Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C17/00—Compasses; Devices for ascertaining true or magnetic north for navigation or surveying purposes
  • G01C17/02—Magnetic compasses
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
  • G01R33/0206—Three-component magnetometers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices

Definitions

• the invention relates to magnetic observatories (of the local terrestrial magnetic field) and more particularly to autonomous magnetic observatories.
• the invention relates to a method for automatically obtaining and self-calibrating the local (terrestrial) magnetic field vector.
• the method of data processing of the magnetometer makes it possible, from the three variations of the components and the absolute value of the modulus of the local magnetic field vector, to obtain a calibration of scale factors making it possible to obtain the variations of the three components of the magnetometer.
• vector local magnetic field in units SL vector local magnetic field in units SL
• a second partially autonomous magnetic observatory is disclosed in "GAUSS: Geomagnetic Automated System” by HU AUSTER, M MANDEA, A. HEMSHORN, KORTE and E. PULZ in PUBLS. INST. Geophys. POL. ACAD. SC., C-99 (398), 2007.
• This publication discloses an apparatus for automatically measuring the orientation of the local magnetic field vector.
• This autonomous observatory also provides a horizontal and vertical self-monitoring by detecting the local vertical.
• one of the objects of the present invention is to provide a device for the autonomous and self-calibrated measurement of the local magnetic field vector, the local magnetic field to be included in the local magnetic field direction of the Earth .
• the magnetic observatory for the measurement of the local magnetic field vector comprises: a scalar magnetometer for the absolute measurement of the local magnetic field vector module, a magnetic variometer recording variations of three components mathematically independent of the vector field local magnetic, a clock, an angular magnetometer and a controller.
• the angular magnetometer comprises a first rotatable support according to a first axis of rotation, said main axis, making it possible to obtain a horizontal orientation of the first orientable support, the so-called first orientable support comprising: a main motorisation for modifying the horizontal orientation of the first support orientable about the main axis, an inclination sensor, a second support orientable along a second axis of rotation called secondary axis, orthogonal to the main axis, for obtaining a vertical orientation of the second orientable support said second support steerable device comprising: a secondary motor for changing the vertical orientation of the second steerable support about the secondary axis, a magnetic sensor for measuring the direction of the local magnetic field vector, a North search device.
• the angular magnetometer further comprises means for controlling the main and secondary motorization and a device for measuring and angularly acquiring the horizontal and vertical orientations of the first and second orientable supports.
• the second steerable support may include the tilt sensor.
• the inclination sensor can be mounted on the first orientable support or on the second orientable support.
• the magnetic observatory is made autonomous and capable of self-calibrating the measurements of the local magnetic field by means of the controller which is configured to automatically control the main motor and the secondary motor, to manage the orientation: of the inclination sensor for the measurement of the direction of the vertical, the North search device for measuring the direction of the geographic North and the magnetic sensor for measuring the direction of the local magnetic field vector. It is also configured to acquire: the angles of the direction of the local magnetic field vector with respect to the geographic North and to the Vertical according to the horizontal and vertical orientations of the first and second orientable supports measured with the measuring device and angular acquisition , the three variations of the local magnetic field vector measured by the variometer and the values of the modulus of the local magnetic field vector measured by the scalar magnetometer. Finally, it is configured to process the data previously acquired to automatically obtain the local magnetic field vector and the measurement errors associated with each instrument.
• the first and second steerable supports, the main and secondary engines, the North search device, the tilt sensor, the control means of the main and secondary motorization and the measurement and acquisition device consist of non-magnetic components, defined such that the magnetic susceptibility of the materials is between -1 and 1, preferably between -10 "1 and 10 " 1 , even more preferably between -10 "3 and 10 " 3 .
• the non-magnetic components are materials selected from: ceramic, aluminum, arcap, titanium, copper, ertalon, nylon, ertacetal, peek.
• the scalar magnetometer is of the type: proton, Overhauser, atomic, optically pumped.
• the magnetic sensor is a sensor type: fluxgate, fluxset, rotating electrical circuit or a scalar magnetometer polarized by a magnetic device.
• the North search device is of the type: GNSS, GPS, target of a target, aim of a star, gyroscope, absolute rotation detector, polarization of sunlight.
• the observatory comprises a plurality of non-magnetic pillars, preferably of concrete, whose average dimensions [thickness, width, length] are between [1, 10, 10] cm and [6, 10, 10] m, preferably, the average dimensions [width, length, depth] are between [10, 20, 20] cm and [1, 2, 2] m, even more preferably between [15, 25, 25] cm and [0.25, 0.5, 0.5] m, said pillars being separated by an average distance of between 0 and 10 m, preferably between 1 and 6 m, even more preferably between 2 and and 4 m.
• the non-magnetic pillars support at least one of the following: the scalar magnetometer, the angular magnetometer, the variometer, the clock (201), the controller.
• the observatory is self-calibrated and can be installed anywhere on the Earth's crust, it is best to install it on a stable structure that is not subject to vibration in order to obtain better quality measurements. .
• the observatory comprises at least one non-magnetic shelter surrounding the plurality of pillars and comprising an insulated wall whose average thickness is between 1 and 60 cm, preferably between 2 and 30 cm, even more preferably between 5 and 10 cm.
• At least one magnetic shelter protects at least one of the following: the scalar magnetometer (MS), the angular magnetometer (AM), the variometer (MV), the clock (201), the controller (202).
• the invention in a second aspect, relates to a method for automatically obtaining the local magnetic field vector.
• This method comprises the step of providing a magnetic observatory as described above. Then, the controller carries out the steps of acquiring values, controlling the engines, calculating values, calibrating measurements and finally calculating the value of the local magnetic field vector.
• the controller acquires measurements of the modulus of said local magnetic field vector measured with the scalar magnetometer at different times ti, and acquires measurements of the three mathematically independent components of the local magnetic field vector dU, dV, dW, measured with the variometer at different times ti.
• the controller controls the main operator to change the horizontal orientation of the first steerable support and, depending on the indications of the inclination sensor, measures the vertical direction, V, it also controls the secondary motorization for modifying the vertical orientation of the second steerable support and, according to the indications of the North search device, measures the direction of the geographic North N, and it controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer and, depending on the indications of the magnetic sensor obtained at different times ti, measures two angles, D * and, corresponding to the direction of the local magnetic field vector.
• the controller calibrates the scaling factors of each mathematically independent component of the variometer and calibrates the orthogonality and spatial orientation of the three mathematically independent components of the variometer and calculates Eulerian rotation matrices.
• the controller calculates the value of the local magnetic field vector by obtaining oriented measurements by applying Eulerian rotations to the three measures of the variometer: dU, dV, and dW, by obtaining scaled measurements by multiplying the three oriented measurements of the variometer by the respective scale factors and by obtaining 3 components of the local magnetic field vector by summing the baselines to the three oriented measures and to scale.
• the method described above makes it possible to obtain a measurement of the local magnetic field automatically and in a self-calibrated manner.
• Automation eliminates the need for a skilled operator, which greatly reduces costs and improves measurement accuracy.
• Auto-calibration improves the quality of measurements.
• This self-calibration also contributes to the reduction of costs because it allows the installation of the autonomous observatory in an inexpensive structure in comparison with traditional observatories.
• the combination of automation and auto-calibration thus brings significant added value to the instruments taken separately.
• the measurement of the two angles, D * and, characterizing the direction of the field in the horizontal and vertical plane respectively is obtained by the controller by modifying the horizontal orientation of the first orientable support (rotation of the support in the plane horizontal) until the magnetic sensor indicates a zero, acquire the first angle D * and changing the vertical orientation of the second orientable support (rotation of the support in the vertical plane) until the magnetic sensor indicates a zero, acquire the second angle I * .
• the measurement of the two angles, D * and is obtained by the controller by modifying the horizontal orientation of the first orientable support so that the magnetic sensor indicates a zero measurement value and acquire the orientation.
• D1 of the first orientable support corresponding to this measurement then modifying the horizontal orientation of the first orientable support of 180 ° and adjusting the horizontal orientation so that the magnetic sensor indicates a zero measurement value and acquire the orientation D2 of the first orientable support corresponding to this measurement, then by modifying the vertical orientation of the second orientable support by 180 ° and by adjusting the horizontal orientation of the first orientable support so that the magnetic sensor indicates a zero measurement value and acquire the orientation D3 of the first orientable support corresponding to this measurement, by modifying the horizontal orientation of the first orientable support of 180 ° and adjusting the horizontal orientation so that the magnetic sensor indicates a zero measurement value and acquire the orientation D4 of the first orientable support corresponding to this measurement and finally calculating the first angle corresponding to the horizontal direction of the magnetic field local average of the four measures
• the measurement of I * is obtained by modifying the horizontal orientation of the first orientable support in the direction D * -90, then performing the same first three steps as for D * in which the horizontal and vertical orientations are reversed. and wherein the roles of the first and second steerable supports are reversed, and thereby acquire the orientations 11, 12, 13, 14 by changing the horizontal orientation of the first steerable support in the direction D * -90 ° and adjusting the orientation vertical of the second orientable support so that the magnetic sensor indicates a value of zero measurement, the orientation 14 of the second orientable support corresponding to this measurement.
• the North search device is an absolute rotation detector type searcher and the controller controls the secondary motorization to modify the vertical orientation of the second orientable support and, according to the indications of the North search device. , measures the direction of the geographic north N by changing the horizontal orientation of the first steerable support until the absolute rotation detector indicates a zero measurement value and acquire the orientation N of the first steerable support corresponding to this measurement.
• the measurement of the direction of the geographic north N is obtained in a manner similar to D * , the magnetic sensor being replaced by the absolute rotation detector and the orientations D1, D2, D3, D4 and D * being replaced by N1, N2, N3, N4 and N in the method described above.
• the controller calculates the base lines of the variometer based on the three measures of the variometer: dU, dV, and dW, the absolute modulus of the local magnetic field vector F, the two angles characterizing the direction of the magnetic field vector.
• local (F) the inclination I and the declination D, and optionally functions gU, gV, gW, making it possible to change the coordinates D, F, I to U, V, W coordinates.
• the controller calculates the baselines U0, V0, W0, according to:
• V 0 gv (F, D, 1) - dV
• W 0 gw (F, D, 1) - dW.
• the controller calibrates the scaling factors of each component of the variometer by measuring the amplitudes of the variations of the baselines with respect to the amplitudes of the signals of each of the three components of the local magnetic field vector (F) during a pre-set time, then multiply three measures of the variometer (MV): dU, dV, and dW by corrective factors, scaling factors, fu, fv, fw, then subtracting the corrected values and calculating baselines according to:
• V 0 gv (F, D, I) - fv * dV
• the controller increases or decreases the respective scale factors to reduce the variations of the respective baselines and corrects the variometer measurements by multiplication by the respective scale factors. These steps may be repeated iteratively until the increase or decrease in scaling factors is less than a predetermined value.
• the controller calibrates the orthogonal ity and the orientation in space of the three mathematically independent components of the variometer and calculates Eulerian E rotation matrices by examining the variation of the baseline of a component. depending on the signal amplitude of the other components for a predetermined period of time, calculating Eulerian rotation matrices E and adjusting the orientations until the variation of the baseline is less than a predetermined value.
• the scalar magnetometer and the variometer perform measurements with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz.
• the measurements of the orientations of the local magnetic field vector are carried out with a frequency of between 10 -7 Hz and 10 -2 Hz, preferably between 10 -6 Hz and 10 -3 Hz, even more preferably between 10 "5 Hz and 10 " 4 Hz.
• the autonomous observatory implements a method as described above.
• the drawings Brief description of the drawings
• FIG. 1 shows a local magnetic field vector and its decomposition into different mathematically independent components.
• FIG. 2 shows an example of an autonomous magnetic observatory according to the invention
• FIG. 3 shows an exemplary embodiment of an angular magnetometer according to the invention
• Figure 4 shows an example of a North search device based on a sighting principle
• FIG. 5 shows an example of a North search device based on an absolute rotation detector principle
• FIG. 6 shows an example of orientation of the sensitive axis of a magnetic sensor with respect to the local magnetic field
• FIG. 7 shows a second example of an autonomous magnetic observatory according to the invention.
• FIG. 8 shows a diagram of an exemplary method for obtaining the local magnetic field vector according to the invention.
• FIG. 9a shows a leveling defect along an east-west axis corresponding to a rotation about an axis X;
• FIG. 9b shows an example of baseline measurement of a Z component of the field presenting a leveling defect in the east-west direction.
• Figure 10 shows an example of an algorithm for the determination of scale factor and variometer orientation defects.
• Figure 1 shows an example of local terrestrial magnetic field vector, hereinafter local magnetic field vector F and its decomposition into different mathematically independent components.
• the local magnetic field F being a vector field, the knowledge of its intensity is not sufficient to characterize it completely. In fact, it must define an orientation with respect to reference directions. These reference directions are preferably the local vertical (or, equivalently, a horizontal plane) and the direction of the geographic North N.
• the magnetic variation "D” is then defined as the angle between the geographic North N and the projection of the magnetic vector in a horizontal plane.
• Magnetic inclination "I” is defined as the angle between the horizontal plane and the magnetic vector in a vertical plane containing this vector.
• FIG. 2 shows an example of autonomous magnetic observatory 200 according to the invention.
• this observatory 200 comprises: a scalar magnetometer MS, an angular magnetometer MA, a variometer MV, a clock 201, and a controller, 202.
• the scalar magnetometer MS serves to measure the intensity of the magnetic field, or in other words to measure the modulus of the local magnetic field vector F.
• the scalar magnetometer can be described as an absolute instrument in the sense that it measures a quantity not relative to a other.
• the scalar term designates a property that remains unchanged during a change of reference system.
• the scalar magnetometer MS is, for example, a magnetometer of the type: proton, overhauser, atomic, optically pumped.
• the measurements of the local magnetic field vector module F are carried out with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz.
• the accuracy of the measurements is such that the measurement error is less than 1 nT, even more preferably less than 0.5 nT, ideally less than 0.2 nT.
• a clock 201 for defining a time reference is used for the synchronization of the measurements of the different instruments.
• This clock is, for example, an internet time server, an atomic clock or a GNSS signal.
• the synchronization of the measurements must be better as the temporal variation of the field is high.
• Figure 3 shows an embodiment of an angular magnetometer MA according to the invention.
• the angular magnetometer makes it possible to determine the direction of the local magnetic field vector F.
• the angular magnetometer MA comprises:
• the first support comprises: a main motor 322 for modifying the orientation of the first orientable support 320 around the main axis, the inclination sensor 313, the second orientable support 310.
• the two axes of rotation 321 and 311 allow a displacement in two planes (horizontal and vertical) essentially orthogonal. These displacements allow to obtain a horizontal orientation of the first orientable support and a vertical orientation of the second orientable support.
• the second steerable support 310 comprises a secondary motorization 312 for modifying the orientation of the second orientable support around the secondary axis 311, a magnetic sensor 323 for measuring the direction of the local magnetic field vector F and a device for searching the North 324.
• the angular magnetometer MA also comprises a control means 340 of the main motor 322 and secondary 312 and an angular measurement and acquisition device 350 of the horizontal and vertical orientations of the first and second orientable supports 320, 310.
• the angular magnetometer has two degrees of freedom in rotation, and is capable of orienting the magnetic sensor 323 in an arbitrary direction. In particular, it is able to orient the magnetic sensor according to the magnetic field.
• the main motor 322 and secondary 312 have a negligible magnetic footprint so that they do not disturb the measurement of the magnetic field.
• these engines are piezoelectric motors.
• the angular measurement and angular acquisition device 350 makes it possible to measure an angle of rotation of the first and second orientable supports 320 and 310 respectively about each of the main axes 321 and secondary 311.
• This device is, for example, an optical encoder. This device can be used as a feedback signal for slave angular displacement.
• the position of the primary axis 321 can be adjusted using screws "calantes".
• Other solutions are possible such as a cardan joint, a ball joint or a suspended pendulum type system.
• the tilt sensor 313 measures the orientation of the primary axis 321 relative to the vertical.
• the inclination sensor makes it possible to obtain a first reference direction.
• This sensor is, for example, an electrolyte level or an accelerometer measuring gravity.
• the inclination sensor is secured to the rotating reference frame with the secondary axis 311 so that it is integral with the North search device 324 and the magnetic sensor 323.
• the inclination sensor can also be placed in a frame integral with the main axis 321.
• the angular magnetometer MA further comprises an auxiliary tilt sensor 314 that can be installed on the fixed part of the instrument.
• the auxiliary tilt sensor has two degrees of freedom.
• the angular magnetometer MA comprises a North 324 search device.
• This North search device makes it possible to determine the direction of the geographic North N and thus to determine a second reference direction.
• the North search device has two degrees of freedom, that is to say it can perform rotations along the main axes 321 and secondary 311.
• the North 324 search device is, for example, a target sighting system as shown in FIG. 4.
• a target sighting system as shown in FIG. 4.
• Such a device points a direction, Target, whose angle, Az, with respect to the geographic north N , is previously known. We thus deduce the direction of geographic North N.
• Such a device may be a laser pointing a retro-reflector.
• the return beam is then captured by a device such as a photocell or a PSD (Position Sensitive Detector).
• PSD Position Sensitive Detector
• Other solutions are also possible as a CCD camera.
• GPS or GNSS beacons can also be used.
• Figure 4 also illustrates the magnetic north direction Nmag, and the declination angle D.
• the North 324 search device may be a sun sighting device (sunshot): it is possible to derive the direction of the geographical north N by pointing the sun at a given moment (and this even if the sun is hidden by clouds, for example thanks to polarizing filters).
• Known methods require knowledge of sidereal time, declination and right ascension of the sun. These data are available in astronomical tables or derived from models.
• Other devices can also be used as an astronomical sighting device: the above principle can be applied to any other visible object.
• a star camera in a fixed reference point relative to the observatory, the stars do not appear fixed in the sky. An image of them at a given moment then makes it possible to deduce the direction of the geographic North N.
• the North 324 search device is an absolute rotation detector as shown in FIG. 5.
• This type of sensor is capable of detecting the rotation of the Earth ⁇ ⁇ .
• This detector measures the projection of the horizontal component 510 of the earth rotation vector along its sensitive axis 520.
• FIG. 5 shows an example of projection for a latitude ⁇ , and an orientation angle of the sensitive axis ⁇ measured relative to Geographical North N (not shown). These two directions are in the same horizontal plane.
• By moving this sensitive axis in a horizontal plane its output describes a sinusoid whose maximums lie in the north-south direction and the zeros in the east-west direction.
• sensors or gyroscopes may be in particular of the mechanical type, optical fiber (FOG), laser (RLG), hemispherical resonance (HRG), MEMS or atomic.
• the magnetic sensor 323 is a directional magnetic field sensor whose sensitive axis can be oriented by the main axis 321 of rotation and the secondary axis 311 of rotation of the angular magnetometer MA.
• This magnetic sensor may be, for example, fluxgate type, fluxset, rotating electrical circuit or a scalar magnetometer polarized by a magnetic device.
• the magnetic sensor 323 may be capable of measuring the intensity of the magnetic field (scalar measurement).
• the scalar magnetometer MS can therefore be included in the angular magnetometer MA.
• Such a sensor may be a full-field fluxgate mono or triaxial probe, a fluxset triaxial probe, or a scalar magnetometer.
• the scalar measurement can also be performed by an auxiliary magnetic sensor included in the instrument.
• the measurements of the orientation of the local magnetic field vector are carried out with a frequency of between 10 -7 Hz and 10 -2 Hz, preferably between 10 -6 Hz and 10 -3 Hz, even more preferably between 10 " 5 Hz and 10 " 4 Hz.
• the accuracy of the angular measurements in the horizontal plane is such that the measurement error is less than 20 arc seconds, even more preferably less than 10 seconds. 'bow, ideally less than 6 arc seconds.
• the accuracy of the angular measurements in the vertical plane is such that the measurement error is less than 10 arc seconds, even more preferably less than 5 arc seconds, ideally less than 1 second of bow.
• the variometer MV also called vector magnetometer, measures the variations of three components mathematically independent of the local magnetic field F at regular intervals.
• these measurements of three components mathematically independent of the local magnetic field F are carried out with a frequency of between 0.01 Hz and 100 Hz, preferably between 0.05 Hz and 10 Hz, even more preferably between 0.1 Hz and 1 Hz.
• the accuracy of the measurements is such that the measurement error is less than 2 nT, and even more preferably less than 1 nT, ideally less than 0.5 nT.
• the variometer MV and the scalar magnetometer MS can be combined in the same instrument.
• the autonomous observatory also comprises a controller 202.
• This controller is configured to:
• magnetic sensor 323 for measuring the direction of the local magnetic field vector F
• step d) processing the data acquired in step c) to automatically obtain the local magnetic field vector F and the measurement errors associated with each instrument.
• the first and second steerable supports 320 and 310 the main motor 322 and secondary 312, the North search device 324, the motor control means 340, and the measuring device and angular acquisition 350 consist of non-magnetic components, that is to say that the magnetic susceptibility of the materials constituting them is between -1 and 1, preferably between -10 "1 and 10 " 1 , even more preferably between - 10 "3 and 10 " 3 .
• the materials are materials selected from: ceramic, aluminum, arcap, titanium, copper, ertalon, nylon, ertacetal, peek.
• Figure 7 shows a second example of autonomous magnetic observatory 700 according to the invention.
• the autonomous magnetic observatory comprises an angular magnetometer MA, a scalar magnetometer MS, a variometer MV, a clock 201, a controller 202 as described above.
• the observatory 700 further comprises a power source 701, and the controller 202 is further capable of acquiring and transferring data from the instruments (MA, MS, MV) to an external network via a communication system 702
• the magnetometers are remote from the different electronics so as not to be disturbed by them.
• the angular magnetometer MA and the variometer MV can be inserted in enclosures with high thermal inertia and insulation. A sandwich structure can also be envisaged.
• the angular magnetometer MA and the variometer MV can also be installed on a stable support such as a pillar anchored on a solid base 730.
• the base may be rock or a slab made of non-magnetic materials.
• two non-magnetic housings 710 surround a plurality of pillars 711 and comprise an insulated wall 720 whose average thickness is between 1 and 60 cm, preferably between 2 and 30 cm, so that even more preferred between 5 and 10 cm.
• the plurality of non-magnetic pillars preferably of concrete, whose average dimensions [thickness, width, length] are between [1, 10, 10] cm and [6, 10, 10] m, preferably the average dimensions [thickness, width, length] are between [10, 20, 20] cm and [1, 2, 2] m, even more preferably between [15, 25, 25] cm and [0.25, 0.5 , 0.5] m, said pillars being separated by an average distance of between 0 and 10 m, preferably between 1 and 6 m, even more preferably between 2 and 4 m.
• one of the shelters 710 comprises the vector magnetometer MV and the other the angular magnetometer MA.
• the observatory comprises a single shelter 710.
• the non-magnetic pillars 711 support at least one of the following elements: the scalar magnetometer MS, the angular magnetometer MA, the variometer MV, the clock 201, the controller 202.
• the non-magnetic shelter protects at least one of the following: the scalar magnetometer MS, the angular magnetometer MA, the variometer MV, the clock 201, the controller 202.
• the autonomous magnetic observatory according to the invention can be installed on land or under water, for example at the bottom of the sea.
• the instruments (MA, MS, MV), the clock 201 and the controller 202 are protected by one or more sealed protective housings.
• the instruments (MA, MS, MV), the clock 201 and the controller 202 are installed in a single housing, or, preferably, in separate housings.
• the housing or cases prevent the external water from damaging or destroying their contents.
• an underwater magnetic observatory can be supplied with electricity via batteries, a submarine cable, a surface buoy connected to the observatory by a conductive cable, a marine electric generator.
• the invention relates to a method for obtaining the local magnetic field vector F.
• FIG. 8 shows a diagram of an exemplary method 800 for obtaining the local magnetic field vector F according to the invention. This method includes the steps:
• S810 provision of a magnetic observatory as described above
• S820 the controller 202 acquires measurements of the local magnetic field vector module, F, measured with the scalar magnetometer MS at different times ti;
• S840 the controller controls the main motor 322 to change the horizontal orientation of the first steerable support 320 and, depending on the indications of the inclination sensor, measures the vertical, V; e) S841: the controller controls the secondary motorization 312 to change the vertical orientation of the second steerable support 310 and, based on the indications of the North search device 324, measures the direction of the geographic North N;
• S842 the controller controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer MA and, according to the indications of the magnetic sensor 323 obtained at different times ti, measures two angles, D * and P, corresponding to the direction of the local magnetic field vector F;
• S873 obtaining the 3 components of the local magnetic field vector F by adding the baselines to the three oriented measurements and to the scale.
• the local magnetic field vector F varies over time, which is why measurements must be made at different times ti.
• the scalar magnetometer MS is an instrument capable of supplying the value of the local magnetic field module F with an accuracy such that the measurement error is generally less than 1 nT, preferably less than 0.5 nT, ideally less than 0.2 nT. .
• the controller 202 of the autonomous observatory is configured to acquire the values provided by the scalar magnetometer MS.
• the controller 202 controls the main motor 322 and secondary 312 to measure the vertical direction V with the tilt sensor.
• the controller is also configured to control the main 322 and secondary 312 motorizations to measure the North Geographic N direction using the North 324 search facility.
• the North 324 search device is an absolute rotation detector and the search for the geographic North N consists of modifying the horizontal orientation of the first orientable support 320 until the absolute rotation detector indicates a zero measurement value.
• the controller 202 then acquires the orientation of the first medium indicating the geographic North N.
• the North search device is an absolute rotation detector and the search for the geographic North N includes the steps:
• N (N1 + N2 + N3 + N4) / 4.
• the measurement of the geographic North N and the vertical V makes it possible to define a reference frame in which the local magnetic field vector F will be expressed.
• the magnetic sensor 323 of the angular magnetometer MA is a directional sensor so that only a component of the local magnetic field parallel to the sensitive axis of the sensor is measured.
• Controller 202 is therefore configured to measure the two angles, D * and, by changing the horizontal orientation of the first steerable support 320 until the magnetic sensor indicates a minimum value which is ideally a zero, which gives a first orientation D * .
• the controller then changes the vertical orientation of the second steerable support 310 until the magnetic sensor indicates a minimum value which is ideally a zero, giving a second orientation I * .
• the component of the magnetic field measured by the magnetic sensor 323 can be expressed as a function of the two degrees of freedom as well as different misalignments using the model known to those skilled in the art of Lauridsen (Lauridsen, KE , 1985. Experiments with the Danish Meteorological Institute Geophysical Papers, R-71):
• T the output of the magnetic sensor 323, T0: a possible offset
• H the horizontal component (in the plane perpendicular to the main axis 321) of the local magnetic field vector F
• Z the vertical component (parallel to the main axis) of the local magnetic field vector F
• D the magnetic declination
• a angle of the angular magnetometer MA in the horizontal plane relative to the geographic north
• ⁇ angle in the vertical plane with respect to vertical
• e defect of alignment of the magnetic sensor 323 with respect to its sensitive axis in a vertical plane
• ⁇ misalignment of the magnetic sensor relative to its sensitive axis in the horizontal plane when the sensitive axis is horizontal.
• the angular directions of the magnetic field are then determined by seeking magnetic sensor orientations substantially perpendicular to the magnetic field.
• the controller 202 controls the main and secondary motorization to modify the horizontal and vertical orientations of the angular magnetometer MA and searches for zeros corresponding to 4 orientations (+/- 180 0 depending on the horizontal and +/- 180 ° depending on the vertical ). For example, for the search for declination D, the controller performs the steps:
• the controller 202 obtains the inclination I by performing the steps:
• a second method consists in looking for a direction in which the magnetic sensor 323 indicates a measurement value close to zero but which has a residue dT.
• the components of the local magnetic field whose orientations are sought can be converted according to the following relationships:
• the controller 202 acquires values of the three components of the local magnetic field vector F: dU, dV, dW, measured with the variometer MV at different times ti.
• the measurement of the magnetic field is preferably also carried out by means of a vector magnetometer, also called MV variometer, which measures the variations of the three components of the magnetic field at regular intervals (eg 10 Hz, 1 Hz). Hz, 0.05 Hz). These are relative measurements against a reference. These measurements must therefore be calibrated using absolute measurements provided by an angular magnetometer MA and a scalar magnetometer MS.
• the values of the three components of the local magnetic field vector F: dU, dV, dW, measured with the variometer MV are component variations around a reference value.
• This reference value generally called the LDB "baseline” is determined by subtracting the temporal variation of the component from its “absolute” or complete value.
• the temporal variation is the relative value of the component measured by the MV variometer.
• C 0 C (t) - 5C (t), with: C 0 : the LDB of a component C supposed to be independent of time, C (t): the "absolute" value of C at time t (determined by the absolute measurements of the angular magnetometer MA and the scalar magnetometer MS) and 5C (t): the variation of C at the instant t measured by the variometer MV.
• LDB depends only on the implementation of MV and not on the magnetic field itself. However, this value may vary over time due, for example, to temperature. Regular absolute measurements then allow its adjustment.
• the calculation of the LDBs can also involve a change of coordinates, for example coordinates D, F, I to any coordinates U, V, W.
• the controller 202 calculates the baselines of the variometer MV on the basis of the three measurements of the variometer MV: dU, dV, and dW, of the absolute modulus of the local magnetic field vector F, of the two angles characterizing the direction of the local magnetic field vector F: the inclination I and the declination D, and, of functions gu, gv, gw, making it possible to change the coordinates D, F, I to U, V, W coordinates.
• the controller calculates the base lines Uo, Vo, Wo, according to:
• V 0 gv (F, D, 1) - dV
• W 0 gw (F, D, 1) - dW.
• the variometer MV may also have a scale factor defect (one per component) which must be calibrated by the controller 202.
• the measurement of the field is generally performed through a chain of acquisition transforming the basic magnetic signal into a signal usable by the controller, for example, a current, a voltage or digital bits.
• a scale factor is generally applied to interpret the output signal of the acquisition chain. The determination of the baseline at sufficiently high resolution makes it possible to highlight this effect and the correct.
• the controller calibrates the scaling factors of each component of the MV variometer by performing the steps of:
• V 0 gv (F, D, I) - fv * dV
• W 0 gw (F, D, 1) - fw * dW.
• steps a) to d) being repeatable until the increase or decrease in scale factors is less than a predetermined value.
• declination D we have:
• the MV variometer may have an orthogonal or orientation defect or be mis-oriented in order to measure "unconventional" components. These defects must be corrected and calibrated by the controller 202. In this case, it is possible to detect a correlation between a component measured by the variometer MV and the LDB of another component by means of regular absolute measurements. Again, the correction tends to minimize this correlation.
• the correction consists of applying a base change matrix to find the basic components of the magnetic field.
• the base change matrix is called matrix Eulerian, E.
• the controller thus calibrates the orthogonality and spatial orientation of the three mathematically independent components of the MV variometer and calculates Eulerian E rotation matrices by performing the steps of:
• the aforementioned defects may be present during the implementation of the observatory, vary or appear in time.
• the MV variometer can, for example, be installed on an unstable pillar. The correction made must then be made as and when the operation of the observatory.
• the present invention can control and correct these defects in real time.
• controller 202 calculates the value of the local magnetic field vector F by performing the steps of:
• S871 obtaining oriented measurements by applying Eulerian E rotations to the three MV variometer measurements: dU, dV, and dW;
• the variometer MV can measure the variations of the components X, Y, Z, H and F generally expressed in nanoTesla (nT) or angular D and I usually expressed in arcmin, arcsec, degrees-minutes-seconds or degrees decimal.
• the modulus of the local magnetic field is given by the scalar magnetometer MS, the absolute values of the angular components, preferably the declination D and the inclination I, are provided by the angular magnetometer MA.
• One possible solution is to perform several absolute measurements per day (eg every 30 min or each hour) and observe a diurnal variation in LDB corresponding to the diurnal variation of its component. We then determine:
• the measurements can be spread over several days and can also be averaged or smoothed.
• the measurements may have a defective orientation.
• one solution is to measure the LDB at a frequency to highlight diurnal variation.
• Figure 9a shows an example of a leveling defect ⁇ along an east-west axis corresponding to a rotation around the X axis.
• the sensor output then becomes (in matrix notation):
• a rotation matrix is found around an X axis. Calculating the LDB as presented above then requires the inverse transformation which is none other than the transpose of the previous matrix.
• ⁇ can be determined empirically or by calculation (possibly iterative) by considering the LDB. For example, we can try to minimize the variance of the baseline or the covariance of the LDB with another component. In the case of a weak orientation fault, we can assume: cos (0) "1 and sin (0)" ⁇ (in radians):
• Z 0 is not directly accessible.
• One solution is to take the variations in Z and Q * ⁇ "peak-to-peak", possibly filtered or smoothed to avoid measuring noise effects:
• Figure 9b shows an example of a baseline measurement, Z 0 of a Z component of the field having a leveling defect in the east-west direction.
• Figure 9b illustrates the case of a leveling defect of 5 ° in the east-west direction.
• the variations of the baseline lines Z 0 (in nano Tesla, nT) uncorrected baseline and corrected baseline are represented over a period of three days (in abscissa, day). Case of an observatory in any orientation
• FIG. 10 a possible algorithm is presented in FIG. 10. It consists of successively applying matrices of rotations Rx and Ry around the X and Y axes, respectively, so as to mathematically correct the Z-axis. These virtual rotations are determined so as to minimize a particular coefficient such as the variance of Z Q * or the covariance of Z Q * and ⁇ or ⁇ or a combination of both.
• the two parameters to be minimized can be determined as follows:
• ⁇ ⁇ jvAR (z * ) 2 + VOC (Z ;, ⁇ Y) 2
• VOC VOC (A, B) the covariance of A and B. Then, a rotation matrix around the Z axis is applied in order to minimize the variance of X Q * or Y Q OR the covariance of these LDBs with ⁇ or ⁇ or a combination of these parameters.
• a similar process can be used upstream or downstream of the correction guidance to adjust scale factors.
• the whole algorithm can have several iterations.
• the variometer MV measures variations of the field components corresponding to a spherical coordinate system.
• the variometer MV measures variations of the local magnetic field in three orthogonal directions, one of which is oriented parallel to the magnetic field and measures its intensity in the same way as in a cartesian configuration.
• the other two directions are in a plane perpendicular to the magnetic field.
• the direction allowing the relative measurement of D lies in a horizontal plane perpendicular to the magnetic field while that allowing the measurement of I lies in the magnetic meridian.
• These two axes record the projection of the magnetic field (H for the measurement of D and F for the measurement of I).
• a transformation similar to that used during the absolute measurements D and I then makes it possible to go from a residual to an angular value.
• MV measurements may be affected by a scale factor error.
• a similar procedure is then used to adjust them, such as: performing high frequency LDB measurements (eg every hour) and adjusting the scale factor of a (relative) component to minimize its LDB variation .
• the linear correlation coefficient can be used.
• a misalignment of MV produces effects that depend on the measured component.
• F and I the effect is not critical because it is the total field F that is projected. So we have a second order relation.
• the relative measure of D uses the projection of H (sensitive axis in the horizontal plane).
• a horizontal defect induces a projection of the Z component of the field.
• LDB dependence of sin (/)
• the variometer MV measures variations of the field components corresponding to a cylindrical coordinate system. She may be considered intermediate to the two previous ones. A generalization can be done in the case of any orientation of MV.
• An autonomous magnetic observatory 200 which comprises a scalar magnetometer MS for measuring the modulus of the local magnetic field vector F, an angular magnetometer MA for measuring the direction of the vertical V, the geographical north direction N, the direction of the vector field local magnetic F, a variometer MV to measure three variations of the vector local magnetic field F, a clock 201, and a controller, 202.
• the controller is configured to control and manage the orientation of sensors, acquire the measures of the variometer MV, the scalar magnetometer MS, the angular magnetometer MA and the variometer MV, process the acquired measurements to automatically obtain the local magnetic field vector F and the measurement errors associated with each instrument.

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