SPECTRAL ANALYSIS OF GRAVITY AND MAGNETIC ANOMALIES DUE TO RECTANGULAR PRISMATIC BODIES

Geophysics ◽  
1977 ◽  
Vol 42 (1) ◽  
pp. 41-50 ◽  
Author(s):  
B. K. Bhattacharyya ◽  
Lei‐Kuang Leu
Keyword(s):  
Spectral Analysis ◽  
Approximation Method ◽  
Magnetic Anomalies ◽  
The Body ◽  
Exponential Approximation ◽  
Prismatic Body ◽  
First Order ◽  
Gravity And Magnetic ◽  
Sums Of Exponentials ◽  
Prismatic Bodies

The spectra of gravity and magnetic anomalies due to a prismatic body can be expressed as sums of exponentials. The complex exponents of these exponentials are functions of frequency and locations of the corners of the body. An exponential approximation method is used for the analysis of the radial spectra of an anomaly and its first order moments for obtaining accurate estimates of the depths to the top and bottom of the body. A method has also been developed for determining approximately the location of the centroid of the body. When the location of the centroid and the depths to the top and bottom are known for the causative body, it is possible to calculate the horizontal dimensions with the help of the spectrum of the anomaly.

A DIRECT COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES CAUSED BY 2- AND 3-DIMENSIONAL BODIES OF ARBITRARY SHAPE AND ARBITRARY MAGNETIC POLARIZATION BY EQUIVALENT‐POINT METHOD AND A SIMPLIFIED CUBIC SPLINE

Geophysics ◽  
1977 ◽  
Vol 42 (3) ◽  
pp. 610-622 ◽  
Author(s):  
Chao C. Ku
Keyword(s):  
Arbitrary Shape ◽  
Gaussian Quadrature ◽  
Magnetic Anomalies ◽  
Cubic Spline ◽  
The Body ◽  
Point Method ◽  
Magnetic Polarization ◽  
3 Dimensional ◽  
Equivalent Point ◽  
Gravity And Magnetic

A computational method, which combines the Gaussian quadrature formula for numerical integration and a cubic spline for interpolation in evaluating the limits of integration, is employed to compute directly the gravity and magnetic anomalies caused by 2-dimensional and 3-dimensional bodies of arbitrary shape and arbitrary magnetic polarization. The mathematics involved in this method is indeed old and well known. Furthermore, the physical concept of the Gaussian quadrature integration leads us back to the old concept of equivalent point masses or equivalent magnetic point dipoles: namely, the gravity or magnetic anomaly due to a body can be evaluated simply by a number of equivalent points which are distributed in the “Gaussian way” within the body. As an illustration, explicit formulas are given for dikes and prisms using 2 × 2 and 2 × 2 × 2 point Gaussian quadrature formulas. The basic limitation in the equivalent‐point method is that the distance between the point of observation and the equivalent points must be larger than the distance between the equivalent points within the body. By using a reasonable number of equivalent points or dividing the body into a number of smaller subbodies, the method might provide a useful alternative for computing in gravity and magnetic methods. The use of a simplified cubic spline enables us to compute the gravity and magnetic anomalies due to bodies of arbitrary shape and arbitrary magnetic polarization with ease and a certain degree of accuracy. This method also appears to be quite attractive for terrain corrections in gravity and possibly in magnetic surveys.

scholarly journals ANALYSIS OF HIGH RESOLUTION AEROMAGNETIC DATA OF SOME PARTS OF BENUE TROUGH, NIGERIA

2020 ◽  
Vol 4 (2) ◽  
pp. 76-85
Author(s):  
Nuraddeen Usman ◽  
Ibrahim Jibril
Keyword(s):  
Spectral Analysis ◽  
Magnetic Anomalies ◽  
Aeromagnetic Data ◽  
Benue Trough ◽  
First Order ◽  
Qualitative Interpretation ◽  
Maximum Value ◽  
Magnetic Source ◽  
Analysis Technique ◽  
Basement Depth

This work is aimed to determine the depth to basement of some magnetic sources in the study area. Four aeromagnetic sheets were acquired from the Nigerian Geological Survey Agency which includes (Bajoga, 131, Gulani, 132, Gombe, 152 and Wuyo, 153). The study area covers an estimated area of about 12100km2 between latitude 90N-110N and longitude 110E-130E. The total magnetic field of the study area have been evaluated. In order to determine the basement depth, spectral analysis technique was applied. Detailed analysis of the aeromagnetic data for the study area was performed. The procedure involved in the analysis include reduction to equator to remove the effect of inclination, contouring of the total magnetic intensity, separation of the regional and residual anomalies using polynomial fitting of first order, qualitative interpretation and quantitative interpretation. The residual field of the study area composes of low magnetic anomalies reaching a minimum value of -158.6nT as observed in the northern and southern parts and high magnetic anomalies reaching a maximum value of 178.1nT as observed in the western part of the study area. The result from the spectral analysis for each block shows that the depths to the magnetic source are 5.20Km for block 1, 5.74Km for block 2, 7.59Km for block 3 and 3.56Km for block 4. The average depth to magnetic source in the study area was found to be 5.52Km. Based on the computed average sedimentary thickness obtained in this study area, hydrocarbon accumulation in the study area is feasible.

REMANENT MAGNETIZATION FROM COMPARISON OF GRAVITY AND MAGNETIC ANOMALIES

Geophysics ◽  
1976 ◽  
Vol 41 (1) ◽  
pp. 56-61 ◽  
Author(s):  
D. H. Shurbet ◽  
G. R. Keller ◽  
J. P. Friess
Keyword(s):  
Remanent Magnetization ◽  
Spatial Relationship ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
The Body ◽  
Gravity And Magnetic

Gravity and magnetic anomalies caused by deeply buried rock bodies in northwest Texas are compared. Interpretation of the gravity anomalies by modeling is used to locate and define the geometry of the body in a way analogous to the use of bathymetry in studies concerned with magnetization of seamounts. The direction of magnetization is then determined from the spatial relationship between the gravity and magnetic anomalies. This procedure amounts to an in‐situ determination of direction of magnetization of the body. In one example direction of magnetization indicates the time of intrusion and in another it indicates regional heating since intrusion.

SPECTRAL ANALYSIS OF TOTAL MAGNETIC ANOMALIES OF STEP MODEL

Geophysics ◽  
1978 ◽  
Vol 43 (3) ◽  
pp. 634-636 ◽  
Author(s):  
M. V. Ramanaiah Chowdary
Keyword(s):  
Magnetic Properties ◽  
Spectral Analysis ◽  
Frequency Analysis ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Model Parameters ◽  
Considerable Success ◽  
Step Model ◽  
Gravity And Magnetic ◽  
Gravity And Magnetic Data

A great deal of interest has been shown in the frequency analysis of gravity and magnetic data originally suggested by Dean (1958). The application of this method for potential field problems has met with considerable success. The purpose of this note is to show that the interpretation of total magnetic anomalies due to a sloping step model, which represents a contact between zones having different magnetic properties in terms of model parameters, is less complicated in the frequency domain than in the spatial domain.

scholarly journals GRAV MAG PRISM: A MATLAB /OCTAVE PROGRAM TO GENERATE GRAVITY AND MAGNETIC ANOMALIES DUE TO RECTANGULAR PRISMATIC BODIES

2013 ◽  
Vol 31 (3) ◽  
pp. 347 ◽  
Author(s):  
Alessandra De Barros e Silva Bongiolo ◽  
Jeferson De Souza ◽  
Francisco José Fonseca Ferreira ◽  
Luís Gustavo De Castro
Keyword(s):  
Potential Field ◽  
Magnetic Anomalies ◽  
Field Modeling ◽  
Gravity And Magnetic ◽  
Ascii Format ◽  
Arbitrary Dimensions ◽  
Prismatic Bodies

ABSTRACT. In this paper we present the GRAV MAG PRISM code to generate synthetic gravity and magnetic anomalies from rectangular prismatic bodies with arbitrary dimensions, densities and magnetizations. The code has been developed in a MATLAB/OCTAVE environment and provides a graphical and a numerical output, as well. The data are written in xyz (ASCII) format and can be contaminated with noise. We also summarize the theory and show some examples of the program’s applications. The code can be used for research and educational purposes and is freely-distributed by the authors on request.Keywords: gravity and magnetic anomalies, potential-field modeling, program. RESUMO. Neste trabalho é apresentado o programa GRAV MAG PRISM, o qual gera anomalias de gravidade e magnéticas sintéticas a partir de corpos prismáticos retangulares com dimensões, densidades e magnetizações arbitrárias. O código foi desenvolvido em ambiente MATLAB/OCTAVE, com saídas gráficas e numéricas. Os dados são escritos no formato xyz (ASCII) e podem ser contaminados por ruídos. Neste trabalho também é apresentado um resumo da teoria e alguns exemplos de aplicação do programa. O código acompanha um tutorial e pode ser utilizado para fins educacionais e de pesquisa, sendo de distribuição gratuita através de solicitação aos autores.Palavras-chave: anomalias gravimétricas e magnéticas, modelagem de campos potenciais, programa.

Spectral analysis of gravity and magnetic anomalies due to a vertical circular cylinder

Geophysics ◽  
1983 ◽  
Vol 48 (2) ◽  
pp. 224-228 ◽  
Author(s):  
A. Soto ◽  
S. K. Singh ◽  
C. Flores
Keyword(s):  
Spectral Analysis ◽  
Circular Cylinder ◽  
Bessel Function ◽  
Magnetic Anomalies ◽  
Gravity And Magnetic ◽  
Amplitude Spectra ◽  
Phase Spectra ◽  
Magnetic Case

Expressions for the spectra of gravity and magnetic anomalies due to a vertical right circular cylinder can be written in terms of a Bessel function and the sum of two exponentials. From the zeros of the amplitude spectra, which are the zeros of the Bessel function, an estimate of the radius of the cylinder can be obtained. The depths to the top and to the bottom enter as exponents of the exponential terms and can be evaluated by taking ratios of the spectra at several frequencies. The density or the intensity of magnetization can then be easily estimated. For the magnetic case the epicenter of the cylinder can be obtained from the slope of the phase spectra. The method has been tested on anomalies of various cylinders and was found to give good results.

COMBINED ANALYSIS OF GRAVITY AND MAGNETIC ANOMALIES

Geophysics ◽  
1951 ◽  
Vol 16 (1) ◽  
pp. 51-62 ◽  
Author(s):  
G. D. Garland
Keyword(s):  
Magnetic Anomalies ◽  
The Body ◽  
Rock Material ◽  
Combined Analysis ◽  
Gravity And Magnetic ◽  
Anomalous Density ◽  
The Relationship

The relationship betwen gravity and magnetic anomalies is investigated. It is shown that the ratio of the anomalous susceptibility to the anomalous density of an unknown body may be determined from gravimeter and vertical magnetometer observations, independent of assumptions as to the depth or form of the body. The use of this ratio in identifying the rock material of the body is discussed, and illustrated by applying the method to a well‐known case.

COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES DUE TO INHOMOGENEOUS DISTRIBUTION OF MAGNETIZATION AND DENSITY IN A LOCALIZED REGION

Geophysics ◽  
1977 ◽  
Vol 42 (3) ◽  
pp. 602-609 ◽  
Author(s):  
B. K. Bhattacharyya ◽  
K. C. Chan
Keyword(s):  
Rock Mass ◽  
Physical Property ◽  
Magnetic Anomalies ◽  
The Body ◽  
Inhomogeneous Distribution ◽  
Uniform Magnetization ◽  
Gravity And Magnetic ◽  
First Vertical Derivative ◽  
Analytical Expressions ◽  
Distribution Of Magnetization

Analytical expressions in the form of a convolution of two functions have been derived for the gravity and magnetic anomalies due to inhomogeneous distribution of magnetization and density in a localized region. These expressions show explicitly that the anomalies are completely determined by the divergence of magnetization and the first vertical derivative of density. It is thus analytically established that when a body with uniform magnetization or density is considered, the impulsive change in the physical property of the rock mass at the boundary of the body is responsible for generating an anomalous magnetic or gravity field. These properties are utilized to derive efficient algorithms for computing gravity and magnetic anomalies. An example is given to demonstrate the usefulness of the derived algorithm in computing the anomaly due to magnetization of an irregularly shaped topography.

CONTINUOUS SPECTRUM OF THE TOTAL‐MAGNETIC‐FIELD ANOMALY DUE TO A RECTANGULAR PRISMATIC BODY

Geophysics ◽  
1966 ◽  
Vol 31 (1) ◽  
pp. 97-121 ◽  
Author(s):  
B. K. Bhattacharyya
Keyword(s):  
Magnetic Field ◽  
Magnetization Vector ◽  
High Amplitude ◽  
The Body ◽  
Two Dimensional ◽  
Near Surface ◽  
Prismatic Body ◽  
Total Magnetic Field ◽  
Dimensional Spectrum ◽  
Prismatic Bodies

The Fourier transform of the total‐magnetic‐field anomaly due to a rectangular prismatic body with arbitrary magnetization yields the two‐dimensional spectrum of the anomaly. In the expression for the spectrum the individual effects of the horizontal and vertical dimensions of the body appear as separate factors. Another factor in the expression takes into account the combined influence of the orientation of the magnetization vector and the dip and declination of the earth’s magnetic field. The expression for the two‐dimensional spectrum is used to obtain analytical formulas of the spectra for magnetic‐field values along profiles parallel to the two horizontal axes of the body. This theoretical study provides a quantitative picture of the shift of the spectrum to the low‐frequency end with increase in either depth or horizontal dimension, or in both, of the magnetized body. It has thus been possible to realize the feasibility of a method for separating the effects of near‐surface high‐amplitude components from those of deep crustal sources in total‐field aeromagnetic maps. Separation of these effects is, however, not unique because of spectral overlap between anomalies due to “shallow” and “deep” sources. A detailed discussion has been made about the characteristics of amplitude and phase spectra of anomalies due to prismatic bodies of differing dimensions. The spectra of anomalies seem to be useful in rapid estimation of the dimensions of a body under suitable conditions. The effect of demagnetization on the fields due to prismatic bodies has been ignored in this paper.

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