CN117310833A - Portable gravity gradient measuring unit and measuring method thereof - Google Patents
Portable gravity gradient measuring unit and measuring method thereof Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/266—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light by interferometric means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/28—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with deflection of beams of light, e.g. for direct optical indication
- G01D5/30—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with deflection of beams of light, e.g. for direct optical indication the beams of light being detected by photocells
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
- G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
- G01P15/093—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by photoelectric pick-up
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
- G01V7/02—Details
- G01V7/04—Electric, photoelectric, or magnetic indicating or recording means
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Abstract
The invention discloses a portable gravity gradient measuring unit and a measuring method thereof. The measuring unit comprises an optical detection structure, an elastic sheet and a supporting structure thereof; the optical detection structure comprises two optical fiber rings with the length L; the front and back sides of the elastic sheet are respectively provided with an optical fiber ring, and the optical fiber ring on the front side is concentric with the optical fiber ring on the back side; the mass block is embedded in the center of the elastic sheet, and the mass of the mass block on the front side is the same as the mass of the mass block on the back side; the mass block is used for generating gravitational acceleration when being acted by the earth gravitational field and generating stress on the elastic sheet; the optical fiber rings are used for receiving the same-frequency optical signals input by the light source and generating stretching and refractive index change under the stress effect conducted by the elastic sheet, so that the optical signals in the two optical fiber rings generate phase change and generate interference signals to be output to the photoelectric detector of the optical detection structure; the photoelectric detector is used for converting the received optical signals into electric signals and sending the electric signals to the master control unit to calculate the gravity gradient.
Description
Technical Field
The invention relates to a portable gravity gradiometer, in particular to a portable gravity gradient measuring unit based on optical fiber sensing and a measuring method thereof, which can be used for measuring earth gravity gradient fields in space, aviation, land and underwater, and are also suitable for researches in petroleum and natural gas exploration, mineral exploration, map making, environment monitoring, military and other application fields and structural geology, hydrogeology, seismology, geodetics, space science and the like.
Background
Gravity gradients are the rate of change of gravitational acceleration in space and can be used to describe the change in the strength and direction of the gravitational field. Generally, a gravitational field is formed by a nearby object in space, and the intensity and direction of the gravitational field are affected by factors such as the shape of the earth, the distribution of materials, and the motion state thereof. In the fields of geophysical exploration, geological disaster early warning, earth circle inversion, earth dynamics research and the like, the information of underground and surface material density, structural characteristics and the like can be obtained by measuring gravity gradient change, and the method has important significance in understanding geophysical laws, predicting geological disasters, exploring mineral resources and the like.
In the fields of geophysical exploration, seismic exploration, resource exploration, etc., it is often necessary to detect relatively complex, uncertain geological structures that are often not in an area that is easily measured. Thus, portability of the gravity gradiometer is one of the very important advantages.
At present, the implementation method of the gravity gradiometer mainly comprises the following steps: gravity gradiometers based on rotational accelerometers, gravity gradiometers based on free fall, and gravity gradiometers based on angular accelerometers.
Gravity gradiometers based on rotary accelerometers are currently the only commercially available gravity gradiometers. This type of gravity gradiometer requires measurement by multiple accelerometers and requires a turntable to impart uniform rotation modulation to pairs of accelerometers that are in the same plane, thus making it difficult to use conveniently in field exploration. In addition, the gravity gradiometer based on the rotary accelerometer has obvious accelerometer consistency problem due to the need of using eight or even twelve accelerometers, which hinders the further development of the precision.
Gravity gradiometers based on free fall adopt a naive idea: after measuring the gravitational acceleration at two points, the rate of change in the gravitational acceleration in space, i.e. the gravitational gradient, is obtained by dividing by the distance between the two points. At present, the gravity gradiometer developed based on the principle mainly adopts two main technical means, namely laser interference and atomic interference. In the development process of the laser interference and atomic interference gravity gradiometer, vacuum equipment is required to realize a vacuum environment, and an additional shock insulation device is required to reduce the influence of ground vibration, so that miniaturization is difficult.
The gravity gradiometer based on angular accelerometer adopts superconducting quantum interference device to sense superconducting current change caused by test mass displacement. The university of maryland, moody et al, devised a three-axis superconducting gravity gradiometer consisting of six linear accelerometers and three angular accelerometers, providing stable test mass levitation and signal coupling using the quantized flux and the meissner effect, and a superconducting quantum interference device providing very low noise amplification of the signal. However, practical superconducting gravity gradiometers with actual performances comparable to those of the rotary accelerometer gravity gradiometers have not been developed at home and abroad, and miniaturization still faces a number of critical technical bottlenecks due to the need for low-temperature environment control equipment to realize superconducting environments.
Disclosure of Invention
Aiming at the technical problems existing in the prior art, the invention aims to provide a portable gravity gradient measuring unit and a measuring method thereof. The invention relates to a gravity gradiometer based on an elastic sheet-mass block structure, and the principle of the gravity gradiometer is shown in figure 1. The normal direction of the elastic sheet is parallel to the gravity gradient direction to be measured. A mass block is embedded in the center of the elastic sheet to make the masses of the mass blocks at two sides of the elastic sheet identical, and an adhesive is used for fixing the optical fibers in the area of the elastic sheet. The mass block is subjected to the gravity field of the earth to generate gravity acceleration, and stress is generated on the elastic sheet, and the stress is conducted to the optical fiber to generate stretching and refractive index change of the optical fiber, so that phase change of an optical signal in the optical fiber is generated. The gravitational acceleration at this point can be obtained by detecting this phase change.
Specifically, the sensing optical fibers are wound outwards from the center of the circle along the circumferences on the front and back surfaces of the elastic sheet respectively to form two independent sensing optical fiber rings, and the optical fibers of different layers (on different circumferences) are closely adjacent to each other. The strain of the mass on the elastic sheet is conducted to the optical fiber. The sensing optical fiber is fixed on the elastic sheet by an adhesive. Two sections of sensing optical fibers bonded on the elastic sheet are L, the refractive index of the optical fibers is n, and the propagation coefficient of light in the optical fibers isWhere λ is the wavelength of the light wave, the phase of the light signal through the sensing fiber on the front or back side is changed to:
φ＝nkL (1)
the formula is fully differentiated to obtain:
the first term is the phase shift caused by the axial elongation of the optical fiber, the second term is the phase shift caused by the stress strain effect (stress causes the refractive index of the optical fiber to change), and the third term is the phase shift caused by the poisson effect (the diameter of the optical fiber to change). Since the poisson effect is negligible compared to the first two effects, there are
Wherein:
wherein: epsilon is the strain of the optical fiber, v f For the Poisson's ratio of optical fiber, p 11 、p 12 Is the photoelastic coefficient of the fiber.
According to the definition of strain, when ΔL is small, there isThereby:
recording deviceThe original type can be simplified as follows:
when the optical fiber is bonded to the stress linear region of the elastic sheet, Δl is proportional to the gravitational acceleration g, that is:
ΔL＝Sg (7)
where S represents a linear scale factor between gravitational acceleration and fiber strain.
Thereby:
the alignment factor can be realized on a standard vibrating tableThereby obtaining a numerical relationship between the gravitational acceleration g and the observed quantity Δφ. The observed quantity Δφ can be measured by Michelson interferometer or Mach-Zehnder interferometer structure shown in FIG. 7, FIG. 8, FIG. 9, and FIG. 10. After solving the observed quantity delta phi, the method can be conductedThe gravitational acceleration g is obtained by the formula (8).
It is now necessary to measure the gravity gradient in the direction z, the coordinate z in the z direction 1 And z 2 Two parallel measuring units are introduced to obtain the coordinate z 1 And z 2 Gravitational acceleration g at 1 And g 2 Gravity gradient can be obtained by the following formula:
the novel optical fiber gravity gradiometer mainly comprises light weight components such as an optical fiber light path, an elastic sheet and the like, has lighter weight and portability. Compared with the traditional gravity gradiometer with larger volume, the gravity gradiometer can be conveniently carried on various platforms, such as carriers of airplanes, satellites, unmanned aerial vehicles and the like. And secondly, the gravity gradiometer adopts the optical fiber transmission and weak optical signal detection technology, has lower power consumption, can use battery power supply or solar power supply, and does not need a high-power supply. In conclusion, the optical fiber gravity gradiometer provided by the invention has good portability, can be suitable for gravity gradient measurement requirements of different occasions, and has important application value.
The technical scheme of the invention is as follows:
the portable gravity gradient measuring unit is characterized by comprising an optical detection structure, an elastic sheet and a supporting structure thereof;
the optical detection structure comprises two optical fiber rings with the length L, which are marked as a first sensing optical fiber ring and a second sensing optical fiber ring;
the front surface and the back surface of the elastic sheet are respectively provided with an optical fiber ring, and the optical fiber ring on the front surface is concentric with the optical fiber ring on the back surface; the mass block is embedded in the center of the elastic sheet, and the mass of the mass block on the front side is the same as the mass of the mass block on the back side;
the mass block is used for generating gravity acceleration when being acted by the earth gravity field and generating stress on the elastic sheet;
the optical fiber rings are used for receiving the same-frequency optical signals input by the optical source of the optical detection structure and generating stretching and refractive index change under the action of stress conducted by the elastic sheet, so that the optical signals in the two optical fiber rings generate phase change and generate interference signals to be output to the photoelectric detector of the optical detection structure; the photoelectric detector is used for converting the received optical signals into electric signals and sending the electric signals to the master control unit, and the photoelectric detector is used for calculating the gravity gradient gamma according to the electric signals.
Further, the master control unit calculates the gravitational acceleration according to the electric signalThen calculating according to the corresponding gravity accelerations g at different positions in the gravity gradient direction to be measured to obtain a gravity gradient gamma of the gravity gradient direction to be measured; wherein, n is the refractive index of the optical fiber, ">v f For the Poisson's ratio of optical fiber, p 11 、p 12 The optical fiber is photoelastic coefficient, phi is the phase change of the optical fiber ring, and delta L is the length change of the optical fiber ring.
Further, the optical detection structure is based on a Michelson interferometer and comprises a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing optical fiber ring 4, a first Faraday rotator 5, a second sensing optical fiber ring 6, a second Faraday rotator 7 and a first photoelectric detector 8; the light emitted by the light source 1 sequentially passes through the first coupling unit 2 and the second coupling unit 3 and is divided into two beams: one beam is input into a first Faraday rotator 5 after passing through a first sensing optical fiber ring 4, reflected back to the first sensing optical fiber ring 4 by the first Faraday rotator 5 and returned to the second coupling unit 3; the other beam is input into a second Faraday rotator mirror 7 through a second sensing optical fiber ring 6, reflected back to the second sensing optical fiber ring 6 through the second Faraday rotator mirror 7 and returned 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.
Further, the optical detection structure is a michelson interferometer optical detection structure based on dual-wavelength compensation, and comprises a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing optical fiber ring 4, a first Faraday rotator 5, a second sensing optical fiber ring 6, a second Faraday rotator 7, a first photoelectric detector 8, a polarization beam splitter/combiner 9 and a second photoelectric detector 10; the dual-wavelength light emitted by the light source 1 sequentially passes through the first coupling unit 2 and the second coupling unit 3 and is divided into two beams: one beam is reflected back to the first sensing optical fiber ring 4 after entering the first Faraday rotator 5 through the first sensing optical fiber ring 4 and returns to the second coupling unit 3; the other beam is reflected back to the second sensing optical fiber ring 6 and returned to the second coupling unit 3 after entering the second Faraday rotator mirror 7 through the second sensing optical fiber ring 6; after being interfered, the two beams of light returning to the second coupling unit 3 enter the polarization beam splitter/combiner 9 through the first coupling unit 2, are split into fast axis light and slow axis light, and are respectively input into the first photoelectric detector 8 and the second photoelectric detector 10.
Further, the optical detection structure is based on a Mach-Zehnder interferometer and comprises a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing optical fiber ring 4, a second sensing optical fiber ring 6 and a first photoelectric detector 8; the 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 optical fiber ring 4, and the other beam is input into the second coupling unit 3 through the second sensing optical fiber ring 6; the two beams of light enter the first photodetector 8 after coming together and interfering with each other in the second coupling unit 3.
Further, the optical detection structure is a Mach-Zehnder interferometer optical detection structure based on dual-wavelength light source compensation, and comprises a light source 1, a first coupling unit 2, a second coupling unit 3, a first sensing optical fiber ring 4, a second sensing optical fiber ring 6, a first photoelectric detector 8, a polarization beam splitter/combiner 9 and a second photoelectric detector 10; the dual wavelength light emitted from 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 optical fiber ring 4, and the other beam is input into the second coupling unit 3 through the second sensing optical fiber ring 6; the two beams of light are converged and interfered by the second coupling unit 3 and then enter the polarization beam splitter/combiner 9, and are split into fast axis light and slow axis light which are respectively detected by the first photoelectric detector 8 and the second photoelectric detector 10.
Further, 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.
A gravity gradient measurement method based on the portable gravity gradient measurement unit, comprising the following steps:
1) Coordinate z in the direction of measurement z 1 、z 2 Respectively introducing one portable gravity gradient measuring unit;
2) By means of the coordinates z 1 The signal output by the portable gravity gradient measuring unit is calculated to obtain a coordinate z 1 Gravitational acceleration g at 1 The method comprises the steps of carrying out a first treatment on the surface of the By means of the coordinates z 2 The signal output by the portable gravity gradient measuring unit is calculated to obtain a coordinate z 2 Gravitational acceleration g at 2 ；
3) According to the formulaThe gravity gradient Γ in the direction z is calculated.
The novel gravity gradiometer provided by the invention has the characteristic of portability, and the accuracy of gravity gradient detection can be effectively improved by utilizing a plurality of gravity gradiometers to form an array. Specifically, the placement position and configuration mode of the gravity gradiometer are determined according to the specific measurement area. The array may take the form of a linear arrangement, a grid-like arrangement, or other suitable format. FIG. 2 shows a schematic diagram of a grid-like arrangement of gravity gradiometers, with cubes representing one gravity gradiometer. And installing each gravity gradiometer according to the array configuration, starting the gravity gradiometer and starting data acquisition. The gravity gradient values at each location are recorded separately in the array area in accordance with a predetermined array configuration. And transmitting the data of the gravity gradiometers to a master controller, and processing and analyzing the collected gravity gradient data by the master controller to obtain information of underground density change.
The invention has the following advantages:
the invention provides a novel gravity gradiometer. Compared with the conventional commercial rotary accelerometer type gravity gradiometer, the rotary accelerometer type gravity gradiometer does not need a turntable and other peripherals which are difficult to miniaturize, and has the advantage of miniaturization; meanwhile, eight or even twelve accelerometers are not needed to be used as measuring units, and inconsistent errors of the multiple measuring units due to actual machining errors and the like are reduced. Compared with a gravity gradiometer based on free falling body and a gravity gradiometer based on angular accelerometer, the optical fiber sensor has the characteristics of small size, light weight, electromagnetic interference resistance, stable chemical property and the like, has the advantages of miniaturization and better environmental adaptability, does not need large-scale environmental control peripheral equipment, does not need large-scale power supply equipment to ensure the operation of the environmental control peripheral equipment, and has the characteristic of portability.
Drawings
FIG. 1 is a flow chart of the principle of a gravity gradiometer based on an elastic sheet-mass structure.
FIG. 2 is a schematic diagram of a gravity gradiometer array wherein the cube represents one gravity gradiometer and the data from multiple gravity gradiometers is transmitted to a master.
FIG. 3 is a diagram of a portable gravity gradiometer consisting of a mass, optical fibers, elastomeric sheets and a peripheral support structure; wherein the peripheral support structure is not shown.
Fig. 4 is a top view of the portable gravity gradiometer.
FIG. 5 is a front view of a portable gravity gradiometer; wherein the peripheral support structure is not shown.
FIG. 6 is a side view of a portable gravity gradiometer; wherein the peripheral support structure is not shown.
Fig. 7 is a michelson interferometer-based optical detection structure.
Fig. 8 is a michelson interferometer optical detection structure based on dual wavelength light source compensation.
Fig. 9 is an optical detection structure based on a mach-zehnder interferometer.
Fig. 10 is a mach-zehnder interferometer optical detection structure based on dual wavelength light source compensation.
The device comprises a 1-light source, a 2-first coupling unit, a 3-second coupling unit, a 4-first sensing optical fiber ring, a 5-first Faraday rotator, a 6-second sensing optical fiber ring, a 7-second Faraday rotator, an 8-first photoelectric detector, a 9-polarization beam splitter/combiner and a 10-second photoelectric detector.
Detailed Description
The invention will now be described in further detail with reference to the accompanying drawings, which are given by way of illustration only and are not intended to limit the scope of the invention.
As shown in fig. 3 to 6, the polarization maintaining optical fibers are wound on the front and back surfaces of the elastic sheet made of beryllium copper along the circumference outwards from the center of the circle, and the optical fibers of different layers (on different circumferences) are closely adjacent to each other. A high-density mass block (such as W-Ni-Cu alloy) is embedded in the center of the elastic sheet to make the mass of the elastic sheet on both sides of the front and back of the elastic sheet identical, and the optical fiber is fixed in the elastic sheet region by using an adhesive such as ultraviolet fiber glue. The elastic sheet is supported by a cylindrical tubular structure. To increase sensitivity, the polarization maintaining fiber may be replaced with a fiber grating.
For the optical detection structure, one of the following schemes may be adopted:
1. the michelson interferometer-based optical detection architecture of fig. 7 was used. The light emitted from the light source 1 sequentially passes through the first coupling unit 2 and the second coupling unit 3, and is then split into two beams: a bundle passes through a first sensing fiber ring 4 bonded to one side of the elastic sheet and then is reflected at a first faraday rotator 5, returns to the first sensing fiber ring 4 and returns to the second coupling unit 3; the other beam passes through a second sensing fiber ring 6 bonded to the other side of the flexible sheet and then reflects at a second faraday rotator mirror 7 back to the second sensing fiber ring 6 and back to the second coupling unit 3. The two beams of light returning to the second coupling unit 3 interfere, and the interference light reaches the first coupling unit 2 and then enters the first photodetector 8. The first coupling unit 2 may be implemented using a circulator or a coupler, the second coupling unit 3 may be implemented using a Y-waveguide or a coupler, and the first photodetector 8 may be implemented using a balanced detector.
2. The michelson interferometer optical detection structure based on dual wavelength compensation of fig. 8 is used. Light of two wavelengths emitted from the light source 1 sequentially passes through the first coupling unit 2, the second coupling unit 3, and then is split into two beams: a bundle passes through a first sensing fiber ring 4 bonded to one side of the elastic sheet and then is reflected at a first faraday rotator 5, returns to the first sensing fiber ring 4 and returns to the second coupling unit 3; the other beam passes through a second sensing fiber ring 6 bonded to the other side of the flexible sheet and then reflects at a second faraday rotator mirror 7 back to the second sensing fiber ring 6 and back to the second coupling unit 3. The two beams of light returning to the second coupling unit 3 interfere, and the interference light enters the polarization beam splitter/combiner 9 after reaching the first coupling unit 2, is split into fast axis light and slow axis light, and is finally detected by the first photoelectric detector 8 and the second photoelectric detector 10 respectively. Compared with the structure shown in fig. 3, the structure can compensate the influence of common mode environmental noise through the same light path by two wavelengths, thereby improving the signal to noise ratio. The first coupling unit 2 may be implemented using a circulator or a coupler, the second coupling unit 3 may be implemented using a Y waveguide or a coupler, and the light source 1 may be implemented using a tunable laser.
3. The mach-zehnder interferometer based optical detection structure of fig. 9 is employed. The light emitted from the light source 1 passes through the first coupling unit 2 and is then split into two beams: one bundle passes through a first sensing fiber ring 4 bonded to one side of the elastic sheet, and the other bundle passes through a second sensing fiber ring 6 bonded to the other side of the elastic sheet; the two light beams meet at the second coupling unit 3 and interfere, and the interference light enters the first photodetector 8 after reaching the second coupling unit 3. Wherein the first coupling unit 2 may be implemented using a Y-waveguide or a coupler, the second coupling unit 3 may be implemented using a coupler, and the first photodetector 8 may be implemented using a balanced detector.
4. The Mach-Zehnder interferometer optical detection structure based on dual-wavelength light source compensation of figure 10 is adopted. The light emitted from the light source 1 passes through the first coupling unit 2 and is then split into two beams: one bundle passes through a first sensing fiber ring 4 bonded to one side of the elastic sheet, and the other bundle passes through a second sensing fiber ring 6 bonded to the other side of the elastic sheet; the two beams of light are converged and interfered in the second coupling unit 3, then enter the polarization beam splitter/combiner 9, are split into fast axis light and slow axis light, and finally are detected by the first photoelectric detector 8 and the second photoelectric detector 10 respectively. Compared with the structure shown in fig. 5, the structure can compensate the influence of common mode environmental noise through the same light path by two wavelengths, thereby improving the signal to noise ratio. The first coupling unit 2 may be implemented using a Y-waveguide or a coupler, the second coupling unit 3 may be implemented using a coupler, and the light source 1 may be implemented using a tunable laser.
For the schemes 2 and 4, to compensate for the temperature drift, a long wavelength fiber grating may be used on one side of the elastic sheet and a short wavelength fiber grating may be used on the other side. At this time, the wavelength shift amounts of the fiber gratings at the two sides are opposite due to the temperature, and the influence of the temperature on the upper and lower surface gratings is consistent due to the proximity of the space, so that the wavelength shift generated by the temperature can be eliminated through the dual structure, thereby inhibiting the temperature environment influence.
For the optical signals returned to the first photoelectric detector 8 or the second photoelectric detector 10, an analog-to-digital converter can be used for converting the analog electric signals into digital electric signals, the digital electric signals are input into a computer, then a phase generation carrier algorithm is adopted for demodulation, delta phi is obtained, and the gravity acceleration at the corresponding position can be obtained through a formula (8). Or the functions are directly integrated on a circuit board, wherein the analog-to-digital converter uses an ADC chip, and the algorithm is finished on an ARM or FPGA chip.
By making it possible to achieve a factor on a standard vibrating tableAnd (3) bringing into formula (8) to obtain the gravitational acceleration g at the measurement cell.
It is now necessary to measure the gravity gradient in the direction z, the coordinate z in the z direction 1 And z 2 Two parallel measuring units are introduced to obtain the coordinate z 1 And z 2 Gravitational acceleration g at 1 And g 2 Gravity gradient can be obtained by the formula (9).
Although specific embodiments of the invention have been disclosed for illustrative purposes, it will be appreciated by those skilled in the art that the invention may be implemented with the help of a variety of examples: various alternatives, variations and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will have the scope indicated by the scope of the appended claims.
Claims (8)
1. The portable gravity gradient measuring unit is characterized by comprising an optical detection structure, an elastic sheet and a supporting structure thereof;
the optical detection structure comprises two optical fiber rings with the length L, which are marked as a first sensing optical fiber ring and a second sensing optical fiber ring;
the front surface and the back surface of the elastic sheet are respectively provided with an optical fiber ring, and the optical fiber ring on the front surface is concentric with the optical fiber ring on the back surface; the mass block is embedded in the center of the elastic sheet, and the mass of the mass block on the front side is the same as the mass of the mass block on the back side;
the mass block is used for generating gravity acceleration when being acted by the earth gravity field and generating stress on the elastic sheet;
the optical fiber rings are used for receiving the same-frequency optical signals input by the optical source of the optical detection structure and generating stretching and refractive index change under the action of stress conducted by the elastic sheet, so that the optical signals in the two optical fiber rings generate phase change and generate interference signals to be output to the photoelectric detector of the optical detection structure; the photoelectric detector is used for converting the received optical signals into electric signals and sending the electric signals to the master control unit, and the photoelectric detector is used for calculating the gravity gradient gamma according to the electric signals.
2. The portable device of claim 1The gravity gradient measuring unit is characterized in that the master control unit firstly calculates the gravity acceleration according to the electric signalThen calculating according to the corresponding gravity accelerations g at different positions in the gravity gradient direction to be measured to obtain a gravity gradient gamma of the gravity gradient direction to be measured; wherein, n is the refractive index of the optical fiber,
ν f for the Poisson's ratio of optical fiber, p 11 、p 12 The optical fiber is photoelastic coefficient, phi is the phase change of the optical fiber ring, and delta L is the length change of the optical fiber ring.
3. The portable gravity gradient measurement unit according to claim 1, wherein the optical detection structure is a michelson interferometer based optical detection structure comprising a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber loop (4), a first faraday rotator (5), a second sensing fiber loop (6), a second faraday rotator (7) and a first photodetector (8); light emitted by the light source (1) sequentially passes through the first coupling unit (2) and the second coupling unit (3) and is divided into two beams: one beam is input into a first Faraday rotator mirror (5) after passing through a first sensing optical fiber ring (4), is reflected back to the first sensing optical fiber ring (4) through the first Faraday rotator mirror (5) and returns to a second coupling unit (3); the other beam is input into a second Faraday rotator mirror (7) through a second sensing optical fiber ring (6), reflected back to the second sensing optical fiber ring (6) through the second Faraday rotator mirror (7) and returned to the second coupling unit (3); the two beams of light returned to the second coupling unit (3) interfere and enter the first photoelectric detector (8) after passing through the first coupling unit (2).
4. The portable gravity gradient measurement unit according to claim 1, wherein the optical detection structure is a michelson interferometer optical detection structure based on dual wavelength compensation, comprising a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber loop (4), a first faraday rotator (5), a second sensing fiber loop (6), a second faraday rotator (7), a first photodetector (8), a polarizing beam splitter/combiner (9), a second photodetector (10); the dual-wavelength light emitted by the light source (1) sequentially passes through the first coupling unit (2) and the second coupling unit (3) and is divided into two beams: one beam is reflected back to the first sensing optical fiber ring (4) after entering the first Faraday rotator mirror (5) through the first sensing optical fiber ring (4) and returns to the second coupling unit (3); the other beam is reflected back to the second sensing optical fiber ring (6) and returned to the second coupling unit (3) after entering the second Faraday rotator mirror (7) through the second sensing optical fiber ring (6); two beams of light returning to the second coupling unit (3) interfere and enter the polarization beam splitter/combiner (9) after passing through the first coupling unit (2), and the two beams of light are split into fast axis light and slow axis light which are respectively input into the first photoelectric detector (8) and the second photoelectric detector (10).
5. The portable gravity gradient measurement unit according to claim 1, wherein the optical detection structure is a mach-zehnder interferometer based optical detection structure comprising a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber loop (4), a second sensing fiber loop (6), a first photodetector (8); 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 optical fiber ring (4), and the other beam is input into the second coupling unit (3) through the second sensing optical fiber ring (6); the two beams of light enter the first photoelectric detector (8) after meeting and interfering with the second coupling unit (3).
6. The portable gravity gradient measurement unit according to claim 1, wherein the optical detection structure is a mach-zehnder interferometer optical detection structure based on dual wavelength light source compensation, comprising a light source (1), a first coupling unit (2), a second coupling unit (3), a first sensing fiber loop (4), a second sensing fiber loop (6), a first photodetector (8), a polarization beam splitter/combiner (9), a second photodetector (10); the dual-wavelength light emitted by the light source (1) is divided into two beams by the first coupling unit (2): one beam is input into the second coupling unit (3) through the first sensing optical fiber ring (4), and the other beam is input into the second coupling unit (3) through the second sensing optical fiber ring (6); the two beams of light are converged and interfered by the second coupling unit (3) and then enter the polarization beam splitter/combiner (9) to be split into fast axis light and slow axis light which are respectively detected by the first photoelectric detector (8) and the second photoelectric detector (10).
7. Portable gravity gradient measurement unit according to claim 4 or 6, characterized in that the first sensing fiber ring (4) employs a long wavelength fiber grating and the second sensing fiber ring (6) employs 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 according to claim 1, comprising the steps of:
1) Coordinate z in the direction of measurement z 1 、z 2 Respectively introducing one portable gravity gradient measuring unit;
2) By means of the coordinates z 1 The signal output by the portable gravity gradient measuring unit is calculated to obtain a coordinate z 1 Gravitational acceleration g at 1 The method comprises the steps of carrying out a first treatment on the surface of the By means of the coordinates z 2 The signal output by the portable gravity gradient measuring unit is calculated to obtain a coordinate z 2 Gravitational acceleration g at 2 ；
3) According to the formulaThe gravity gradient Γ in the direction z is calculated.
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