On the Kursk Magnetic Anomaly Based on the Studies of 19211

P. P. Lazarev.

In Advances in the Physical Sciences (Vol. II, p. 61) information was given about the most interesting anomaly of geomagnetism in the world, previously studied by Prof. E. E. Leist, subsequently studied more thoroughly by a Commission of the Academy of Sciences, and, finally, at present being investigated in all respects by the Special Commission for the Study of the Kursk Anomaly under the Presidium of the Supreme Council of National Economy.

Here we shall cite only those works which belong to the magneto-gravitational division, directed by Academician P. P. Lazarev.

The winter campaign of 1921 yielded a quite definite picture of the distribution of the magnetic anomaly, which proved to break off sharply in the region of Orel Province (in the northwest) and to extend in the form of a narrow strip (2 versts wide), as had already been noted by E. E. Leist, through the northern and northwestern part of Kursk Province, passing through the Shchigry, Tim, and Oskol districts and heading into the region of Voronezh Province.

The reconnaissance survey further showed that outside this sharp strip of anomaly there exists a weak anomalous field, but it is of less practical interest and its survey does not require such enormous technical resources as the survey of the region of maximum anomaly.

Further, during the reconnaissance it was found that, beginning southeast of the considerable Shchigry maximum of the anomaly, carefully studied in 1920 and presented in the form of maps in the reports,1 in the direction toward Tim district there is a significant decrease of the anomaly. In the regions of Oskol district bordering on Voronezh Province (in the southeast of the anomaly), a second sharp maximum of geomagnetism is again found, giving almost the same values for the vertical component \(Z\), the horizontal component \(H\), and the declination \(D\) as the Shchigry maximum.

The difference consists in the fact that the Shchigry maximum corresponds to a single belt, whereas the Oskol (Oskol-Voronezh) maximum is a series of parallel belts (five), extending in a direction from west to east or from northwest to southeast and separated by weaker anomalous fields.

The summer investigations of 1921 in the field of the study of magnetic phenomena consisted in a detailed survey of the indicated places, which was carried out at more than 7,000 points (including also the observations of 1919–1920).

Thus the density of the network of magnetic observations considerably exceeded the density of Leist’s network, who made observations at 4,500 points for the entire Kursk province. In the region of the northern anomaly Leist had fewer than 2,000 points.

Incidentally, the appearance in the eastern part of the anomaly of a number of anomalous belts was not noted by Leist.

For future investigations there remain to be studied only two small unexamined places of the northern anomaly with comparatively weak magnetism1 and the southern belt of the anomaly, extending through Belgorod, Nepkhaevo, and Kochetovka into the region of Kharkov province. These works, if there are no insurmountable external hindrances, the Commission proposes to complete in two years.

Alongside the practical work of surveying, a number of scientific works have been carried out in Moscow which are of importance for the solution of technical questions. Thus, Prof. V. A. Kostitsyn2 has made calculations concerning the position and depth of magnetic masses, under the assumption that we are dealing with a cylinder of magnetic substance magnetized by the action of the earth’s field and lying underground parallel to the surface. Such assumptions make it possible to solve the problem rigorously, assuming the cross-section of the cylinder to be elliptical, with its axis placed at any angle to the horizon, and make it possible to show that, in order for the position of the isolines and the magnitudes of the force to correspond to what we have in nature, the depth of occurrence must be about 200–250 sazhen.

Observations made during drilling showed strong magnetization of the pipes and the bit, the cause of which, up to the present, as follows from the report of Prof. Ya. V. Samoilov concerning analogous observations in America, cannot be clarified. To study the conditions of magnetization, I. A. Sokolov undertook at the Physical Institute, under the direction of P. P. Lazarev, an investigation of this phenomenon on models.

Finally, A. I. Zaborovsky made observations over the anomaly at different heights above the ground and obtained data on the assumption that the magnetization is caused by a series of belts placed along the axial line of the anomaly.

From the data of Zaborovsky the depth of occurrence was calculated; in this connection \(Z\) was taken equal to \(\frac{A}{r}\), where \(A\) is a constant and \(r\) is the distance from the poles: knowing the additional height underground for a number of points situated along one vertical, one can find the depth of the deposit below the surface of the earth. It turned out to be about 150–200 sazhen. We see that by this method figures are obtained close to those indicated by Prof. V. A. Kostitsyn. Prof. A. D. Arkhangelsky also arrives at approximately the same figures on the basis of geological considerations1.

In concluding the review of works on geomagnetism, it must be pointed out that the head of the theoretical section of the gravitational subsection, Acad. V. A. Steklov, reported to the Academy of Sciences2 the work “On determining from 3–4 observations the depth of occurrence of a magnetic layer and the distance of the edges of its contour from the main observation point,” which is of substantial importance for determining the depth of the magnetic layer that interests the Commission.

In view of the considerations of Acad. A. F. Ioffe, A. N. Krylov, and P. P. Lazarev on the possibility of the same magnetization maps both for a definite underground ridge of magnetic substance and for a rupture and vertical displacement of a magnetized layer parallel to the earth’s surface, gravitational observations were undertaken in 1921 on the instructions of the magnetic department. Their task was to determine the variations of the horizontal component of gravity (Prof. P. M. Nikiforov) and to measure the vertical component (acceleration \(g\)) (Prof. A. A. Mikhailov)3.

At the present time A. A. Mikhailov’s investigation has been completed; he showed that in the region of the anomaly, not quite coinciding with the maximum of the vertical component of the magnetic force, there is also a maximum of the gravitational force.

The whole picture indicates that we are dealing with an underground ridge obliquely situated beneath the earth’s surface.

The totality of the data obtained now makes it possible to put forward more or less probable assumptions about the causes of the anomaly.

The gravitational and magnetic data indicate that the body causing the anomaly must differ from magnetic iron ore both in density and in magnetic permeability. It must be heavier in specific gravity and possess stronger magnetic properties. Thus the question arises whether we are not dealing here with the penetration of masses of metallic iron, of which the earth’s core must consist4,

through a rupture of the rocks into the more superficial layers. Such a hypothesis explains all the phenomena, both magnetic and gravitational, and does not contradict the geological facts carefully studied by Prof. A. D. Arkhangelsky.

Whether this hypothesis is justified can be decided only in further investigations by means of deep drilling.

In concluding the review, it is necessary to mention a series of computational works being carried out at present in Moscow in the computation bureau directed by P. P. Lazarev.

These works had, first of all, the task of checking the accuracy of the observations in the field by calculating the field of the potential of the horizontal component \(H\).

The calculations showed that the areas obtained in 1920, within the limits of observational error, agree with the computations. As for the work of 1919, there is a small area where, evidently, military operations did not permit determinations to be made with sufficient accuracy and where the discrepancies reach a considerable magnitude. This area will be re-examined in 1922.

Finally, computations were made of the accuracy of the observations of the magnetic quantities and of the error from plotting them on the map. These two quantities agree fairly closely with one another in the work of the commission, although it is nevertheless necessary to increase somewhat the accuracy of the geodetic survey in order to make full use of the accuracy of the magnetic measurements.

Something quite different is found in a critical treatment of Leist’s data: in his case the discrepancy between the accuracy of the magnetic data and that of the geodetic data reaches enormous magnitudes, and in order for one to correspond to the other it would have been possible to conduct magnetic observations 50 times less accurately than Leist did, or else it would have been necessary to take maps of an entirely different scale1.

Clarification of the nature of the southern anomaly and a more detailed study of gravity will constitute the further task set for the expedition for the summer of 1922.

Moscow, February 1922.

1. For Kursk Governorate at the time of Leist’s work there was only a 3-verst map; he, however, needed to have a map of about 100 and even 50 sazhen to the inch in order to make full use of the accuracy of the determinations of \(H\). ↩↩↩↩↩

2. At the meeting of 26 Oct. 1921. ↩↩

3. These observations had already been planned in 1919. ↩

4. See the collection of abstracts and translated articles on geodetic questions: article by E. Wiechert, pp. 11–111 (appendix to part 66 of the proceedings of the Military-Topographical Department). ↩