Distributed optical fiber drilling test system and method

Abstract

The invention discloses a distributed optical fiber drilling test system and method. The test system comprises a distributed sound wave sensing demodulation instrument, a structural sensing optical cable, a sound wave transducer console, a plastic sleeve, a sound wave transducer, a connecting cable, a coupling medium and a computer. The structural sensing optical cable and the plastic sleeve are implanted into the drill hole, the plastic sleeve is located in the center of the drill hole, the structural sensing optical cable is located between the plastic sleeve and the hole wall of the drill hole, and the coupling medium is backfilled. The distributed optical fiber drilling test system and the distributed optical fiber drilling test method are an in-situ monitoring technology which can be put into operation in time in engineering survey, construction and operation processes, and are used for acquiring drilling sound wave data and determining the type and the state of a rock-soil body in a drilling depth range. The system and the method have the characteristics of high sensitivity and good durability, and can realize in-situ and long-term stratigraphic structure information acquisition and stratigraphic disturbed state scanning with ultrahigh density or extremely high spatial resolution.

Description

Technical Field

[0001] The invention belongs to the technical field of geotechnical engineering testing, and in particular relates to a distributed optical fiber drilling testing system and method. Background Art

[0002] In the field of engineering survey, detection and monitoring, the strata will be disturbed during the construction process, such as crack formation or density changes. Therefore, long-term monitoring of changes in the structure and physical state of the strata is crucial. This monitoring not only helps to assess the degree of disturbance of the strata by underground space development, but also provides an important basis for engineering safety assessment. Through systematic monitoring, we can better understand the response mechanism of the strata and its relationship with underground space design, construction, maintenance and management, thereby optimizing the project implementation process and ensuring the long-term safe and reliable operation of the project.

[0003] Sound waves can propagate in rock and soil media. The physical and mechanical properties of rock and soil and their structural characteristics can be obtained by using information such as the wave velocity, attenuation coefficient and spectral characteristics of sound waves. In recent years, with the continuous improvement of my country's infrastructure, the demand for efficient and accurate underground testing technology has been growing. When carrying out underground engineering construction and construction, the disturbance effect on the stratum and surrounding rock is crucial. In order to effectively perceive the state of the stratum and its changes over time, an in-situ monitoring method that can work for a long time is required. This kind of monitoring can not only timely discover the geological and engineering risks that may be caused by stratum changes, but also provide reliable data support for engineering design and construction processes, ensuring the safety and sustainable development of underground projects.

[0004] The rock and soil wave velocity test is a geophysical exploration technology that uses the acoustic characteristics of sound waves propagating in the strata to study geological profiles and evaluate the quality of rock and soil. This method is based on the influence of the strata on the velocity of sound waves. By measuring the propagation characteristics of sound waves in the strata, the physical properties of the strata can be inferred. Usually, this measurement is generated by a transmitter and captured by two or more receivers distributed along the axial direction of the borehole. The time difference between the arrival of the sound waves is recorded by two receivers to obtain the time required for the sound waves to pass through the strata between the two receivers.

[0005] The propagation of sound waves in the formation is affected by many factors, including the elastic modulus, density, porosity, degree of fracture development, and properties of the fluid in the pores. When the sound waves emitted by the sound source propagate in the formation, they will excite various types of wave modes inside the formation, mainly including longitudinal waves (P waves) and transverse waves (S waves). These body waves reflect the physical state and mechanical properties of the formation itself. In addition, interface waves such as pseudo-Rayleigh waves and Stoneleigh waves are generated on the side walls of the borehole, which carry rich formation information. Combined with modern data processing technology and interpretation models, acoustic wave test data can be used to accurately depict the geological conditions at depths of several meters to kilometers underground, providing a reliable scientific basis for resource development and engineering construction.

[0006] Although traditional rock and soil wave velocity testing technology has been widely used in engineering, it still has some limitations, such as high cost, limited measurement range, low data acquisition efficiency and insufficient adaptability to complex strata.

[0007] The development of Distributed Acoustic Sensing (DAS) technology provides a new solution for wave velocity testing of rock and soil. This technology uses optical fiber as a sensor and can perform continuous measurement over the entire length of the fiber, thereby significantly improving the spatial resolution of the test and the efficiency of data acquisition. Compared with traditional sensors, distributed fiber optic sensors have the advantages of small size, corrosion resistance, anti-electromagnetic interference, and high cost performance, making them particularly suitable for harsh underground environments.

[0008] Although distributed fiber optic sensing technology has significant advantages in theory, there are still some technical problems to be solved in the field of geotechnical wave velocity testing, including how to collect effective acoustic signals, how to improve the signal-to-noise ratio of signals, and how to process and analyze large amounts of data to extract useful information. In addition, one of the key issues is to develop a highly adaptable and reliable borehole-based geotechnical wave velocity testing method for specific formation conditions and testing purposes. Summary of the invention

[0009] In view of the deficiencies in the prior art, the present invention proposes a distributed optical fiber drilling test system and method, which achieves high-precision measurement of the acoustic wave characteristics of rock and soil bodies and provides a reliable basis for the evaluation of engineering geological conditions.

[0010] In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0011] A distributed optical fiber drilling test system comprises a distributed acoustic sensor (DAS) demodulation instrument, a structural sensing optical cable, an acoustic transducer console, a plastic sleeve, an acoustic transducer, a connecting cable, a low-speed coupling medium and a computer. During the test, the structural sensing optical cable and the plastic sleeve are implanted in the test borehole, the plastic sleeve is located in the center of the borehole, the structural sensing optical cable is located between the plastic sleeve and the borehole wall, and the borehole is backfilled with the coupling medium.

[0012] Furthermore, the structural sensing optical cable includes at least one sensing optical fiber on which a large number of weak gratings are engraved.

[0013] Furthermore, the structural sensor optical cable is loose-tube packaged.

[0014] Furthermore, the structural sensing optical cable can be wound in a spiral shape. The use of a spirally wound sensing optical cable can further shorten the track spacing, thereby improving the spatial resolution of the test system.

[0015] The acoustic wave transducer console is connected to the acoustic wave transducer through a connecting cable. During testing, the acoustic wave transducer is placed in a water-filled plastic casing, and the acoustic wave transducer is lifted uniformly at a certain rate from the bottom of the hole to the hole mouth or lifted point by point at certain spatial intervals. The acoustic wave transducer console controls the acoustic wave transducer to emit acoustic wave signals during operation.

[0016] Furthermore, the connecting cable is a signal transmission cable connecting the acoustic wave transducer console and the acoustic wave transducer, the connecting cable has a length mark and has a depth measurement function.

[0017] Furthermore, the low-speed coupling medium is selected according to the properties of the formation around the borehole. The structural sensing optical cable, the plastic casing and the formation are effectively coupled by backfilling the borehole with the low-speed coupling medium. The coupling medium should be a material with a lower sound wave velocity, thereby ensuring that the sliding longitudinal wave becomes the first arrival wave.

[0018] A method for testing using the above-mentioned distributed optical fiber acoustic logging system is characterized by comprising the following steps:

[0019] Step 1) Select a test area and drill a hole, lay a plastic casing in the center of the hole, lay a structural sensing optical cable in the space between the plastic casing and the inner wall of the hole, and fill it with a low-speed coupling medium to ensure that the structural sensing optical cable is tightly coupled with the surrounding strata;

[0020] Step 2) connecting the distributed acoustic wave sensor demodulation instrument on the ground to the structural sensor optical cable buried in the borehole;

[0021] Step 3) connecting the ground acoustic wave transducer console to the acoustic wave transducer, and lowering the acoustic wave transducer to the bottom of the hole;

[0022] Step 4) controlling the acoustic wave transducer to emit ultrasonic signals through the acoustic wave transducer console; receiving waveform signals of each sensing channel along the structural sensing optical cable through a distributed acoustic wave sensor demodulation instrument;

[0023] Step 5) lifting the acoustic wave transducer uniformly at a certain rate from the bottom of the hole to the hole mouth or lifting it point by point at certain spatial intervals;

[0024] Step 6) Part of the sound wave energy is absorbed by the medium during propagation, so the intensity of the sound wave will decay during propagation in the medium. The law of sound wave intensity decay is:

[0025] J=J0e -2αl (1)

[0026] In formula (1), J represents the sound intensity after the sound wave propagates a distance of l, J0 represents the sound intensity of the sound power per unit area, and α represents the absorption coefficient of the medium. The absorption coefficient of the medium decreases with the increase of the density and sound velocity of the medium, and increases with the increase of the sound wave frequency.

[0027] In the same medium, the ratio of the velocity of longitudinal waves to that of transverse waves is:

[0028]

[0029] Among them, vp represents the longitudinal wave velocity, vs represents the shear wave velocity;

[0030] Between different media, the incident longitudinal wave is divided into four types of waves after passing through the interface: reflected longitudinal wave, reflected shear wave, refracted longitudinal wave and refracted shear wave. Due to the characteristics of shear wave itself, it cannot propagate in liquid or gas phase media. For the present invention, the medium includes the low-speed coupling medium in the borehole and the stratum outside the borehole.

[0031] For refracted longitudinal waves, the propagation speed is related to the angle of incidence and the angle of refraction. According to Snell's law, the following relationship exists for refracted longitudinal waves:

[0032]

[0033] Among them, v p1 is the longitudinal wave velocity of the coupling medium in the borehole, v p2 is the P-wave velocity in the formation. From formula (3), we can see that the incident angle θ is proportional to the change in the refraction angle θ1. As the refraction angle θ1 increases, it can reach a maximum of 90°. At this time, the incident angle is recorded as the critical angle θ * , we can get the following formula:

[0034]

[0035] When the incident angle θ of the incident longitudinal wave reaches the first critical angle θ *When the incident angle θ continues to increase and exceeds the first critical angle θ, the generated refracted longitudinal wave will no longer refract and will slide along the borehole wall, i.e., a sliding longitudinal wave. * After that, the incident longitudinal wave will be totally reflected along the borehole wall, and there will be no refracted longitudinal wave in the formation. When the incident angle θ of the incident longitudinal wave reaches the second critical angle When the refracted shear wave is generated, it will no longer refract and will slide along the borehole wall, which is called sliding shear wave.

[0036] Taking advantage of the fast propagation speed of the glide wave, by analyzing the records of different sensor channels, selecting appropriate sensor channels and receiving intervals, and distinguishing between glide waves, direct waves, and single and multiple reflected waves, it is convenient for waveform extraction and identification.

[0037] Step 7) Using multiple adjacent sensor channel records to achieve single-transmit multiple-receive, the wave velocity distribution of different strata within a certain range is obtained. The calculation formula for each stratum is as follows:

[0038]

[0039] In formula (5), v i represents the acoustic wave propagation velocity of the stratum between the i-th track and the i-1-th track, L represents the track spacing of the structural sensing optical cable (2), Δt i is the difference in the arrival time of the first wave between the i-th track and the i-1-th track;

[0040]

[0041] In formula (6), t i and t i-1 They represent the time when the first wave reaches the i-th and i-1-th tracks respectively, v1 represents the propagation speed of the sound wave in the low-speed coupling medium, and v2 represents the speed at which the sound wave slides along the borehole wall in the formation. When v1 is known, the wave velocity v2 of the formation can be determined according to formula (6).

[0042] Step 8) Use the relevant algorithm to further process and interpret the waveform signal obtained in step 7) to obtain the elastic parameters or viscoelastic parameters of the formation around the borehole, as well as the lithology, porosity, permeability, and type and saturation of the fluid in the pores of the formation around the borehole. The relationship between the shear wave and longitudinal wave velocity of the formation and the formation parameters such as medium density and elastic parameters is as follows:

[0043]

[0044] Among them, v p is the longitudinal wave velocity, v s is the shear wave velocity, E is Young's modulus, μ is Poisson's ratio, ρ b is the volume density of the medium.

[0045] The process of further processing and interpreting the velocity data and waveform signals of the formation around the borehole in step 8) is as follows: for the optical fiber sensing data with clear waveforms, the acoustic wave velocity of the formation where the two adjacent sensing channels are located is directly obtained through the channel spacing and the difference in the first wave arrival time of the two adjacent sensing channels; for the optical fiber sensing data with unclear waveforms, the software system uses multi-channel wave train signals to calculate the cross-correlation function of a certain section of the wave train based on the cross-correlation algorithm, and uses the principle of the cross-correlation function to calculate the group velocity of a certain wave in the wave train; through the far and near waveforms recorded by the full wave train logging of the two acoustic waves, the starting window is determined according to the first arrival of the longitudinal wave The window length is determined according to the difference in propagation time of longitudinal waves and transverse waves in the stratum. Then, the near waveform is fixed and the far waveform is moved. The cross-correlation functions of different window shift values ​​are calculated. The window shift value corresponding to the maximum value of the cross-correlation function is obtained, and then the longitudinal wave time difference of the two sensor channels is determined using the window shift value. Similarly, the propagation time difference of various wave trains such as transverse waves, pseudo-Rayleigh waves, and Stoneley waves can be obtained. The wave velocity of the corresponding wave train in the stratum where the sensor channel is located is obtained by dividing the channel spacing with the propagation time difference of each wave train of two adjacent sensor channels. The software system uses the difference in acoustic characteristics of sound waves propagating in rock and soil to invert the distribution and disturbance state of the rock and soil.

[0046] The present invention discloses a distributed optical fiber drilling test system and method based on distributed optical fiber acoustic wave sensing (DAS), which uses a highly sensitive structural sensor optical cable to replace the monopole, dipole or multipole transistor acoustic wave receiving transducer in a conventional acoustic wave scanning logging device with a relatively high price, and uses a borehole to detect rock and soil bodies within a depth of several meters to several thousand meters. The system and method use a structural sensor optical cable as a receiving sensor downhole, without any electronic devices, and solve the problem that the downhole receiving sensor and its supporting amplifier, analog-to-digital conversion and data storage device and data transmission module are easy to corrode and difficult to insulate. By burying the structural sensor optical cable in the borehole and connecting it to the DAS modem on the ground, the signal received by the structural sensor optical cable can be analyzed in real time, solving the bottleneck problem that a large amount of data signals collected by the acoustic wave scanning test device are difficult to achieve high-speed return. By slowly pulling the acoustic wave transducer upward in the plastic casing in the borehole and controlling the transducer to continuously excite the acoustic signal, the wave velocity and state of the rock and soil body within the depth range of the borehole can be detected. This system and method can greatly reduce the cost of a single test of acoustic logging, achieve the effect of long-term multiple reuse, and realize underground wave velocity testing with high-efficiency acquisition, ultra-high density or extremely high spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a schematic diagram of the layout of a distributed optical fiber drilling test system of the present invention;

[0048] Figure 2 is a cross-sectional schematic diagram of a drilling well of the present invention;

[0049] Figure 3 It is a schematic diagram of the principle of the drilling sonic test of the present invention;

[0050] Figure 4 It is a schematic diagram of the structure of the spiral structure type sensor optical cable of the present invention.

[0051] Figure 5 It is a sensing optical cable including a weak grating optical fiber engraved with it.

[0052] Among them: 1—distributed acoustic wave sensor demodulation instrument, 2—structural sensor optical cable, 3—acoustic wave transducer console, 4—plastic sleeve, 5—acoustic wave transducer, 6—connecting cable, 7—coupling medium, 11—reflected wave, 12—refracted wave, 13—direct wave, 14—gliding longitudinal wave, 15—calculated velocity distribution curve, 16—theoretical velocity distribution curve, 17—waveform diagram of the sensor channel changing with time, 21—weak grating, 22—sensing optical fiber. DETAILED DESCRIPTION

[0053] A distributed optical fiber drilling test system and method proposed in the present invention are described in detail below in conjunction with the accompanying drawings; in the description of the present invention, it should be understood that the terms "left side", "right side", "upper", "lower", "bottom" and the like indicating directions or positional relationships are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and "A", "B", "C" and the like do not indicate the importance of the components, and therefore cannot be understood as limitations on the present invention; the specific dimensions used in this embodiment are only for illustrating the technical solution, and do not limit the scope of protection of the present invention.

[0054] like Figure 1 and 2 As shown, an embodiment of the present invention provides a distributed optical fiber drilling test system and method, including a distributed acoustic sensor (DAS) demodulator 1, a structural sensor optical cable 2, an acoustic transducer console 3, a plastic sleeve 4, an acoustic transducer 5, a connecting cable 6, a low-speed coupling clay ball 7 and a computer; during testing, the structural sensor optical cable 2 and the plastic sleeve 4 are implanted in the test borehole, the plastic sleeve 4 is located in the center of the borehole, and the structural sensor optical cable 2 is arranged in a U shape and is tightly attached to the plastic sleeve 4.

[0055] The present invention relates to a simplified formation model. Figure 3 As shown in the figure, the stratum is divided into three layers, with 5m thick mudstone, 5m thick dolomite and 5m thick mudstone in the upper, middle and lower layers. The longitudinal wave velocity of mudstone is 2000m / s, and that of dolomite is 5000m / s. The low-speed coupling medium in the model is a clay ball, and after saturation, its longitudinal wave velocity is about 1000m / s.

[0056] Selection of structural sensor optical cable 2 Figure 4 The spirally wound structural sensing optical cable 2 shown in the figure includes at least one sensing optical fiber, and the sensing optical cable is loose-tube packaged. The track spacing of the spirally wound structural sensing optical cable 2 is 0.5m.

[0057] The acoustic transducer console 3 is connected to the acoustic transducer 5 through a connecting cable 6. During the test, the acoustic transducer 5 is placed in a water-filled plastic casing 4, and the acoustic transducer 5 is lifted uniformly at a certain rate from the bottom of the hole to the hole mouth or lifted point by point at a certain spatial interval. The acoustic transducer console 3 controls the acoustic transducer 5 to emit an ultrasonic sinusoidal signal with a frequency of 22kHz during operation, and uses a distributed acoustic wave sensor demodulation instrument 1 to collect the optical signal of the structural sensor optical cable. The connecting cable 6 is a signal transmission cable connecting the acoustic transducer console 3 and the acoustic transducer 5. The connecting cable 6 has a scale and has a depth measurement function.

[0058] According to the properties of the formation around the borehole, clay balls 7 are selected as low-speed coupling media. The structural sensing optical cable, plastic casing and underground rock and soil are effectively coupled by backfilling the borehole with low-speed coupling media. The saturated clay balls are a type of rock and soil material with low acoustic wave velocity, and their wave velocity is lower than that of the mudstone and dolomite around the borehole, thereby ensuring that the sliding longitudinal wave in the formation becomes the first arrival wave.

[0059] A method for testing using the above-mentioned distributed optical fiber drilling test system is characterized by comprising the following steps:

[0060] Step 1) Select a test area and drill a hole, lay a plastic casing 4 in the center of the hole, lay a structural sensing optical cable 2 in the space between the plastic casing 4 and the inner wall of the hole, and backfill a low-speed coupling medium 7 to ensure tight coupling between the structural sensing optical cable 2 and the surrounding strata;

[0061] Step 2) connecting the distributed acoustic wave sensor demodulation instrument 1 on the ground to the structural sensor optical cable 2 buried underground;

[0062] Step 3) connecting the ground acoustic wave transducer console 3 with the acoustic wave transducer 5, and lowering the acoustic wave transducer 5 to the bottom of the hole;

[0063] Step 4) Control the acoustic wave transducer 5 to emit an ultrasonic signal through the acoustic wave transducer console 3, and the ultrasonic signal frequency is 22kHz; receive the waveform signal of each sensing channel along the structural sensing optical cable 2 through the distributed acoustic wave sensor demodulation instrument 1;

[0064] Step 5) lifting the acoustic wave transducer 5 point by point from the bottom of the hole to the hole mouth at a spatial interval of 0.1 m; the lifting rate is 1 to 9 m / min, the minimum interval is 0.1 m, and generally does not exceed 2 m;

[0065] Step 6) A portion of the sound wave of the sound wave transducer is directly propagated to the structural sensor optical cable 2 as a direct wave 13, and a portion of the sound wave energy is absorbed by the medium during the propagation process, so the intensity of the sound wave will attenuate during the propagation process in the medium. The law of sound wave intensity attenuation is:

[0066] J=J0e -2αl (1)

[0067] In formula (1), J represents the sound intensity after the sound wave propagates a distance of l, J0 represents the sound intensity of the sound power per unit area, and α represents the absorption coefficient of the medium. The absorption coefficient of the medium decreases with the increase of the density and sound velocity of the medium, and increases with the increase of the sound wave frequency.

[0068] In the same medium, the ratio of the velocity of longitudinal waves to that of transverse waves is:

[0069]

[0070] Between different media, the incident longitudinal wave is divided into four types of waves after passing through the interface: reflected wave 11 (divided into reflected longitudinal wave and reflected transverse wave) and refracted wave 12 (divided into refracted longitudinal wave and refracted transverse wave). Due to the characteristics of the transverse wave itself, it cannot propagate in liquid or gas phase media. For the present invention, the medium includes the low-speed coupling medium in the borehole and the stratum outside the borehole.

[0071] For refracted longitudinal waves, the propagation speed is related to the angle of incidence and the angle of refraction. According to Snell's law, for refracted longitudinal waves:

[0072]

[0073] Among them, v p1 is the longitudinal wave velocity of the saturated clay ball in the borehole, v p2 is the P-wave velocity in the formation. From formula (3), we can see that the incident angle θ is proportional to the change in the refraction angle θ1. As θ increases, the maximum value can reach 90°. At this time, the incident angle is recorded as the critical angle θ * , we can get the following formula:

[0074]

[0075] When the incident angle θ of the incident longitudinal wave reaches the first critical angle θ * When the incident angle θ continues to increase, it exceeds the first critical angle θ. * After that, the incident P-wave will be totally reflected along the interface, and there will be no refracted P-wave in the stratum. When the incident angle θ of the incident P-wave reaches the second critical angle When the refracted shear wave is generated, it will no longer refract and will slide along the borehole wall, which is called sliding shear wave.

[0076] Taking advantage of the fast propagation speed of the glide wave, by analyzing the records of different sensor channels, selecting appropriate sensor channels and receiving intervals, and distinguishing between glide waves, direct waves, and single and multiple reflected waves, it is convenient for waveform extraction and identification.

[0077] Step 7) Using multiple adjacent sensor channels to record, single transmission and multiple reception are realized. The sensor channels receive a waveform graph 17 that changes with time, and the wave velocity distribution curve 15 of different strata within a certain range is obtained by calculation. The calculation formula for each stratum is as follows:

[0078]

[0079] In formula (5), v i represents the acoustic wave propagation velocity of the stratum between the i-th track and the i-1-th track, L represents the track spacing of the structural sensing optical cable (2), Δt i is the difference in the arrival time of the first wave between the i-th track and the i-1-th track;

[0080]

[0081] In formula (6), t i and t i-1 They represent the time when the first wave arrives at the i-th track and the i-1-th track, respectively, and v p1 represents the propagation speed of sound waves in the low-speed coupling medium, v p2 Indicates the speed at which the sound wave travels along the borehole wall in the formation. In this example, v p1 =1000m / s, according to formula (6), the wave velocity v of the formation can be determined p2 In actual engineering, due to the complexity of the formation conditions and calculation errors, the actual calculated velocity distribution curve 15 and the theoretical velocity distribution curve 16 may have small deviations and fluctuations.

[0082] Step 8) Use the relevant algorithm to further process and interpret the waveform signal obtained in step 7) to obtain the elastic parameter characteristics or viscoelastic parameter characteristics of the formation around the borehole, as well as the lithology, porosity, permeability, and type and saturation of the fluid in the pores of the formation around the borehole. The relationship between the shear wave and longitudinal wave velocity of the formation and the formation parameters such as medium density and elastic parameters is as follows:

[0083]

[0084] Among them, v p is the longitudinal wave velocity, v s is the shear wave velocity, E is Young's modulus, μ is Poisson's ratio, ρ b is the volume density of the medium.

[0085] The process of further processing and interpreting the velocity data and waveform signals of the formation around the borehole in step 8) is as follows: for the optical fiber sensing data with clear waveforms, the acoustic wave velocity of the formation where the two adjacent sensing channels are located is directly obtained through the channel spacing and the difference in the first wave arrival time of the two adjacent sensing channels; for the optical fiber sensing data with unclear waveforms, the software system uses multi-channel wave train signals to calculate the cross-correlation function of a certain section of the wave train based on the cross-correlation algorithm, and uses the principle of the cross-correlation function to calculate the group velocity of a certain wave in the wave train; through the far and near waveforms recorded by the full wave train logging of the two acoustic waves, the starting window is determined according to the first arrival of the longitudinal wave The window length is determined according to the difference in propagation time of longitudinal waves and transverse waves in the stratum. Then, the near waveform is fixed and the far waveform is moved. The cross-correlation functions of different window shift values ​​are calculated. The window shift value corresponding to the maximum value of the cross-correlation function is obtained, and then the longitudinal wave time difference of the two sensor channels is determined using the window shift value. Similarly, the propagation time difference of various wave trains such as transverse waves, pseudo-Rayleigh waves, and Stoneley waves can be obtained. The wave velocity of the corresponding wave train in the stratum where the sensor channel is located is obtained by dividing the channel spacing with the propagation time difference of each wave train of two adjacent sensor channels. The software system uses the difference in acoustic characteristics of sound waves propagating in rock and soil to invert the distribution and disturbance state of the rock and soil.

[0086] Compared with conventional detector arrangements, the characteristics of distributed fiber optic acoustic wave sensing technology are simple construction, high efficiency, and high cost performance. There is no need to set up a shock-absorbing and noise-reducing structure (sound insulator) for conventional acoustic system probes in the structural sensing optical cable. Compared with open hole wells, the transmitter slides in the plastic casing, and the low-speed coupling medium is filled outside the plastic casing, so the wellbore has little effect. The distributed acoustic system composed of structural sensing optical cables can record the propagation time difference of various wave trains such as longitudinal waves, transverse waves, pseudo-Rayleigh waves, and Stoneley waves, and realize the wave velocity test of the rock and soil body with a full wave train.

[0087] Based on the description of the preferred embodiments of the present invention, it should be clear that the present invention defined by the attached claims is not limited to the specific details set forth in the above description, and many obvious changes to the present invention that do not depart from the spirit or scope of the present invention may also achieve the purpose of the present invention.

Claims

1. A distributed optical fiber drilling test system and method, characterized in that: The invention comprises a distributed acoustic wave sensing demodulation instrument (1), a structural sensing optical cable (2), an acoustic wave transducer console (3), a plastic sleeve (4), an acoustic wave transducer (5), a connecting cable (6), a low-speed coupling medium (7) and a computer. During testing, the structural sensing optical cable (2) and the plastic sleeve (4) are implanted in a test borehole, the plastic sleeve (4) is located in the center of the borehole, the structural sensing optical cable (2) is located between the plastic sleeve (4) and the borehole wall, and the borehole is backfilled with the coupling medium (7).

2. The distributed optical fiber drilling test system according to claim 1, characterized in that: The structural sensing optical cable (2) comprises at least one sensing optical fiber on which a large number of weak gratings are inscribed.

3. The distributed optical fiber drilling test system according to claim 1, characterized in that: The structural sensor optical cable (2) is loose-tube packaged.

4. The distributed optical fiber drilling test system according to claim 1, characterized in that: The acoustic wave transducer console (3) is connected to the acoustic wave transducer (5) via a connecting cable (6). During testing, the acoustic wave transducer (5) is placed in a water-filled plastic sleeve (4), and the acoustic wave transducer (5) is lifted uniformly at a certain rate from the bottom of the hole to the hole mouth or lifted point by point at certain spatial intervals. The acoustic wave transducer console (3) controls the acoustic wave transducer (5) to emit an acoustic wave signal.

5. The distributed optical fiber drilling test system according to claim 4, characterized in that: The connecting cable (6) is marked with a length.

6. The distributed optical fiber drilling test system according to claim 1, characterized in that: The low-speed coupling medium (7) is selected according to the properties of strata at different depths of the borehole.

7. A method for testing using the distributed optical fiber drilling test system according to any one of claims 1 to 6, characterized in that: The following steps are involved: Step 1) Select a test area and drill a hole, lay a plastic casing in the center of the hole, lay a structural sensing optical cable in the space between the plastic casing and the inner wall of the hole, and fill it with a low-speed coupling medium to ensure that the structural sensing optical cable is tightly coupled with the surrounding strata; Step 2) connecting the distributed acoustic wave sensor demodulation instrument on the ground to the structural sensor optical cable buried in the borehole; Step 3) connecting the ground acoustic wave transducer console to the acoustic wave transducer, and lowering the acoustic wave transducer to the bottom of the hole; Step 4) controlling the acoustic wave transducer to emit ultrasonic signals through the acoustic wave transducer console; receiving waveform signals of each sensing channel along the structural sensing optical cable through a distributed acoustic wave sensor demodulation instrument; Step 5) lifting the acoustic wave transducer uniformly at a certain rate from the bottom of the hole to the hole mouth or lifting it point by point at certain spatial intervals; Step 6) Part of the sound wave energy is absorbed by the medium during propagation, so the intensity of the sound wave will decay during propagation in the medium; the law of sound wave intensity decay is: J=J0e -2αl (1) In formula (1), J represents the sound intensity after the sound wave propagates a distance of l, J0 represents the sound intensity of the sound power per unit area, and α represents the absorption coefficient of the medium. The absorption coefficient of the medium decreases with the increase of the density and sound velocity of the medium, and increases with the increase of the sound wave frequency. In the same medium, the ratio of the velocity of longitudinal waves to that of transverse waves is: Among them, v p represents the longitudinal wave velocity, v s represents the shear wave velocity; Between different media, the incident longitudinal wave is divided into four types of waves after passing through the interface: reflected longitudinal wave, reflected transverse wave, refracted longitudinal wave and refracted transverse wave; due to the characteristics of the transverse wave itself, it cannot propagate in liquid and gas phase media; for the present invention, the medium includes the low-speed coupling medium in the borehole and the stratum outside the borehole; For refracted longitudinal waves, the propagation speed is related to the angle of incidence and the angle of refraction. According to Snell's law, the following relationship exists for refracted longitudinal waves: Among them, v p1 is the longitudinal wave velocity of the low-speed coupling medium in the borehole, v p2 is the P-wave velocity in the formation; from formula (3), we can see that the incident angle θ is proportional to the change in the refraction angle θ1. As θ increases, the maximum value can reach 90°. At this time, the incident angle is recorded as the critical angle θ * , we can get the following formula: When the incident angle θ of the incident longitudinal wave reaches the first critical angle θ * When the incident angle θ continues to increase and exceeds the first critical angle θ, the generated refracted longitudinal wave will no longer refract and will slide along the borehole wall, i.e., the sliding longitudinal wave. * After that, the incident P-wave will be totally reflected along the interface, and there will be no refracted P-wave in the stratum. When the incident angle θ of the incident P-wave reaches the second critical angle When the refracted shear wave is , it will no longer refract and will slide along the hole wall, which is called sliding shear wave. Taking advantage of the fast propagation speed of the glide wave, by analyzing the records of different sensor channels, selecting the appropriate sensor channel and receiving spacing, distinguishing the glide wave, direct wave and single and multiple reflected waves, and performing waveform extraction and recognition; Step 7) Using multiple adjacent sensor channel records to achieve single-transmit multiple-receive, the wave velocity distribution of different strata within a certain range is obtained. The wave velocity calculation formula for each stratum is as follows: In formula (5), v i represents the acoustic wave propagation velocity of the stratum between the i-th track and the i-1-th track, L represents the track spacing of the structural sensing optical cable (2), Δt i is the difference in the arrival time of the first wave between the i-th track and the i-1-th track; In formula (6), t i and t i-1 They represent the time when the first wave reaches the i-th track and the i-1-th track respectively, v1 represents the propagation speed of the sound wave in the low-speed coupling medium, and v2 represents the speed of the sound wave sliding along the borehole wall in the formation. When v1 is known, the wave velocity v2 of the formation can be determined according to formula (6); Step 8) uses a related algorithm to further process and interpret the waveform signal obtained in step 7) to obtain elastic parameters or viscoelastic parameters of the formation around the borehole, as well as the lithology, porosity, permeability, type and saturation of the fluid in the pores of the formation around the borehole; the relationship between the shear wave and longitudinal wave velocity of the formation and the formation parameters such as medium density and elastic parameters is as follows: Among them, v p is the longitudinal wave velocity, v s is the shear wave velocity, E is Young's modulus, μ is Poisson's ratio, ρ b is the volume density of the medium.

8. The method for testing using a distributed optical fiber drilling test system according to claim 7, characterized in that: The process of further processing and interpreting the velocity data and waveform signals of the formation around the borehole in step 8) is as follows: for the optical fiber sensing data with clear waveforms, the acoustic wave velocity of the formation where the two adjacent sensing channels are located is directly obtained by directly using the channel spacing and the difference in the first wave arrival time of the two adjacent sensing channels; for the optical fiber sensing data with unclear waveforms, the software system uses multi-channel wave train signals to calculate the cross-correlation function of a certain section of the wave train based on the cross-correlation algorithm, and uses the principle of the cross-correlation function to calculate the group velocity of a certain wave in the wave train; the starting point is determined according to the first arrival of the longitudinal wave by logging the far and near waveforms recorded by the full wave train of the two acoustic waves. Window value, determine the window length according to the difference in propagation time of longitudinal waves and transverse waves in the stratum, then fix the near waveform and move the far waveform, calculate the cross-correlation function of different window shift values, find the window shift value corresponding to the maximum value of the cross-correlation function, and then use the window shift value to determine the longitudinal wave time difference of the two sensor channels; similarly, the propagation time difference of various wave trains such as transverse waves, pseudo-Rayleigh waves, and Stoneley waves can be obtained; by dividing the channel spacing with the propagation time difference of each wave train of two adjacent sensor channels, the wave velocity of the corresponding wave train in the stratum where the sensor channel is located is obtained; the software system uses the difference in acoustic characteristics of sound waves propagating in rock and soil to invert the distribution and disturbance state of the rock and soil.

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