Spiral winding optical fiber distributed acoustic sensing surface wave forward modeling method

Abstract

The invention discloses a spiral winding optical fiber distributed sound sensing surface wave forward modeling simulation method, and relates to the technical field of geophysical exploration, and the method comprises the steps: S1, building a spiral winding optical fiber local coordinate system mapping operator, and mapping a spiral winding optical fiber from a global coordinate to a local coordinate; s2, constructing a mapping relation between the medium speed and the axial strain rate of the spiral winding optical fiber, and converting a mass point speed field into an axial strain rate field of the spiral winding optical fiber; s3, calculating a spirally wound optical fiber DAS surface wave record, and carrying out spirally wound optical fiber DAS surface wave forward modeling; s4, converting space coordinates of the spirally wound optical fiber DAS surface wave record, returning the space coordinates of each discrete equivalent scattering unit in the spirally wound optical fiber to a global coordinate system where a medium is located, realizing spirally wound optical fiber DAS surface wave simulation, and obtaining an influence rule of key parameters on characteristics of the spirally wound optical fiber DAS seismic record. And a theoretical foundation is laid for the application of the spirally wound optical fiber DAS in practical engineering.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of geophysical exploration, and particularly relates to a spiral-wound optical fiber distributed acoustic sensing surface wave forward simulation method. BACKGROUND

[0002] Distributed acoustic sensing (DAS) is a kind of vibration sensing technology taking optical fiber as a sensing medium and optical pulse as an information carrier, which is widely applied in natural earthquake monitoring, near-surface exploration and oil and gas resource exploration and other fields. The axial sensitivity of the optical fiber makes the conventional straight optical fiber DAS only obtain single-component seismic wave information along the optical fiber axial direction, and it is difficult to completely collect the complete vibration information at the position of the optical fiber. The spiral winding of the optical fiber around the axis can change the laying mode of the optical fiber, enhance the sensitivity of the DAS to the vibration signals of the medium in different directions, and increase the sensitivity of the optical fiber to the seismic wave signals with wider azimuth incidence. However, the contact mode of the optical fiber with the stratum is changed after the winding, so that the DAS optical fiber response is more complicatedly influenced by the parameters such as the pitch length and the winding angle. SUMMARY

[0003] The purpose of the application is to overcome the defects in the prior art. The spiral-wound optical fiber distributed acoustic sensing surface wave forward simulation method provided by the application obtains the influence law of the key parameters on the characteristics of the spiral-wound optical fiber DAS seismic record, and promotes the application of the spiral-wound optical fiber DAS in fine surface wave exploration.

[0004] To solve the above technical problems, the application adopts the following technical scheme: a spiral-wound optical fiber distributed acoustic sensing surface wave forward simulation method, comprising the following steps:

[0005] S1: mapping operator of spiral-wound optical fiber local coordinate system is established, and the spiral-wound optical fiber is mapped from the global coordinate to the local coordinate;

[0006] S2: a mapping relationship between the medium velocity and the axial strain rate of the spiral-wound optical fiber is constructed, and the particle velocity field is converted into the axial strain rate field of the spiral-wound optical fiber;

[0007] S3: spiral-wound optical fiber DAS surface wave record is calculated, and spiral-wound optical fiber DAS surface wave forward simulation is carried out;

[0008] S4: spiral-wound optical fiber DAS surface wave record spatial coordinate conversion is carried out, and the spatial coordinates of each discrete equivalent scattering unit in the spiral-wound optical fiber are returned to the global coordinate system in which the medium is located.

[0009] Preferably, in step S1, the initial winding point of the optical fiber is placed on the surface of the horizontal spiral-wound optical cable, and the coordinate mapping operator of the global coordinate system and the local coordinate system of the horizontally uniform spiral-wound optical fiber is expressed as:

[0010]

[0011] In the above formula, R i (i = 1, 2, 3) is the rotation matrix defined according to the right-hand screw rule, and T, N, and B are the tangential vector, normal vector, and binormal vector at a point on the helically wound fiber, respectively. The specific expression is as follows:

[0012]

[0013] θ is the rotation angle, which is the angle between the initial winding point and the actual winding point in the cross-section of the spirally wound optical cable relative to the axis.

[0014] Preferably, in step S1, it is specifically expressed as follows:

[0015]

[0016] In the above formula, r is the winding radius, α is the winding angle, s is the arc length of the optical fiber starting from the winding start point, the superscript T indicates transpose, and the vectors T, N, and B together constitute the local coordinates (T, N, B) of the point on the helically wound optical fiber.

[0017] Preferably, in step S2, the medium velocity field is mapped from the global coordinate system to the local coordinate system using the aforementioned local coordinate system mapping operator for helically wound optical fiber with an arbitrary rotation angle θ. The medium strain rate field is obtained through the medium velocity-strain rate relationship, and an expression for the optical fiber strain rate under the local coordinate system of the helically wound optical fiber is established to calculate the medium strain rate e. l :

[0018] e l =Γ·e·Γ T (4)

[0019] In the above equation, e represents the six-component medium strain rate tensor obtained using the three-component particle velocity field of an isotropic elastic wave:

[0020]

[0021] In the above formula, Represents the strain rate field of the medium with different components, where v x v y v z Let x, y, and z represent the particle velocity fields of the elastic wave, respectively. Representing the spatial partial derivative operator, based on the axial vibration sensitivity of optical fibers, by setting vectors N and B in the operator Γ to zero, the axial strain rate e of the helically wound optical fiber is obtained. h :

[0022] e h =a·e; (6)

[0023] In the above formula, 'a' represents the mapping coefficient tensor of the helical wound fiber.

[0024] Preferably, the expression for the mapping coefficient tensor of the helically wound fiber is:

[0025]

[0026] The specific coefficients are expressed as follows:

[0027]

[0028] A mapping relationship between the medium velocity and the axial strain rate of the helical wound fiber is constructed, and the particle velocity field is converted into the axial strain rate field of the helical wound fiber.

[0029] Preferably, in step S3, formula (8) is rewritten as a linear matrix equation, and the DAS gauge length response is introduced:

[0030] d = AGM; (9)

[0031] In the above formula, This represents the diagonal matrix formed by the gauge length response operators. Δl is the DAS channel spacing, L G Represents gauge length, vector The matrix G represents the six-component medium strain rate of the channel in the i-th seismic record, where G = diag[G1...G i [ ] is a diagonal matrix composed of the mapping coefficients of a helically wound optical fiber, and its submatrices It consists of the coefficients corresponding to the i-th channel of the wound optical fiber.

[0032] Preferably, in step S4, based on the geometric relationship between the wound optical fiber and the helical wound optical cable in the global coordinate system, the spatial coordinates of each discrete equivalent scattering unit in the helical wound optical fiber are normalized to the global coordinate system of the medium. The spatial coordinate normalization relationship is as follows:

[0033] x f =s·sinα; (10)

[0034] In the above formula, x f The horizontal distance of the DAS surface wave seismic record after the spiral-wound fiber optic cable has been repositioned.

[0035] Beneficial Effects: The present invention discloses a forward modeling method for distributed acoustic sensing surface waves in helically wound optical fibers. By establishing a mapping relationship between the global coordinate system and the local coordinate system of the helically wound optical fiber, the helically wound optical fiber is mapped from the global coordinate system to the local coordinate system. Furthermore, a mapping relationship between the medium velocity and the axial strain rate of the helically wound optical fiber is constructed, converting the particle velocity field into the axial strain rate field of the helically wound optical fiber. The surface wave record of the helically wound optical fiber is calculated, and forward modeling of the surface wave of the helically wound optical fiber is carried out, thus forming a forward modeling method for the surface wave of the helically wound optical fiber. This provides theoretical support for examining the influence of the winding angle on the surface wave characteristics of the helically wound optical fiber and lays the foundation for the inversion and interpretation of the helically wound optical fiber DAS data. Attached Figure Description

[0036] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0037] In the attached diagram:

[0038] Figure 1 This is a flowchart of the spiral-wound fiber optic distributed acoustic sensing surface wave forward modeling method of the present invention;

[0039] Figure 2 This is a schematic diagram of the multi-core spiral wound optical fiber of the present invention;

[0040] Figure 3 This is a schematic diagram of three different spiral wound optical fibers with different winding angles and rotation angles according to the present invention.

[0041] Figure 4 This is a schematic diagram of the medium velocity and DAS strain rate of the helically wound optical fiber of the present invention;

[0042] Figure 5 The Rayleigh wave dispersion map and Love wave dispersion map of this invention. Detailed Implementation

[0043] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following text is only used to describe an implementation method of a spiral wound fiber distributed acoustic sensing surface wave forward modeling method of the present invention, and does not strictly limit the scope of protection specifically claimed by the present invention.

[0044] Furthermore, the technical solutions of the various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0045] Example 1: A forward modeling method for surface wave acoustic sensing in a helically wound fiber optic distributed acoustic sensor, such as... Figure 1 As shown, it includes the following steps:

[0046] S1: Establish a local coordinate system mapping operator for helical wound optical fiber to map the helical wound optical fiber from global coordinates to local coordinates;

[0047] S2: Construct a mapping relationship between the medium velocity and the axial strain rate of the spirally wound fiber, and convert the particle velocity field into the axial strain rate field of the spirally wound fiber.

[0048] S3: Calculate the DAS surface wave record of helically wound fiber and carry out forward modeling of helically wound fiber DAS surface wave;

[0049] S4: Spatial coordinate transformation of DAS surface wave recording in helical wound fiber, which normalizes the spatial coordinates of each discrete equivalent scattering unit in the helical wound fiber to the global coordinate system of the medium.

[0050] In Embodiment 1, in step S1, the initial winding point of the optical fiber is placed on the surface of the horizontally spirally wound optical cable. The coordinate mapping operator between the global coordinate system and the local coordinate system of the horizontally uniformly spirally wound optical fiber is expressed as:

[0051]

[0052] In the above formula, R i (i = 1, 2, 3) is the rotation matrix defined according to the right-hand screw rule, and T, N, and B are the tangential vector, normal vector, and binormal vector at a point on the helically wound fiber, respectively. The specific expression is as follows:

[0053]

[0054] In the above formula, θ is the rotation angle, which is the angle between the initial winding point and the actual winding point in the cross-section of the spirally wound optical cable relative to the axis.

[0055] In Embodiment 1, step S1 is specifically represented as follows:

[0056]

[0057] In the above formula, r is the winding radius, α is the winding angle, s is the arc length of the optical fiber starting from the winding start point, the superscript T indicates transpose, and the vectors T, N, and B together constitute the local coordinates (T, N, B) of the point on the helically wound optical fiber.

[0058] In Example 1, in step S2, the medium velocity field is mapped from the global coordinate system to the local coordinate system using the aforementioned local coordinate system mapping operator for helically wound optical fiber with an arbitrary rotation angle θ. The medium strain rate field is obtained through the medium velocity-strain rate relationship, and an expression for the fiber strain rate under the local coordinate system of the helically wound optical fiber is established to calculate the medium strain rate e. l :

[0059] e l =Γ·e·Γ T (4)

[0060] In the above equation, e represents the six-component medium strain rate tensor obtained using the three-component particle velocity field of an isotropic elastic wave:

[0061]

[0062] In the above formula, Represents the strain rate field of the medium with different components, where v x v y v z Let x, y, and z represent the particle velocity fields of the elastic wave, respectively. Representing the spatial partial derivative operator, based on the axial vibration sensitivity of optical fibers, by setting vectors N and B in the operator Γ to zero, the axial strain rate e of the helically wound optical fiber is obtained. h :

[0063] e h =a·e; (6)

[0064] In the above formula, 'a' represents the mapping coefficient tensor of the helical wound fiber.

[0065] In Example 1, the expression for the mapping coefficient tensor of the helical wound fiber is:

[0066]

[0067] The specific coefficients are expressed as follows:

[0068]

[0069] A mapping relationship between the medium velocity and the axial strain rate of the helical wound fiber is constructed, and the particle velocity field is converted into the axial strain rate field of the helical wound fiber.

[0070] In Example 1, in step S3, formula (8) is rewritten as a linear matrix equation, and the DAS gauge length response is introduced:

[0071] d = AGM; (9)

[0072] In the above formula, This represents the diagonal matrix formed by the gauge length response operators. Δl is the DAS channel spacing, L G Represents gauge length, vector The matrix G represents the six-component medium strain rate of the channel in the i-th seismic record, where G = diag[G1...G i [ ] is a diagonal matrix composed of the mapping coefficients of a helically wound optical fiber, and its submatrices It consists of the coefficients corresponding to the i-th channel of the wound optical fiber.

[0073] In Example 1, in step S4, based on the geometric relationship between the wound optical fiber and the helical wound optical cable in the global coordinate system, the spatial coordinates of each discrete equivalent scattering unit in the helical wound optical fiber are normalized to the global coordinate system where the medium is located. The spatial coordinate normalization relationship is as follows:

[0074] x f =s·sinα; (10)

[0075] In the above formula, x f The horizontal distance of the DAS surface wave seismic record after the spiral-wound fiber optic cable has been repositioned.

[0076] Example 2: Using the helical wound fiber distributed acoustic sensing surface wave forward modeling simulation method provided by this invention, forward modeling simulation of the strain rate field of helical wound fiber DAS is carried out, such as... Figure 2 As shown, a uniformly wound optical fiber is installed, wherein... Figure 2 a is a schematic diagram of the spiral optical fiber winding in global coordinates. Figure 2 b is a schematic diagram of the winding radius of the helical optical fiber. Figure 2 c represents the arc length of the optical fiber starting from the winding initiation point, and s represents the arc length of the optical fiber. (See diagram). Figure 2 As shown in Figure a, the black dots represent the starting points of the optical fiber winding, located at the origin of the global coordinate system. Rotation angles θ of 0° and 180° correspond to winding angles α of 20° and 70°, respectively. Figure 2 As shown in b, the spiral fiber has a winding radius r of 0.01m and a gauge length L of 2m.

[0077] In Example 2, taking a two-layer horizontal layered medium as an example, the thickness of the first layer is 5m, and the longitudinal wave velocity is v. p = 663.3 m / s, shear wave velocity v s =200m / s, density ρ=1800kg / m³ 3 The second layer is a uniform half-space medium with a longitudinal wave velocity v. p =1224.7 m / s, shear wave velocity v s =500m / s, density ρ =1900kg / m³ 3 ,like Figure 3As shown, spiral-wound optical fibers with different winding angles and rotation angles are configured, where... Figure 3 In the optical fiber a, the rotation angle θ is 0° and the winding angle α is 20°; Figure 3 In b, the rotation angle θ of the optical fiber is 0° and the winding angle α is 70°; Figure 3 In c, the optical fiber has a rotation angle θ of 90° and a winding angle α of 70°.

[0078] In Example 2, as Figure 4 a, Figure 4 b and Figure 4 c shows the three-component particle velocity records of a two-layer horizontal layered medium. x v y and v z surface wave records, Figure 4 d represents the surface wave record of the DAS strain rate of a helically wound optical fiber with θ = 0° and α = 20°. Figure 4 e represents a surface wave record of the DAS strain rate of a helically wound optical fiber with θ = 0° and α = 70°. Figure 4 Surface wave recording of DAS strain rate of helically wound optical fiber with f = 180° and α = 20°. Figure 4 The black curve in (df) represents the sum of the mapping coefficients ∑a of the corresponding spiral-wound optical fiber, as shown in Figure e. Figure 4 The single red rectangle in f indicates half a cycle, when the rotation angle θ = 0° and α is 20° and 70° respectively, as shown below. Figure 4 d and Figure 4 As shown in e, the DAS surface wave records of helical wound fiber all exhibit obvious spatial periodic variations, which is consistent with the periodicity of the mapping coefficient ∑a of the helical wound fiber.

[0079] In Example 2, as Figure 5 As shown, Figure 5 a is the radial component of the medium velocity v x The surface wave dispersion spectrum, Figure 5 b is the tangential component of the medium velocity v y The surface wave dispersion spectrum, Figure 5 c is the vertical component of the medium velocity v. z The surface wave dispersion spectrum, Figure 4 d represents the dispersion spectrum of the DAS strain rate of the helically wound optical fiber with θ = 0° and α = 20°. Figure 4 e is the dispersion spectrum of the DAS strain rate of a helically wound optical fiber with θ = 0° and α = 70°. Figure 4 f is the dispersion spectrum of the strain rate of a helically wound fiber DAS with θ = 180° and α = 20°. The surface wave of the helically wound fiber DAS mainly consists of vertical components Rayleigh waves and Love waves. Figure 5 The white dotted line in the image represents the Rayleigh wave dispersion curve. Figure 5The black dotted line in the image represents the Love wave dispersion curve. Using theoretical Rayleigh wave dispersion curves and Love wave dispersion curves, such as... Figure 5 As shown, the fundamental mode energy of the dispersion spectrum contains significant Love wave information, while the higher-order mode energy mainly contains Rayleigh wave information. With increasing angle, i.e., at α = 70°, the surface wave of the helically wound fiber DAS is mainly composed of the horizontal component Rayleigh wave, and its dispersion spectrum is similar to that of the horizontal component Rayleigh wave. However, due to the notch effect of the gauge length, energy drops occur in some frequency bands of the dispersion spectrum. According to... Figure 5 As shown in the theoretical Rayleigh wave dispersion curve, the dispersion spectrum of the surface wave of the helically wound fiber DAS mainly contains horizontal Rayleigh wave information. When α = 70°, as... Figure 4 e and Figure 4 As shown in f, the corresponding spiral-wound fiber mapping coefficients ∑ are given when θ is 0° and 180°, respectively. a The period remains unchanged, differing by only half a period. The two curves respectively correspond to the spatial periodicity of the DAS surface wave recording in the corresponding helically wound fiber, such as... Figure 5 e and Figure 5 As shown in f, the characteristics of the surface wave dispersion spectra of helically wound fiber DAS corresponding to θ=0° and θ=180° are almost identical, that is, both dispersion spectra are similar to the horizontal component Rayleigh wave dispersion spectra. According to the forward modeling simulation method of helically wound fiber distributed acoustic sensing surface wave disclosed in this invention, the influence of fiber winding angle and rotation angle on the surface wave characteristics of DAS and the influence law on the characteristics of helically wound fiber DAS seismic records can be simulated, laying the foundation for the inversion and interpretation of helically wound fiber DAS data.

[0080] The above description only describes the present invention and its embodiments. This description is not restrictive. Those skilled in the art will realize that the embodiments described herein are to help readers understand the principles of the present invention and should be understood as not limiting the scope of protection of the present invention to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in the present invention without departing from the essence of the present invention, and these modifications and combinations are still within the scope of protection of the present invention.

Claims

1. A method for forward modeling of surface waves in a helically wound fiber-optic distributed acoustic sensing system, characterized in that, Includes the following steps: S1: Establish a local coordinate system mapping operator for helical wound optical fiber to map the helical wound optical fiber from global coordinates to local coordinates; S2: Construct a mapping relationship between the medium velocity and the axial strain rate of the spirally wound fiber, and convert the particle velocity field into the axial strain rate field of the spirally wound fiber. S3: Calculate the DAS surface wave record of helically wound fiber and carry out forward modeling of helically wound fiber DAS surface wave; S4: Spatial coordinate transformation of DAS surface wave recording in helical wound fiber, which normalizes the spatial coordinates of each discrete equivalent scattering unit in the helical wound fiber to the global coordinate system of the medium.

2. The method for forward modeling of surface waves in a helically wound optical fiber distributed acoustic sensing system according to claim 1, characterized in that: In step S1, the initial winding point of the optical fiber is placed on the surface of the horizontally spirally wound optical cable. The coordinate mapping operator between the global coordinate system and the local coordinate system of the horizontally uniformly spirally wound optical fiber is expressed as: In the above formula, R i (i = 1, 2, 3) is the rotation matrix defined according to the right-hand screw rule, and T, N, and B are the tangential vector, normal vector, and binormal vector at a point on the helically wound fiber, respectively. The specific expression is as follows: In the above formula, θ is the rotation angle, which is the angle between the initial winding point and the actual winding point in the cross-section of the spirally wound optical cable relative to the axis.

3. The method for forward modeling of surface waves in a helically wound fiber optic distributed acoustic sensing system according to claim 2, characterized in that: In step S1, specifically, it is represented as follows: In the above formula, r is the winding radius, α is the winding angle, s is the arc length of the optical fiber starting from the winding start point, the superscript T indicates transpose, and the vectors T, N, and B together constitute the local coordinates (T, N, B) of the point on the helically wound optical fiber.

4. The method for forward modeling of surface waves in a helically wound optical fiber distributed acoustic sensing system according to claim 3, characterized in that: In step S2, the medium velocity field is mapped from the global coordinate system to the local coordinate system using the aforementioned local coordinate system mapping operator for helically wound optical fiber with arbitrary rotation angle θ. The medium strain rate field is obtained through the medium velocity-strain rate relationship, and the fiber strain rate expression under the local coordinate system of the helically wound optical fiber is established to calculate the medium strain rate e. l : e l =C·e·C T (4) In the above equation, e represents the six-component medium strain rate tensor obtained using the three-component particle velocity field of an isotropic elastic wave: In the above formula, Represents the strain rate field of the medium with different components, where v x v y v z Let x, y, and z represent the particle velocity fields of the elastic wave, respectively. Representing the spatial partial derivative operator, based on the axial vibration sensitivity of optical fibers, by setting vectors N and B in the operator Γ to zero, the axial strain rate e of the helically wound optical fiber is obtained. h : And h =a·e; (6) In the above formula, 'a' represents the mapping coefficient tensor of the helical wound fiber.

5. The method for forward modeling of surface waves in a helically wound optical fiber distributed acoustic sensing system according to claim 4, characterized in that: The expression for the mapping coefficient tensor of helical wound fiber is: The specific coefficients are expressed as follows: A mapping relationship between the medium velocity and the axial strain rate of the helical wound fiber is constructed, and the particle velocity field is converted into the axial strain rate field of the helical wound fiber.

6. The method for forward modeling of surface waves in a helically wound optical fiber distributed acoustic sensing system according to claim 5, characterized in that: In step S3, equation (8) is rewritten as a linear matrix equation, and the DAS gauge length response is introduced: d = AGm; (9) In the above formula, This represents the diagonal matrix formed by the gauge length response operators. Δl is the DAS channel spacing, L G Represents gauge length, vector The matrix G represents the six-component medium strain rate of the channel in the i-th seismic record, where G = diag[G1...G i [ ] is a diagonal matrix composed of the mapping coefficients of a helically wound optical fiber, and its submatrices It consists of the coefficients corresponding to the i-th channel of the wound optical fiber.

7. The method for forward modeling of surface waves in a helically wound optical fiber distributed acoustic sensing system according to claim 6, characterized in that: In step S4, based on the geometric relationship between the wound optical fiber and the helical wound optical cable in the global coordinate system, the spatial coordinates of each discrete equivalent scattering unit in the helical wound optical fiber are normalized to the global coordinate system of the medium. The spatial coordinate normalization relationship is as follows: x f =s·sinα; (10) In the above formula, x f The horizontal distance of the DAS surface wave seismic record after the spiral-wound fiber optic cable has been repositioned.

Images