Interpretation of magnetic anomalies due to dikes: The complex gradient method

Geophysics ◽  
1981 ◽  
Vol 46 (11) ◽  
pp. 1572-1578 ◽  
Author(s):  
D. Atchuta Rao ◽  
H. V. Ram Babu ◽  
P. V. Sanker Narayan
Keyword(s):  
Magnetic Anomaly ◽  
Magnetic Anomalies ◽  
Half Width ◽  
Amplitude Function ◽  
Maximum Value ◽  
Magnetic Inclination ◽  
Vector Quantity ◽  
Characteristic Points ◽  
Depth Of Burial ◽  
Complex Gradient

A method to interpret the magnetic anomaly due to a dipping dike using the resultant of the horizontal and vertical gradients of the anomaly is suggested. The resultant of both the gradients is a vector quantity and is defined as the “complex gradient.” A few characteristic points defined on the amplitude and phase plots of the complex gradient are used to solve for the parameters of the dike. For a dike uniformly magnetized in the earth’s magnetic field, the amplitude plot is independent of [Formula: see text], the index parameter, which depends upon the strike and dip of the dike and the magnetic inclination of the area. The phase plot of the complex gradient is an antisymmetric curve with an offset value equal to [Formula: see text]. For a dike whose half‐width is greater than its depth of burial, two maxima at equal distances on either side of a minimum value appear on the amplitude plot. For a dike whose half‐width is equal to or less than its depth of burial, the amplitude plot is a bell‐shaped symmetric curve with its maximum appearing directly over the origin. In the case of a thin dike, the amplitude function falls off to half its maximum value at the same point on the abscissa where the phase function reaches, i.e., [Formula: see text]. A combined analysis of the amplitude and phase plots of the complex gradient yields all the parameters of the dike. The method is applicable for the magnetic anomaly in either the total, vertical, or horizontal field. A field example is included to show the applicability of the method.

Analysis of Magnetic Anomaly Characteristics of Underground Non-Coplanar Cross-buried Iron Pipelines

2020 ◽  
Vol 25 (2) ◽  
pp. 223-233
Author(s):  
Pan Wu ◽  
Minghui Wei
Keyword(s):  
Magnetic Susceptibility ◽  
Magnetic Anomaly ◽  
Magnetic Dipole ◽  
Geomagnetic Field ◽  
Magnetic Anomalies ◽  
Buried Pipelines ◽  
Magnetic Declination ◽  
Magnetic Inclination ◽  
Basic Characteristics ◽  
Geomagnetic Intensity

The non-coplanar cross-buried pipelines are a common way of pipeline wiring. In order to investigate the magnetic anomaly characteristics of the non-coplanar cross-buried pipelines and guide the site operation, the influences of a series of factors on the magnetic anomaly of the non-coplanar cross-buried pipelines are analyzed. Based on the principle of magnetic dipole construction, a forward model is established for the magnetic anomaly characteristics of the subsurface non-coplanar cross-buried pipelines. The basic characteristics of magnetic anomaly for the non-coplanar cross-buried pipelines are defined. The influences of geomagnetic parameters (geomagnetic intensity, geomagnetic inclination, and geomagnetic declination), pipeline parameters (thickness, magnetic susceptibility), and cross angle of pipelines on the characteristics of magnetic anomalies are analyzed. The results show that the shape of the total magnetic anomaly is mainly affected by the magnetic inclination, and the curve of magnetic anomaly at ± I site shows some symmetry. The amplitude is approximately linearly affected by the total geomagnetic field, magnetic declination, pipeline thickness, material magnetic susceptibility, and pipeline cross angle. There is a periodic change of the amplitude with the increase of geomagnetic inclination (−90°–>90°). The crest-trough distance is mainly affected by geomagnetic inclination, magnetic declination, thickness, magnetic susceptibility, and pipeline cross angle. A more accurate measurement can be achieved if the direction of the pipelines is roughly measured and then the number of measurement points is augmented near the intersection of pipelines and the measurement lines. Present work obtains the equivalent magnetic dipole units by segmenting pipelines. The magnetic anomaly characteristics of non-coplanar crossed iron pipelines are successfully simulated. The numerical results are in accordance with the experimental analysis.

QUANTITATIVE INTERPRETATION OF VERTICAL MAGNETIC ANOMALIES OVER VEINS

Geophysics ◽  
1950 ◽  
Vol 15 (4) ◽  
pp. 667-686 ◽  
Author(s):  
Kenneth L. Cook
Keyword(s):  
Magnetic Field ◽  
Magnetic Anomaly ◽  
Magnetic Induction ◽  
Vertical Component ◽  
Magnetic Anomalies ◽  
Quantitative Interpretation ◽  
Magnetic Intensity ◽  
Magnetic Inclination ◽  
Magnetic North ◽  
The Magnetic Field

By using ordinary magnetic induction methods of analysis, Haalck, Heiland, and others have developed formulas which express the magnetic anomaly over a vertical or inclined vein of tabular shape as a function of the susceptibility, dimensions, shape, and disposition of the vein, and of the strength and direction of the earth’s magnetic field. On the basis of these fundamental formulas, other formulas for the vertical component of the magnetic field are derived in the present paper for such veins in intermediate northern magnetic latitudes. Special emphasis is given to the orientation of the veins relative to the magnetic north direction. Several families of vertical magnetic intensity curves for veins with different strikes and dips are given. All theoretical curves for veins striking magnetic north are plotted in terms of a parametric unit so that, once plotted, they can be used repeatedly in different districts, provided a proper multiplying factor is chosen for the observed curve. The importance of the transverse horizontal magnetization effect under certain conditions of orientation is demonstrated. It is shown mathematically that small vertical magnetic anomalies are to be expected for thin veins striking east and dipping south at an angle equal to, or approximately equal to, the complement of the angle of magnetic inclination.

Magnetic interpretation using a least-squares, depth-shape curves method

Geophysics ◽  
2005 ◽  
Vol 70 (3) ◽  
pp. L23-L30 ◽  
Author(s):  
El-Sayed M. Abdelrahman ◽  
Khalid S. Essa
Keyword(s):  
Least Squares ◽  
Magnetic Anomaly ◽  
Least Squares Method ◽  
Random Noise ◽  
Synthetic Data ◽  
Theoretical Data ◽  
Magnetic Anomalies ◽  
Shape Factors ◽  
Characteristic Points ◽  
Horizontal Cylinders

We have developed a least-squares approach to depth determination from residual magnetic anomalies caused by simple geologic structures. By normalizing the residual magnetic anomaly using three characteristic points and their corresponding distances on the anomaly profile, the problem of determining depth from residual magnetic anomalies has been transformed into finding a solution to a nonlinear equation of the form z = f(z). Formulas have been derived for spheres, horizontal cylinders, thin dikes, and contacts. The method is applied to synthetic data with and without random noise. We have also developed a method using depth-shape curves to simultaneously define the shape and depth of a buried structure from a residual magnetic anomaly profile. The method is based on determining the depth from the normalized residual anomaly for each shape factor using the least-squares method mentioned above. The computed depths are plotted against the shape factors on a graph. The solution for the shape and depth of the buried structure is read at the common intersection of the depth-shape curves. The depth-shape curves method was successfully tested on theoretical data with and without random noise and applied to a known field example from Ontario.

THE SEPARATION OF REMANENT FROM INDUCED MAGNETISM IN SITU

Geophysics ◽  
1966 ◽  
Vol 31 (4) ◽  
pp. 779-796 ◽  
Author(s):  
N. E. Goldstein ◽  
S. H. Ward
Keyword(s):  
Magnetic Anomaly ◽  
Field Experiments ◽  
Magnetic Anomalies ◽  
Static Field ◽  
Observational Evidence ◽  
Field Direction ◽  
Dynamic Field ◽  
In Situ Separation

Remanent and induced magnetism both contribute to static field magnetic anomalies whereas only induced magnetism contributes to dynamic field magnetic anomalies. The theory whereby this phenomenon may be used to advantage for in‐situ separation of remanent from induced magnetism is presented as a prelude to observational evidence confirming the phenomenon. Four field experiments on Western States magnetic anomalies prove that it is possible to predict whether or not a given static field magnetic anomaly is primarily due to remanent or to induced magnetism. The limitations of the method include variability of micropulsation field direction, ellipticity, and intensity.

A comparative study of the relation figures of magnetic anomalies due to two‐dimensional dike and vertical step models

Geophysics ◽  
1982 ◽  
Vol 47 (6) ◽  
pp. 926-931 ◽  
Author(s):  
H. V. Ram Babu ◽  
A. S. Subrahmanyam ◽  
D. Atchuta Rao
Keyword(s):  
Comparative Study ◽  
Major Axis ◽  
Magnetic Anomalies ◽  
The Body ◽  
Two Dimensional ◽  
Depth Ratio ◽  
Characteristic Points ◽  
Vertical Step ◽  
Bottom To Top ◽  
Body Parameters

Magnetic anomalies in vertical and horizontal components, when plotted one against the other in polar form, result in a curve called the relation figure (Werner, 1953). In this paper, a comparative study of the relation figures of magnetic anomalies due to two‐dimensional (2-D) dike and vertical step models is made. The relation figures for these two models are found to be ellipses with different properties. The tangent at the origin to the ellipse is parallel to the major axis of the ellipse for the dike model, whereas it is perpendicular to the major axis for the vertical step. This property may be used to distinguish whether the source is a dike or a vertical step. For both of the models, the angle made by the axis of symmetry of the ellipse with the coordinate axis is equal to θ, the combined magnetic angle. The ratio between the lengths of the major and minor axes of the ellipse is directly related to the width‐to‐depth ratio of the dike or the bottom‐to‐top depth ratio of the vertical step. A few characteristic points defined on the ellipse are used to evaluate the body parameters. The major portion of the ellipse is obtained in the close vicinity of the source. Because of symmetry, the ellipse may be extrapolated easily outside the data length, and hence the effect of noise caused by adjacent objects is kept at a minimum.

Magnetic anomaly of a circular lamina

Geophysics ◽  
1979 ◽  
Vol 44 (1) ◽  
pp. 102-107 ◽  
Author(s):  
S. K. Singh ◽  
R. Castro E. ◽  
M. Guzman S.
Keyword(s):  
Circular Cylinder ◽  
Closed Form ◽  
Gravity Anomaly ◽  
Magnetic Anomaly ◽  
Magnetic Anomalies ◽  
Hankel Transform ◽  
Poisson’S Equation ◽  
Poisson's Equation ◽  
Gravity And Magnetic

Closed form expressions for the gravity anomaly of a circular lamina and the gravity and magnetic anomalies due to a vertical right circular cylinder have been obtained previously (Singh, 1977a; Singh, 1977b; Singh and Sabina, 1978) by a method which avoids complicated integrations commonly used in deriving such solutions (e.g., Nabighian, 1962; Rao and Radhakrishnamurty, 1966). The method involves use of the Fourier‐Hankel transform of Poisson’s equation. The final expressions are obtained in closed form by employing certain tabulated integrals.

Analysis of the Czech magnetic anomaly data obtained by ground-based and airborne magnetic surveys

2020 ◽  
Author(s):  
Pavel Hejda ◽  
Dana Čápová ◽  
Eva Hudečková ◽  
Vladimír Kolejka
Keyword(s):  
Magnetic Anomaly ◽  
Magnetic Anomalies ◽  
Data File ◽  
Ore Deposits ◽  
Substantial Contribution ◽  
Magnetic Survey ◽  
Contour Maps ◽  
Anomaly Map ◽  
Ground Magnetic ◽  
Airborne Survey

<p>The modern epoch of ground magnetic surveying activity on the Czech territory was started by the Institute of Geophysics by setting up a fundamental network of the 1<sup>st</sup> order in 1957-58. It consists of 199 points and was reoccupied in 1976-78 and 1994-96. The anomaly maps were constructed by subtraction of the IGRF model.</p><p>Extensive aeromagnetic measurements have been performed from 1959 to 1972 by permalloy probe of Soviet provenience. The accuracy of the instrumentation was about (and often above) 10 nT. The second period of airborne survey started in 1976. Thanks to the deployment of proton precession magnetometer, the accuracy improved to ~ 2 nT. Since 2004 the measurements were carried out by caesium magnetometer. The data were digitized, known anthropogenic anomalies were cleared away and data were transformed to the regular grid with step 250 m. The final data file of magnetic anomalies ΔT, administered by the Czech Geological Survey, represents a substantial contribution to the exploration of ore deposits and to the structure geology in general.</p><p>In view of the fact that data file of magnetic anomalies was compiled from data acquired by heterogeneous methods in the course of more than 50 years, our recent study is aimed at looking into the homogeneity of the data by comparison them with ground-based magnetic survey. A simple comparison of the contour maps showed good similarity of the large regional anomalies. For more detailed analysis, the variation of ΔT in the neighbourhood of all points of the fundamental network was inspected and the basic statistic characteristics were computed. Summary results as well as several examples will be presented accordingly as the INSPIRE compliant services and eventually as the user-friendly web map application and made available on the CGS Portal http://mapy.geology.cz/ and on the updated web of the CzechGeo/EPOS consortium www.czechgeo.cz. Incorporating the map into the World Digital Magnetic Anomaly Map (WDMAM – IAGA) is also under consideration. This data will also be interesting for the EPOS.</p>

COMMENTS ON “INTERRELATIONSHIPS BETWEEN MAGNETIC ANOMALY COMPONENTS”

Geophysics ◽  
1959 ◽  
Vol 24 (2) ◽  
pp. 366-369 ◽  
Author(s):  
Aivars Celmins
Keyword(s):  
Magnetic Field ◽  
Magnetic Anomaly ◽  
Simple Formula ◽  
Magnetic Anomalies ◽  
Horizontal Component ◽  
Two Dimensional ◽  
Interesting Behavior ◽  
Normal Magnetic Field

On page 748 of the above named paper, Affleck (1958) mentions an interesting behavior of magnetic anomalies which are caused by homogeneous magnetized two‐dimensional bodies. He states that in these cases the airborne magnetometer anomaly can be treated as either the vertical or horizontal component anomaly if the true magnetization is replaced by a pseudo‐magnetization of other direction and intensity. It may be of some interest to formulate this behavior more precisely, so much the more as the interdependence between the magnetization directions and the direction of a normal magnetic field can be expressed by a rather simple formula.

Gravity and magnetic anomalies of the Sutton Mountains region, Quebec and Vermont: expressions of rift volcanics related to the opening of Iapetus

1981 ◽  
Vol 18 (4) ◽  
pp. 680-692 ◽  
Author(s):  
P. S. Kumarapeli ◽  
A. K. Goodacre ◽  
M. D. Thomas
Keyword(s):  
Gravity Anomaly ◽  
Magnetic Anomaly ◽  
Triple Junction ◽  
Three Dimensional ◽  
Surface Expression ◽  
Magnetic Anomalies ◽  
Iapetus Ocean ◽  
Metavolcanic Rocks ◽  
Gravity And Magnetic ◽  
Narrow Belt

Prominent, nearly coincident, positive gravity and magnetic anomalies occur in the Sutton Mountains region, centered about 100 km east of Montreal, Quebec. Several lines of evidence indicate that the gravity anomaly stems from two principal sources: a deep (mid and lower crustal) source of speculative origin and a shallow source identifiable with a narrow belt of late Precambrian – early Cambrian metavolcanic rocks, the Tibbit Hill volcanics. The magnetic anomaly seems to be produced mainly by the metavolcanic rocks. Three-dimensional modelling of a residual gravity anomaly, supplemented by two-dimensional modelling of the magnetic anomaly, shows that the seemingly minor belt of metavolcanic rocks constitutes the surface expression of a thick (maximum thickness ~8 km) pile of dominantly mafic volcanics, which are only slightly exposed at the present level of erosion.The Tibbit Hill volcanics are regarded as products of rift-related volcanism that occurred at an rrr triple junction developed during the early stages of the opening of the Iapetus Ocean. The Ottawa graben is probably the failed arm of this triple junction. The emplacement of the Grenville dike swarm whose trend is nearly coincident with that of the Ottawa graben was probably coeval with the volcanism in the Sutton Mountains region. The present work shows that the volcanism in the region was on a much larger scale than hitherto recognized.

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