CC BY
11EARTH SCIENCES
POSSIBILITIES OF UNDERGROUND MAGNETIC EXPLORATION
Isgandarov E.,
Associate professor,
Department of "Geophysics" Azerbaijan State Oil and Industrial University (ASOIU), Azerbaijan, Baku
Fetullayeva A.
Master student,
Department of "Geophysics" Azerbaijan State Oil and Industrial University (ASOIU), Azerbaijan, Baku
DOI: 10.5281/zenodo.7560373
ABSTRACT
The article is devoted to the question of the possibility of using underground magnetic prospecting for prospecting and exploration of mineral deposits, in particular, magnetized ore bodies. As is known, recently in the world practice of applying geophysical methods, digital instruments are widely used, which provide high accuracy of measured geophysical data. Underground magnetic prospecting covers magnetometric work in wells and mines and other underground structures and is usually carried out in conditions of approaching geological objects, which makes it possible to clarify the location of the sought-for minerals and, in particular, magnetized ore bodies.
Keywords: underground magnetic prospecting, borehole magnetic prospecting, magnit anomaly, mine magnetic prospecting, vertical component of magnetic field intensity, full vector of magnetic field strength, magnetized ore body.
Introduction
In the search and exploration of mineral deposits and ore deposits, underground magnetic prospecting is increasingly being used. This includes borehole and mine magnetic prospecting. Magnetometric observations in mines are carried out to search for missed and deep-lying ore bodies, the anomalous effect of which on the day surface is insignificant. A number of works in world practice have been devoted to the issue of applying magnetometric work in underground conditions (Bergdahl, 1963; Afanasiev, 1969; Nikitsky and Glebovsky, 1990; Mochales and etc., 2008; Mukhamet-shin and etc.,2012; Beloglazova, 2014; Guo and etc. 2015; Xu and etc., 2019; Jianjun and Dandan, 2021; Kai and etc., 2021). In all these works, the results of the magnetometric work carried out in underground workings as well as in borehole conditions are presented. These results, as well as the studies performed on modeling magnetometric data, show the importance of underground magnetometric work.
The purpose of our research is to study magneto-metric materials obtained in the field on certain areas in world practice (since such work has not yet been carried out in our country) and, based on the interpretation and processing of these materials in a modern interface, show the possibility and efficiency of carrying out mag-netometric work. in underground conditions.
Means and methods
It should be noted that field work carried out in underground conditions, including in wells, has its own specifics, but the measured values can be said to remain the same. As you know, when magnetometric work is performed on the surface of the earth, magnetic anomalies are ultimately calculated. A magnetic anomaly in magnetic exploration is the difference between the observed (measured) value of an element of the geomagnetic field and its normal value for a given area. Based on the results of field measurements, the following anomalies are calculated:
1. Anomaly of the full vector of magnetic field intensity (Logachev and Zakharov, 1979; Revyakin and etc., 1986) [8,13]:
ATa = T - Tn (3)
Here T are the observed values of the total vector of the magnetic field strength; Tn is the normal value of the total vector of the magnetic field strength.
2. Anomaly of the vertical component of the full vector of the magnetic field strength:
AZa = Z - Zn (4)
Where Z are the observed values of the vertical component of the magnetic field strength; Zn is the normal value of the vertical component of the magnetic field strength. The anomaly of the horizontal component of the total magnetic field vector can also be calculated.
When calculating magnetic anomalies, it is necessary to introduce corrections into the observed values of the elements of the geomagnetic field. The main one is the correction for changes in the geomagnetic field over time. It is calculated on the basis of materials of continuous registration in time of elements of the geomagnetic field by special variational magnetic stations. With high-precision magnetic surveys, it may be necessary to introduce a correction for the excess of observation points relative to sea level.
In the case of borehole magnetic exploration, measurements are made along the borehole using a borehole magnetometer with a certain step of moving the device from top to bottom. In this case, the magnetic susceptibility of rocks x along the well is measured, as well as the total magnetic field strength vector Ta and the vertical component of the total magnetic field strength vector Za, which make it possible to identify magnetically active geological ore bodies in the borehole environment. Let us consider the possibilities of borehole magnetic prospecting using the example of a model of a vertically magnetized ball with a radius of 1 km located at a distance of 2 km from the well at a depth
of 6 km. As is known, the vertical component of the magnetic field strength vector is calculated by the following formula (Lanza and Meloni, 2006; Serkerov, 1999; Tafeev and Sokolov, 1981) [7,11,14]:
Z = M (1)
(x2 + h2)2
Where M is the magnetic moment of a vertically magnetized ball: M=J-V=50 4/3 rc-r3=200 gamm; x is the coordinate of the center of the magnetized ball along the x axis; V is the volume of the ball in m3 units; J - intensity of magnetization; r is the radius of the ball. Based on this formula, the Za values along the well were calculated on the computer (Fig. 1) and the
corresponding curve was plotted (Fig. 2). As can be seen, from the wellhead to a depth of 5 km, the Za values decrease from 60 gamm to zero, and then reaches its maximum negative value of -100 gamm opposite the ball center at a depth of 6 km. Then at a depth of 7 km, the Za curve reverses its sign and increases to a value of 60 gamm and then continuously decreases. Thus, it is possible to estimate the depth of occurrence of a magnetized geological body. The same picture is observed in the results of field observations along the wells crossing the magnetized bodies (Bergdahl, 1963; Afanasiev, 1969; Nikitsky and Glebovsky, 1990) [1-3].
a g i • ill ?? |m
J SM1025 4 u tm 1 »7»« J JSJttM
1 ».mi e -loo
•I B.7771
2 JJ3SJM 1 M73CM 4 tatm ■J sinca
Figure 1. Results of calculation by Excel.
Figure 2. Constructed curve Za from a magnetized sphere along the well.
In the case of underground magnetic prospecting carried out in mines or mine workings, observations are carried out along a straight line at a certain depth (Guo and etc.,2015; Jianjun and Dandan, 2021; Kai and etc., 2021; Mukhametshin, and Anisimov,2012;Mochales and etc.,2008; Xu and etc., 2019) [4-6,9-10,15]. However, it should be noted that as the observation depth increases, the value of the magnetic field strength increases as the observation point approaches the geological ore body. In addition, measurements in mine workings provide additional material for interpretation, allowing more accurate determination of body parameters. Let us consider an example of interpretation of the curves of the vertical component of the magnetic field strength vector Za observed on two parallel profiles in a mine at different depths (Nikitsky and Glebovsky, 1990) [12], (Fig. 3). As can be seen, one observation line passes above the ore body (№ 1), and the second line passes below this body (№ 2). Both curves show Za maxima, which are shifted relative to each other in the horizontal direction, which may correspond to the inclined position of the ore magnetized
body. For a more visual representation of the desired ore body, a digital transformation of the Za values observed at various depths was performed.To do this, Za values were first taken at the characteristic points for each of the curves and the coordinates of the observation points were digitized in the "digid" mode of the SURFER graphic program and a "data" file was obtained (Table 1). Based on this file, a "grid" file was created, with the help of which a Za map was built along a section crossing a model magnetized ore deposit (Fig. 4). Then this map was compared with the original vertical section of the magnetized ore body passing through the observed profiles № 1 and № 2 (Fig. 5). As can be seen from the figure, this map more clearly reflects the shape and direction of the most magnetized ore body. However, it should be noted that when the observation profile approaches the observation line passing through the center of gravity of the ore body, the sign of the Za value changes to the opposite. But this does not apply to our case, when the observation profiles are located above and below the magnetized ore body (see Fig. 3).
Table 1
Za values and coordinates of observation points
X, inc Y, inc Za, Gamm
0.564905761 126.3500949 -0.7
11.53216666 128.2574446 -0.8
24.40677729 125.39642 -1
35.37403819 126.3500949 -1.2
50.63283597 125.39642 -1
63.5074466 125.39642 0
75.90521979 127.7806072 2
90.68718014 124.9195826 3
106.4228153 125.8732575 2.9
121.2047757 125.8732575 2
137.8940858 125.39642 1.5
156.0139081 124.4427452 1
172.2263808 123.4890703 0.5
51.1096734 39.56568253 0
50.63283597 40.99619483 0
63.5074466 38.61200767 0.1
75.42838236 38.13517024 0.3
88.77983042 39.56568253 0.6
105.9459779 38.61200767 1
122.635288 38.13517024 1.5
139.3245981 38.13517024 2
156.0139081 37.65833281 2.5
171.2727059 37.18149538 3
187.0083411 36.22782052 3.2
201.313464 36.70465795 2
217.5259367 35.27414566 -1
229.9237099 35.27414566 -1.2
244.2288328 34.3204708 -1
Figure 4. Magnetic map along a section intersecting magnetized ore deposit.
Figure 5. Comparison of the Za map with a section of a magnetized ore body
Conclusion:
1. An analysis of the results of underground mag-netometric work in mines and wells carried out in various regions and areas was carried out. Shown the possibility and efficiency of using underground magnetic
prospecting for delineation of ore bodies and other accumulations of mineral deposits.
2. A model magnetic field along the well for a vertically magnetized homogeneous geological body in the form of a ball was calculated and built on a computer.
It is shown that the Za curve changes its sign from positive to negative and reaches its maximum negative value opposite the center of gravity of the magnetized geological body.
3. Digital processing of the observed values of gravity was performed on the example of a model magnetized ore deposit. A map of the section of the vertical component of the full magnetization vector was built using a graphical editor in a modern interface, which more clearly shows the shape and direction of the desired magnetized ore body in comparison with the profile of mine magnetic observations.
4. Magnetic prospecting is an economically advantageous geophysical method and therefore it is recommended to use underground gravity prospecting in many areas, including in Azerbaijan.
References
1. Afanasiev, B. M. (represented by Prof. Mikov, D. S.). Techniques for interpreting borehole vector magnetic survey data using a palette for calculating anomalies from two-dimensional bodies. Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov, Volume 196, 1969, p. 2.
2. Bergdal, S.G. Magnetometer for wells and its use for magnetic prospecting. Quarterly at the Colorado School of Mines. Volume. 58, 1963, pp. 253-258.
3. Beloglazova, N.A. and etc. On the issue of determining the magnetization of rocks based on the results of three-component borehole measurements. Ural Geophysical Bulletin No. 2(24), 2014, p. 22-26.
4. Guo, Z., Liu, D., Pan, Q. and etc. Vertical magnetic field and its analytic signal applicability in oil field underground pipeline detection. Journal of Geophysics and Engineering, 2015, №12, pp.340-350.
5. Jianjun, X. and Dandan C. Technology of detecting deep underground metal pipeline by magnetic gradient method. The 9th International Conference on Environmental and Engineering Geophysics, 2021, pp.1-7., doi:10.1088/1755-1315/660/1/012074.
6. Kai, Z., Jiuchuan, W., Linsong, Y. and etc. Calibration method of petroleum underground prospects based on high precision gravity and magnetic exploration. Earth Sciences Research Journal. 2021, vol. 24, №3, Bogotá Apr 24, pp. 1-6. Print version ISSN 17946190.
7. Lanza, R., Meloni, A. The Earth's Magnetism: An Introduction for Geologists. - Berlin: Springer, 2006, 267 p.
8. Logachev, A.A. and Zakharov, V.P. Magnetic prospecting. - L.: Nedra, 1979, 353 pages.
9. Mukhametshin, A.M. and Anisimov, V.M. Geoinformation system on the example of underground vector magnetic prospecting, Mining Information and Analytical Bulletin (scientific and technical journal). 2012, №4, pp.1-5. https://cyberleninka. ru/arti-cle/n/geoinformatsionnaya-sistema.
10. Mochales, T., Casas A.M., Pueyo, E. L. and etc. Detection of underground cavities by combining gravity, magnetic and ground penetrating radar surveys: A case study from the Zaragoza area, NE Spain Environmental Earth Sciences, January 2008, 53(5):pp.1067-1077, D0I:10.1007/s00254- 007-0733.
11. Serkerov, S.A. Gravity exploration and magnetic exploration. - M.: Nedra, 1999, 437 pages.
12. Nikitsky, V.E. and Glebovsky, Y.S. Magnetic prospecting. Handbook of geophysics. - M.: Nedra, -1990, 469 pages.
13. Revyakin, P.S., Brodovoy, V.V., Revyakina, E.A. High-precision magnetic prospecting. - M.: Nedra, 1986. - 272 p.
14. Tafeev, G.P., Sokolov, K.P. Geological interpretation of magnetic anomalies. Leningrad: Nedra, 1981. - 327 p.
15. Xu, W., Guo, Z., Wu, P. and etc. Characteristic of Magnetic Fluctuation of Underground Passing Pig Labeled with Magnets. Hindawi Journal of Sensors Volume 2019, p.11, Article, ID 2374246, https://doi.org/10.1155/2019/2374246.
