompanying mineralized zones with faults, (iii) describing geological deformation events, (iv) monitoring the subsurface shear zones, (v) defining the environmental effects of the faults before organizing any investments, and (vi) imaging subsurface faults for different scientific studies. Finally, we show the method applied to two theoretical models including the influence of the regional background and the multi-fault effect and to real field examples from Australia and Turkey. Available geologic and geophysical information corroborates our interpretations.

Magnetic source parameters of two‐dimensional structures using extended Euler deconvolution

Geophysics ◽  
2001 ◽  
Vol 66 (3) ◽  
pp. 814-823 ◽  
Author(s):  
Martin F. Mushayandebvu ◽  
P. van Driel ◽  
Alan B. Reid ◽  
James Derek Fairhead
Keyword(s):  
Homogeneous Function ◽  
A Priori ◽  
Source Parameters ◽  
Thin Sheet ◽  
Source Location ◽  
Euler Deconvolution ◽  
True Source ◽  
Susceptibility Contrast ◽  
Magnetic Profile ◽  
Constrained Solutions

The Euler homogeneity relation expresses how a homogeneous function transforms under scaling. When implemented, it helps to determine source location for particular potential field anomalies. In this paper, we introduce an additional relation that expresses the transformation of homogeneous functions under rotation. The combined implementation of the two equations, called here extended Euler deconvolution for 2-D structures, gives a more complete source parameter estimation that allows the determination of susceptibility contrast and dip in the cases of contact and thin‐sheet sources. This allows for the structural index to be correctly chosen on the basis of a priori knowledge about susceptibility and dip. The pattern of spray solutions emanating from a single source anomaly can be attributed to interfering sources, which have their greatest effect on the flanks of the anomaly. These sprays follow different paths when using either conventional Euler deconvolution or extended Euler deconvolution. The paths of these spray solutions cross and cluster close to the true source location. This intersection of spray paths is used as a discriminant between poor and well‐constrained solutions, allowing poor solutions to be eliminated. Extended Euler deconvolution has been tested successfully on 2-D model and real magnetic profile data over contacts and thin dikes.

Interpretation of buried basement in the southwestern Athabasca Basin, Canada, from integrated geophysical and geological datasets

2020 ◽  
Vol 21 (1) ◽  
pp. geochem2019-061
Author(s):  
Victoria Tschirhart ◽  
Sally Pehrsson ◽  
Colin Card ◽  
Eric G. Potter ◽  
Jeremy Powell ◽  
...  
Keyword(s):  
Potential Field ◽  
Physical Property ◽  
Drill Hole ◽  
Airborne Gravity ◽  
Content Type ◽  
Athabasca Basin ◽  
Basement Rocks ◽  
Gravity And Magnetic ◽  
Geophysical Surveys ◽  
Lineament Analysis

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield.Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

scholarly journals The Horizontal Loop Electromagnetic (HLEM) Response of Ifewara Transcurrent Fault,Southwestern Nigeria: A Computational Results

2021 ◽  
Vol 3 (3) ◽  
Author(s):  
A. A. Adepelumi ◽  
O. B. Olayiwola ◽  
D. E. Falebita ◽  
D. E. Falebita ◽  
O. Afolabi ◽  
...  
Keyword(s):  
Complex Terrain ◽  
Mineral Exploration ◽  
Forward Modeling ◽  
Basement Complex ◽  
Forward Solution ◽  
Southwestern Nigeria ◽  
Geological Interpretation ◽  
Overburden Thickness ◽  
Dip Angle ◽  
Depth Of Burial

The need to accurately interpret geological models that approximate mineralized zones in a Basement Complex terrain necessitate the development of horizon loop electromagnetic method (HLEM) forward modeling solutions for such scenarios. The focus of the present work is on finding rapid forward modeling solutions for synthetic HLEM data as an aid in exploration for moderate to deep conductive mineral exploration targets.The main thrust is obtaining idealized HLEM models that are required for geological interpretation of the subsurface in such environment. The original HLEM equations developed by Wesley were extended to represent a horizontally stratified earth with a conductive approximated by shear zone. From these equations a computer program was written to calculate the HLEM responses for optimal conductor model with known values of coil separations (L), depth of burial (z) and angle of dip of the target.The thin conductive model was used because it is simple and suitable for different geological scenarios. The accuracy of the approximate forward solution has been confirmed for HLEM systems with various geometric ranges, frequencies and conductivities. Three models having varying overburden thickness, dip angle of target and source-receiver separation were used in the forward modeling. The effect of varying the dip angle,overburden thickness and coil separation was studied in all the three models used. The result obtained from the forward modeling showed that variation of the dip angle gave rise to changes in the amplitudes of the anomalies generated, while that of overburden and coil separation gave rise to changes in anomaly shape. Also, the geometry and position of the causative body were precisely delineated.

Systematics of time‐migration errors

Geophysics ◽  
1994 ◽  
Vol 59 (9) ◽  
pp. 1419-1434 ◽  
Author(s):  
James L. Black ◽  
Matthew A. Brzostowski
Keyword(s):  
General Scheme ◽  
Velocity Variation ◽  
Lateral Velocity ◽  
Depth Migration ◽  
Time Migration ◽  
Simple Rules ◽  
Rules Of Thumb ◽  
Velocity Changes ◽  
Dip Angle ◽  
Depth Of Burial

Even if the correct velocity is used, time migration mispositions events whenever the velocity changes laterally. These errors increase with lateral velocity variation, depth of burial, and dip angle θ. Our analyses of two model types, one with an implicit gradient and one with an explicit gradient, yield simple “rules of thumb” for these errors to first order in the lateral gradient. The x error is [Formula: see text], and the z error is [Formula: see text], where the quantity A = A(x, z) contains the information about depth of burial and magnitude of lateral gradient. These rules can be used to determine when depth migration is needed. Further analysis also shows that the image‐ray correction to time migration is accurate only at small dip. For dipping events, the image‐ray correction must be supplemented by a shift in x of the form [Formula: see text] and a shift in z given by [Formula: see text]. These time‐migration corrections take the same form for both the models we have studied, suggesting a general scheme for correcting time migration, which we call “remedial migration.”

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