A method for actively detecting concentrated stress on the leading working surface using variable frequency ultrasonic waves

Abstract

The present invention discloses a method for actively detecting concentrated stress of an advance working face by using variable frequency ultrasonic waves, comprising the following steps: arranging coring boreholes in a working face track chute and a transport chute respectively; calibrating the linear relationship between stress and wave velocity of coal samples in a laboratory; installing variable frequency ultrasonic probes on site; real-time acquisition and transmission of ultrasonic signals; signal processing and determination of stress; based on the principle of wave reflection, inverting the ultrasonic wave velocity around the borehole by the time difference between variable frequency ultrasonic emission and reception signals, and comparing it with the wave velocity of the cored coal samples measured in the laboratory to calculate the stress distribution around the borehole. The method can determine the stress change of the advance working face in real time according to the change of wave velocity, thereby achieving the purpose of rapid and accurate monitoring of stress in the advance working face.

Description

Technical Field

[0001] The invention relates to a method for detecting concentrated stress on an advanced working face of a coal mine, and in particular to a method for actively detecting concentrated stress on an advanced working face using variable frequency ultrasonic waves. Background Art

[0002] At present, the main methods of mine pressure monitoring used on site include drill cuttings monitoring, surrounding rock deformation monitoring, borehole stress monitoring, electromagnetic radiation monitoring, acoustic emission monitoring, charge monitoring, vibration monitoring, and active / passive inversion of vibration waves. These methods have played a positive role in monitoring and early warning of rock burst in coal mines. However, most of these monitoring methods are passive monitoring and are greatly affected by geological changes. For example, the most promising microseismic monitoring technology is mostly post-recording and cannot reflect stress changes in real time; drill cuttings monitoring is convenient and fast, but the monitoring is discontinuous and the location and lithology of the drill cuttings point have a great impact on stress monitoring. Therefore, it is urgent to seek and develop new stress monitoring technologies. In recent years, with the rapid development of ultrasonic detection technology in well logging, good results have been achieved due to its controllable frequency and fewer factors affecting the propagation within a small range. Summary of the invention

[0003] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method for actively detecting concentrated stress on the advance working surface by using variable frequency ultrasonic waves, which can determine the stress changes on the advance working surface in real time according to the changes in wave velocity, thereby achieving the purpose of rapid and accurate monitoring of stress in the advance working surface.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0005] The present invention provides a method for actively detecting concentrated stress on an advanced working surface using variable frequency ultrasonic waves, comprising the following steps:

[0006] S1. Arrange several core drilling holes in the track chute and transport chute of the roadway working face on both sides of the advance working face;

[0007] S2. Laboratory calibration of the linear relationship between stress and wave velocity of coal samples; the coal samples taken out from the coring holes are marked with the coring positions and directions, and the set stress is applied to the coal samples and experimental tests are carried out. The ultrasonic probe is used to actively transmit ultrasonic waves with the same frequency as the field, and ultrasonic waves are transmitted at one end of the sample and received at the other end of the sample. By recording the time difference between the transmitted and received signals, the propagation speed v of the ultrasonic wave in the coal sample is calculated, see formula (1), and then several groups of different stress values ​​are applied to the coal samples to repeat the above experimental tests to calibrate the linear relationship between stress and wave velocity:

[0008] v=h / (tj-tr) (1)

[0010] Where: h is the height of the cored coal sample;

[0011] t j —The time when the ultrasonic wave is received;

[0012] t r - Ultrasonic emission time;

[0013] S3. Install a variable frequency ultrasonic probe in the borehole;

[0014] S4, real-time acquisition and transmission of ultrasonic signals; control the frequency conversion ultrasonic probe to transmit and receive ultrasonic waves through the ultrasonic processing controller, record the time difference between transmitting and receiving ultrasonic waves in the same frequency band, and transmit them in real time through Ethernet;

[0015] S5, signal processing and stress determination; the ultrasonic processing controller processes the time difference between the transmitted and received reflected ultrasonic signals, and calculates the propagation velocity v of the ultrasonic wave in the coal body according to the principle of formula (2), and then compares the wave velocity value measured on site with the wave velocity under different stress states measured experimentally. If the measured wave velocity is greater than the wave velocity value measured experimentally, it means that the stress inside the area is greater than the stress applied to the cored coal sample during the experiment; if it is less than, it means that the stress is less than the stress value applied during the experiment; at the same time, according to the corresponding relationship between the experimentally calibrated stress and wave velocity, the coal body stress value around the borehole is calculated in real time;

[0016] v=2h / (tj-tr) (2)

[0018] Where: h is the distance the ultrasonic wave propagates in the borehole;

[0019] t j —The time when the ultrasonic wave is received;

[0020] t r —Ultrasonic emission time.

[0021] Preferably, in step S1, the core drilling holes are arranged within 100 to 120 m of the advanced working face of the coal mine, and one core drilling hole is arranged along the center line of the tunnel every 20 to 30 m, with a drilling depth of 10 m and a diameter of 150 mm.

[0022] Preferably, in step S3, the variable frequency ultrasonic probe is installed in the borehole through a bracket, and an inflatable bag is installed on the part of the bracket where the variable frequency ultrasonic probe is installed, and the variable frequency ultrasonic probe is stabilized by coupling the inflated inflatable bag with the borehole wall.

[0023] Preferably, a pneumatic coupling device is provided at one end of the stent away from the inflatable bag, for inflating and suctioning the inflatable bag, to ensure that the stent and the variable frequency ultrasonic probe can be retrieved at the end of each monitoring stage.

[0024] Preferably, the variable frequency ultrasonic probe is selected to be a model that can transmit and receive ultrasonic signals, and can realize free switching between active and passive modes; when switched to active mode, the variable frequency ultrasonic probe can generate ultrasonic waves of different frequency bands; when switched to passive mode, it is similar to a "microseismic" probe, collecting vibration information in the coal and rock media around the probe.

[0025] The beneficial effects of the present invention are:

[0026] 1. Compared with the prior art, the present invention is based on the positive correlation between the coal rock medium and the stress it is subjected to; when the variable frequency ultrasonic wave propagates through the structural or lithological change area, the wave velocity will be abnormal, and the wave velocity gradient is high; according to the change of the wave velocity monitored on site, and at the same time, the stress and wave velocity correspondence obtained by the experiment are compared to determine the stress value in front of the working face;

[0027] 2. Since the variable frequency ultrasonic probe can switch freely between active and passive modes and the frequency of ultrasonic waves is easy to control, stress and geological structure detection can be carried out by emitting ultrasonic waves of different frequencies, thereby improving the accuracy of monitoring.

[0028] 3. Finally, control the mining speed or implement pressure relief measures based on the specific monitoring stress data to ensure safe production in the coal mine. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 is a schematic diagram of the plane layout of the advance working surface provided by an embodiment of the present invention;

[0031] Figure 2 is a schematic diagram of a field monitoring system provided by an embodiment of the present invention;

[0032] Figure 3 It is a schematic diagram of on-site installation of a variable frequency ultrasonic probe provided by an embodiment of the present invention.

[0033] In the figure: 1. Track chute; 2. Transport chute; 3. Advance working surface; 4. Variable frequency ultrasonic probe; 5. Ultrasonic processing controller; 6. Pneumatic coupling device; 6-1 inflatable bag; 7. Bracket. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] like Figures 1 to 3 As shown, a method for actively detecting concentrated stress on the leading working surface with variable frequency ultrasonic waves is provided.

[0036] The following steps are involved:

[0037] S1. Arrange several core drilling holes in the track drift 1 and the transport drift 2 of the tunnel working face on both sides of the advance working face 3; the core drilling holes are arranged within a range of about 100 to 120m from the coal mine advance working face 3, and the spacing r between each core drilling hole arranged along the center line of the tunnel is about 20 to 30m, the drilling depth h = 10m, and the drilling diameter D = 150mm.

[0038] S2. Laboratory calibration of the linear relationship between stress and wave velocity of coal samples; the coal samples taken out from the coring holes are marked with the coring positions and directions, and the set stress is applied to the coal samples and experimental tests are carried out. The ultrasonic probe is used to actively transmit ultrasonic waves with the same frequency as the field, and ultrasonic waves are transmitted at one end of the sample and received at the other end of the sample. By recording the time difference between the transmitted and received signals, the propagation speed v of the ultrasonic wave in the coal sample is calculated, see formula (1), and then several groups of different stress values ​​are applied to the coal samples to repeat the above experimental tests to calibrate the linear relationship between stress and wave velocity:

[0039] v=h / (tj-tr) (1)

[0040] Where: h is the height of the cored coal sample;

[0041] t j —The time when the ultrasonic wave is received;

[0042] t r - Ultrasonic emission time;

[0043] S3. Install the variable frequency ultrasonic probe 4 in the borehole; the variable frequency ultrasonic probe 4 is installed in the borehole through the bracket 7, and the inflatable bag 6-1 is installed on the part of the bracket 7 where the variable frequency ultrasonic probe 4 is installed, and the inflatable bag 6-1 is inflated and coupled with the borehole wall to stabilize the variable frequency ultrasonic probe 4. The end of the bracket 7 away from the inflatable bag 6-1 is provided with a pneumatic coupling device 6, which is used to inflate and suck air into the inflatable bag 6-1 to ensure that the bracket 7 and the variable frequency ultrasonic probe 4 can be retrieved at the end of each monitoring stage.

[0044] S4, real-time acquisition and transmission of ultrasonic signals; the ultrasonic processing controller 5 controls the frequency conversion ultrasonic probe 4 to transmit and receive ultrasonic waves, records the time difference between transmitting and receiving ultrasonic waves of the same frequency band, and transmits them in real time via Ethernet;

[0045] The variable frequency ultrasonic probe 4 is a model that can transmit and receive ultrasonic signals, and can realize free switching between active and passive modes; when switched to active mode, the variable frequency ultrasonic probe 4 can generate ultrasonic waves in different frequency bands; when switched to passive mode, it is similar to a "microseismic" probe, collecting vibration information in the coal and rock media around the probe.

[0046] S5, signal processing and stress determination; the ultrasonic processing controller 5 processes the time difference between the transmitted and received reflected ultrasonic signals, and calculates the propagation velocity v of the ultrasonic wave in the coal body according to the principle of formula (2), and then compares the wave velocity value measured on site with the wave velocity under different stress states measured experimentally. If the measured wave velocity is greater than the wave velocity value measured experimentally, it means that the stress inside the area is greater than the stress applied to the cored coal sample during the experiment; if it is less than, it means that the stress is less than the stress value applied during the experiment; at the same time, according to the corresponding relationship between the experimentally calibrated stress and wave velocity, the coal body stress value around the borehole is calculated in real time;

[0047] v=2h / (tj-tr) (2)

[0048] Where: h is the distance the ultrasonic wave propagates in the borehole;

[0049] t j —The time when the ultrasonic wave is received;

[0050] t r —Ultrasonic emission time.

[0051] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for actively detecting concentrated stress on the leading working surface using variable frequency ultrasonic waves, characterized in that: The following steps are involved: S1. Arrange several core drilling holes in the track chute and transport chute of the roadway working face on both sides of the advance working face; S2. Laboratory calibration of the linear relationship between stress and wave velocity of coal samples; the coal samples taken out from the coring holes are marked with the coring positions and directions, and the set stress is applied to the coal samples and experimental tests are carried out. The ultrasonic probe is used to actively transmit ultrasonic waves with the same frequency as the field, and ultrasonic waves are transmitted at one end of the sample and received at the other end of the sample. By recording the time difference between the transmitted and received signals, the propagation speed v of the ultrasonic wave in the coal sample is calculated, see formula (1), and then several groups of different stress values ​​are applied to the coal samples to repeat the above experimental tests to calibrate the linear relationship between stress and wave velocity: v=h / (tj-tr) (1) Where: h is the height of the cored coal sample; t j —The time when the ultrasonic wave is received; t r - Ultrasonic emission time; S3. Install a variable frequency ultrasonic probe in the borehole; S4, real-time acquisition and transmission of ultrasonic signals; control the frequency conversion ultrasonic probe to transmit and receive ultrasonic waves through the ultrasonic processing controller, record the time difference between transmitting and receiving ultrasonic waves in the same frequency band, and transmit them in real time through Ethernet; S5, signal processing and stress determination; the ultrasonic processing controller processes the time difference between the transmitted and received reflected ultrasonic signals, and calculates the propagation velocity v of the ultrasonic wave in the coal body according to the principle of formula (2), and then compares the wave velocity value measured on site with the wave velocity under different stress states measured experimentally. If the measured wave velocity is greater than the wave velocity value measured experimentally, it means that the stress inside the area is greater than the stress applied to the cored coal sample during the experiment; if it is less than, it means that the stress is less than the stress value applied during the experiment; at the same time, according to the corresponding relationship between the experimentally calibrated stress and wave velocity, the coal body stress value around the borehole is calculated in real time; v=2h / (tj-tr) (2) Where: h is the distance the ultrasonic wave propagates in the borehole; t j —The time when the ultrasonic wave is received; t r —Ultrasonic emission time.

2. A method for actively detecting concentrated stress on the leading working surface by using variable frequency ultrasonic waves as claimed in claim 1, characterized in that: In step S1, the core drilling holes are arranged within 100 to 120 m of the advanced working face of the coal mine, and one core drilling hole is arranged along the center line of the tunnel every 20 to 30 m, with a drilling depth of 10 m and a diameter of 150 mm.

3. The method for actively detecting concentrated stress on the leading working surface by using variable frequency ultrasonic waves as claimed in claim 1, characterized in that: In step S3, the variable frequency ultrasonic probe is installed in the borehole through a bracket, and an inflatable bag is installed on the part of the bracket where the variable frequency ultrasonic probe is installed. The variable frequency ultrasonic probe is stabilized by coupling the inflated inflatable bag with the borehole wall.

4. The method for actively detecting concentrated stress on the leading working surface by using variable frequency ultrasonic waves as claimed in claim 1, characterized in that: A pneumatic coupling device is provided at one end of the stent away from the inflatable bag, which is used to inflate and suction the inflatable bag, ensuring that the stent and the variable frequency ultrasonic probe can be retrieved at the end of each monitoring stage.

5. The method for actively detecting concentrated stress on the leading working surface by using variable frequency ultrasonic waves as claimed in claim 1, characterized in that: The variable frequency ultrasonic probe is a model that can transmit and receive ultrasonic signals, and can realize free switching between active and passive modes. When switched to active mode, the variable frequency ultrasonic probe can generate ultrasonic waves in different frequency bands. When switched to passive mode, it is similar to a "microseismic" probe, collecting vibration information in the coal and rock media around the probe.

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