FEATURES OF THE MAGNETIC FIELD

IN THE AREA OF THE DUTURUL FAULT

(BALEI DISTRICT, EASTERN TRANSBAIKALIA)

V. A. KORTUNOV, I. P. NOVIKOV

(Presented by the scientific seminar of the Department of Geophysical Prospecting Methods)

A geophysical party of the Tomsk Polytechnic Institute carried out high-precision magnetometric investigations in order to determine the possibility of identifying and tracing the Duturul tectonic zone within the periphery of the Balei graben.

The survey profiles were laid out across the expected strike of the structure under study. The observation network was adopted as \(100 \times 10\) m. For control and to achieve high observational accuracy, various methodological procedures were used. In order to improve the accuracy of tying the reference network, the number of tie instrument-runs was not limited, and the reference network was established according to the method of repeated observations by two independent instrument-runs. The root-mean-square error in establishing the reference network was brought to \(2.8\gamma\). To improve the accuracy of the routine survey, most of the profiles were worked by two instruments; the time between visits to reference points did not exceed thirty minutes. This methodology made it possible to carry out reliable linear interpolation of the correction for zero-point displacement. The root-mean-square error of observations at routine points was \(5\gamma\).

Based on the results of the magnetic survey, a map of \(\Delta Z\) graphs was constructed, onto which the geological situation of the area and the strike lines of the structure under study, according to geological and magnetometric data, were plotted. Figure 1 presents a part of the map of \(\Delta Z\) graphs along the Duturul fault in the area of Mount Krestovaya.

Taking into account the complexity of the geological-geophysical setting in the area of the magnetometric investigations, we carried out mainly only a qualitative interpretation, which consisted in explaining the nature of the disturbances in the magnetic-field intensity and in revealing regularities in the relationship of the observed anomalies to geological objects.

In analyzing the magnetic-survey data, the following conclusions were made:

1. On all profiles the Duturul fault is traced and correlated by negative minima of the magnetic-field intensity with an intensity from \(-10\) to \(-60\gamma\). Moreover, on the first three profiles (PR0, PR1, PR2) the inferred axis of the fault according to magnetic-prospecting data coincides with the fault axis on the geological base. Beginning with profile three, the correlated axis of the minima lies somewhat to the southeast, shifting by 100–150 m from the line of tectonic disturbance shown on the geological map. This deviation can be explained, in our opinion, by certain errors in the topographic tying of the maps, and mainly, appa—

dimensionality, and the inaccuracy of the location of the fault plotted from geological data, owing to the poor exposure of the area, where a cover of alluvial–deluvial deposits up to 50 m thick is developed almost everywhere.

1. The inconstancy of the composition of the metamorphic and volcanogenic-sedimentary sequences, as well as the presence of dikes, stocks, and small subordinate tectonic disturbances, has led to a sharp diversity of the magnetic field within the area described.

2. The quantitative interpretation carried out on the most characteristic profiles shows that the disturbing body is the weathering crust of the fault, created by selective weathering, with a depth of occurrence from 37.5 to 90 m and with a magnetization angle of 28°.

In order to determine the possibility of clearly separating the metamorphic formations and volcanogenic-sedimentary deposits in contact along the fault, certain quantities were used that characterize the so-called “dissectedness” of the \(\Delta Z\) graphs (3).

Fig. 1. Map of \(\Delta Z\) graphs. 1—numbers of profiles and pickets; 2—graphs; 3—metamorphic deposits; 4—volcanogenic-sedimentary formations; 5—fault axis according to magnetometric data; 6—fault axis according to geological data.

The sample variance was calculated:

\[ S^{2}=\frac{1}{n-1}\cdot\sum_{i=1}^{n}(\Delta Z_i-\Delta Z)^2, \tag{1} \]

where

\[ \Delta Z=\frac{\Delta Z_1+\Delta Z_2+\ldots+\Delta Z_n}{n}, \]

the coefficient of variation,

\[ K=\frac{\sqrt{S^{2}}}{\Delta Z}, \tag{2} \]

and the mean value of the magnetic-field gradient:

\[ y_1=\left|\frac{\partial Z}{\partial x}\right| =\frac{\sum_{i=1}^{n}\left|\Delta Z_{i+1}-\Delta Z_i\right|}{(n-1)\Delta x}, \tag{3} \]

where \(\Delta Z_i\) and \(\Delta Z_{i+1}\) are the magnetic-field values measured at neighboring points of the profile; \(\Delta x\) is the distance between points; \(n\) is the number of points in the selected interval.

These quantities were calculated for the areas of distribution of volcanogenic-sedimentary formations, and also for metamorphic rocks. The calculations were made for individual profiles and compared for each profile. From Table 1 it is evident that all the quantities characterizing the “dissectedness” of the \(\Delta Z\) graphs are greater for the volcanogenic-sedimentary rocks than for the metamorphic rocks.

Using the method of averaging neighboring points, we constructed a plan of \(\Delta Z\) isolines, which subsequently served as the initial material for transformation of the magnetic field. As a result of mathematical

Fig. 2. Map of the first vertical derivative of the magnetic field.
1—zero isolines of the \(\Delta Z_z\) field; 2—positive isolines of the \(\Delta Z_z\) field; 3—negative isolines of the \(\Delta Z_z\) field; 4—areas of occurrence of negative \(\Delta Z_z\) values less than \(500\ \gamma/\text{km}\); 5—axes of tectonic disturbances according to magnetometric data; 6—axis of a tectonic disturbance according to geological data.

Table 1

processing of magnetic observations on the M-20 electronic digital computer [1], the most interesting picture was obtained in computing the vertical gradients of the \(\Delta Z\) values, from which a plan of \(\Delta Z_z\) isolines was constructed (Fig. 2). This made it possible to obtain a clear geophysical picture of the area under study, with sharply expressed narrow local minima whose distribution corresponds to the position of the fault being studied. Within the described area, a number of small transverse tectonic disturbances are also revealed on the \(\Delta Z_z\) isoline plan.

Thus, the described methodology for conducting magnetometric work makes it possible to identify and trace structures of the Duturul type and to map contacts of weakly and moderately magnetic rocks, whereas other geophysical investigations carried out in this area do not yield positive results.

The application of such methods for transforming magnetic anomalies as various averaging techniques, transformation of potential fields into the upper and lower half-spaces, computation of dispersion, the coefficient of variation, higher derivatives, etc., leads to the isolation of particular features of the original maps and substantially facilitates analysis of the source material, while carrying additional information.

LITERATURE

1. Temporary manual for the mathematical processing of gravitation and magnetic observations on the M-20 electronic digital computer. Novosibirsk, 1962.

2. G. V. Gladkii. Gravity Prospecting and Magnetic Prospecting. “Nedra,” 1964.

3. L. L. Lyakhov, G. A. Solov’ev. The use of statistical methods in geophysical prospecting for rare-metal deposits in Western Transbaikalia. Geology and Prospecting, No. 12, 1968.