Possibilities of Recalculating the Gravity Field into the Lower Half-Space

(Case Study of the Baley District, Eastern Transbaikalia)

V. A. Kortunov, I. P. Novikov

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

In interpreting data from gravity investigations, the spatial distribution of potential fields is now widely used. Analysis of the spatial distribution of potential fields is one of the principal methods for solving inverse problems in many areas of geophysics. The use of such a method of transforming gravity anomalies, as continuation into the lower half-space, makes it possible to identify the main structural elements of the area under study: geological blocks, tectonic disturbances, dikes, etc.

There is an opinion that analytic continuation of gravity fields (or, more precisely, of any potential fields) into the lower half-space has no meaning, since as the depth of recalculation increases, errors appear whose magnitude is equal to the value of the useful component.

Indeed, when recalculating into the lower half-space to a depth \(h > z\) (\(h\) is the depth of field recalculation, \(z\) is the depth of occurrence of singular points), in the trigonometric series representing the gravity anomaly, errors having a sinusoidal character along the profile will predominate. Moreover, the sinusoidal component, supplemented by errors due to inaccuracies of observations and the influence of other anomalies, will suppress the useful component (V. M. Berezkin, 1968). However, when recalculating the gravity field into the lower half-space, the \(\Delta g\) curves behave relatively calmly up to the upper edge of the disturbing body, since the continuation process has not yet separated into two components; then, after passing through the singular points of the body, a variable sinusoidal component appears, which becomes predominant as depth increases. When passing through the singular points, the \(\Delta g\) field undergoes substantial changes. This circumstance indicates that we already have certain fictitious values of the gravity field (\(\Delta g\) fictitious), analogous, let us say, to values of apparent resistivity (it should be noted that in most cases, in geophysical investigations, one has to operate with apparent quantities).

We have empirically established that the distortion of the \(\Delta g\) field with the depth of recalculation has a general character for all recalculation points, while the expression of the disturbing object in this fictitious field has the same regularity.

The principal task of geology, as well as of geophysics, is the analysis of con-

Fig. 1. Comparison of the variation of the gravity field along the vertical with the geological section: 1–5 — Jurassic–Cretaceous deposits; 6 — Jurassic deposits; 7 — Ulichin granitoids; 8 — stratigraphic boundaries of rocks; 9 — tectonic zones according to geological data; 10 — loose deposits; 11 — lines of equal values \((\Delta F\ \text{fictitious})\); 12 — tectonic faults identified from gravity-survey data.

configurations of various maps, sections, and graphs, using methods of comparison and similarity of the configurations of the above-listed materials for the corresponding areas.

One of the methods we used in interpreting the gravity studies of the Baley district was the method of recalculating the gravity field into the lower half-space to depths \(h=50, 100, 150, 200, 250, 300\) m. Recalculation of the field \(\Delta g\) into the lower half-space made it possible for us not only to identify a number of disturbing objects, but also to determine their position in space. From the transformed values of the gravity field, gravity sections were constructed. Such sections were constructed by plotting the computed values of \(\Delta g\) at the recalculation points, followed by drawing lines of equal values.

When comparing geological sections with the corresponding gravity sections (an example of such a comparison is shown in Fig. 1), the following may be noted.

1. All tectonic zones clearly established from drilling data or mine workings find their expression also in the constructions of gravity sections. In the figure presented, such reliably recorded zones include the tectonic disturbances of the western and eastern parts of the geological section.

2. A number of tectonic disturbances (in particular, two disturbances in the central part of the geological section) are not reflected in the fictitious gravity field. As our analyses show, in the area of these disturbances not a single borehole was drilled that could have recorded and confirmed their existence. Apparently, surface mining works revealed small local zones of fracturing, measured at several centimeters in thickness, which served as the unjustified reason for plotting these tectonic zones.

3. In similarly poorly studied areas by drilling and mining works, according to our data, in the \(\Delta g\) field (fictitious) there are a number of well-traced anomalous zones, which most probably correspond to zones of crushing, kaolinization, cataclasis, and other types of rock alteration not detected by geologists.

REFERENCES

1. V. M. Berezkin. Method of analytic continuation of the complete vertical gradient of gravity for studying the distribution of disturbing masses in the thickness of the Earth’s crust. Geologiya i razvedka, 1968, No. 2.

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

3. G. V. Gladkii. Gravity prospecting and magnetic prospecting. Nedra Publishing House, 1964.

4. V. V. Kolobkov, M. I. Lapina. Review of methods for solving the direct and inverse problems of magnetic prospecting. Academy of Sciences of the USSR. Moscow, 1960.