Techniques for interpreting borehole vector magnetic survey data using a template for calculating anomalies from two-dimensional bodies
Abstract Generated abstract
This paper addresses interpretation of borehole vector magnetic surveys, particularly for two-dimensional magnetized bodies measured with a three-component borehole magnetometer. It proposes two practical uses of D. S. Mikov’s point nomogram: quantitative fitting of the total anomaly curve along a borehole, optionally combined with the vertical component to infer magnetization direction, and graphical construction of anomaly vectors to compare calculated and observed vector patterns. The author argues that applying both methods improves interpretation of ore-body shape and position, and cautions that the common practice of locating ore-body edges from convergent and divergent vector fans is generally unreliable except for highly elongated bodies magnetized along their long axis.
Full Text
TECHNIQUES FOR INTERPRETING BOREHOLE VECTOR MAGNETIC SURVEY DATA USING A NOMOGRAM FOR CALCULATING ANOMALIES FROM TWO-DIMENSIONAL BODIES
B. M. Afanas'ev
(Presented by Prof. D. S. Mikov)
Borehole vector magnetic surveying is carried out with the TSM-3-VITR apparatus, which makes it possible to measure three mutually perpendicular components of the total anomalous vector \(T_a\); in this case two components are located in the plane of the section (curvature) of the borehole \((XZ)\), and one is in the plane perpendicular to the section plane \((YZ)\). Interpretation of vector magnetic-survey data is generally performed qualitatively, on the basis of studying the character of the distribution of vectors, with the following proposition taken as fundamental: a divergent fan of vectors issues from the lower edge of the ore body, while a convergent fan indicates the direction toward the upper edge of the ore body.
The first attempts at interpreting materials from vector magnetic surveying carried out in boreholes of magnetite deposits of the Eastern Sayan showed the expediency of using, for the interpretation of borehole vector magnetic surveying (in the case of two-dimensional bodies), the point nomogram of D. S. Mikov. In this case the following methods of using this nomogram are possible.
Method I.
It is known that the magnitude of the total anomaly vector for any two-dimensional body does not depend on the direction of magnetization. This is the great advantage of the \(T_a\) curve along a borehole, obtained by the method of borehole vector magnetic surveying, over the \(Z_a\) curve obtained by the method of magnetic-field logging.
Quantitative interpretation of the \(T_a\) curve is carried out as follows:
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At each point of the borehole where \(T\) was measured, the point nomogram is used to determine separately the values of \(H_a\) and \(Z_a\) from the assumed ore body for any conditionally chosen direction of magnetization of the body, with an intensity corresponding to the magnetization intensity of the sought body.
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After this, \(T_a=\sqrt{Z_a^2+H_a^2}\) is determined at each point, and from these values a graph of \(T\) along the borehole from the assumed body is constructed. By varying the location and shape of the body by the usual trial-and-error method, agreement between the calculated and measured \(T\) curve in the borehole is achieved.
It should be borne in mind that parallel interpretation of the curves \(T_a\) and \(Z_a\) along the borehole opens up certain possibilities for determining
directions of magnetization of ore bodies. To do this: 1) from the \(T_a\) curve the shape of the ore body is found by trial selection; 2) for the ore-body shape found (from \(T_a\)), by the trial method one finds such a direction of magnetization of this body as would satisfy the \(Z_a\) curve.
Method II.
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By a procedure analogous to that described in Method I, \(Z_a\) and \(H_a\) are determined at each point of the borehole (each component with its own sign from the assumed ore body).
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The vectors \(T_a\) are constructed graphically (more precisely, the projections of \(T_a\) onto the plane of the section).
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The body is selected until the pattern of vector distribution approximates the measured one.
Naturally, it is advisable to apply both methods: first Method I, and then Method II.
In connection with the presentation of the second method, it is appropriate to note that the method currently used in practice for determining the lower and upper edges of an ore body from the point of intersection, respectively, of diverging and converging fans does not correspond to reality, as is clearly seen from Fig. 1. Such a method will be suitable only for bodies that are very elongated in section and magnetized along the long axis.
Fig. 1. Distribution of the vectors \(T_a\) in the \(XZ\) plane (obtained from \(Z\) and \(H\), calculated with a point palette).
In each specific case, the position of the edges of the ore body will depend on the direction of magnetization of the ore body, its shape, and its distance from the borehole axis. This once again proves the necessity of using the methods proposed here (in particular, the second) for interpreting the results of borehole vector magnetic surveying.
REFERENCES
- D. S. Mikov. Methods for Interpreting Magnetic Anomalies. Publishing House of Tomsk State University, 1962.
- Technical Description and Operating Instructions for the TSM-3-VITR Three-Component Borehole Magnetometer. State Geological Committee under the Council of Ministers of the USSR, VITR, Leningrad, 1963.