A method for the interpretation of magnetic anomalies of dipping doublets

Summary A simple method for the interpretation of the vertical magnetic anomalies of dipping doublets is presented. In the first step, the line integrals of the anomalies along theX direction, which represent the anomalies of a vertical line double are utilised to obtain the depths to the two poles. In the second step, the profile alongY, through the point of intersection of the doublet with theX axis is used to determine the dip of the dipole.     

Fair function minimization for interpretation of magnetic anomalies due to thin dikes, spheres and faults

A new method is proposed to interpret magnetic anomalies due to a thin dike, a sphere, and a fault like structure, where depth, horizontal location, effective magnetization intensity and effective magnetization inclination of a buried structure are simultaneously obtained. The proposed method is based on Fair function minimization and also on stochastic optimization modeling. This new technique was firstly tested on a theoretical synthetic data randomly generated by a chosen statistical distribution from a known model with different random noises components. This mathematical simulation shows a very close agreement between the assumed and the estimated parameters. The applicability and validity of this method are thereafter applied to magnetic anomaly data taken from United States, Australia, India, and Brazil. The agreement between the results obtained by the new method and those obtained by other interpretative methods is good and comparable. Moreover, the depth obtained by such a method is found to be in high accordance with that obtained from drilling information.     

重磁异常解释的归一化局部波数法

局部波数法是进行重磁数据解释的常用方法之一.本文提出归一化局部波数法,该方法在不需要任何关于地质体信息的前提下能有效地完成异常的反演工作,且给出了不同归一化方式的应用效果.理论模型试验表明归一化局部波数法能准确地完成异常的反演,且通过对比发现其他归一化方式（中值、几何平均和调和平均）的计算结果相对算术平均归一化结果具有更高的分辨率.将该方法应用于实测磁异常的解释,获得了未知地质体的空间位置.     

Grandient measurements in the interpretation of magnetic anomalies due to two-dimensional prism-shaped bodies

Summary A direct method of interpreting magnetic anomalies due to two-dimensional prisms is presented here. Measurement of the first vertical derivative supplies additional information concerning the parameters. The normal ground magnetic profile, as well as the gradient profile will have symmetrical points whose distances from the origin are related to the width, depth and height of a two dimensional prism shaped body.     

Interpretation of magnetic anomalies of dikes using correlation factors

The magnetic anomaly due to a buried dike consists of the sum of two easily separated elementary functions. These functions, which have simple symmetry, are called even and odd functions. The correlation factors (r _0,1 for the even andr _0,2 for the odd function) between least-squares residual anomalies from even and odd functions are computed. Correlation values are used to determine the depth to the top and the half-width of the dike. The method also includes the determination of the index parameter and the amplitude coefficient. The validity of the method is tested against a theoretical and a field example where the parameters of the latter were determined by other investigators in comparing the results.     

Characteristic curves for interpreting the vertical magnetic anomalies of dipping doublets

Summary Standard charts from which a complete solution of the vertical magnetic anomalies of dipping doublets can be obtained very rapidly are derived and presented. The most important distances to be scaled from the field profile are the distances between the maximum and minimum anomalies, half maximum anomalies and one-third maximum anomalies only. The use of the charts for claculating the various parameters and locating the origin is also incorporated.     

A Fourier transform method for the interpretation of self-potential anomalies due to two-dimensional inclined sheets of finite depth extent

The self-potential anomaly due to a two-dimensional inclined sheets of finite depth extent has been analysed in the frequency domain using the Fourier transform. Expression for the Fourier amplitude and phase spectra are derived. The Fourier amplitude and phase spectra are analysed so as to evaluate the parameters of the sheet. Application of this method on two anomalies (synthetic and field data) has given good results.     

Analogous method for interpretation of gravitational anomalies

Summary From a corollary ofGreen's theorem is derived, that a thin layer of variable density can produce the same gravitational effect as a certain mass distribution below this layer. To this thin layer of variable density is analogous a sheet illuminated of variable intensity. The intensity of illumination is detected with a Philips LDR (light dependent resistor) cell. It is pointed out further that the method can be applied to determine the vertical gravity gradient too.

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Zusammenfassung Aus einem Folgesatz des Green-Theorems wird abgeleitet, dass eine dünne Schicht mit veränderlicher Dichte die gleiche Gravitationswirkung verursacht wie eine bestimmte — unter dieser Schicht liegende — Massenverteilung. Zwischen dieser dünnen Schicht mit veränderlicher Dichte und einer beleuchteten Fläche mit veränderlicher Lichtstärke besteht eine Analogie. Die Intensität der Beleuchtung wird mit einer Philips-LDR-Zelle gemessen. Es wird weiter darauf hingewiesen, dass die Methode auch für Bestimmung des vertikalen Gravitationsgradienten anwendbar ist.

     

Quantitative interpretation of self-potential anomalies due to spherical ore bodies

Summary A quantitative method of interpreting self-potential anomaly caused by a spherical ore body using downward continuation method is presented. Master curves to determine the depth, radius and angle of polarization have been prepared.     

Hilbert transform in the interpretation of magnetic anomalies of various components due to a thin infinite dike

The Hilbert transformH(x) applicable to vertical (Z), horizontal (H), and total (T) magnetic anomalies due to a thin dike of infinite depth extent is derived from the generalised expression of magnetic effectF(x). The depth and dip of the dike is extracted by a simple procedure making use ofF(x) andH(x). A modified version of the amplitude of the analytic signal is given to locate the origin. The abscissa of the point of intersection ofF(x) and the discrete Hilbert transformH(1.x) directly yields the depth to the top. An example for each case is considered theoretically to illustrate the process. Applicability of the method is examined on the vertical component of the well-known magnetic anomaly at Kiirunavaara in northern Sweden, originally described by Von Carlheim Gyllenskjold, as well as on total magnetic anomaly of Bensons Mines, U.S.A.     

Short note: A simple nomogram for interpretation due to magnetic horizontal cylinders

The magnetic anomaly ΔF due to a long horizontal cylinder yields three points related to its extrema. A nomogram is presented for the determination of some source parameters of the causative body.     

On the interpretation of magnetic anomalies for strong remanent magnetizations

In this paper some aspects concerning the interpretation of magnetic anomalies are treated, particularly when the remanent magnetization intensity is strong. In this case, since total and induced magnetization vectors can have very different directions, a correct anomaly interpretation strictly depends on the knowledge of their declinations and inclinations.Thus, a specific procedure is described to determine such parameters from well-known semi-empirical techniques and vectorial relations.Furthermore, the classical definition of apparent susceptibility is shown to be inadequate to this problem and a more general formulation is suggested, which is not only related to the true susceptibility and to the Koenisberger ratio, but also to the declinations and inclinations of the induced and remanent magnetization vectors.The two apparent susceptibilities are then compared for some synthetic magnetic anomalies and significant differences are found.     

Comments on the interpretation of linear magnetic anomalies

Summary In response to the recent interpretation of marine magnetic anomalies in terms of the crustal spreading hypothesis, a summary is made of other factors including structural, initial cooling, post cooling, and possible spontaneous processes which may cause at least some of the characters of linear magnetic anomalies. Arguments are developed by presentation ofQ values for 571 specimens from fourteen Icelandic lavas and six dikes, and by theQ value, stability factor, and oxidation variation between the cooling faces of one of these lavas. None of the processes described can alone readily account for the observed symmetry of magnetic anomalies with respect to mid-oceanic ridges, but are presented to encourage examination and development of mechanisms other than geomagnetic polarity changes superimposed on a crustal generating system, as the causes of at least some oceanic magnetic anomalies.     

A second derivative method of interpretation of gravity anomalies of anticlines

Summary A simple method for the interpretation of second derivative gravity anomalies of an anticline is presented. The method utilises only three readily measurable distances, from the origin to the points of half maximum second derivative anomaly, zero anomaly and minimum second derivative anomaly. Charts for the computation of the various parameters are constructed.     

A method for rapid evaluation of magnetic anomalies over thin sheets

A method for rapid evaluation of magnetic anomalies over thin sheets is presented. This method is based on characteristic distancesX _3/4,X _1/2,X _1/4 at which the anomaly falls off to 3/4, 1/2, and 1/4 of the total (peak to peak) amplitude, respectively. Simple mathematical relations using these characteristic distances are presented to estimate the death and dip of the sheet. This method is fast and does not require prior knowledge of the base level and the origin. A few field examples are included to show the applicability and efficacy of the method.     

Interpretation of gravity and magnetic anomalies caused due to spherical bodies

Summary The method of continuation has been used to obtain the master curves for gravity and magnetic anomalies caused by spherical bodies. The procedure to calculate the depth of burial and radius of spherical bodies has been outlined.     

An analytic signal approach to the interpretation of total field magnetic anomalies1

The analytic signal (AS) is defined as the square root of the sum of the squares of the vertical and the two horizontal derivatives of the total magnetic field ΔT. This paper verifies theoretically that peaks of the AS correlate directly with their magnetic causative bodies and are positioned symmetrically over them, i.e. the main feature of the AS is that it is independent of the inclination of the magnetic field. This avoids the difficulties that are often faced in the conventional process of reduction to pole for ΔT, when the direction of magnetization of the causative bodies is not known. In addition, the AS has characteristics similar to the derivative features of the magnetic field, so that it is very sensitive to edge effects of the causative magnetic bodies. The theoretical derivations are tested by comparison with calculations on models, and, in a field example from Hunan Province, China, the AS is applied successfully to the interpretation of ΔT, whereas the conventional process of reduction to pole fails, due to the reverse magnetization of the causative body.     

Tectonic interpretation of the magnetic anomalies southwest of Vancouver Island

Summary A tectonic interpretation of the magnetic anomalies off the coast of California, Oregon and Washington between 40° and 52° north latitude shows that in the area surveyed the oceanic crust is cut by seven major dislocation zones which divide the region in eight areas. For five of these areas the original connection can be reconstructed. The existence of a window of young crust surrounded by older crust and of a short, isolated length of active oceanic ridge southwest of Vancouver Island as proposed recently by different authors is not confirmed.     

Future of the interpretation theory of gravity and magnetic anomalies

The general methodology of the interpolation theory of potential fields (gravity and magnetic anomalies), and, primarily, the three fundamental ideas underlying this methodology—(I) approximation, (II) linearization, and (III) optimization-are the matter of the utmost importance. The rationale of these ideas and the ways of their practical implementation are discussed in this paper.