FR2946759A1 - GRAVIMETRY MEASUREMENT METHOD AND SYSTEM IMPLEMENTING THE METHOD - Google Patents

Abstract

The invention relates to a gravitational field measurement method comprising at least two steps. A first step (101) determines, using at least one measurement antenna, a table of measurements representative of the gravitational field generated in the coverage area of the antenna, said antenna being composed of a plurality of gravimetric sensors, a gravimetric sensor consisting of at least one high resolution three-axis gravimeter. A second step (102) determines an image of the mass density distribution in the coverage area of the antenna by inverting the measured gravitational field and contained in the measurement table. The invention also relates to a system implementing the method as described by the claims.

Description

The invention relates to a gravimetric measurement method and to a system implementing the method. The invention can be used in particular in the fields of detection of tunnels, buried structures, geological and petroleum research, detection of the presence and movement of land, air and space mobiles. The invention can also be used for the implementation of anti-collision devices, for example embedded in submarines, and guidance or proximity rockets embedded in torpedoes or missiles.

The detection of objects of interest and their movements is usually carried out using sonars for underwater detection and radar for aerial detection. In the remainder of the description, a target designates an object of interest, for example an airplane, a land vehicle or a submarine. Radars are based on the reflection of radio waves to detect the position and movement of targets in their field while sonars are based on the reflection of sound waves, or on the listening of noise when they are passive . These detection methods have certain limitations. Indeed, they can be rendered ineffective when countermeasures techniques are used, in particular by scrambling or decoy. The measurement of the gravitational field, in particular of its radial component is used in applications related to the detection, in particular of the framework of the geological exploration, as described in the work of Guillaume Martelet entitled modeling of the crustal structure and the mechanical behavior lithosphere from gravity anomalies. Applications to the Himalayas and the granitic massif of Mt Lozère, Cevennes 1999, Thesis of doctorate, Institute Physique of the Globe.

Profile measurements can also be used for this purpose, as described in particular in the article by A. Gerard and P. Griveau entitled Quantitative interpretation in gravimetry or magnetism from transformed maps of vertical gradient, 33rd Congress of the E.A.E.G. Hannover, June 1971. In addition, detectors using gravitational field or gradient sensors have been used, in embedded applications on satellite, on aircraft as well as on land vehicles. Gravitational field measurements are also used for the implementation of underwater navigation methods using gravitational field correspondence techniques inspired by TERCOM systems, an acronym derived from the English expression TErrain COntour Matching. These techniques are described in Controlling the Quantum World: The Science of Atoms, Molecules, and Photons, National Academy Press, 2007. Existing detection techniques based on the gravitational field measurement cited above are now used for observation or navigation purposes and not in the context of the detection of presence and movement of targets because of their lack of precision for such applications.

An object of the invention is in particular to overcome the aforementioned drawbacks. For this purpose the invention relates to a gravitational field measurement method comprising at least two steps. A first step determines, using at least one measurement antenna, a table of measurements representative of the gravitational field generated in the coverage area of the antenna, said antenna being composed of a plurality of gravimetric sensors, a gravimetric sensor being composed of at least one high resolution three-axis gravimeter. A second step determines, for example, an image of the mass density distribution in the coverage area of the antenna by inverting the measured gravitational field and contained in the measurement table. According to one aspect of the invention, the inversion of the gravitational field is performed by applying a linear transform to said array.

According to another aspect of the invention, gravimeters gravimetric sensors constituting the measuring antenna are of one or more types among which: gravimeters material waves, cold atoms, optical wedge, parallelepiped, sphere electrostatic.

The gravimetric sensors composing the measurement antenna comprise, for example, a three-axis gyrometer with a resolution compatible with the gravimeters included in the gravimetric sensors. According to one embodiment, the measurement table corresponds to a finite grid of elementary boxes grouping the image of a given, finite or infinite volume of the observable universe. According to another embodiment, the gravimetric sensors constituting the measurement antenna are aligned and arranged at regular intervals. The inversion in the finite grid of elementary dishes is carried out, for example, using a Radon-Abel transform. According to one aspect of the invention, the arrangement of the gravimetric sensors making up the measurement antenna is not flat. The inversion performed in the second step of the measured gravitational field contained in the measurement table is performed, for example, using a suitable generalized transform. The inversion performed during the second step of the measured gravitational field contained in the measurement table is performed, for example, by inverse projection of the measurements in the finite grid of selected elementary boxes. The method may comprise a third step, said step using the mass distribution images produced successively during the application of the first two steps of the method and analyzing the differences between at least two distinct mass distribution images and deducing from this difference the trajectory of moving targets. MTI type measurements are applied, for example, to differences detected between at least two distinct mass distributions. The invention also relates to a gravimetric measurement system. The system comprises a measurement antenna, said antenna being composed of a plurality of gravimetric sensors, a sensor being composed of at least one high-resolution three-axis gravimeter and comprising means for implementing the method described above. According to one embodiment, the measurement system comprises means for towing the measurement antenna.

The invention has the particular advantage of providing a method of measurement and detection that is completely passive and impossible to scramble. In addition, unlike a radar or sonar, a fixed or moving target positioned behind an obstacle can be detected, since any material is perceived to be transparent when this method is used. In addition, detection is not affected by weather elements such as precipitation and temperature. Furthermore, the detection method according to the invention operates in a linear manner which results in the possibility of differential measurements. It also makes it possible to visualize the ground included in the field of observation as well as mobile targets such as submarines, ships, planes and helicopters.

Other characteristics and advantages of the invention will become apparent with the aid of the description which follows, which is given by way of nonlimiting illustration and with reference to the appended drawings in which:

Figure 1 illustrates the detection method according to the invention; FIG. 2 gives an example of a device implementing the invention. Figure 1 illustrates the gravimetric measurement method according to the invention. The method is composed of at least three steps 101, 102, 103. Depending on the targeted application, the third step 103 may or may not be implemented, the latter may be used for detection purposes. target. The objective of the first two steps is to obtain an image of the mass density distribution in the sensor environment implementing the method, a sensor designating in the following description the association of a series of sensors. assembled into a measuring antenna and coupled to a suitable processing system. The image thus obtained makes it possible to distinguish local fluctuations of gravitational field. An antenna composed of a plurality of associated accurate gravimetric sensors is used to obtain successive series of gravitational field measurements. A number of these series of measurements corresponding to a sliding time window are extracted, and are combined to give a local image of the gradient of the gravitational field along the trajectory of the wearer and the width of the antenna. This local field gradient is then inverted by a suitable transform, during the second step detailed later in the description, in order to pass from a local vector gravitational field to the mass density distribution giving rise to the measured gravitational field. . The number of sensors must be sufficient to create inversion constraints avoiding ambiguities related to a purely gradiometric technique by relegating them to a lower order than the requested resolution. It must also not have plane symmetry so as not to lead to an ambiguous positioning with respect to a measurement plan. The method thus makes it possible to obtain, without emitting anything and without being identifiable by itself, an image of the environment of the sensor and of all the targets in this environment by traversing any material present. The various steps of the method are detailed in the following description. The method can be implemented in embedded devices, for example in boats, submarines but also in fixed installations and mobile installations, towable or transportable.

When the method according to the invention is activated 100, a first process 101 aims to continuously acquire measurements of the gravitational field. These measurements are performed by at least one measuring antenna capable of providing information on its environment, said antenna being composed of a plurality of gravimetric sensors. A large number of sensors must be used, for example 96. The gravimetric sensors composing the measurement antenna 35 comprise, for example, a high-resolution three-axis gravimeter associated with a three-axis gyrometer of compatible resolution. These sensors can be arranged in different configurations. Said sensors may be distributed using, for example, a planar, linear, voluminal or synthetic configuration. In this configuration, the sensors can be distributed regularly or irregularly. The method according to the invention is applicable to all types of gravimetric sensors on the measuring antenna since it does not lead to a plane symmetry of the measurement grid. The final resolution grid of the sensor depends on the arrangement of the sensors, said provision is, therefore, an implementation choice to be taken into account during the design of the system implementing the method according to the invention. The gravimetric sensors may for example be made using a technology exploiting material waves, but any other sensor providing a resolution and sufficient accuracy can be used. Thus, gravidometers with cold atoms, optical wedge, parallelepiped or electrostatic sphere can be used. The use of gyrometers within sensors is not mandatory. These can be replaced by the use of any other technique, for example optical, allowing a determination to the required accuracy of the relative directions of measurement of the axes of the gravimetric sensors in order to determine the orientation of the measurement mark. It is also possible for each gravity sensor to measure the gravity field on Na simultaneous axes, Na being greater than 3 and the axes being arranged so that each subset of axes taken from 3 to 3 constitutes a three-dimensional reference (non-coplanar axes). ). This arrangement makes it possible to solve, thanks to the additional independent equations thus obtained, a system of equations defining the field without using a coupled gyrometer. The orientation of a given gravitational sensor can be determined by calibration, for example, by slowly rotating it in a uniform gravitational field. An alternative is to calibrate the sensor in its entirety, the antenna being stationary relative to the earth. This calibration is performed by exploiting the gravitational field of the earth, the moon and the sun for a sufficient period of time, for example from 6 to 24 hours, in order to obtain the required accuracy. Any other method for calculating with the required accuracy the relative orientations of the axes of the various sensors constituting the antenna can be used in the context of the invention.

The simulation of observation and detection coverage is simplified because of the intrinsic properties of the sensor whose behavior is totally and exclusively defined by a geometry in 1 / r 2. In the measurement acquisition process 101, each of the N sensors performs a series of P measurements of the gravitational field in which it is immersed.

The measurement acquisition process 101 includes, an extraction task whose purpose is to periodically generate an array of measurements of the gravitational field, ie a vector array. For this, a successive series of P measurements is used. It is also possible

Use a sliding window of p measurements or any other combination providing a system of equations of n x p measures where n is an integer less than or equal to N and p an integer less than or equal to P.

Each measurement at time t for the sensor i provides for each axis of each sensor a value given by the projection on the measurement axis 20 of an integral that can be expressed by the following expression: r `+ ft F ( p, i) xm (p, T (p, t)) dvdt, i (i, t) = M (i) JJ / universe t IY (p'i) I observable in which:

I i is the index of the sensor;

M (i) represents the gain matrix of the sensor i on the axes of the marker used; p represents a point traversing the observable universe on which the integration is made, this point being surrounded by an elementary volume dv without overlap with the neighboring point;

T represents the local time at the place where the measurement is made;

T (p, t) is the corrected time of the propagation delay of the gravitational field created by the point p and corrected for the relativistic effects due to non-collocalisation (1) of the point p and the sensor. In the remainder of the description, it is considered as a first approximation that T (p, t) = t; A more precise formula of the type T (p, t) = l @ (p'i) I + t or c is the speed of c

propagation of the gravitational potential may be substituted for the first approximation cited above;

m (p, T (p, t)) is the mass density at point p at time T; in the case of fixed targets and the fixed environment this function is independent of t; in the case of mobile target or mobile environment (waves, forest) the function depends on t;

R (p, i) is the vector joining the sensor i at the point p considered; takes into account the movement of the sensor itself. The first integral of expression (1) expresses the fact that the sensor is influenced by the entire observable universe. The second integral expresses the

The measurement is carried out for a certain period dt, at time t. The integrated term corresponds to Newton's law, said term indicating that the gravitational field created by a given element is directed along the axis joining the element to the observation point and that it is proportional to its mass, inversely proportional. squared the distance, and does not depend on any

20 another factor. The gravitational field is independent of any obstacle that may be on the propagation line. This point is a significant difference from a sonar or a radar, the latter being disturbed by the environment in which they move.

The law of Newton recalled previously is additive and linear. The

A measurement table obtained for the sum of two material distributions is the sum of the tables of each distribution. This property causes the possibility of differential measurements. In addition, the table of measurements obtained by multiplying the distribution of material by a factor x is x times the measurement table of the initial distribution.

In what follows, the rigid term is understood as rigid on the scale of the precision required.

The antenna used is, for example, rigid and can be moved by a carrier or towed by a vehicle. The array of vectors measured will be according to the extrusion of the figure constituted by all the sensors of the sensor according to the trajectory of the antenna fixed by the carrier or the vehicle. It is also possible to use non-rigidly connected sensors and to measure the own motion for each sensor, or by integrating the 5 values of g measured previously.

A second measurement processing process 102 uses, in particular, as input the array of measurements obtained during the acquisition step 101. The array of measured vectors can then be transformed into an image of the environment which generated this gravitational field. on the sensor, that is to say an image of the mass density distribution in the sensor environment, and this by applying to this table an inverse linear transform of the expression (1). It is preferable not to carry out this operation directly because the information collected by the sensors in view of their accuracy, their resolution and their measurement noise does not contain a quantity of information equal to that contained in the universe. observable. On the other hand, the integral being continuous, the volume of calculations to be performed would be infinite. The processing can be reduced to a finite case by defining a finite grid of elementary boxes grouping the image of a certain volume of the observable universe, said boxes being called graxels in the remainder of the description. Thus, each graxel is the image of a given volume of observable space. The observation and the very fact that we existed shows that the field of gravitation measured on earth is not infinite and that it is very close to the earth's own field. The mass distribution beyond the edge of the image provides only a finite contribution to the measured gravitational field which will vary little with the displacement of the observation volume of the sensor, said volume being usually called the instrumented range. Treatment by integration or by periodic calibration makes it possible to overcome it. It is possible to choose a grid of defined measures with a step in r2 so that the amount of information used for each graxel is the same. The outermost layer of the grid will be extended to infinity, thus covering the entire observable universe. This layer will not be exploited later and serves only to set the boundary conditions. Since the transform is linear in the sense defined above, the total amount of information available in the measurement table is retained after transformation. The use of a grid consistent with the amount of information except for its last layer which groups the

contribution from the rest of the universe, allows to work at constant precision for each useful graxel.

The use of a grid of n measurements makes it possible to express the gravitational field by using the following expression: '+ at F (p, i) xm (pn, T (pn, t)) vndt g (l, t ) = gO + m (l) J ns (2) Grid t IY (pn, l) I n where: gO is the gravitational field created by the set of points on the outside of the grid; this can be evaluated, for example, by an initial measurement followed by an update at each measurement cycle;

n denotes the graxel index in the calculation grid; pn represents the point situated at the center of gravity of the graxel considered; vn is the volume of graxel; 20 m is, in this expression, a function of average density of the graxel considered. The sum obtained is discrete and finite and can then be inverted with a finite volume of calculations.

If the sensors are aligned and arranged at regular intervals, a carrier moving the antenna perpendicular to this axis, then the measurements constitute a plane matrix which is the image of the observable universe. The inversion in the grid of graxels is, for example, by a so-called Radon-Abel transform.

In the case where the arrangement of the measuring points is not flat, the inversion can be done, for example, by a suitable generalized Radon transform or by inverse projection of the measurements in the chosen image grid. In this case, the tracking of the deformations of the antenna which conditions the relative positioning of the measuring points can be done by performing a double integral of the acceleration measured by the sensor in order to obtain the relative displacement with respect to its position. initial. The result of this second step of the process according to the invention is an image of the mass distribution.

A third post-processing process 103 exploits the image of the mass distribution resulting from the measurement processing step 102. This third process analyzes the difference between two successive mass distributions and makes it possible to track the moving targets. This is a consequence of the linearity of the sensor with respect to the mass distribution as defined above. This third step is optional and may not be implemented for navigation sensor applications for ships and submarines, for searching for tunnel or buried structures, for geological research, and for any other application requiring knowledge of the terrain surrounding the sensors. In addition, extraction and tracking techniques can be applied to the differential image to extract the trajectories of the targets, to improve their detection and to reduce the rates of false alarms. Techniques derived from the radar, such as the application of differential filtering to MTI-type measurements, an acronym derived from the Anglo-Saxon expression "Moving Target Indicator" can be advantageously applied. They also have the advantage of eliminating the natural gravitational field to keep only moving objects. In order to extend the speed domain accessible to the sensor for a given resolution, a corrective algorithm can be applied at the time of integration by making a series of hypotheses of displacement of a possible target on the graxels. This arrangement makes it possible to reveal the moving targets whose displacement during the acquisition of the measurements exceeds the dimensions of a graxel. This effect, usually called distance migration in the context of radars, also affects the detection method according to the invention.

Techniques based on a comprehensive test of travel hypotheses for a given domain are also applicable. The techniques of detections used with long integration used in particular in the sonars can be applied to during this third stage. Detection techniques described in US5432753 entitled Target Detector and Localizer for Passive Sonar.

FIG. 2 gives an example of a system implementing the gravimetric detection method. This system comprises an antenna 201, said antenna being composed of a series of sensors 202 aligned and fixed on a bar 208. Other arrangements of said sensors can also be envisaged for this antenna. The antenna is towed by a ship 200. For this, a cable 209 may be used, said cable connecting the ship to a first submerged wing 203, the first wing being connected to a second wing also immersed 204, for example by one or several solid rods 210. The antenna is fixed on said solid rod 210 so as to move at shallow depth at the same speed as the boat 200. If the bar 208 is fixed to the solid rod 210 so as to move in following the direction of movement 211 of the boat while remaining parallel to the surface of the sea and perpendicular to said rod, the displacement of the antenna makes it possible to generate a planar matrix of measurements 207 defining the image grid mentioned previously in the description. These measurements can be, for example, transmitted to a processing center located in the boat 200, said processing center for exploiting the measurements as described above in the description. Other implementations of the system according to the invention can be considered, the antenna being able to be positioned, for example, on the blank of a submarine, on a line of sensors at the front of the boat or carried by a plane or glider.

Claims (14)

CLAIMS1- Gravitational field measurement method comprising at least two steps characterized in that: a first step (101) determining, using at least one measurement antenna, a table of measurements representative of the gravitational field generated in the coverage area the antenna, said antenna being composed of a plurality of gravimetric sensors, a gravimetric sensor being composed of at least one high resolution three-axis gravimeter; a second step (102) determining an image of the mass density distribution in the coverage area of the antenna by inverting the measured gravitational field and contained in the measurement table. 2. The method as claimed in claim 1, characterized in that the inversion of the gravitational field is performed by applying a linear transform to said array. 3- A method according to any one of claims 1 or 2 characterized in that the gravimeters gravimetric sensors constituting the measuring antenna are of one or more types among which: gravimeters material waves, cold atoms, optical wedge, with parallelepiped, with electrostatic sphere. 4. Method according to any one of the preceding claims, characterized in that the gravimetric sensors comprising the measuring antenna comprise a three-axis gyrometer with a resolution compatible with the gravimeters included in the gravimetric sensors. 5. Method according to any one of the preceding claims, characterized in that the array of measurements corresponds to a finite grid of elementary boxes grouping the image of a given volume, finite or infinite of the observable universe. 6. Process according to any one of the preceding claims, characterized in that the gravimetric sensors constituting the measuring antenna are aligned and arranged at regular intervals. 7- Process according to claims 5 and 6 characterized in that the inversion in the finite grid of elementary boxes is carried out using a Radon-Abel transform. 8- Method according to any one of claims 1 to 5 characterized in that the arrangement of the gravimetric sensors constituting the measuring antenna is not flat. 9- Method according to claim 8 characterized in that the inversion performed during the second step of the measured gravitational field contained in the table of measurements is performed using a suitable generalized transform. 20 10-Process according to claims 5 and 8 characterized in that the inversion, performed during the second step, the measured gravitational field contained in the measurement table is performed by inverse projection of the measurements in the finite grid of elementary boxes 25 chosen. 11- Method according to any one of the preceding claims, characterized in that a third step (103) uses the mass distribution images produced successively during the application of the first two steps of the method (101, 102) and analysis. the differences between at least two distinct mass distribution images and deduces from this difference the trajectory of moving targets. 12- Method according to claim 11 characterized in that MTI type measurements are applied to the differences detected between at least two separate mass distributions. 13- gravimetric measuring system characterized in that it comprises a measuring antenna (208), said antenna being composed of a plurality of gravimetric sensors, a sensor being composed of at least one high resolution three-axis gravimeter and comprising means for implementing the method according to any one of ~ o claims 1 to 11. 14- Measuring system according to claim 13 characterized in that it comprises means for towing (200) the measuring antenna (208).