Beat frequency sensing structure based on active multi-core fiber grating and two-dimensional vector sensing method

Abstract

The invention discloses a beat frequency sensing structure based on an active multi-core fiber grating and a two-dimensional vector sensing method, the sensing structure comprises seven active multi-core fibers, except a central fiber core, even number of more than 4 fiber cores in the active multi-core fibers are inscribed with first gratings at first positions and second gratings at second positions, the reflectivity of the first grating and the second grating on the same fiber core is different, an F-P resonant cavity is formed and used for amplifying or reflecting light with a specific wavelength, each fiber core is changed into a single-frequency laser channel, and when the F-P resonant cavity senses that seismic wave signals deform, laser passing through the fiber cores changes, and the frequency of the single-frequency laser channel changes. The two single-frequency laser channels in central symmetry can realize beat frequency, at least two groups of beat frequency signals are selected at will, the beat frequency value is measured through the frequency counter, the magnitude of the vibration signal is obtained by analyzing the frequency change of the beat frequency signals, and the azimuth angle of the vibration signal is obtained through the geometric position relation of the selected single-frequency laser channels.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing, and in particular relates to a beat frequency sensing structure and a two-dimensional vector sensing method based on active multi-core optical fiber gratings. Background Art

[0003] Seismic technology is the core supporting technology for deep oil and gas exploration. Every innovation in global seismic technology has promoted breakthroughs in deep oil and gas exploration. In the borehole seismic exploration method, the detector is close to the geological formation, which retains the high-frequency seismic signal components and eliminates the interference of the ground environment. Therefore, it can provide a higher signal-to-noise ratio and a wider measurement frequency band. It is the method with the highest resolution and the most detailed description of oil and gas reservoirs among all seismic methods.

[0004] Fiber optic sensing is an emerging detection technology. Since the material of optical fiber is silicon dioxide, it is inherently resistant to high temperatures, corrosion, and safety. In principle, optical measurement has higher sensitivity, higher resolution, and stronger signal transmission capability than electromagnetic instruments. It meets the stringent requirements of exploration and development in extreme environments such as deep wells and ultra-deep wells, and is a very promising application solution for future oil and gas resource exploration and development. The key to solving the problem of picking up and enhancing weak signals in deep and ultra-deep oil and gas seismic exploration is to improve the measurement accuracy of the sensor. The strain of the fiber grating (FBG) caused by the coupling of deep seismic waves to the sensor is extremely weak (≤με). At present, the sensitivity of wavelength modulation and demodulation detectors is generally around 10 3 pm / g level, for optical instruments with pm resolution, only mg-level seismic vibration acceleration can be measured, so it is gradually difficult to obtain seismic wave signals extending to deep depths by directly observing the wavelength drift of the FBG reflection center. Especially when the wavelength (frequency) drift caused by the weak signal coupling to the fiber Bragg grating is less than 0.02nm, it is impossible to achieve accurate measurement by observing the spectrum of the traditional broadband light source FBG through a spectrometer. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a beat frequency sensing structure and a two-dimensional vector sensing method based on active multi-core fiber gratings that are reasonably designed, simple to operate, and accurate in measuring weak signals.

[0006] The technical solution adopted to solve the above technical problems is: a beat frequency sensing structure based on active multi-core fiber grating, which includes an active multi-core optical fiber. In the active multi-core optical fiber, except for the central core, an even number of cores of more than 4 cores that are centrally symmetrically distributed are engraved with a first grating at the first position and a second grating at the second position. The reflectivities of the first grating and the second grating on the same core are different, forming an FP resonant cavity to amplify or reflect light of a specific wavelength.

[0007] Preferably, the reflectivity of the first grating is 90% to 99%, and the reflectivity of the second grating is 60% to 70%.

[0008] Preferably, the distance between the first grating and the second grating is 1-10 cm.

[0009] A two-dimensional vector sensing method for a beat frequency sensing structure uses a pump light source to provide energy for the beat frequency sensing structure. Each FP resonant cavity of the beat frequency sensing structure amplifies or reflects light of a specific wavelength to form a single-frequency laser channel, and a total of more than 4 single-frequency laser channels with an even number are formed; each FP resonant cavity senses a seismic wave signal and generates deformation, and the single-frequency laser output changes. Two centrally symmetrical single-frequency laser channels are divided into one group, and at least two groups are selected. Each group of single-frequency laser channels is beat to obtain a beat frequency signal. The magnitude of the vibration signal is obtained by measuring the frequency change of the beat frequency signal, and the azimuth of the vibration signal is obtained by the geometric position relationship of the selected single-frequency laser channels.

[0010] Preferably, the method for determining the azimuth angle of the vibration signal is:

[0011] Step 1. Define the reference direction, number the fiber cores, and select the fiber cores with the smallest angle to the reference direction and the angle value within The fiber core within the range is defined as core No. 1, and the other fiber cores are numbered in turn in the counterclockwise direction, where the k-th fiber core and the k+(m / 2)-th fiber core are taken for frequency beating, k∈[1,m / 2] and is an integer, m is the total number of fiber cores except the central fiber core, m is an integer, the k+j-th fiber core and the k+j+(m / 2)-th fiber core are taken for frequency beating, (k+j)∈[1,m / 2] and is an integer, j is an integer;

[0012] Step 2. Obtain the azimuth angle θ of the vibration signal according to the following formula: v , 0°≤θ v ≤180°;

[0013] θ v =θ k -β

[0014]

[0015] In the formula, θ k is the angle between the selected kth fiber core and the reference direction, Δv k-[k+(m / 2)] is the beat frequency of the kth fiber core and the k+(m / 2)th fiber core, π is 180°, Δv (k+j)-[k+j+(m / 2)] is the beat frequency value of the k+jth fiber core and the k+j+(m / 2)th fiber core.

[0016] The beneficial effects of the present invention are as follows:

[0017] The present invention uses an FP resonant cavity composed of an active multi-core fiber grating to amplify or reflect light of a specific wavelength, thereby improving the measurement accuracy of weak seismic wave signals. By performing beat frequency detection on the single-frequency laser outputted by the centrally symmetrical core in the beat frequency sensing structure, the weak seismic wave signals can be effectively picked up and enhanced. At the same time, with the help of the space division multiplexing structure of the multi-core optical fiber, the azimuth reconstruction algorithm of the vibration vector sensing is realized, thereby realizing the accurate detection of parameters such as the azimuth, amplitude, and frequency of the seismic wave signal. Finally, the oil and gas distribution information of the deep geological formation is obtained through the inversion algorithm.

[0018] Compared with the prior art, the present invention changes the observed quantity from wavelength drift to frequency difference component, and obtains weak seismic signals through the frequency change of single-frequency laser beat frequency signal. It has the characteristics of high sensitivity and high resolution and can detect seismic wave vibration acceleration signals with μg-level resolution. Compared with the existing fiber grating technology that can only measure mg-level seismic wave vibration acceleration signals, the sensitivity is improved by 3 orders of magnitude. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural schematic diagram of the beat frequency sensing structure of the active multi-core fiber Bragg grating.

[0020] Figure 2 It is a geometric principle diagram of obtaining the azimuth angle of the vibration signal according to the present invention.

[0021] Among them: active multi-core optical fiber 1; first grating 2; second grating 3. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below in conjunction with the accompanying drawings and examples, but the present invention is not limited to the following embodiments.

[0023] exist Figure 1 In the embodiment, the beat frequency sensing structure based on active multi-core fiber grating includes a seven-core active multi-core optical fiber 1. In the active multi-core optical fiber 1, except for the central core, the first positions of the cores of more than 4 cores that are centrally symmetrically distributed are all engraved with a first grating 2, and the second positions are all engraved with a second grating 3. In this embodiment, 6 cores are selected for engraving. The reflectivities of the first grating 2 and the second grating 3 on the same core are different, forming an FP resonant cavity for amplifying or reflecting light of a specific wavelength, wherein the reflectivity of the first grating 2 is 90% to 99%, and the reflectivity of the second grating 3 is 60% to 70%. The distance between the first grating 2 and the second grating 3 is 5 cm, and can also be 1 cm, or 10 cm.

[0024] Since the FP resonant cavity on each fiber core amplifies or reflects light of a specific wavelength, each fiber core becomes a single-frequency laser channel. When the FP resonant cavity senses deformation due to the seismic wave signal, the laser passing through the fiber core changes, and two centrally symmetrical single-frequency laser channels can achieve beat frequency.

[0025] A two-dimensional vector sensing method using a beat frequency sensing structure uses a pump light source to provide energy for the beat frequency sensing structure. Each FP resonant cavity of the beat frequency sensing structure amplifies or reflects light of a specific wavelength to form a single-frequency laser channel, forming a total of more than 4 even-numbered single-frequency laser channels. This embodiment has 6 single-frequency laser channels. Each FP resonant cavity senses a seismic wave signal and produces deformation, and the single-frequency laser output changes. Two centrally symmetrical single-frequency laser channels are divided into a group, and at least two groups are arbitrarily selected. This embodiment selects two groups, and each group of single-frequency laser channels is beat to obtain a beat frequency signal. The beat frequency value is measured by a frequency counter, and the magnitude of the vibration signal is obtained by analyzing the frequency change of the beat frequency signal. The azimuth of the vibration signal is obtained by the geometric position relationship of the selected single-frequency laser channels, such as Figure 2 , as follows:

[0026] Step 1. Define the reference direction, number the fiber cores, and select the fiber cores with the smallest angle to the reference direction and the angle value within The fiber core within the range is defined as core No. 1, and the other fiber cores are numbered in turn in the counterclockwise direction, where the k-th fiber core and the k+(m / 2)-th fiber core are taken for beating, k∈[1,m / 2] and is an integer, m is the total number of fiber cores except the central fiber core, that is, the total number of side cores, m is an integer, the k+j-th fiber core and the k+j+(m / 2)-th fiber core are taken for beating, (k+j)∈[1,m / 2] and is an integer, j is an integer;

[0027] In this embodiment, k=2, m=6, j=1.

[0028] Step 2. Obtain the azimuth angle θ of the vibration signal according to the following formula: v , 0°≤θ v ≤180°;

[0029] θ v =θ k -β

[0030]

[0031] In the formula, θ k is the angle between the selected kth fiber core and the reference direction, Δv k-[k+(m / 2)] is the beat frequency of the kth fiber core and the k+(m / 2)th fiber core, π is 180°, Δv (k+j)-[k+j+(m / 2)] is the beat frequency value of the k+jth fiber core and the k+j+(m / 2)th fiber core.

Claims

1. A beat frequency sensing structure based on active multi-core fiber Bragg grating, characterized in that: The sensing structure includes an active multi-core optical fiber. In the active multi-core optical fiber, except for the central core, the first positions of the cores of more than 4 cores that are centrally symmetrically distributed are all engraved with a first grating, and the second positions are all engraved with a second grating. The reflectivities of the first grating and the second grating on the same core are different, forming an FP resonant cavity to amplify or reflect light of a specific wavelength.

2. The beat frequency sensing structure based on active multi-core fiber Bragg grating according to claim 1, characterized in that: The reflectivity of the first grating is 90% to 99%, and the reflectivity of the second grating is 60% to 70%.

3. The beat frequency sensing structure based on active multi-core fiber Bragg grating according to claim 1, characterized in that: The distance between the first grating and the second grating is 1-10 cm.

4. A two-dimensional vector sensing method using the beat frequency sensing structure as claimed in claim 1, characterized in that: A pump light source is used to provide energy for the beat frequency sensing structure as claimed in claim 1, and each FP resonant cavity of the beat frequency sensing structure amplifies or reflects light of a specific wavelength to form a single-frequency laser channel, forming a total of more than 4 and an even number of single-frequency laser channels; each FP resonant cavity senses the seismic wave signal and produces deformation, and the single-frequency laser output changes, and two centrally symmetrical single-frequency laser channels are divided into a group, and at least two groups are selected, and each group of single-frequency laser channels is beat to obtain a beat frequency signal, and the magnitude of the vibration signal is obtained by measuring the frequency change of the beat frequency signal, and the azimuth of the vibration signal is obtained through the geometric position relationship of the selected single-frequency laser channels.

5. The method according to claim 4, characterized in that: The azimuth angle of the vibration signal is determined by: Step 1. Define the reference direction, number the fiber cores, and select the fiber cores with the smallest angle to the reference direction and the angle value within The fiber core within the range is defined as core No. 1, and the other fiber cores are numbered in turn in the counterclockwise direction, where the k-th fiber core and the k+(m / 2)-th fiber core are taken for frequency beating, k∈[1,m / 2] and is an integer, m is the total number of fiber cores except the central fiber core, m is an integer, the k+j-th fiber core and the k+j+(m / 2)-th fiber core are taken for frequency beating, (k+j)∈[1,m / 2] and is an integer, j is an integer; Step 2. Obtain the azimuth angle θ of the vibration signal according to the following formula: v , 0°≤θ v ≤180°; i v =θ k -b In the formula, θ k is the angle between the selected kth fiber core and the reference direction, Δv k-[k+(m / 2)] is the beat frequency of the kth fiber core and the k+(m / 2)th fiber core, π is 180°, Δv (k+j)-[k+j+(m / 2)] is the beat frequency value of the k+jth fiber core and the k+j+(m / 2)th fiber core.

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