EXPERIENCE IN APPLYING SPATIAL STATISTICAL CHARACTERISTICS TO THE ANALYSIS OF THE MAGNETIC FIELD OF THE APRELKOVSKOYE GOLD ORE DEPOSIT

A. T. Sysoev

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

In the practice of studying the structure of magnetic fields of gold ore deposits, one encounters problems of separating anomalies that practically do not differ from one another in their parameters (period of variation, amplitude, anisotropy), but that have different origins. Low-frequency variations of the vertical component of the magnetic field may serve as an example. These variations are caused by individual petromagnetic differences of rocks, ore bodies and other large structural elements, as well as by the relief of the ground surface. It should be noted that, while having identical parameters, these variations are complicated by high-frequency anomalies. The use in this case of traditional methods of anomaly separation by means of averaging, recalculations into the upper and lower half-spaces, calculations of higher derivatives of the magnetic potential, and integral transformation is excluded, since these methods are based on the assumption that the fields being transformed are produced by perturbing objects of different sizes. In this case the effectiveness of field separation will be the greater, the more the anomalies differ from one another in their parameters.

For separating anomalies that have identical parameters into certain groups according to their nature, a method may be applied for investigating spatial statistical relationships with other fields and with geological and geomorphological features. Thus, by detailed studies of the structure of magnetic fields of gold ore deposits, we were able to establish that low-frequency variations of the magnetic field caused by the relief of the ground surface are statistically closely related to changes in elevations [2]. A statistical relationship between “relief” anomalies and the elevations of the ground surface is observed in frequency, amplitude, and anisotropy.

A close statistical relationship at low frequencies of the vertical component of the magnetic field with the relief of the ground surface occurs only in the case of a homogeneous geological structure of the area under study, when low-frequency variations are caused only by the relief of the ground surface. A disruption of the correlation relationship between low-frequency variations of the magnetic field and changes in elevations will indicate that in the structure of the magnetic field—

there are low-frequency variations that are not associated with relief, but are caused by geological inhomogeneities of the deposit. The spatial regularity under consideration in the observed fields does not appear in an obvious form, since it is complicated by high-frequency variations and can be revealed by means of appropriate mathematical processing.

The frequency relation is established by means of correlation transformation, which makes it possible to distinguish low-frequency variations and suppress high-frequency ones. For this purpose, autocorrelation functions of the relief and magnetic field are calculated along individual profiles (across the incipient periodicities in the variation of elevation marks) [1]. As a rule, the period of the low-frequency component of the relief and of the magnetic field is directly recorded from the correlation functions, without an additional assessment of the spectrum by Fourier transformation.

To establish the correlation relation of low-frequency variations in the increment of the vertical component of the magnetic field with elevation marks in amplitude, the entire area is divided into separate sites. Within the sites, mean values of the vertical component of the magnetic field and of the elevation marks are determined. Finding mean values within fixed dimensions of areas makes it possible to eliminate the influence of high-frequency variations. The averaging dimensions can be estimated from the spectrum of low-frequency variations obtained by means of correlation transformation. From the mean values obtained, a correlation field is constructed for the dependence of the magnetic-field increment on the elevation marks, and the correlation coefficient is calculated.

The relation in anisotropy between low-frequency variations of intensity and the relief of the locality is established by comparing diagrams of the directionality of isolines of low-frequency variations of the vertical component of the magnetic field and of relief contours. To construct a plan of isolines of low-frequency variations of the vertical component of the magnetic field, values of the intensity increment averaged over areas are used.

The described method for separating low-frequency anomalies into “relief-related” ones and those caused by the special morphogenetic structure of the ore field was applied at the Aprelkovskoye gold-ore deposit (Eastern Transbaikalia). The ore field is confined to a diorite massif of Middle Paleozoic age. Through the central part of the ore field, from south to north, passes a valley dividing the ore field into left-bank and right-bank parts. The diorite massif within the entire deposit had been considered homogeneous in its structure. We studied the magnetic field within the ore field over an area of 17 sq. km on a $25 \times 5$ m grid, with a root-mean-square error of $\pm 10$ gammas. In the left-bank and right-bank parts of the ore field, low-frequency variations of magnetic-field intensity are observed, expressed in a periodic alternation of positive and negative (relative to the conventional zero) fields. The positive fields in both areas possess high values of variance and entropy, which characterize the structure of high-frequency variations. For the negative fields, a regular decrease in the numerical values of these parameters is characteristic. According to the results of studying the statistical relation of low-frequency variations of the magnetic field with the relief of the ground surface, it was established that the low-frequency variations of the magnetic field in the left-bank part of the deposit are caused by the relief of the ground surface. In Fig. 1, a line spectrum of quasiperiodic…

Fig. 1. a — spectrum of low-frequency variations in elevation marks and the magnetic field of the left bank of the ore field; b — spectrum of low-frequency variations in elevation marks and the magnetic field of the right bank of the ore field.

Fig. 2. a — correlation field of the dependence of the increment of the vertical component of the magnetic field on elevation marks in the left-bank part of the ore field; b — correlation field of the dependence of the increment of the vertical component of the magnetic field on elevation marks in the right-bank part of the ore field.

Fig. 3. a — diagram of the orientation of isolines of low-frequency variations of the magnetic field on the left bank of the ore field; b — diagram of the orientation of relief isolines on the left bank of the ore field; c — diagram of the orientation of isolines of low-frequency variations of the magnetic field on the right bank of the ore field; d — diagram of the orientation of relief isolines on the right bank of the ore field

low-frequency variations of elevations (solid lines) and of the magnetic field (dashed line).

The correlation field of the relationship between the increment of the magnetic field and elevations for this same area is shown in Fig. 2, a. Averaging of the field and relief was carried out over areas of \(200 \times 200\) m. The correlation coefficient is 0.715. Figure 3 presents diagrams of the orientation of isolines of low-frequency variations of the magnetic field (a) and relief isolines (b). From the results presented it may be concluded unambiguously that the low-frequency variations of the intensity in the left-bank part of the ore field are caused by the relief of the ground surface.

Let us now turn to the statistical characteristics of the low-frequency variations of the magnetic field and of the elevations of the relief in the right-bank part of the ore field. Frequency analysis of the magnetic field and the terrain relief (Fig. 1, b) shows that within this area there exists a group of anomalies that does not coincide in frequency with the relief and, consequently, is not caused by it. Another group of anomalies clearly coincides in frequency with the variation in elevations. The amplitude of these variations practically coincides with the amplitude of the low-frequency variations of the left-bank part of the ore field. In the event that this group of anomalies is caused by the terrain relief, we should observe, as in the left-bank part of the ore field, a directly proportional relationship between the amplitude of these variations of the magnetic field and the elevations.

Figure 2, b shows the correlation field of the relationship between the mean values of the magnetic-field intensity and the relief elevations for the area within which a distinct frequency relationship is observed. No amplitude relationship between these low-frequency variations of the field and the elevations is observed in this area. The correlation coefficient is equal to zero. The low-frequency variations of the right-bank part of the ore field are anisotropic. The principal direction of the isolines is NW (Fig. 3, c). In the diagram of relief isolines for this same area, only the nearly latitudinal and NW directions stand out.

Thus, the presence of a general NW direction of the isolines, with the complete absence of this direction in the terrain relief, and the absence of a directly proportional relationship between the amplitude of the low-frequency variations of the magnetic field and the elevations, even when the frequencies coincide, indicate that low-frequency variations of NW strike, unrelated to the relief, are present in the structure of the magnetic field, and

caused by the heterogeneous structure of the diorite massif in the right-bank part of the ore field.

Additional studies have established that this heterogeneity is a separate single-root ore node.

REFERENCES

1. E. S. Wentzel, Probability Theory. Moscow, “Nauka,” 1969.

2. L. Ya. Erofeev, G. K. Avten’ev. On statistical regularities in the relationship of terrain relief with the magnetic field at gold-ore deposits. Questions of the Geography of Kuzbass and Gorny Altai. Issue 4, Novokuznetsk, Kemerovo Pedagogical Institute, 1971.