Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM FOR ACQUIRING SEISMIC DATA WITH SIX COMPONENTS
Field of the invention
The invention relates to a seismic data acquisition system designed to
perform the measurement of movements in translation and rotation.
More precisely, the invention relates to a seismic data acquisition
system that is designed to perform the measurement of movements in
translation according to three independent components in space, and on
rotation around these three components.
More precisely therefore, the invention relates to a seismic data
acquisition system that is capable of performing measurements according to
six components in space, namely three in translation and three in rotation.
Such a system is therefore known as a six-component system or as a
full-wave system.
The invention also relates to a method to perform this type of
measurement.
Prior art
The current seismic data acquisition systems use seismic sensors,
buried in the ground, such as a geophone or indeed like an accelerometer,
which are inertial sensors of the mass-spring type. These systems are used to
measure the vertical component of a wave that is reflected by the different
layers of the earth's sub-surface, following a ground disturbance occurring at
the surface, using a suitable resource.
The systems used conventionally in the seismic industry therefore most
often allow measurement of a movement according to a single, vertical
component.
Moreover, the current systems measure the effects of a movement that
may be associated with a movement in translation, or with a movement in
rotation, or indeed with a movement associating the two, without being able to
make any distinction.
In order to improve the existing devices, those skilled in the art have
proposed the use in these data acquisition systems of a motion sensor with
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three orthogonal components, and which is used to measure a movement of
the environment that one is seeking to quantify according to these three
components, that is according to a vertical component and two horizontal
components that are mutually orthogonal.
These devices are advantageous to the extent that, by suitable digital
processing, they allow us to eliminate the surface waves acquired by the said
is systems, and therefore take account only of the waves actually emitted by a
ground disturbance and reflected by the earth's sub-surface layers.
It has been observed however, that elimination of the undesirable
surface waves is not always perfect, despite the digital processing. In fact
in
certain cases, these systems no longer meet the current requirements for
precision and efficiency.
In particular, these systems remain incomplete, to the extent that they
never allow one to distinguish between the movements in the environment
that are associated with movement in rotation and translation.
Now the surface waves (the undesirable waves) are of the "Rayleigh
wave" type, one of whose characteristics is, in particular, the tilting
(movement
in rotation) of the environment in which they are propagating.
The current solutions must therefore be improved.
In order to improve elimination of the undesirable waves, one possible
course of action is therefore to have full knowledge of the movement of the
environment in which the sensor module is located.
An objective of the invention is therefore to propose a system that will
simultaneously allow measurement of movements in translation according to
three preferably orthogonal components in space, and in rotation about these
three same components in space.
In order to ascertain the movements in rotatiofl, we are also aware of
different measurement systems, such as gyroscopes, which we mention only
by way of an example. 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 relatively costly and/or bulky however.
Summary of the invention
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In order to attain at least one of these objectives, the present invention
provides a seismic data acquisition system, comprising a plurality of seismic
sensors which are buried in the ground and are able to measure a seismic
wave that is reflected by the different layers of the earth's sub-surface,
following a ground disturbance caused at the surface by a suitable resource,
including a module housing the seismic sensors, which are sensible to three
independent translation motion components of the seismic wave in the ground
and to three independent motion rotation components of the seismic wave in
the ground around each of the three independent translation motion
components.
The seismic data acquisition system according to the invention can also
have at least one of the following characteristics:
-the sensor include at least six motion sensors;
- the motion sensors are motion sensors in translation;
-the motion sensors are arranged in the module, in a substantially
random manner in position and in direction, so that vector in representing the
three independent translation motion components measured by the sensors
and the three independent motion rotation components measured by the
sensors is linked to a vector '15 representing the actual movements of the
ground and having three independent rotation components around the
independent spatial components, by an invertible matrix (A);
-the motion sensors are arranged in pairs so that the motion sensors in
a pair of sensors are positioned in relation to each other in a symmetrical
manner in relation to the geometrical centre of the module;
-the motion sensors are arranged so that the geometrical axis passing
through the motion sensors in a pair of sensors is orthogonal to the
geometrical axis passing through the motion sensors in another pair of
sensors;
-each motion sensor includes a sensitive axis, the motion sensors in
one pair of sensors being arranged so that their sensitive axes are parallel;
-the motion sensors are arranged so that the parallel sensitive axes of
one pair of sensors are orthogonal to the parallel sensitive axes of another
pair of sensors;
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-the module is cubic shape;
-the means forming a six-component system include at least three
sensors in translation and at least three sensors in rotation.
The present invention also provides a method for acquisition of seismic
data, wherein a suitable resource emits at the ground surface a ground
disturbance causing a seismic wave to be reflected by the different layers of
the earth's sub-surface, wherein the seismic wave that is reflected is
measured by a plurality of seismic sensors which are buried in the ground,
and wherein the seismic sensors are housed in a module and are able to
measure three independent translation motion components of the seismic
wave in the ground and three independent motion rotation components of the
seismic wave in the ground around each of the three independent translation
motion components.
Brief description of the drawings
Other characteristics, aims and advantages of the present invention will
appear on reading the detailed description that follows, and with reference to
the appended drawings, which are given by way of non-limiting examples and
in which:
-figure 1 schematically presents, in cross section view, a data
acquisition system according to the prior art;
-figure 2a schematically presents, in a cross section view, a preferred
embodiment, given by way of a non-limiting example, of a data acquisition
system according to the invention, the said system being subjected to a
movement in translation;
-figure 2b presents the data acquisition system of figure 2a, according
to the same cross section view, subjected to a movement in rotation about an
axis that is orthogonal to the direction of the movement in io translation of
figure 2a;
-figure 3 schematically presents, in a perspective view, a preferred
embodiment, given by way of a non-limiting example, of a data acquisition
system according to the present invention.
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-figure 4 schematically presents, in a perspective view, a data
acquisition system according to one alternative and generalised embodiment
of the invention.
5 Detailed description of the invention
Figure 1 presents a cross section view of a data acquisition system according
to the prior art, and formed of a module 1 that includes at its centre a
motion
sensor 2 with a sensitive axis 3, used to perform measurements on
movements according to a single component in space. More precisely, this
motion sensor 2 is designed to measure a movement according to the vertical
component (z axis) of the wave reflected in the different sub-surface layers
following a disturbance of the ground reproduced at the surface. This motion
sensor 2 is typically a geophone measuring the speed, or an accelerometer.
Such a seismic data acquisition system with a device of the mass-
spring type, sensitive to linear acceleration (translation or any movement
comparable locally to translation), is never capable of identifying
acceleration
arising from a movement in rotation, and therefore to distinguish between
acceleration associated with a movement in rotation from acceleration
associated with a movement in translation.
In a cross section view, figures 2a and 2b present a pair of motion
sensors in a six-component seismic data acquisition system according to a
preferred embodiment of the invention. The description given with reference to
these figures is used to represent only one part of the six-component system
according to the invention, since the system is represented only partially by
the cross section view.
This pair of motion sensors in fact can be used to measure movements
in translation according to a first component in space, and movements in
rotation about another component in space, which is orthogonal to the first
component. For its part, the complete system will be described later, with
reference to figure 3.
On these figures 2a and 2b, the seismic data acquisition system
represents a module 10 and two motion sensors 20 and 30, which are motion
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sensors in translation. The motion sensors 20, 30 are according to a motion
sensor that is used according to the prior art, and each includes at least one
sensitive axis, and preferably one only, shown respectively as 41 and 42.
The motion sensors 20, 30 are positioned in relation to each other in a
symmetrical manner in relation to the geometrical centre 0 of the module 10.
The sensors 20, 30 are attached to 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 sense.
The module is of parallelepiped shape and preferably cubic shape.
More precisely, the sensors 20, 30 are attached respectively to two
parallel and opposite faces 11, 12 of the module 10 of the data acquisition
system, the sensitive axes 41 and 42 belonging respectively to the planes
formed by these faces 11 and 12.
More precisely again, the sensors 20, 30 are attached to the centre of
the faces 11, 12 of the module 10.
The motion sensors 20, 30, can be speed sensors or acceleration
sensors. In the remainder of the description however, and for reasons of
simplification, only the case where the motion sensors 20, 30 are speed
sensors is described.
In figure 2a, the system according to the present invention, represented
partially with a pair of motion sensors 20, 30, is subjected to a movement in
translation in the plane XZ of an environment 50.
In the case of a movement in translation, represented by the arrow 51
of figure 2a, the two sensors 20 and 30 are each used to acquire a
measurement of the speed associated with a movement in translation, that is
of the speed of movement of the environment in which the sensors are
located, and more generally the data acquisition module 10.
The first sensor 20 is used to acquire speed V1 and, for its part, the
second sensor 30 is used to acquire speed V2. The speed in translation Vt of
the environment 50 is then estimated by half the sum of the values obtained
by each of the sensors 20 and 30, namely a speed Vt = (V1+V2)/2.
Incidentally, in such conditions, the use of two sensors instead of a single
sensor results in an increase of 3dB in the instrument signal-to-noise ratio.
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In figure 2b, this same system, also represented partially with the pair
of motion sensors 20, 30, is subjected to a movement in rotation of the same
environment 50 around the component Y, orthogonal to plane XZ, and
passing through the centre 0 of the module 10.
In the case of a movement in rotation, represented by arrows 52, 53, 54
and 55, the two sensors 20, 30 are each used to acquire a measurement of
the speed in rotation associated with the movement in rotation of the
environment 50.
In fact, under the effect of a movement in rotation, the sensors 20 and
30 undergo a movement in the opposite direction. Thus, for a speed in
rotation co of the environment 50, the sensors 20 and 30 allow us to obtain a
speed, V1 and V2 respectively, where V1 = Rco and V2 = -Rco, R being the
radius of the circle of centre 0 and passing through the respect ve centres
C1,
C2 of the motion sensors 20 and 30, where the centre 0 of the circle coincides
with the centre of the module and centres C1 and C2 coincide respectively with
the centres of the faces 11 and 12 of the module 10. The distance separating
the centres C1 and C2 of the motion sensors 20 and 30 therefore comes to
2R.
By subtraction of the values V1 and V2, acquired by the sensors 20 and
30, it is then possible to know the value of the speed in rotation w of the
environment 50, then determined by the relation co = (V1-V2) /2R.
The movement in rotation that is acquired by the motion sensors 20, 30
of the data acquisition system can be determined with a chosen degree of
precision. In fact, the greater the distance separating the centres C1 and C2
of
the motion sensors 20 and 30, the greater the sensitivity to a movement in
rotation of the environment 50 is. It is therefore quite possible to envisage
adapting the dimensions of the data acquisition system as a function of the
precision desired in ascertaining the speed in rotation of the environment.
In order to obtain measurements of movements in translation according
to three first independent components in space, and of movements in rotation
around these three independent components, that is to form a data acquisition
system with six components according to the invention, it turns out to be
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necessary to employ at least six motion sensors in translation, arranged in
pairs, as described above.
Figure 3 presents such a system in diagram form, and in a perspective
view, according to a preferred embodiment.
The data acquisition system with six components includes means that
are constituted by three pairs of motion sensors, referenced 20, 30, and 21,
31 and 22, 32 built into a module 10. These motion sensors are motion
sensors in translation.
The motion sensors in a pair of sensors are positioned in relation to
each other in a symmetrical manner in relation to the geometrical centre 0 of
the module 10.
The module 10 employed is of parallelepiped shape, and preferably
cubic shape.
Given the shape of the module 10, each of the motion sensors 20, 30,
21, 31, 22, 32 is more preferably placed respectively at the centre of the
face
11, 12, 13, 14, 15, 16.
The geometrical axis Y1 passing through the motion sensors in a pair
of sensors 20, 30 is orthogonal to the geometrical axis X1 or Z1 passing
through the motion sensors in another pair of sensors 21, 31 or 22, 32.
For each pair of motion sensors, the respective sensitive axes of the
motion sensors that form the pair concerned are parallel and preferably
oriented in the same direction and the same sense.
The parallel sensitive axes 41, 42 of a pair of sensors 20, 30 are
orthogonal to the parallel sensitive axes, firstly 43, 44 and secondly 45, 46,
of
another pair of sensors.
More precisely, in this preferred embodiment, motion sensors 20 and
are respectively attached to two parallel and opposite faces 11 arid 12 of
the module belonging to the plane XZ, motion sensors 21 and 31 are
respectively attached to parallel and opposite faces 13 and 14 of the module
30 10
belonging to the plane YZ, and motion sensors 22 and 32 are respectively
attached to two parallel and opposite faces 15 and 16 of the module 10
belonging to the plane XY, and for each of these pairs of motion sensors, in
accordance with the description given with reference to figures 2a and 2b.
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Each motion sensor 20, 30, 21, 31, 22, 32 is preferably a motion sensor
with a single sensitive axis, shown respectively as 41, 42, 43, 44, 45, 46.
With such an arrangement, it is possible to know simultaneously the
speeds in translation and in rotation of the environment for which one is
seeking to obtain the dynamic behaviour according to the different
components in space, by performing, for each pair of motion sensors, a
calculation of half the sums and half the differences of the speeds measured
for each of these components in space, as described above with reference to
figures 2a and 2b.
However, in order to obtain an exact measurement of the speeds in
translation and in rotation of the environment, it is necessary to perform a
calibration, without which the value measured directly would be inaccurate. In
fact, there can exist manufacturing faults on the module (orthogonality of the
walls, for example), inaccuracies in the positioning of the sensors, or
inaccuracies associated with the sensitivity of the sensors.
The movements measured by the sensors, and the actual movements
of the environment in which the module is located with its sensors, can each
be represented by a six-line column matrix, respectively entitled matrices in
and V. These two column matrices are related by a square and invertible
matrix A, according to the relation V = 14ñ (El) with the coefficients of this
matrix A dependent mainly on the sensitivity of the motion sensors and on
their location within the module, that is on the distance of the centre of the
sensors from the centre of the module and on the angle of their sensitive axis
in relation to the axes of the module.
As a consequence, this calibration is most often carried out after the
manufacture of the module 10 including its movement sensors.
The calibration consists of obtaining the coefficients of matrix A.
In order to obtain these coefficients, it is possible to envisage, for
example, applying to the module obtained after manufacture, an exclusive
translation according to an axis X, and to acquire the measured data, to
repeat the same operations in an exclusive manner according to an axis Y
and an axis Z, and to continue with exclusive rotations around these three
axes X, Y and Z, where axes X, Y, Z are mutually orthogonal in pairs.
=
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Once the coefficients have been obtained, that is once matrix A is
known, by solution of equation (El), one then only has to invert matrix A and
to store the coefficients of this matrix A-1, preferably in the module.
With this calibration done, the motion sensors do not need to have very
5 precise sensitivity, since any sensitivity errors can be corrected. By
means of
this calibration, the geometrical quality of the support, as well as the
positioning of the sensors on this support, do not require extreme rigour,
since
any faults are compensated for by the method. The additional cost associated
with the calibration step is clearly offset by the reduction in manufacturing
10 constraints.
The description given with reference to figures 2a, 2b and 3 concern a
preferred embodiment of the invention. Many variants can be envisaged
however.
For example, an alternative embodiment, as illustrated in figure 4, can
consist of a module and motion sensors that are arranged so that the sensors
are placed in a substantially random manner in the volume of the module 10,
that is at different distances in relation to the centre 0 of the module 10
and
with any orientation and any sense.
To the extent that this module is calibrated according to the method of
calibration described above, it is possible to find the six components of
movement of the environment to which the module is subject in a manner that
is as accurate as that using a parallelepiped module as described previously,
if the relation (El) is applied.
To this end, the only requirement is that calibration matrix A should be
capable of inversion, which excludes a small number of singular positions of
the motion sensors, such as two sensors coinciding, for example, or all of the
sensors being in one or two planes.
In particular, this allows one to envisage of modules of non-
parallelepiped shape, and in general not representing orthogonal walls, or
indeed not placing the motion sensors in the same pair at the centre of the
faces of a module of parallelepiped shape. This generalisation therefore
relaxes the constraints governing the form factor of the module.
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The description given with reference to figures 2a, 2b, 3 and 4
concerns means 20, 30, 21, 31, 22, 32 that are motion sensors in translation,
but as motion sensors, one can also envisage employing at least three motion
sensors in translation and at least three motion sensors in rotation.
Finally, the system according to the invention can also include means
that are used to form a six-component system, other means such as, for
example, one or more sensors selected, as a non-limiting example, among a
pressure sensor and/or a temperature sensor.
