Methods and techniques of seismic exploration of metal mines

Xu Mingcai Gao Jinghua Chai Mingtao Wang Guangke

(Institute of Geophysical and Geochemical Exploration, Ministry of Geology and Minerals, Langfang, Hebei, 12849)

The research on seismic method and technology of metal mines is a very complicated subject. This paper starts with the study of seismic wave theory and model experiment under complex wave field conditions, and introduces the methods of data acquisition, data processing and comprehensive interpretation of seismic exploration of metal mines in combination with the experimental research work in Zouping copper mine area, Shandong Province. The research results show that in the seismic exploration of metal deposits, the ore-controlling structure in the survey area can be studied and geological mapping can be carried out by using the multiple stacking technique. Using seismic scattering wave field, we can study the heterogeneity of underground media related to ore bodies.

key words: the seismic method of metal ore exploration superposes the inhomogeneity of scattered wave of ore-controlling structure for many times

1 Introduction

The geological prospecting work is developing in the direction of finding deep hidden blind ore. Because the ore body is buried deeply, the geophysical anomalies caused by the ore body become weaker, which makes the problem of multiple solutions in the prospecting work more prominent. In this case, the seismic method developed by using the kinematic and dynamic characteristics of elastic waves fully shows its potential and advantages. Many successful experiences and achievements have been made in the application of seismic method to find sedimentary stratabound deposits such as oil and coal fields. However, there are still many difficulties in the application of seismic method to find deep concealed metal deposits, mainly in the complex shape of metal deposits, small ore body scale and poor continuity of stratigraphic interface, which often cannot meet the specular reflection conditions on which the existing seismic methods are based. The complexity of surface conditions and surface structure makes the obtained seismic records have low signal-to-noise ratio and serious interference factors. There are also many problems in processing and interpreting these low signal-to-noise ratio seismic records. With the increasing difficulty of geological prospecting, seismic methods with large detection depth, high resolution and less multi-solution have once again attracted people's attention. During the Eighth Five-Year Plan period, we systematically carried out the research on seismic methods and techniques in metal ore exploration, and put forward the methods and techniques of studying underground ore-controlling structures by reflected wave method and inhomogeneous bodies related to ore bodies by scattered wave method. This method has been applied to the experimental study of Zouping copper mine in Shandong Province, and achieved good results.

2 Theory and model of scattered wave field

2.1 Basic theory

If the velocity of underground medium is not uniform, the velocity of medium c () can be expressed as follows:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Where is any point in three-dimensional space, C () is the background velocity, and F () is the scattering potential of medium.

in the case of inhomogeneous underground medium, if the seismic wave field approximately satisfies the acoustic wave equation,

in the geophysical formula of Volume 2 of the Proceedings of the 3th International Geological Congress,

Geophysics of Volume 2 of the Proceedings of the 3th International Geological Congress

is the total wave field, u (,,t) is the background wave field related to the source and background velocity, usc (,

by Fourier transform of equation (2) relative to t, and taking into account equations (1) and (3), the acoustic wave equation in frequency domain can be obtained:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

When the scattered wave field usc (in frequency domain That is: usc (,,w) < < U (,,w), approximated by Born, it can be concluded that:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

where: is the location of the receiving point. G (,,w) is the Green's function of the background medium, and it satisfies the following requirements:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

The Green's function G (,,w) can be written explicitly when C () is constant, but it is generally more complicated when C () is not constant. For convenience, the first-order asymptotic approximation of geometric optics can be used to give:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Where: the propagation time function (,) satisfies eikonal equation (function equation):

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Geometric diffusion function A (, ) satisfies the transport equation (transport equation):

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Substitute formula (8) into formula (6), and make:

Geophysics, Volume 2, Proceedings of the 3th International Geological Congress

Formula (6) can be written as:

Proceedings of the 3th International Geological Congress. Available:

Geophysics in Volume 2 of the Proceedings of the 3th International Geological Congress

is set as a point in the underground medium, and in Equation (1), t= (,) stands for an isochronous interface. If the source and receiver are fixed, the isochronous plane is unique.

according to equation (1), as long as f( )≠, that is, the underground medium is inhomogeneous, the scattered wave field usc will be generated with the excitation of the seismic source, and the strength of the scattered wave is proportional to the size of f (), that is to say, the more serious the heterogeneity of the underground medium is, the stronger the scattered wave field will be, and the stronger the heterogeneity of the underground medium will be. Based on this, we can study which waves are produced by mineralization zone and which scattered waves are produced by ore body according to the strength of scattered waves on seismic time profile in metal mine exploration. When the underground medium is uniform, the medium velocity c () is the same as the background velocity C (), and it can be seen from Equation (1) that in this case, no scattered wave is generated. Therefore, the distribution of surrounding rocks can also be studied according to the wave resistance characteristics on seismic profiles.

2.2 model experiment

In general, the geological model structure encountered in metal ore exploration is complicated. Theoretically, many complex geological models can basically be decomposed into the sum of some vertical and horizontal plate models. Therefore, it is of great significance to study the seismic response of vertical plate geological model.

vertical plate model in fig. 1

seismic response of vertical plate model in fig. 2

for the convenience of calculation, we designed a two-dimensional model of vertical plate, and the model parameters are shown in fig. 1. Figure 2 is the theoretical calculation result of this model. The source is located at 48 o'clock on the record. The diffraction wave field at 1ms is generated by the top interface of the vertical plate, and its amplitude is very strong around the plate. After leaving the plate, the amplitude is obviously attenuated. In the lower part of diffracted wave, there is a seismic wave field with weak amplitude but high frequency, and its occurrence range is basically consistent with the position of vertical plate in the horizontal direction. These weak amplitude high frequency waves are scattered waves generated at the boundary of vertical plate. This scattered wave is different from high frequency interference, which is random and irregular, while the scattered wave has certain coherence.

fig. 3 experimental results of dispersed block model (a) and its physical model (b)

when the geological model is complex, the physical model experiment can better simulate the seismic response of the complex medium model. Fig. 3a shows the geological model of a dispersed block, and fig. 3b shows the results of ultrasonic physical model test. In this model experiment, the working mode of self-excitation and self-collection is adopted. As can be seen from Figure 3b, the scattered waves generated by the dispersed block have a high frequency and are distributed in an orderly manner, which is similar to the thin-layer reflection generated by the near-horizontal layer.

Through the model experiment and analysis of the results of the model experiment, it is known that the scattered wave field generated by the underground inhomogeneous body is characterized by high frequency and orderly distribution in a certain range. These characteristics of scattered waves are different from high-frequency interference waves and reflected waves. In general, high-frequency interference waves are chaotic, and the reflected waves (or refracted waves, etc.) generated by the impedance interface of underground waves have good coherence in a large range.

3 methods and techniques of mining area test

Zouping copper deposit in Shandong Province is spatially controlled by volcanic structures, the main ore-controlling structure is caldera passage, the main ore-bearing rock mass is Shi Ying syenite diorite in the caldera passage, the direct surrounding rock of ore-bearing porphyry is volcanic rock series dominated by clastic rocks and clastic lava, and the ore-bearing rock mass in the survey area is located about 1m below the Quaternary strata.

Before the seismic method test, we measured the physical parameters of a small number of core samples in the test area, and the results are shown in Table 1:

Table 1 Table 1 Table of core physical parameters in the test area

From Table 1, it can be seen that there is obvious wave impedance difference between volcanic rocks and Shi Ying syenite and monzonite, and the interface wave impedance difference between Quaternary strata and underlying bedrock is stronger.

according to the determination results of physical parameters, combined with the requirements of geological exploration tasks, and on the basis of comprehensive analysis of geological and geophysical and geochemical exploration data in the experimental area, experimental research work was carried out in known areas and unknown areas respectively. In order to study the prospecting structure and underground inhomogeneities related to ore bodies in the survey area, we have carried out the research of reflected wave method and high-fidelity scattered wave method with multiple coverage along the same survey line, and also carried out refraction seismic survey along the survey line to obtain formation velocity.

through experiments, the selected working parameters of multiple coverage reflection wave method are: the track spacing is 1m, the offset distance is 1m, each track is received by 12 4Hz geophones buried together, the seismic source is explosives, the recording length is 124ms, the sampling rate is .5ms, the instrument receiving passband is 35-5hz, the number of receiving channels is 24, and the coverage times are 12. High-fidelity scattered wave method adopts 3m channel spacing and 15m offset, and other working parameters are the same as reflected wave method.

The data processing is carried out on a 486 microcomputer, and special processing software suitable for seismic data of metal mines is used. The processing of reflected seismic data lies in comprehensively improving the signal-to-noise ratio and resolution of seismic records, including: de-coding, editing, amplitude recovery, static correction, bidirectional deconvolution, frequency filtering, dip differential filtering, velocity analysis, dynamic correction, stacking and three-instant processing. The processing of high-fidelity scattered wave seismic data includes: decoding, editing, amplitude recovery, broadband filtering, dynamic correction, dip differential filtering and scattered wave enhancement. After processing, high quality seismic time profile is obtained.

4 work results

fig. 4 shows the reflected seismic time profile passing through a geophysical anomaly in Zouping, Shandong Province, and fig. 5 is the instantaneous frequency profile corresponding to fig. 4. The stratum shape reflected in Figure 4 is relatively gentle and inclines eastward. T1 and T2 wave resistances are reflected waves from different strata interfaces in Quaternary, and T3 wave resistance is reflected from the bottom interface of Quaternary, with strong energy and good continuity, and its interface depth is 13 ~ 143 m; There is an angle between T4 wave resistance and T3 wave resistance, and at CDP235, T4 wave resistance is broken. According to the geological data of the survey area and the results of gravity and magnetic inversion, the T4 wave resistance is interpreted as the interface reflection wave between volcanic rocks and intermediate-acid concealed rock bodies invaded along the volcanic channel in the later period. T4 wave resistance breaks at CDP 95 ~ CDP 148, corresponding to the high frequency block on the instantaneous frequency profile shown in Figure 5. According to the reflected seismic time profile shown in Figure 4 and the instantaneous frequency profile shown in Figure 5, the location of the crater can be accurately determined.

fig. 4 seismic time profile of line 5 reflection of Zouping copper mine earthquake in Shandong province

fig. 5 corresponds to the instantaneous frequency profile of fig. 4

fig. 6 is the seismic scattered wave profile corresponding to the seismic time profile of fig. 4. The difference is that the distance between adjacent CDP on the reflection seismic profile is 5m, and that on the scattered wave profile is 6m (note: the distance between adjacent CDP on the original scattered wave profile is 1.5m, and the original scattered wave profile is displayed by drawing channels for the convenience of comparison). The seismic scattered wave profile shown in Figure 6 has not been subjected to any mixing processing including horizontal stacking. Therefore, the position and inhomogeneity of underground inhomogeneity can be reflected without distortion.

fig. 6 seismic scattered wave profile

On the scattered wave profile, some quasi-layered high-frequency seismic waves with good coherence appear at the position corresponding to the crater on the reflection seismic profile (fig. 4). In order to better study and explain the high-frequency waves, the section A shown on the section of fig. 6 is enlarged and shown in fig. 7.

section a of fig. 7 corresponding to fig. 6 is enlarged and displayed.

by analyzing the seismic scattered wave section shown in fig. 7 (CDP spacing is 1.5m), the reflected seismic wave resistance characteristics are completely different from the superimposed section shown in fig. 4 even in the corresponding crater. At the outer edge of the crater (CDP: 29 ~ 36, CDP: 44 ~ 46), the seismic waves are only weakly distributed in disorder.

according to the theoretical research and model experiment results of seismic wave field under complex conditions and the geological conditions of the test area, we think that the high-frequency seismic wave on the section in fig. 7 is a weak scattering wave generated by the inhomogeneity in the crater. According to Equation (1), the more inhomogeneous the underground medium is, the greater the scattering potential f () superimposed on the background velocity, and the stronger the resulting scattered wave is. On the contrary, according to the intensity of scattered waves on the profile shown in Figure 7, the inhomogeneity of underground medium lithology can be inferred.

according to the geological data of the known mining area, the copper deposit exists in the crater, which is an asymmetric funnel-shaped inclined rock bead, which is nearly round in the horizontal section near the surface. The lithology in the caldera is Shi Ying syenite diorite, which has obvious velocity and density difference with the copper ore body invaded in the volcanic passage structure. In general, the inhomogeneity of mineralized zone is weaker than that of ore-bearing rock mass, so the scattered wave intensity generated by mineralized zone is lower than that generated by ore-bearing rock mass. Therefore, the mineralized zone and ore-bearing rock mass can be tentatively distinguished according to the intensity of scattered waves.

In addition, the location of the crater detected by seismic method is consistent with the gravity and magnetic anomalies and geo-electrochemical anomalies in the survey area, but the range of the crater inferred by gravity and magnetic anomalies is wide, and the range of the crater determined by seismic method is narrow, and the location is accurate and specific, which reflects that the geo-electrochemical anomalies of the concealed ore body are different from the inhomogeneities detected by earthquake.

according to the metallogenic regularity of the survey area, the wave resistance characteristics on the seismic profile and the comprehensive analysis of the geophysical and geochemical data in the survey area, we think that the medium inhomogeneity between CDP 37 and CDP37～42 on the profile in fig. 7 is serious, and there may be a good copper ore body. In fact, the Quaternary bottom boundary depth and mineralization zone revealed by borehole ZK92E3-1 in the section of Figure 7 have confirmed the accuracy of seismic inference, but no copper ore body is found. The location of the drilling hole deviates from the non-uniform block detected by seismic method. If drilling on the inhomogeneous block inferred from seismic data is verified, it is very likely to find new concealed deposits.

5 Conclusion

Through the experimental study of seismic method in Zouping copper mine area, Shandong Province, we can draw the following conclusions:

(1) In metal ore exploration, using reasonable methods and techniques, we can use seismic method to find deep buried blind ore.

(2) In metal ore exploration, seismic reflection wave method (multiple horizontal stacking) can better study ore-controlling structures and divide strata for three-dimensional mapping; Seismic scattering wave method can better study the heterogeneity related to underground ore bodies, and combine geophysical and geochemical anomalies to study the location of ore deposits.

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