A method for detecting electromagnetic waves in boreholes in gas-rich areas of coal seams

Abstract

The present invention belongs to the technical field of coal mine gas-rich area detection, and specifically relates to a method for detecting borehole electromagnetic waves in coal seam gas-rich areas, which mainly includes rock physics experiments, data collection and gas content prediction processes, and its specific implementation steps are as follows: the first step: processing multiple groups of coal samples extracted by drilling holes in the predicted tunnel into standard coal samples, and cutting or hollowing them out and injecting gas within a certain range and a certain step length, debugging the experimental environment, using a pulse electromagnetic wave transmitting and receiving device to closely adhere to the coal sample for experiments, analyzing the received signal data and calculating the coal rock attenuation coefficient and the amplitude and phase attenuation values; the second step: selecting multiple groups of pre-extraction boreholes in the tunnel of the area to be tested as collection boreholes. The present invention uses the amplitude and phase of electromagnetic waves as control parameters, compares them with the field measured data through rock physics experiments, and analyzes the signal values ​​of the two, which is conducive to accurately delineating the content ratio of the gas area.

Description

Technical Field

[0001] The invention belongs to the technical field of coal mine gas-enriched area detection, and in particular relates to a borehole electromagnetic wave detection method for coal seam gas-enriched areas. Background Art

[0002] Gas is a gaseous geological body that is flammable, explosive, colorless and odorless. The special properties of gas make it the biggest source of disaster in my country's coal mine production. The gas problem is the top priority for coal mine safety production, which is related to the safety of life and property. Moreover, as my country's coal mining technology gradually improves and the mining depth increases, gas disasters will become more serious, and the problem of coal and gas outbursts will become more severe. For a considerable period of time in the future, coal, as my country's primary main energy source, will continue to dominate my country's energy. Therefore, whether it is possible to accurately predict gas-rich areas is of great significance to coal mining safety.

[0003] At present, the geophysical detection technologies for detecting gas-rich areas mainly include three-dimensional seismic exploration technology, seismic interferometry imaging technology, audio magnetotelluric method, transient electromagnetic detection technology and other technical means. Among them, three-dimensional seismic exploration is a technology that detects the burial depth and shape of the underground rock interface based on the propagation route and time of artificially excited seismic waves in the underground rock formation, recognizes the underground geological structure and then searches for oil and gas reservoirs; seismic interferometry imaging technology is a detection technology that uses background noise imaging to characterize the three-dimensional distribution of isotropic and anisotropic shear wave velocities of underground media and their changes in time, and infers the distribution of cracks and gas based on the sensitivity of seismic shear wave velocity to the presence of cracks and gas; audio magnetotelluric method is a detection technology that uses the natural magnetotelluric field as a field source to determine the electrical parameters of underground rocks, and achieves geological purposes such as understanding geological structures by studying the changes in the section of the location.

[0004] In today's underground coal mine detection, the main purpose is to prevent underground disasters, so advanced prediction and refined processing have become the main development direction of underground detection. Among the existing geophysical detection methods and instruments in China, slot wave exploration and borehole electromagnetic wave detection can better meet the current needs.

[0005] Among them, borehole electromagnetic wave detection is a broadband high-frequency electromagnetic wave detection technology developed based on underground working conditions. Its detection principle is no different from that of traditional ground penetrating radar. After the broadband electromagnetic wave is transmitted from the transmitting antenna to the underground medium around the hole, it is reflected or refracted at the junction of media with different wave impedances. The reflected electromagnetic wave will return to the receiving antenna and then be received and recorded. Then the refracted electromagnetic wave will continue to propagate around and repeatedly reflect and refract until the energy is dissipated.

[0006] Existing borehole electromagnetic wave detection has the advantage of small size, and can use existing boreholes such as underground inspection holes, prediction holes, and drainage holes for detection. On-site detection or sampling back to the physical laboratory for detection has a single detection method, and there are certain differences between the two, which makes the detection accuracy of coal seam gas-rich areas not high. Therefore, we propose a method for precise borehole electromagnetic wave detection to achieve prediction of gas-rich areas, coal seam roof and floor elevations, abnormal geological bodies near the borehole, and coal seam occurrence. Summary of the invention

[0007] The purpose of the present invention is to provide a method for detecting coal seam gas-rich areas through borehole electromagnetic wave, which can use the amplitude and phase of the electromagnetic wave as control parameters, compare the rock physics experiment with the field measured data, analyze the signal values ​​of the two, and help to accurately delineate the content ratio of the gas area.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for detecting coal seam gas-rich areas by borehole electromagnetic waves mainly includes rock physics experiments, data collection and gas content prediction processes, and its specific implementation steps are as follows:

[0010] Step 1: Process multiple groups of coal samples extracted from the predicted tunnel drilling into standard coal samples, and perform cutting or hollowing and gas injection operations within a certain range and a certain step length on them, debug the experimental environment, use a pulse electromagnetic wave transmitting and receiving device to conduct experiments close to the coal samples, analyze the received signal data and calculate the coal rock attenuation coefficient and amplitude and phase attenuation values;

[0011] Step 2: Select multiple groups of pre-extraction boreholes as acquisition boreholes in the laneway of the test area, place the pulse electromagnetic wave transmitter and receiver in the hole with a fixed interval of 0.5m, and during the measurement, operate the host to transmit and receive electromagnetic wave signals while pushing the push rod forward at a regular interval of 0.1m, and monitor the data collection in real time. After the collection is completed, record the detection environment and drilling position;

[0012] Step 3: After a series of preliminary processing, the original data collected by the host are compensated for all channel signals and the electromagnetic wave amplitude and phase attenuation values ​​are recorded; according to the travel time of the electromagnetic wave, the transmitted electromagnetic wave signal is attenuated in combination with the coal rock attenuation coefficient, so as to obtain the electromagnetic wave amplitude and phase values ​​in the coal rock medium, and the absolute value difference is obtained from the measured data and compared with the experimental data to obtain the proportion of the gas area.

[0013] The technical effects achieved by the present invention are:

[0014] The electromagnetic wave detection method for drilling in gas-rich areas of coal seams of the present invention uses the amplitude and phase of the electromagnetic wave as control parameters, compares the rock physics experiment with the field measured data, and analyzes the signal values ​​of the two, which is conducive to accurately delineating the content ratio of the gas area and provides an important reference basis for avoiding gas outburst accidents during coal mining.

[0015] The present invention provides a method for detecting electromagnetic waves in boreholes in coal seam gas-rich areas. The measured data is initially processed by converting its format, and then the data is pre-processed by time zero correction, bad track removal, DC elimination and direct wave elimination to eliminate the influence caused by instrument hardware reasons. After completion, a specified frequency range is selected for bandpass filtering to facilitate subsequent further processing.

[0016] The present invention provides a method for detecting electromagnetic waves in boreholes in coal seam gas-rich areas. For the design of tunnel boreholes, the present invention selects pre-extraction boreholes for repeated use, which greatly reduces the collection time and labor costs and greatly improves the convenience of collecting multiple sets of data.

[0017] The invention provides a method for detecting coal seam gas-rich areas through borehole electromagnetic waves. During a rock physics experiment, the environmental data collected in the tunnel is used to accurately calibrate the experimental environment, thereby reducing errors caused by external factors in the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the data collection process of the present invention;

[0019] Figure 2 It is a schematic diagram of the travel time and amplitude of the reflected signal of the present invention;

[0020] Figure 3 is a flow chart of data processing of the present invention;

[0021] Figure 4 It is a schematic diagram of the standard coal sample of the present invention. DETAILED DESCRIPTION

[0022] In order to make the purpose and advantages of the present invention more clearly understood, the present invention is specifically described below in conjunction with embodiments. It should be understood that the following text is only used to describe one or several specific embodiments of the present invention, and does not strictly limit the scope of protection of the specific claims of the present invention.

[0023] like Figure 1-4 As shown, a method for detecting coal seam gas-rich areas by drilling electromagnetic waves includes rock physics experiments, data acquisition and gas content prediction, as follows:

[0024] A. Rock physics experiments:

[0025] 1) Preliminary preparation: Twelve identical coal samples extracted from the predicted tunnel drilling are processed into standard coal samples. The standard coal sample diagram is as follows: Figure 4 As shown, five complete standard coal samples were reserved as the first group of experimental samples, and their longest sides were cut at 0%, 20%, 40%, 60%, and 80% in turn; the remaining seven samples were hollowed out at a fixed volume in the center and injected with gas in proportion according to the gas content of 1%, 3%, 5%, 8%, 10%, 15%, and 20% in turn, and set up as the second group of experimental samples; the measurement environment was calibrated according to the tunnel temperature and humidity recorded in advance;

[0026] 2) Experimental process: The experiment uses a transmitting and receiving device with a main frequency of 100MHz. When measuring two groups of experimental samples, the two are placed close to the coal sample surface for 1 # Kneading noodles 2 # Place; then the experiment begins, the coal sample is placed in the calibrated environment, the device is arranged and operated to transmit and receive electromagnetic waves, and the electromagnetic waves of the experimental sample are monitored in real time to prevent incomplete signals, and the above steps are repeated for the remaining coal samples;

[0027] B. Data Collection:

[0028] The data acquisition system consists of transmitting and receiving antennas, transmission optical fibers and a host computer. The measurement method is single-hole reflection measurement, as follows:

[0029] a) Borehole selection: In coal and gas outburst mines or high-gas mines, the detailed rules for preventing and controlling coal and gas outbursts should be followed. Periodic drilling and pre-extraction gas monitoring should be carried out in the mining area. The pre-extraction boreholes can be reused in combination with monitoring data and geological data. In the drilling design stage, considering the geological structure and the spatial orientation of the borehole detection, multiple boreholes should be set up in non-same planes, and the drilling direction should be along both sides of the roadway. At the same time, the distance between the boreholes should be less than the maximum measurement range of the transmitting and receiving antennas;

[0030] b) Instrument layout: The antenna main frequency is 100MHz, the interval is 0.5m, the sampling space interval is 0.1m, the time sampling interval is 0.25ns, and the drilling depth is 15m; the transmitting antenna and the receiving antenna are bound to the push rod head at a fixed interval, and the push rod is placed in the borehole to be tested. The push rod is connected to the host computer with optical fiber, and the host computer is debugged to ensure that the instrument is in good condition. The specific layout is as follows: Figure 1 As shown;

[0031] c) In-hole data collection: The push rod is pushed slowly into the hole close to the hole wall, and the host is operated to emit and collect electromagnetic wave signals, and the integrity of the data collected by the host is monitored. After the data collection of a single hole is completed, the push rod is taken out and the drilling position, drilling temperature and humidity of the data group are recorded; the above steps are repeated for the remaining holes to be measured, and the collected reflection signal data is transmitted to the computer after the measurement is completed;

[0032] C. Gas content prediction:

[0033] Experimental data analysis, preliminary processing of measured data, signal gain, and data interpretation are as follows:

[0034] 1) Experimental data analysis: The amplitude and phase of the electromagnetic wave are used as the main parameters for analysis;

[0035] In general, electromagnetic waves propagate forward in the form of a sphere, and the formula for the sphere is:

[0036] S=4πr 2 (1)

[0037] Since electromagnetic waves propagate in the medium, they will produce absorption attenuation, projection loss and signal changes caused by geometric diffusion. The principle is as follows Figure 2 As shown, combined with formula (1), we can get:

[0038] A1=A0S λ =A0(4πr 2 ) λ =A0(4π(vt) 2 ) λ =4 λ π λ A0 2λ t 2λ (2)

[0039] Among them, A1 is the amplitude after change, A0 is the initial amplitude of the emitted electromagnetic wave, λ is the change direction, r is the propagation distance, t is the propagation time, and v is the wave speed;

[0040] From formula (2), we can see that the amplitude change is positively correlated with time, as follows:

[0041]

[0042] Among them, α1 is the linear variation coefficient, β1 is the exponential variation coefficient;

[0043] Similarly, the phase change is also positively correlated with time, as follows:

[0044]

[0045] in, is the phase after the change, is the initial phase of the emitted electromagnetic wave, α2 is the linear variation coefficient, and β2 is the exponential variation coefficient;

[0046] Based on the signal data collected from the rock physics experiment, the electromagnetic wave field value of the transmitting antenna of the i-th group (i = 1, 2, 3, ..., 12) of the experimental samples is calculated. and the electromagnetic wave field value of the receiving antenna Among them A i is the amplitude value of the electromagnetic wave transmitted and received by the i-th group, is the phase of the electromagnetic wave transmitted and received by the i-th group, t i is the propagation time of the i-th group of electromagnetic waves;

[0047] The first group of experimental coal samples were cut into five groups of control samples, and the corresponding propagation distances r of electromagnetic waves were 20cm, 16cm, 12cm, 8cm and 4cm respectively; the electromagnetic wave field value Y of each group of transmitting and receiving antennas was i and Z i Substitute into formula (3) respectively and Formula (4) is used to obtain the expression of the amplitude and phase variation with time of electromagnetic wave propagation in coal seam medium, and the coal rock attenuation coefficient α is derived by combining the Maxwell equations;

[0048] The seven sets of electromagnetic wave data received from the second group of experimental samples were compared with the electromagnetic wave field values ​​received by the uncut coal samples. The amplitude A1 and phase The amplitude of the electromagnetic wave received by the coal sample containing gas and Phase Calculate the amplitude variation of electromagnetic waves under different gas contents and phase variables

[0049] 2) Preliminary processing of measured data: The total number of channels of this group of data is 150. Since the collected raw data is stored in binary, it should be converted into a new format first, and then the data should be pre-processed by time zero correction, bad track removal, DC elimination and direct wave elimination to eliminate the influence of instrument hardware. After completion, the specified frequency range should be selected for bandpass filtering to facilitate further processing;

[0050] 3) Signal gain: In order to accurately determine the gas content in the measured area, signal gain is performed on all channel data to compensate for the signal attenuation caused by absorption attenuation, projection loss and geometric diffusion, and compared with the reflected electromagnetic wave signal under the propagation of coal and rock media without abnormal bodies. The specific implementation steps are as follows:

[0051] According to the coal rock attenuation coefficient α, all channel signal data are compensated to remove the attenuation caused by propagation in lossy media. The amplitude value A of the effective electromagnetic wave after compensation is x (x=1,2,3,……) and phase value At the same time, the time after the effective electromagnetic wave appears in each data is recorded as the travel time t τ (τ=1,2,3,……,x);

[0052] After recording, according to the coal rock attenuation coefficient α and travel time t τ , quantitatively attenuate the transmitted electromagnetic wave signal and record the amplitude value A of the attenuated electromagnetic wave y (y=1,2,3,……,x) and phase value

[0053] 4) Data interpretation: The amplitude value A x , Phase value and amplitude A y , Phase value According to the travel time t τ By performing absolute difference calculation one by one, the change values ​​of the amplitude and phase of the electromagnetic wave signal in the gas-rich area in this region can be obtained as A γ (γ=1,2,3,……,x), Finally passed A γ and 2×A z , and Among them, A z and It is the change value under one-way attenuation, so it needs to be multiplied by 2 when compared with the measured data. By comparison, the range proportion of gas in the gas-enriched area can be calculated. If the gas proportion in this range needs to be accurately measured, the experimental samples should be increased and the step length should be reduced during the rock physical test.

[0054] In summary, the present invention uses the amplitude and phase of electromagnetic waves as control parameters, compares them with field measured data through rock physics experiments, and analyzes the signal values ​​of the two, which is conducive to accurately delineating the content ratio of the gas area and provides an important reference basis for avoiding gas outburst accidents during coal mining.

[0055] The above is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principles of the present invention, and these improvements and modifications should also be considered as the protection scope of the present invention. The structures, devices and operating methods not specifically described and explained in the present invention shall be implemented according to the conventional means in the art unless otherwise specified and limited.

Claims

1. A method for detecting coal seam gas-rich areas by drilling electromagnetic waves. It is characterized in that The following steps are involved: Step 1: Rock physics experiment: Drill and extract the predicted tunnel holes and process them into several standard coal samples, place them one by one in the calibrated environment, transmit and receive electromagnetic waves through the transmitting and receiving device, and monitor in real time; Step 2: Data collection: The data collection system is composed of transmitting and receiving antennas, transmission optical fibers and a host computer. Single-hole reflection measurement is performed in coal and gas outburst mines or high-gas mines. Step 3: Gas content prediction: perform experimental data analysis, preliminary processing of measured data, signal gain and data interpretation in sequence to deduce the gas range ratio in the gas-rich area; The experimental data analysis is performed with the amplitude and phase of the electromagnetic wave as the main parameters; Assume that the electromagnetic wave propagates forward in the form of a sphere, and the formula for the sphere is: S＝4πr2 (1) Since electromagnetic waves propagate in a medium, they will produce absorption attenuation, projection loss and signal changes caused by geometric diffusion. Combining formula (1), we can get: A 1 =A 0 S λ =A 0 (4πr 2 ) λ =A 0 (4π(vt) 2 ) λ =4 λ π λ A 0 v 2λ t 2λ (2) Among them, A 1 is the amplitude after the change, A 0 is the initial amplitude of the emitted electromagnetic wave, λ is the change direction, r is the propagation distance, t is the propagation time, and v is the wave speed; From formula (2), we can see that the amplitude change is positively correlated with time, as follows: (3) where α 1 is the linear variation coefficient, β 1 is the coefficient of exponential variation; Similarly, the phase change is also positively correlated with time, as follows: in, is the phase after the change, is the initial phase of the emitted electromagnetic wave, α 2 is the linear variation coefficient, β 2 is the coefficient of exponential variation; Based on the signal data collected from the rock physics experiment, the electromagnetic wave field value of the transmitting antenna of the i-th group (i=1, 2, 3, ..., 12) of the experimental samples is calculated. and the electromagnetic wave field value of the receiving antenna Among them A i is the amplitude value of the electromagnetic wave transmitted and received by the i-th group, is the phase of the electromagnetic wave transmitted and received by the i-th group, t i is the propagation time of the i-th group of electromagnetic waves; In the first step, at least twelve standard coal samples are extracted from the predicted tunnel boreholes, of which five complete standard coal samples are set as the first group of experimental samples, and their longest sides are cut at a ratio of 0%, 20%, 40%, 60%, and 80% in sequence; the remaining seven samples are hollowed out at a fixed volume in the center according to the ratio of gas content of 1%, 3%, 5%, 8%, 10%, 15%, and 20%, and gas is injected in proportion, and set as the second group of experimental samples; The experimental data analysis also includes: The first group of experimental coal samples were cut into five groups of control samples, and the corresponding propagation distances of electromagnetic waves were 20cm, 16cm, 12cm, 8cm and 4cm respectively; the electromagnetic wave field value Y of each group of transmitting and receiving antennas was i and Z i Substituting into equation (3) and equation (4) respectively, we can obtain the expressions of the amplitude and phase changing with time when electromagnetic waves propagate in coal seam medium, and derive the coal rock attenuation coefficient α in combination with Maxwell equations; The experimental data analysis also includes: The seven sets of electromagnetic wave data received from the second group of experimental samples were compared with the electromagnetic wave field values ​​received by the uncut coal samples. The amplitude A of the electromagnetic wave received by the uncut coal samples was 1 and Phase The amplitude of the electromagnetic wave received by the coal sample containing gas and Phase Calculate the amplitude variation of electromagnetic waves under different gas contents and phase variables The measured data is initially processed by first converting the format and then preprocessing the data to eliminate the influence of instrument hardware. After completion, a specified frequency range is selected for bandpass filtering; The signal gain performs signal gain on all trace data to compensate for the signal attenuation caused by absorption attenuation, projection loss and geometric diffusion, and compares it with the reflected electromagnetic wave signal propagated in the coal rock medium without abnormal body. The specific steps are as follows: First, all signal data are compensated according to the coal rock attenuation coefficient α to remove the attenuation caused by propagation in lossy media. The amplitude value A of the effective electromagnetic wave after compensation is x (x=1, 2, 3, ...) and phase value At the same time, the time after the effective electromagnetic wave appears in each data is recorded as the travel time t τ (τ=1, 2, 3,...,x); Secondly, after recording, according to the coal rock attenuation coefficient α and travel time t τ , quantitatively attenuate the transmitted electromagnetic wave signal and record the amplitude value A of the attenuated electromagnetic wave y (y=1, 2, 3, ..., x) and phase value The data interpretation is to convert the amplitude value A x , Phase value and amplitude A y , Phase value According to the travel time t τ Perform absolute difference calculations one by one to obtain the amplitude and phase changes of the electromagnetic wave signal in the gas-rich area of ​​the region, respectively: A γ (γ=1, 2, 3,...,x), Finally passed A γ and 2×A z , and Among them, A z and It is the change value under one-way attenuation. When compared with the measured data, it is multiplied by 2. The range proportion of gas in the gas-enriched area can be calculated by comparison.

2. A method for detecting coal seam gas-rich areas by drilling electromagnetic waves according to claim 1, Features: The data collection also includes drilling selection, as follows: First, in coal and gas outburst mines or high-gas mines, periodic drilling and pre-extraction gas monitoring is carried out in the mining area, and the pre-extraction boreholes are reused based on the monitoring data and geological data; Secondly, during the drilling design stage, the geological structure and the spatial orientation of the in-hole detection are taken into consideration, and multiple boreholes are set up in different planes, with the drilling direction along both sides of the tunnel. At the same time, the distance between the boreholes is less than the maximum measurement range of the transmitting and receiving antennas.

3. A method for detecting coal seam gas-rich areas by drilling electromagnetic waves according to claim 1, Features: The data collection also includes drilling instrument deployment, as follows: First, the transmitting antenna and receiving antenna have a main frequency of 100MHz, a spacing of 0.5m, a sampling space interval of 0.1m, a time sampling interval of 0.25ns, and a drilling depth of 15m; Secondly, bind the transmitting antenna and the receiving antenna to the push rod head at a fixed distance, place the push rod in the borehole to be tested, connect the push rod to the host computer with optical fiber, and debug the host computer to ensure that the instrument is in good condition.

4. A method for detecting coal seam gas-rich areas by drilling electromagnetic waves according to claim 1, Features: The data collection also includes in-hole collection, as follows: First, push the push rod close to the hole wall and slowly push it into the hole. At the same time, operate the host to emit and collect electromagnetic wave signals, and monitor the integrity of the data collected by the host. Secondly, after the single hole acquisition is completed, the push rod is taken out and the drilling position, drilling temperature and humidity information of the data set are recorded; Finally, repeat the above steps for the remaining holes to be measured, and transmit the collected reflection signal data to the computer after the measurement is completed.

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