WO2007074168A1

WO2007074168A1 - System for acquiring seismic data with six components

Info

Publication number: WO2007074168A1
Application number: PCT/EP2006/070254
Authority: WO
Inventors: Jean-Paul Menard; Jérôme Laine
Original assignee: Sercel
Priority date: 2005-12-29
Filing date: 2006-12-28
Publication date: 2007-07-05

Abstract

The invention relates to a system for acquiring seismic data, characterized in that it comprises means (20, 30, 21, 31, 22, 32) devised in such a way that they make it possible to measure translational movements of a medium along three independent components in space and rotational movements about each of these three independent components, thus forming a system with six components.

Description

Six-component seismic data acquisition system

Field of the invention

The invention relates to a seismic data acquisition system capable of measuring translational and rotational movements.

The invention relates more precisely to a seismic data acquisition system able to perform measurements of translation movements along three components independent of space, and rotation around these three components.

The invention thus relates more precisely to a seismic data acquisition system capable of performing measurements according to six components of space, namely three of translation and three of rotation.

Such a system is therefore called a six-component system or a complete wave system.

The invention also relates to a method for performing this type of measurements.

Prior art

Current seismic data acquisition systems use seismic sensors, planted in the ground, such as a geophone or as an accelerometer, which are mass-spring type inertial sensors. These systems make it possible to measure the vertical component of a wave reflected by the different layers of the subsoil following a shaking of the terrain caused by a suitable means.

The systems conventionally used in the seismic industry therefore make it possible most often to measure a movement according to a single, vertical component. In addition, current systems measure the effects of a movement that can be linked to a translational movement, to a rotation movement or to a movement associating the two, without being able to make any distinction.

In order to improve the existing devices, one skilled in the art has proposed to implement in these data acquisition systems a three-component orthogonal motion sensor and to measure a movement of the medium that one seeks to to know better according to these three components; that is to say according to a vertical component and two horizontal components orthogonal to each other.

These devices are advantageous, insofar as they make it possible, by a suitable digital processing, to eliminate the surface waves picked up by said systems, thus taking into account only the waves actually emitted by a shaking of the ground and reflected by the layers. from the basement.

However, it has been noticed that the elimination of unwanted surface waves is not always perfect, despite the digital processing. Indeed, these systems do not meet, in some cases, the current requirements of precision and efficiency. In particular, these systems remain incomplete, insofar as they in no way make it possible to distinguish movements of the medium related to a rotation or to a translation.

However, surface waves (unwanted waves) are of the "Rayleigh wave" type, one of the characteristics of which is in particular the tilting (rotational movement) of the medium in which they propagate.

Current solutions need to be improved.

In order to improve the elimination of unwanted waves, a possible way is therefore to have a complete knowledge of the movement of the medium to which the sensor module is fixed.

An object of the invention is therefore to propose a system that allows both measurements of translation movements according to three components of space, preferably orthogonal, and of rotation around these same three components of space

In order to know the rotational movements, various measuring systems are known, such as, for example, gyroscopes. These gyroscopes are either inertial, using a wheel rotating at high speed, or optical, as in the case of a Sagnac interferometer. These components are however relatively expensive and / or bulky.

Summary of the invention

To achieve at least one of these objectives, the present invention provides a seismic data acquisition system and method, the system being characterized in that it comprises means arranged so that they make it possible to measure translational movements of a medium according to three independent components of space and rotational movements around each of these three independent components, thus forming a six-component system. The seismic data acquisition system according to the invention may furthermore have at least one of the following characteristics: the means forming a six-component system comprise at least six motion sensors; the motion sensors are translational movement sensors; the system comprises a module housing the motion sensors, which are arranged in almost any position and direction, so that a matrix A, connecting a vector m representing the movements measured by the sensors and a vector v representing the real movements of the medium, is invertible; the motion sensors are arranged in pairs so that the motion sensors of a pair of sensors are arranged relative to each other symmetrically with respect to the geometric center of the module; the motion sensors are arranged so that the geometric axis passing through the motion sensors of a pair of sensors is orthogonal to the geometric axis passing through the motion sensors of another pair of sensors; each motion sensor comprises a sensitive axis, the motion sensors of a pair of sensors being arranged so that their sensitive axes are parallel; the motion sensors are arranged so that the parallel sensitive axes of a pair of sensors are orthogonal with the parallel sensitive axes of another pair of sensors; the module is of cubic form; the means forming a six-component system comprise at least three translation sensors and at least three rotation sensors.

Brief description of the drawings

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the accompanying drawings, given by way of non-limiting examples and in which: • Figure 1 shows schematically, in sectional view, a data acquisition system according to the prior art;

FIG. 2a shows schematically and in sectional view, a preferred embodiment, given by way of non-standard example limiting, a data acquisition system according to the invention, said system being subjected to a translational movement; Figure 2b shows the data acquisition system of FIG. 2a, according to the same sectional view, subjected to a rotational movement about an axis orthogonal to the direction of the translational movement of FIG. 2a;

FIG. 3 is a schematic and perspective view of a preferred embodiment, given by way of non-limiting example, of a data acquisition system according to the present invention.

FIG. 4 schematically and perspectively shows a data acquisition system according to an alternative and generalized embodiment of the invention.

Detailed description of the invention

FIG. 1 shows a sectional view of a data acquisition system according to the prior art and formed of a module 1 comprising at its center a motion sensor 2 with a sensitive axis 3 and making it possible to carry out measurements of movements according to a single component of space. More precisely, this motion sensor 2 is able to measure a movement according to the vertical component (z axis) of the wave reflected in the different sub-layers following a shaking of the terrain caused on the surface. This motion sensor 2 is typically a geophone measuring the speed or an accelerometer.

Such a seismic data acquisition system comprising a spring mass device sensitive to a linear acceleration (translation or movement assimilable locally to a translation), is in no way able to identify accelerations resulting from a rotational movement. and therefore to distinguish accelerations related to a rotational movement accelerations related to a translational movement. Figures 2a and 2b show in sectional view a pair of motion sensors of a six-component seismic data acquisition system according to a preferred embodiment of the invention. The description made in support of these figures makes it possible to represent only part of the six-component system according to the invention, the system being only partially represented by the sectional view.

This pair of motion sensors makes it possible to measure translational movements according to a first component of the space and rotational movements around another component of the space, orthogonal to the first component. The complete system will be described later, in support of Figure 3.

In these figures 2a and 2b, the seismic data acquisition system represents a module 10 and two motion sensors

20 and 30, which are translation movement sensors. The motion sensors 20, 30 are in accordance with a motion sensor used according to the state of the art and each comprise at least one sensitive axis, preferably only one, referenced respectively 41 and 42.

The motion sensors 20, 30 are arranged relative to one another symmetrically with respect to the geometric center O of the module 10.

The sensors 20, 30 are fixed on the module 10 of the data acquisition system so that their respective sensitive axes 41, 42 are parallel and preferably with a common direction and direction.

The module is of parallelepipedal shape and preferably of cubic form. More precisely, the sensors 20, 30 are respectively fixed on two parallel and opposite faces 11, 12 of the module 10 of the data acquisition system, the sensitive axes 41 and 42 respectively belonging to the planes defined by these faces 11 and 12. More precisely, the sensors 20, 30 are fixed at the center of the faces 11, 12 of the module 10.

The motion sensors 20, 30 may be speed or acceleration sensors. However, in the following description, and for the sake of simplification, only the case where the motion sensors 20, 30 are speed sensors is described.

In FIG. 2a, the system according to the present invention partially shown with a pair of motion sensors 20, 30 is translated in the XZ plane of a medium 50.

In the case of a translational movement, represented by the arrow 51 of FIG. 2a, the two sensors 20 and 30 each make it possible to obtain a measurement of the speed linked to a translational movement, that is to say the speed of displacement of the medium in which the sensors are located, and more generally the data acquisition module 10.

The first sensor 20 makes it possible to obtain a speed Vi and the second sensor 30 makes it possible for it to obtain a speed V ₂ . The translation speed V _t of the medium 50 is then estimated by the half sum of the values obtained by each of the sensors 20 and 30, namely a speed V _t = (Vi + V ₂ ) / 2. Incidentally, under such conditions, the use of two sensors instead of a single sensor makes it possible to gain 3dB on the instrumental signal-to-noise ratio.

In FIG. 2b, this same system, also partially represented with the pair of motion sensors 20, 30 is subjected to a rotational movement of the same medium 50 around the Y component orthogonal to the XZ plane passing through the center O of the module. 10

In the case of a rotational movement represented by the arrows 52, 53, 54 and 55, the two sensors 20 and 30 each make it possible to obtain a measurement of the speed of rotation related to the rotational movement of the medium 50. Indeed, the sensors 20 and 30 undergo, under the effect of a rotational movement, a displacement in the opposite direction. Thus, for a rotational speed ω of the medium 50, the sensors 20 and 30 make it possible to obtain a speed Vi and V ₂ respectively, where Vi = Rω and V ₂ = - Rω, R being the radius of the circle of center O and passing through the respective centers Ci and C ₂ of the motion sensors 20 and 30, the center O of the circle coinciding with the center of the module 10 and the centers Ci and C ₂ coinciding with the centers of the faces 11 and 12 of the module 10 respectively. The distance separating the centers Ci and C ₂ of the motion sensors 20 and 30 is therefore 2R.

By subtraction of the values Vi and V ₂ acquired by the sensors 20 and 30, it is therefore possible to know the value of the rotational speed ω of the medium 50, then determined by the relationship ω = (Vi-V ₂ ) / 2R .

The rotational movement which is acquired by the motion sensors 20, 30 of the data acquisition system can be achieved with a chosen precision. Indeed, the greater the distance separating the centers

Ci and C ₂ motion sensors 20 and 30 is important, the sensitivity to a rotational movement of the medium 50 is large. It is therefore quite possible to adapt the size of the data acquisition system according to the desired accuracy on the knowledge of the speed of rotation of the medium.

In order to obtain measurements of translation movements according to three first independent components of space and rotational movements around these three independent components, ie to form a six-component data acquisition system according to the invention, it is necessary to employ at least six translation movement sensors arranged in pairs, as described above.

FIG. 3 presents such a system schematically and for perspective view, according to a preferred embodiment.

The six-component data acquisition system comprises means constituted by three pairs of motion sensors referenced 20, 30, and 21, 31 and 22, included in a module 10. Motion sensors are translational motion sensors.

The motion sensors of a pair of sensors are arranged relative to one another symmetrically with respect to the geometric center O of the module 10.

The module 10 employed is of parallelepipedal shape, and preferably of cubic form.

Given the shape of the module 10, each of the motion sensors 20, 30, 21, 31, 22, 32 is more preferably disposed in the center of the face 11, 12, 13, 14, 15, 16 respectively.

The geometric axis Y1 passing through the motion sensors of a pair of sensors 20, 30 is orthogonal to the geometric axis X1 or Z1 passing through the motion sensors of another pair of sensors 21, 31 or 22, 32 For each pair of motion sensors, the respective sensitive axes of the motion sensors forming the pair in question are parallel and preferably oriented in the same direction and in the same direction.

The parallel sensitive axes 41, 42 of a pair of sensors 20, 30 are orthogonal with the parallel sensitive axes 43, 44 on the one hand and 45, 46 on the other hand of another pair of sensors.

More specifically, in this preferred embodiment, the motion sensors 20 and 30 are respectively fixed on two parallel and opposite faces 11 and 12 of the module 10 and belonging to the plane XZ, the motion sensors 21 and 31 are respectively fixed on two parallel and opposite faces 13 and 14 of the module 10 and belonging to the plane YZ and the motion sensors 22 and 32 are respectively fixed on two parallel and opposite faces 15 and 16 of the module 10 and belonging to the plane XY, and this according to each of these pairs of motion sensors to the description made in support of Figures 2a and 2b. Each motion sensor 20, 30, 21, 31, 22, 32 is preferably a motion sensor with a single sensitive axis referenced respectively 41, 42, 43, 44, 45, 46.

With such an arrangement, it is possible to know both the translational and rotational speeds of the medium whose dynamic behavior is sought to be obtained according to the different components of space by performing, for each pair of motion sensors, the calculating the half-sums and half-differences of the speeds measured for each of these components of the space, calculation described above in support of FIGS. 2a and 2b.

However, in order to obtain an accurate measurement of the translation and rotation speeds of the medium, it is necessary to perform a calibration without which the directly measured value would be inaccurate. Indeed, there may be manufacturing defects on the module (orthogonality of the walls for example), imprecision positioning sensors, or inaccuracies related to the sensitivity of the sensors.

The movements measured by the sensors and the real movements of the medium in which the module is located with its sensors can each be represented by a column matrix with six lines, namely the matrices m and v respectively. These two column matrices are connected by a square matrix A and invertible according to the relation v = A ^{~ λ} m (El), the coefficients of this matrix A depending mainly on the sensitivity of the motion sensors and their implementation within the module, that is to say the distance from the center of the sensors relative to the center of the module and the angle of their sensitive axis with respect to the axes of the module.

Therefore, this calibration is most often performed after the manufacturing step of the module 10 comprising its motion sensors.

Calibration consists in obtaining the coefficients of matrix A. To obtain these coefficients, it can for example be envisaged to apply to the module obtained after manufacturing an exclusive translation along an axis X and to acquire the measured data, to repeat the same operations exclusively along a Y axis and a Z axis , and continue with exclusive rotations around these three axes X, Y and Z; the X, Y, Z axes being orthogonal to each other in pairs.

Once the coefficients obtained, that is to say once the matrix A is known by solving the equation (El), it suffices to invert the matrix A and to memorize the coefficients of this matrix A ^"1 , of preference in the module.

With this calibration, the motion sensors do not need to have a very precise sensitivity, since these sensitivity errors can be corrected. Thanks to this calibration, the geometric quality of the support as well as the positioning of the sensors on this support do not require extreme rigor since the defects are compensated by the method. The additional cost related to the calibration step is clearly offset by lower manufacturing constraints.

The description made in support of Figures 2a, 2b and 3 relates to a preferred embodiment of the invention. Many variations can however be envisaged.

For example, an alternative embodiment, as illustrated in FIG. 4, can provide a module and motion sensors arranged so that the sensors are arranged almost anywhere in the volume of the module 10, ie distances from the center O of the module 10 and with any orientation and direction.

Since this module is calibrated according to the calibration method described above, it is possible to find the six motion components of the medium to which the module is subjected as precisely as with a parallelepipedal module as described above. if we apply the relation (El).

For that, it suffices that the matrix A of calibration is invertible, which excludes a small number of singular positions of the sensors of movements, such as two sensors combined or all sensors in one or two planes.

This allows in particular to consider non-parallelepiped shape modules, and not generally having orthogonal walls or not to have the motion sensors of the same pair in the center of the faces of a parallelepiped shaped module. This generalization therefore relaxes the constraints on the form factor of the module.

The description made in support of all of FIGS. 2a, 2b, 3 and 4 relates to means 20, 30, 21, 31, 22, 32 which are translational movement sensors, but it may be envisaged to use as motion sensors at least three translational motion sensors and at least three rotational motion sensors. Finally, the system according to the invention can also provide means for forming a six-component system, other means such as for example one or more sensors selected from, by way of non-limiting example, a pressure sensor and / or a temperature sensor. The present invention pertaining to the six-component seismic data acquisition system is not limited to the embodiments described above but extends to any embodiment within its spirit.

By any embodiment in keeping with its spirit, any embodiment of a seismic data acquisition system that makes it possible to obtain six independent measurements corresponding to the three translation components along the three directions of space and to the three rotational movements around three other components of space, ie around the three directions of space.

Claims

1. A seismic data acquisition system, characterized in that it comprises means (20, 30, 21, 31, 22, 32) arranged so that they make it possible to measure translational movements of a medium according to three independent components of space and rotational movements around each of these three independent components, thus forming a six-component system.

2. System according to claim 1, characterized in that the means (20, 30, 21, 31, 22, 32) forming a six-component system comprise at least six motion sensors.

3. System according to claim 2, characterized in that the motion sensors are translational motion sensors.

4. System according to one of claims 2 or 3, characterized in that it comprises a module (10) housing the motion sensors (20, 30, 21, 31, 22, 32), which are arranged, virtually any position and direction, so that a matrix A, connecting a vector m representing the movements measured by the sensors and a vector v representing the real movements of the medium, is invertible.

5. System according to one of claims 2 to 4, characterized in that the motion sensors (20, 30, 21, 31, 22, 32) are arranged in pairs so that the motion sensors of a pair of sensors (20, 30) are arranged symmetrically with respect to each other with respect to the geometric center of the module (10).

6. System according to claim 5, characterized in that the motion sensors (20, 30, 21, 31, 22, 32) are arranged so that the geometric axis passing through the motion sensors of a pair of sensors (20, 30) is orthogonal to the passing geometric axis by the motion sensors of another pair of sensors ((21, 31), (22, 32)).

7. System according to one of claims 4 to 6, characterized in that each motion sensor comprises a sensitive axis (41, 42, 43, 44, 45, 46), the motion sensors of a pair of sensors ( 20, 30) being arranged so that their sensitive axes (41, 42) are parallel.

8. System according to claim 7, characterized in that the motion sensors are arranged so that the parallel sensitive axes (41, 42) of a pair of sensors (20, 30) are orthogonal with the parallel sensitive axes ( 43, 44), (45, 46)) of another pair of sensors ((21, 31), (22, 32)).

9. seismic data acquisition system according to one of claims 4 to 8, characterized in that the module (10) is cubic shape.

10. System according to claim 2, characterized in that the means (20, 30, 21, 31, 22, 32) forming a six-component system comprises at least three translational sensors and at least three rotation sensors.

11. A method of acquiring seismic data, characterized in that measuring translational movements of a medium according to three independent components of space and rotational movements around each of these three independent components.