A portable gravity gradient measurement unit and its measurement method

Abstract

This invention discloses a portable gravity gradient measurement unit and its measurement method. The measurement unit includes an optical detection structure, an elastic sheet, and its supporting structure. The optical detection structure includes two fiber optic loops of length L. A fiber optic loop is disposed on the front and back surfaces of the elastic sheet, with the loop on the front surface and the loop on the back surface being concentric. A mass block is embedded in the center of the elastic sheet, with the mass on the front side being the same as the mass on the back side. The mass block generates gravitational acceleration under the influence of Earth's gravitational field, thus generating stress on the elastic sheet. The fiber optic loops receive optical signals of the same frequency from a light source, and under the stress transmitted from the elastic sheet, they undergo stretching and refractive index changes, thereby causing phase changes in the optical signals in the two fiber optic loops and generating interference signals that are output to the photodetector of the optical detection structure. The photodetector converts the received optical signals into electrical signals and sends them to the central control unit to calculate the gravity gradient.

Description

Technical Field

[0001] This invention relates to a portable gravity gradiometer, and more particularly to a portable gravity gradient measurement unit and its measurement method based on fiber optic sensing. It can be used for measuring the Earth's gravity gradient field in space, air, land, and underwater, and is also applicable to fields such as oil and gas exploration, mineral exploration, cartography, environmental monitoring, and military applications, as well as research in structural geology, hydrogeology, seismology, geodesy, and space science. Background Technology

[0002] The gravitational gradient is the rate of change of gravitational acceleration in space, used to describe changes in the strength and direction of the gravitational field. Generally, a gravitational field is formed by nearby objects in space, and its strength and direction are influenced by factors such as the Earth's shape, material distribution, and its motion. In fields such as geophysical exploration, geological disaster early warning, Earth sphere inversion, and geodynamics research, measuring changes in the gravitational gradient can provide information such as the density and structural characteristics of underground and surface materials, which is of great significance for understanding geophysical laws, predicting geological disasters, and exploring mineral resources.

[0003] In fields such as geophysical exploration, seismic exploration, and resource exploration, it is often necessary to detect complex and uncertain geological structures, which are often located outside of easily measurable areas. Therefore, the portability of gravity gradiometers is one of their most significant advantages.

[0004] Currently, the main methods for implementing gravity gradiometers include: gravity gradiometers based on rotational accelerometers, gravity gradiometers based on free fall, and gravity gradiometers based on angular accelerometers.

[0005] Gravity gradiometers based on rotating accelerometers are currently the only commercially available type. These gravimeters require multiple accelerometers for measurement and a turntable to apply uniform rotational modulation to pairs of accelerometers placed in the same plane, making them inconvenient for field exploration. Furthermore, the use of eight or even twelve accelerometers in rotating accelerometer-based gravity gradiometers leads to significant accelerometer inconsistency issues, hindering further improvements in accuracy.

[0006] Gravity gradiometers based on free fall employ a simple concept: after measuring the gravitational acceleration at two points, dividing by the distance between the two points yields the rate of change of gravitational acceleration in space, i.e., the gravitational gradient. Currently, gravity gradiometers developed based on this principle mainly utilize two major technical methods: laser interferometry and atomic interferometry. The development of laser interferometry and atomic interferometry gravity gradiometers requires vacuum equipment to achieve a vacuum environment and additional vibration isolation devices to reduce the impact of ground vibrations, thus making miniaturization difficult.

[0007] Gravity gradiometers based on angular accelerometers employ superconducting quantum interference devices (SQUs) to sense changes in superconducting current caused by the displacement of the test mass. Moody et al. at the University of Maryland designed a triaxial superconducting gravity gradiometer consisting of six linear accelerometers and three angular accelerometers. It utilizes quantized flux and the Meissner effect to provide stable test mass levitation and signal coupling, while the SQU provides extremely low-noise amplification of the signal. However, no practical superconducting gravity gradiometer with performance comparable to rotating accelerometer-based gravity gradiometers has yet been developed domestically or internationally. Furthermore, miniaturization still faces many critical technical bottlenecks due to the need for cryogenic environmental control equipment to achieve the superconducting environment. Summary of the Invention

[0008] To address the technical problems existing in the prior art, the present invention aims to provide a portable gravity gradient measurement unit and its measurement method. This invention is a gravity gradiometer based on an elastic sheet-mass block structure, the principle of which is as follows: Figure 1 As shown, the normal direction of the elastic sheet is parallel to the direction of the gravitational gradient to be measured. A mass block is embedded in the center of the elastic sheet, with the same mass on both sides. An optical fiber is fixed in the elastic sheet region with adhesive. The mass block is subjected to gravitational acceleration by the Earth's gravitational field, which generates stress on the elastic sheet. This stress is transmitted to the optical fiber, causing stretching and a change in the refractive index, resulting in a phase change in the optical signal. By detecting this phase change, the gravitational acceleration at that point can be obtained.

[0009] Specifically, sensing optical fibers are wound along the circumference of the elastic sheet on both the front and back sides, starting from the center and moving outwards, forming two independent sensing fiber loops. The fibers in different layers (on different circumferences) are closely spaced. The strain exerted by the mass block on the elastic sheet is transmitted to the optical fibers. The sensing fibers are fixed to the elastic sheet by adhesive. Let the length of the two segments of sensing optical fibers bonded to the elastic sheet be L, the refractive index of the fiber be n, and the light propagation coefficient in the fiber be... Where λ is the wavelength of the light wave, the phase change of the optical signal as it passes through the sensing fiber on the front or back is:

[0010] φ=nkL (1)

[0011] Taking the total differential of this equation, we get:

[0012]

[0013] The first term represents the phase shift caused by the axial elongation of the fiber, the second by the stress-strain effect (stress causing a change in the fiber's refractive index), and the third by the Poisson effect (change in fiber diameter). Since the Poisson effect is negligible compared to the first two effects, we have...

[0014]

[0015] in:

[0016]

[0017] Where: ε is the fiber strain, v f p represents the Poisson's ratio of the optical fiber. 11 p 12 is the photoelastic coefficient of the optical fiber.

[0018] According to the definition of strain, when ΔL is very small, we have thereby:

[0019]

[0020] remember The original expression can be simplified to:

[0021]

[0022] When the optical fiber is bonded to the linear stress region of the elastic sheet, ΔL is proportional to the gravitational acceleration g, that is:

[0023] ΔL=Sg (7)

[0024] Where S represents the linear scaling factor between gravitational acceleration and fiber strain.

[0025] thereby:

[0026]

[0027] Factors can be evaluated on a standard vibration table. The calibration is performed to obtain the numerical relationship between gravitational acceleration g and the observed value Δφ. The observed value Δφ can be obtained through... Figure 7 , Figure 8 , Figure 9 , Figure 10 The Michelson interferometer or Mach-Zehnder interferometer structure shown is used for measurement. After the observed value Δφ is calculated, the gravitational acceleration g can be obtained through formula (8).

[0028] Suppose we need to measure the gravitational gradient in the z-direction. We introduce two parallel measurement units at coordinates z1 and z2 in the z-direction. This allows us to obtain the gravitational accelerations g1 and g2 at coordinates z1 and z2. The gravitational gradient can then be obtained using the following formula:

[0029]

[0030] The novel fiber optic gravity gradiometer proposed in this invention is lightweight and portable, primarily composed of lightweight components such as fiber optic paths and elastic sheets. Compared to traditional, bulky gravity gradiometers, it can be easily mounted on various platforms, such as aircraft, satellites, and drones. Furthermore, this gravity gradiometer employs fiber optic transmission and weak light signal detection technology, resulting in low power consumption. It can be powered by batteries or solar energy, eliminating the need for a high-power power supply. In summary, the fiber optic gravity gradiometer proposed in this invention offers excellent portability, is suitable for gravity gradient measurement needs in various situations, and has significant application value.

[0031] The technical solution of this invention is as follows:

[0032] A portable gravity gradient measurement unit, characterized in that it includes an optical detection structure, an elastic sheet and its support structure;

[0033] The optical detection structure includes two fiber optic loops of length L, referred to as the first sensing fiber optic loop and the second sensing fiber optic loop.

[0034] The elastic sheet has an optical fiber ring on its front and back sides, and the optical fiber ring on the front side is concentric with the optical fiber ring on the back side; a mass block is embedded in the center of the elastic sheet, and the mass block has the same mass on the front side and the same mass on the back side.

[0035] The mass block is used to generate gravitational acceleration when subjected to the Earth's gravitational field, thereby generating stress on the elastic sheet;

[0036] The fiber optic ring is used to receive the same frequency optical signal input from the light source of the optical detection structure, and to generate tension and refractive index change under the stress transmitted by the elastic sheet, thereby causing phase change of the optical signals in the two fiber optic rings and generating an interference signal output to the photodetector of the optical detection structure; the photodetector is used to convert the received optical signal into an electrical signal and send it to the central control unit, which is used to calculate the gravity gradient Γ based on the electrical signal.

[0037] Furthermore, the central control unit first calculates the gravitational acceleration based on the electrical signal. Then, the gravitational gradient Γ along the direction of the gravitational gradient to be measured is calculated based on the gravitational acceleration g at different positions along the direction of the gravitational gradient to be measured; where, n is the refractive index of the optical fiber. v f p represents the Poisson's ratio of the optical fiber. 11 p 12 φ is the photoelastic coefficient of the optical fiber, φ is the phase change of the optical fiber loop, and ΔL is the change in length of the optical fiber loop.

[0038] Furthermore, the optical detection structure is an optical detection structure based on a Michelson interferometer, including a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing fiber optic ring 4, a first Faraday rotator mirror 5, a second sensing fiber optic ring 6, a second Faraday rotator mirror 7, and a first photodetector 8. The light emitted by the light source 1 passes through the first coupling unit 2 and the second coupling unit 3 in sequence and is split into two beams: one beam passes through the first sensing fiber optic ring 4 and enters the first Faraday rotator mirror 5, is reflected back to the first sensing fiber optic ring 4 by the first Faraday rotator mirror 5, and returns to the second coupling unit 3; the other beam passes through the second sensing fiber optic ring 6 and enters the second Faraday rotator mirror 7, is reflected back to the second sensing fiber optic ring 6 by the second Faraday rotator mirror 7, and returns to the second coupling unit 3; the two beams of light returning to the second coupling unit 3 interfere and enter the first photodetector 8 after passing through the first coupling unit 2.

[0039] Furthermore, the optical detection structure is a Michelson interferometer optical detection structure based on dual-wavelength compensation, including a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing fiber ring 4, a first Faraday rotator 5, a second sensing fiber ring 6, a second Faraday rotator 7, a first photodetector 8, a polarization beam splitter / combiner 9, and a second photodetector 10. The dual-wavelength light emitted by the light source 1 passes through the first coupling unit 2 and the second coupling unit 3 in sequence and is split into two beams: one beam passes through the first sensing fiber ring 4, enters the first Faraday rotator 5, is reflected back to the first sensing fiber ring 4, and returns to the second coupling unit 3; the other beam passes through the second sensing fiber ring 6, enters the second Faraday rotator 7, is reflected back to the second sensing fiber ring 6, and returns to the second coupling unit 3. The two beams returning to the second coupling unit 3 interfere and then enter the polarization beam splitter / combiner 9 after passing through the first coupling unit 2, splitting into fast-axis light and slow-axis light, which are respectively input to the first photodetector 8 and the second photodetector 10.

[0040] Furthermore, the optical detection structure is an optical detection structure based on a Mach-Zehnder interferometer, including a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing fiber ring 4, a second sensing fiber ring 6, and a first photodetector 8; the light emitted by the light source 1 is split into two beams through the first coupling unit 2: one beam enters the second coupling unit 3 through the first sensing fiber ring 4, and the other beam enters the second coupling unit 3 through the second sensing fiber ring 6; the two beams of light merge and interfere in the second coupling unit 3 before entering the first photodetector 8.

[0041] Furthermore, the optical detection structure is a Mach-Zehnder interferometer optical detection structure based on dual-wavelength light source compensation, including a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing fiber ring 4, a second sensing fiber ring 6, a first photodetector 8, a polarization beam splitter / combiner 9, and a second photodetector 10. The dual-wavelength light emitted by the light source 1 is split into two beams by the first coupling unit 2: one beam is input to the second coupling unit 3 via the first sensing fiber ring 4, and the other beam is input to the second coupling unit 3 via the second sensing fiber ring 6. The two beams of light converge and interfere in the second coupling unit 3 before entering the polarization beam splitter / combiner 9, where they are split into fast-axis light and slow-axis light, which are detected by the first photodetector 8 and the second photodetector 10, respectively.

[0042] Furthermore, the first sensing fiber ring 4 uses a long-wavelength fiber grating, and the second sensing fiber ring 6 uses a short-wavelength fiber grating; or the first sensing fiber ring 4 uses a short-wavelength fiber grating, and the second sensing fiber ring 6 uses a long-wavelength fiber grating.

[0043] A gravity gradient measurement method based on the portable gravity gradient measurement unit includes the following steps:

[0044] 1) Introduce one of the portable gravity gradient measurement units at coordinates z1 and z2 in the measurement direction z;

[0045] 2) The gravitational acceleration g1 at coordinate z1 is calculated using the signal output by the portable gravity gradient measurement unit at coordinate z1; the gravitational acceleration g2 at coordinate z2 is calculated using the signal output by the portable gravity gradient measurement unit at coordinate z2.

[0046] 3) According to the formula The gravitational gradient Γ in the z-direction is calculated.

[0047] The novel gravity gradiometer proposed in this invention is portable, and the accuracy of gravity gradient detection can be effectively improved by using multiple gravity gradiometers in an array. Specifically, the placement and configuration of the gravity gradiometers are determined according to the specific measurement area. The array can be arranged linearly, in a grid pattern, or in other suitable forms. (See appendix) Figure 2 A schematic diagram of a grid-like array of gravity gradiometers is shown, where each cube represents a gravity gradiometer. The gravity gradiometers are installed according to the array configuration, activated, and begin data acquisition. Within the array area, gravity gradient values ​​are recorded at each location according to the predetermined array configuration. Data from multiple gravity gradiometers is transmitted to a central control unit, where the collected gravity gradient data is processed and analyzed to obtain information on changes in subsurface density.

[0048] The advantages of this invention are as follows:

[0049] This invention proposes a novel gravity gradiometer. Compared to existing commercially available rotating accelerometer-type gravity gradiometers, this invention eliminates the need for peripherals such as turntables, which are difficult to miniaturize, thus offering a miniaturization advantage. Furthermore, it eliminates the need for eight or even twelve accelerometers as measurement units, reducing inconsistencies caused by manufacturing errors and other factors. Compared to gravity gradiometers based on free fall and those based on angular accelerometers, this invention, employing a fiber optic sensing scheme, features small size, light weight, resistance to electromagnetic interference, and chemical stability. It offers miniaturization advantages and better environmental adaptability, eliminating the need for large environmental control peripherals or large power supply equipment to ensure their operation, thus providing portability. Attached Figure Description

[0050] Figure 1 This is a flowchart illustrating the principle of a gravity gradient meter based on an elastic sheet-mass block structure.

[0051] Figure 2 This is a schematic diagram of a gravity gradiometer array, where a cube represents one gravity gradiometer, and data from multiple gravity gradiometers is transmitted to the central control unit.

[0052] Figure 3 This is a structural diagram of a portable gravity gradiometer, consisting of a mass block, optical fiber, elastic sheet, and an outer support structure; however, the outer support structure is not shown.

[0053] Figure 4 This is a top view of a portable gravity gradiometer.

[0054] Figure 5 This is a front view of a portable gravity gradiometer; the outer support structure is not shown.

[0055] Figure 6 This is a side view of a portable gravity gradiometer; the outer support structure is not shown.

[0056] Figure 7 This is an optical detection structure based on a Michelson interferometer.

[0057] Figure 8 This is an optical detection structure for a Michelson interferometer based on dual-wavelength light source compensation.

[0058] Figure 9 This is an optical detection structure based on a Mach-Zehnder interferometer.

[0059] Figure 10 This is an optical detection structure for a Mach-Zehnder interferometer based on dual-wavelength light source compensation.

[0060] Among them, 1-light source, 2-first coupling unit, 3-second coupling unit, 4-first sensing fiber ring, 5-first Faraday rotator, 6-second sensing fiber ring, 7-second Faraday rotator, 8-first photodetector, 9-polarization beam splitter / combiner, 10-second photodetector. Detailed Implementation

[0061] The present invention will now be described in further detail with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0062] As attached Figure 3 To be continued Figure 6 As shown, polarization-maintaining fibers are wound along the circumference of a beryllium copper elastic sheet on both the front and back sides, from the center outwards. Fibers in different layers (on different circumferences) are tightly packed together. A high-density mass block (such as a tungsten-nickel-copper alloy) is embedded in the center of the elastic sheet, ensuring its mass is the same on both sides. The fibers are then fixed in the elastic sheet area using adhesives such as UV fiber adhesive. The elastic sheet is supported by a cylindrical structure. To increase sensitivity, the polarization-maintaining fiber can be replaced with a fiber Bragg grating.

[0063] For optical detection structures, one of the following solutions can be adopted:

[0064] 1. Use attachment Figure 7 An optical detection structure based on a Michelson interferometer is described. Light emitted from light source 1 passes sequentially through a first coupling unit 2 and a second coupling unit 3, then splits into two beams: one beam passes through a first sensing fiber loop 4 bonded to one side of an elastic sheet, is reflected at a first Faraday rotator mirror 5, returns to the first sensing fiber loop 4, and then returns to the second coupling unit 3; the other beam passes through a second sensing fiber loop 6 bonded to the other side of the elastic sheet, is reflected at a second Faraday rotator mirror 7, returns to the second sensing fiber loop 6, and then returns to the second coupling unit 3. The two beams returning to the second coupling unit 3 interfere, and the interfering light reaches the first coupling unit 2 and enters the first photodetector 8. The first coupling unit 2 can be implemented using a circulator or coupler, the second coupling unit 3 can be implemented using a Y-waveguide or coupler, and the first photodetector 8 can be implemented using a balanced detector.

[0065] 2. Use attachments Figure 8The optical detection structure of the Michelson interferometer based on dual-wavelength compensation is described. Two wavelengths of light emitted from light source 1 pass sequentially through the first coupling unit 2 and the second coupling unit 3, and are then split into two beams: one beam passes through the first sensing fiber loop 4 bonded to one side of the elastic sheet, is reflected at the first Faraday rotator 5, returns to the first sensing fiber loop 4, and then returns to the second coupling unit 3; the other beam passes through the second sensing fiber loop 6 bonded to the other side of the elastic sheet, is reflected at the second Faraday rotator 7, returns to the second sensing fiber loop 6, and then returns to the second coupling unit 3. The two beams returning to the second coupling unit 3 interfere. The interfering light reaches the first coupling unit 2 and enters the polarization beam splitter / combiner 9, splitting into fast-axis and slow-axis beams, which are finally detected by the first photodetector 8 and the second photodetector 10, respectively. Compared to the attached... Figure 3 The structure shown can compensate for the effects of common-mode environmental noise by using two wavelengths through the same optical path, thereby improving the signal-to-noise ratio. The first coupling unit 2 can be implemented using a circulator or coupler, the second coupling unit 3 can be implemented using a Y-waveguide or coupler, and the light source 1 can be implemented using a tunable laser.

[0066] 3. Use attachments Figure 9 An optical detection structure based on a Mach-Zehnder interferometer is described. Light emitted from light source 1 passes through a first coupling unit 2 and is then split into two beams: one beam passes through a first sensing fiber loop 4 bonded to one side of an elastic sheet, and the other beam passes through a second sensing fiber loop 6 bonded to the other side of the elastic sheet. The two beams merge and interfere at the second coupling unit 3, and the interfering light enters the first photodetector 8 after reaching the second coupling unit 3. The first coupling unit 2 can be implemented using a Y-waveguide or a coupler, the second coupling unit 3 can be implemented using a coupler, and the first photodetector 8 can be implemented using a balanced detector.

[0067] 4. Use attachments Figure 10 The optical detection structure of the Mach-Zehnder interferometer based on dual-wavelength light source compensation is described. Light emitted from light source 1 passes through the first coupling unit 2 and is then split into two beams: one beam passes through the first sensing fiber ring 4 bonded to one side of the elastic sheet, and the other beam passes through the second sensing fiber ring 6 bonded to the other side of the elastic sheet. The two beams merge and interfere in the second coupling unit 3, then enter the polarization beam splitter / combiner 9, splitting into fast-axis and slow-axis beams, which are finally detected by the first photodetector 8 and the second photodetector 10, respectively. Compared to the attached... Figure 5 The structure shown can compensate for the effects of common-mode ambient noise by using two wavelengths through the same optical path, thereby improving the signal-to-noise ratio. The first coupling unit 2 can be implemented using a Y-waveguide or a coupler, the second coupling unit 3 can be implemented using a coupler, and the light source 1 can be implemented using a tunable laser.

[0068] For schemes 2 and 4, to compensate for temperature drift, a long-wavelength fiber grating can be used on one side of the elastic sheet, and a short-wavelength fiber grating on the other side. In this case, the wavelength shift caused by temperature on the two fiber gratings is opposite. Due to spatial proximity, the effect of temperature on the upper and lower surface gratings is consistent. This dual structure can eliminate the wavelength drift caused by temperature, thereby suppressing the influence of the temperature environment.

[0069] For the optical signal returning to the first photodetector 8 or the second photodetector 10, the analog electrical signal can be converted into a digital electrical signal using an analog-to-digital converter, input into a computer, and then demodulated using a phase-generating carrier algorithm to obtain Δφ. The gravitational acceleration at the corresponding position can then be obtained using formula (8). Alternatively, the above functions can be directly integrated onto a single circuit board, where the analog-to-digital converter uses an ADC chip, and the algorithm is implemented on an ARM or FPGA chip.

[0070] Factors can be adjusted on a standard vibration table. The calibration is then substituted into formula (8) to obtain the gravitational acceleration g at the measurement unit.

[0071] Suppose that we need to measure the gravitational gradient in the z direction. We introduce two parallel measurement units at coordinates z1 and z2 in the z direction to obtain the gravitational accelerations g1 and g2 at coordinates z1 and z2. The gravitational gradient can be obtained by formula (9).

[0072] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A portable gravity gradient measurement unit, characterized in that, It includes an optical detection structure, an elastic sheet and its supporting structure; The optical detection structure includes two fiber optic loops of length L, referred to as the first sensing fiber optic loop and the second sensing fiber optic loop. The elastic sheet has an optical fiber ring on its front and back sides, and the optical fiber ring on the front side is concentric with the optical fiber ring on the back side; a mass block is embedded in the center of the elastic sheet, and the mass block has the same mass on the front side and the same mass on the back side. The mass block is used to generate gravitational acceleration when subjected to the Earth's gravitational field, thereby generating stress on the elastic sheet; The fiber optic ring is used to receive the same frequency optical signal input from the light source of the optical detection structure, and to generate tension and refractive index change under the stress transmitted by the elastic sheet, thereby causing phase change of the optical signals in the two fiber optic rings and generating an interference signal output to the photodetector of the optical detection structure. The photodetector is used to convert the received optical signal into an electrical signal and send it to the central control unit, which is used to calculate the gravity gradient Γ based on the electrical signal.

2. The portable gravity gradient measurement unit according to claim 1, characterized in that, The central control unit first calculates the gravitational acceleration based on the electrical signal. Then, the gravitational gradient Γ along the direction of the gravitational gradient to be measured is calculated based on the gravitational acceleration g at different positions along the direction of the gravitational gradient to be measured; where, n is the refractive index of the optical fiber. ν f p represents the Poisson's ratio of the optical fiber. 11 p 12 φ is the photoelastic coefficient of the optical fiber, φ is the phase change of the optical fiber loop, and ΔL is the change in length of the optical fiber loop.

3. The portable gravity gradient measurement unit according to claim 1, characterized in that, The optical detection structure is an optical detection structure based on a Michelson interferometer, including a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber ring (4), a first Faraday rotator (5), a second sensing fiber ring (6), a second Faraday rotator (7), and a first photodetector (8). The light emitted by the light source (1) passes through the first coupling unit (2) and the second coupling unit (3) in sequence and is split into two beams: one beam passes through the first sensing fiber ring (4) and enters the first Faraday rotator (5), is reflected back to the first sensing fiber ring (4) by the first Faraday rotator (5), and returns to the second coupling unit (3); the other beam passes through the second sensing fiber ring (6) and enters the second Faraday rotator (7), is reflected back to the second sensing fiber ring (6), and returns to the second coupling unit (3); the two beams of light returning to the second coupling unit (3) interfere and enter the first photodetector (8) after passing through the first coupling unit (2).

4. The portable gravity gradient measurement unit according to claim 1, characterized in that, The optical detection structure is a Michelson interferometer optical detection structure based on dual-wavelength compensation, including a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber ring (4), a first Faraday rotator (5), a second sensing fiber ring (6), a second Faraday rotator (7), a first photodetector (8), a polarization beam splitter / combiner (9), and a second photodetector (10); the dual-wavelength light emitted by the light source (1) is split into two beams after passing through the first coupling unit (2) and the second coupling unit (3) in sequence: one beam passes through... The first sensing fiber ring (4) is incident on the first Faraday rotator (5) and reflected back to the first sensing fiber ring (4) and returned to the second coupling unit (3); another beam is incident on the second Faraday rotator (7) through the second sensing fiber ring (6) and reflected back to the second sensing fiber ring (6) and returned to the second coupling unit (3); the two beams returning to the second coupling unit (3) interfere and then enter the polarization beam splitter / combiner (9) through the first coupling unit (2), and are divided into fast-axis beam and slow-axis beam, which are respectively input to the first photodetector (8) and the second photodetector (10).

5. The portable gravity gradient measurement unit according to claim 1, characterized in that, The optical detection structure is an optical detection structure based on a Mach-Zehnder interferometer, including a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber ring (4), a second sensing fiber ring (6), and a first photodetector (8). The light emitted by the light source (1) is split into two beams through the first coupling unit (2): one beam enters the second coupling unit (3) through the first sensing fiber ring (4), and the other beam enters the second coupling unit (3) through the second sensing fiber ring (6). The two beams of light meet and interfere in the second coupling unit (3) before entering the first photodetector (8).

6. The portable gravity gradient measurement unit according to claim 1, characterized in that, The optical detection structure is a Mach-Zehnder interferometer optical detection structure based on dual-wavelength light source compensation, including a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber ring (4), a second sensing fiber ring (6), a first photodetector (8), a polarization beam splitter / combiner (9), and a second photodetector (10). The dual-wavelength light emitted by the light source (1) is split into two beams by the first coupling unit (2): one beam is input into the second coupling unit (3) through the first sensing fiber ring (4), and the other beam is input into the second coupling unit (3) through the second sensing fiber ring (6). The two beams of light meet and interfere in the second coupling unit (3) and then enter the polarization beam splitter / combiner (9), where they are split into fast-axis light and slow-axis light, which are detected by the first photodetector (8) and the second photodetector (10), respectively.

7. The portable gravity gradient measurement unit according to claim 4 or 6, characterized in that, The first sensing fiber ring (4) adopts a long-wavelength fiber grating, and the second sensing fiber ring (6) adopts a short-wavelength fiber grating; or the first sensing fiber ring (4) adopts a short-wavelength fiber grating, and the second sensing fiber ring (6) adopts a long-wavelength fiber grating.

8. A gravity gradient measurement method based on the portable gravity gradient measurement unit of claim 1, comprising the following steps: 1) Introduce one of the portable gravity gradient measurement units at coordinates z1 and z2 in the measurement direction z; 2) The gravitational acceleration g1 at coordinate z1 is calculated using the signal output by the portable gravity gradient measurement unit at coordinate z1; the gravitational acceleration g2 at coordinate z2 is calculated using the signal output by the portable gravity gradient measurement unit at coordinate z2. 3) According to the formula The gravitational gradient Γ in the z-direction is calculated.

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