WO2003008903A1

WO2003008903A1 - Method and device for seismic surveillance

Info

Publication number: WO2003008903A1
Application number: PCT/FR2002/002557
Authority: WO
Inventors: Bernard Hodac
Original assignee: Osmos Sa
Priority date: 2001-07-18
Filing date: 2002-07-18
Publication date: 2003-01-30
Also published as: FR2827670A1

Abstract

The invention concerns a method and a device (1) for monitoring deformations of a sample of the earth crust (13), with several measurement lines (2) for instantaneously measuring deformations, coupled in deformation with the crust. The method consists in simultaneously analysing (10) the measured deformations and in deducing therefrom a modelling of the sample deformation and/or a modelling of stresses in the sample. Such a method and such a device enable to measure deformations among others of null frequency or of less than 0.1 Hertz and are particularly adapted to earthquake forecasting.

Description

"Seismic monitoring process and device"

The present invention relates to a seismic monitoring method. It also relates to a device for applying this method.

Earth's crust, also called Earth's crust, tends to move and distort. If movement or deformation is locally prevented, the energy that caused it accumulates in the form of stresses in the affected portion of the Earth's crust. The more or less brutal release of these constraints is a main cause of earthquakes. When released, the stresses at least partially restore the accumulated energy. If the energy returned is important, it takes the form of tremors, vibrations, that is to say, destructive waves. In order to best protect people and property, the early warning signs of an earthquake should be detected as soon as possible.

Known seismographs are generally devices analogous to accelerometers and they are sensitive to waves whose frequencies are greater than 0.1 Hz. They can react to small releases of energy. It is the analysis of weak tremors which generally makes it possible to predict larger tremors. These seismographs make it possible to detect only restitutions of energy, they cannot give information on the quantity of residual stresses, ie those remaining accumulated in the earth's crust in the vicinity of the place of installation of the seismograph. These stresses, and therefore the corresponding energy, can be significant and suddenly be released. It is desirable to be able to provide these energy returns, in particular to be able to make arrangements which are necessary in anticipation of a sudden and sudden release. It is therefore desirable to be able to evaluate the accumulated energy before it is released. On the other hand, some of the movements of the Earth's crust of very long period, for example half a day, can be representative of the accumulation of a significant energy. These movements cannot however be detected by known seismographs. A measurement line is a linear device for measuring the distance or the variation of the distance between two ends of the linear device. The measurement line can be a conductive wire whose resistance variation is measured as a function of its elongation, an optical fiber which transmits light differently according to its stretch, or even a light beam whose journey time between the two ends is measured .

Patent EP A - 0 264 622 teaches to measure a variation in length along a measurement line by means of a light wave guide, for example an optical fiber. It is also known to compare a constraint in an optical fiber in connection with a geological structure, with reference to an unconstrained optical fiber, using the device described by US patent 5,750,901. However, in seismic matter, a variation in length along a line is impossible to interpret in terms of stress. For example, slippages can cause very significant elongations even if the residual stresses are zero.

The object of the invention is to propose a method and a device for seismic monitoring capable of giving relevant indications in terms of seismic risks.

According to the invention, in such a seismic monitoring method, analysis of deformation measurement signals emitted by at least one measurement line mechanically coupled to the earth's crust is analyzed, and this method is characterized in that signals are collected. measurement of variable deformation, emitted by at least two measurement lines coupled with the Earth's crust being sufficiently spaced from each other to define an at least two-dimensional sample of said Earth's crust, and in that a correspondence between substantially instantaneous measurement signals produced substantially simultaneously by the measurement lines.

By “substantially instantaneous measurement signals” is meant signals reflecting a value measured over a much shorter duration than a vibratory wave period of the earth's crust, typically of the order of a tenth of a Hertz to a few Hertz . By “signals produced substantially simultaneously” is meant that the instantaneous signals put in correspondence correspond to instants of measurement which are at most offset by a duration much shorter than a vibratory wave period of the Earth's crust, such as defined above.

One thus obtains, at each instant or intermittently, an indication on the deformations along several lines of the sample and this is indicative of the stress levels in the sample. In particular, very different levels of deformation along two measurement lines at the same time indicate a priori a high level of stress. We can content ourselves with visually analyzing the correspondences between the instantaneous measurements. It is also possible to carry out further automatic processing.

The process makes it possible to observe and model in real time the deformations of a significant sample of the Earth's crust on a sufficiently large scale. We can also assess the constraints in the sample and monitor the evolution of these constraints to predict earthquakes. The process also has a more general scientific scope for the study of the Earth's crust. The deformations of the sample being known, they make it possible, from at least one elastic modulus of this sample, to deduce therefrom the stresses accumulated between two consecutive measurements. It is also possible to detect variations in a displacement of at least part of the sample. Decreasing displacements, in particular if it can be determined that the causes of these displacements remain unchanged, can be significant from the appearance of a blocking prior to the appearance of stresses in the sample. Such a method is therefore particularly useful for understanding the behavior of the earth's crust and, a fortiori, for predicting earthquakes and in this case constituting an alert device.

Each of the measurement lines is connected by a transmission line to a signal processing device. To measure elongations in the measurement lines, it is possible, for example, to send with the processing device an adapted source signal in each of the measurement lines concerned, respectively, and the same processing device is received, as a measurement signal, an image signal, originating from the source signal, returned by the measurement line.

In the processing device, the measurement signal is analyzed. The respective length variation measurements in the measurement lines are carried out substantially simultaneously and instantaneously. The analysis of the image signal is also done simultaneously for all of the measurements carried out, but can be postponed over time. For example, each instantaneous value received can be associated with a time indication, then the values associated with the same time indication can be matched later.

We deduce a local modeling, that is to say in the sample, of instantaneous deformations of the Earth's crust. We can also deduce an evaluation of the constraints and therefore the energy accumulated within the sample between two consecutive or non-consecutive measurements. We can still extrapolate this evaluation to the whole of a neighboring area of the sample.

The sample is preferably as large as possible under economically reasonable conditions. In practice, the size of the sample is such that the length of a measurement line and / or the distance between two neighboring lines goes from the order of the decameter to the order of the kilometer, as well as in terms of the horizontal dimension (s) only with regard to the vertical dimension. It is generally preferred that the significant dimensions of the sample are of an order of magnitude comparable. By "significant dimension" is meant the two large dimensions of a two-dimensional sample or the three dimensions of a three-dimensional sample.

The difference in length between two transmission lines connecting two measurement lines to the same processing device will have little or no influence. Thus, a difference of ten kilometers between two optical fibers each forming a transmission line corresponds to a difference of 1/30000 of a second to be compared to a period of 1/1 0 of a second. If greater precision is required, it is always possible to provide substantially identical transmission line lengths or any other means to equalize transmission times.

According to another aspect of the invention, it relates to a seismic monitoring process in which instantaneous deformations of a sample of the earth's crust are modeled and, preferably, a modeling of the stresses in the sample is deduced therefrom , or even a monitoring of the constraints in relation to thresholds.

By "modeling the deformations" is meant "typically describing by digital values a possible deformation" of the sample, taking into account the deformations noted along all the measurement lines. The real deformation is slightly different and more complex than the modeled deformation. For example, a line that has elongated may have remained straight or, on the contrary, may have bent to the point that its two ends have come close to each other. The other measurement lines will partially lift this uncertainty, especially if there are many. We can consider software that develops several models and retains only the most critical in terms of seismic risk.

According to a third aspect of the invention, the device for implementing the method comprises strain measurement lines and means for processing signals emitted by the measurement lines. It is characterized in that the processing means are means for establishing a correspondence between instantaneous and simultaneous values of the variable signals of at least two measurement lines coupled to the earth's crust at sites spaced apart from one other. The measurement lines are coupled in movement with the earth's crust, for example by a matrix in a borehole or a trench, the matrix being deformable with the earth's crust. It is particularly advantageous to provide at least three non-coplanar measurement lines in order to be able to model the deformations in several directions. Thus, three non-coplanar and non-parallel measurement lines define for the sample a frame of reference for deformations in the three dimensions. As an example, the measurement lines can be arranged along the vertical edges of a parallelogram or along the edges of a tetrahedron. In this last example, we obtain a triangulation of the sample which is all the more efficient when we define in the Earth's crust a greater number of tetrahedrons having two by two at least three common edges. The measurement lines can be arranged on converging or parallel edges, if it has any, of the polyhedron.

To have the measurement lines along edges inclined with respect to the vertical one can use the technique of directed drilling. This is particularly the case for placing the measurement lines horizontally where a trench is too deep. This is for example the case for placing them along the edges of a tetrahedron, a base of which is substantially parallel to the surface of the ground and a point opposite this base is oriented downwards. To subdivide a sample and increase the accuracy of detection and modeling, the device can include measurement lines arranged along edges of several polyhedra. Some of these polyhedra can be substantially proportional to each other. A first polyhedron can be contained in a second and / or be homothetic between them. A polyhedron for the device may not be flush with a point or face with the surface of the earth's crust.

In addition to measurement lines arranged in boreholes, the device can include lines arranged by other means and particularly surface measurement lines, arranged in trenches or overhead. The measurement lines and / or the transmission lines can advantageously consist of optical fibers. In the accompanying drawings:

FIG. 1 is a partial and schematic representation of a device according to the invention of which only one measurement line is shown,

FIG. 2 is a representation of a device according to the invention, the polyhedron of which is a cube flush with the surface, FIG. 3 is a representation of another device according to the invention comprising several adjacent cubes,

FIG. 4 is a perspective view of a device according to the invention comprising three cubes nested one inside the other, FIG. 5 is a top view of the device of FIG. 4,

FIG. 6 is a section of the device in FIG. 4 along V-V,

- Figure 7 is a perspective view of a device according to the invention comprising two tetrahedrons nested one inside the other.

- Figure 8 is a perspective view of a device defined by two parallel planes.

- Figures 9 and 10 are two illustrations of deformations for a cube.

In Figures 2 to 8 are mainly represented the measurement lines, in solid lines and assuming the transparent earth crust. Construction lines, for example edges which do not include measurement lines, are drawn in dotted lines. In the examples shown, optical fibers are used as measurement lines and as transmission lines.

FIG. 1 schematically represents a device 1 according to the invention of which only one of the measurement lines has been drawn 2. For the study of a sample of the earth's crust, other lines not shown are used. The measurement line 2 is connected by a transmission line 3 to a signal processing device 4. The other measurement lines are connected to it in the same way. Inside the processing device, the transmission line is subdivided into an input line 6 for a signal emitted by a source 8 and an output line 7 for a signal reflected by the measurement line 1 to a receiver 9. The signal processing device further comprises control means 1 0 for controlling the emission of the signal, and in particular ensuring the simultaneity of the measurements in each of the measurement lines of the device. It also includes analysis means 11 which make it possible to compare the reflected signals with the signals transmitted for each of the measurement lines, then to deduce therefrom the deformation of the lines of measure and then model the strains and / or stresses in the sample studied.

The reflected signal is an image of the signal emitted by the source and possibly distorted by the measurement line before being returned by the latter. It is the study of the signal deformations which allow to deduce a variation in length of the measurement line. Methods described in the documents cited in can be used for these operations. In particular, it is advantageous for the measurement line to be provided prelonged when it is put in place. It is thus easy to measure elongations and / or shortenings. We can then use the term elongation to designate a shortening, considered as a negative lengthening.

A borehole 1 2 was carried out in the earth's crust 1 3 for the establishment of the measurement line 2. The measurement line is sealed in the borehole using a sealing material 14, forming a matrix. This material must have a low inherent cohesion in order not to oppose any resistance to its deformation jointly with that of the earth's crust in its vicinity. The cohesion of the sealing material will however be sufficient to be able to transmit this deformation to the measurement line which is sealed there. We can use, for example, a grout with a low dosage of cement. To ensure its sealing, the measurement line may, for example, include on its exterior a corrugated surface or other means of attachment and sealing.

In the example of Figure 1, the proximal end 1 6 of the measurement line should not be flush with the surface 1 7 instead of drilling. A sheath 18 has therefore been provided to isolate the transmission line 3 from the sealing material 14. Thus, the transmission line is not subject to deformations of the earth's crust and only the variation in the length of the fiber. optics corresponding to the measurement line are taken into account.

With reference to Figures 2 to 8 we will examine different possible arrangements for the measurement lines. In Figures 3 to 8, only the measurement lines are shown. FIG. 2 represents lines of measurements 2 arranged on vertical edges of a polyhedron 1 9, which is a cube. The proximal ends 1 6 of each of the measurement lines are flush with the ground 1 7. Each of the proximal ends is connected by an independent transmission line 3 to a processing device 4. The processing device thus allows simultaneous signal processing to or from each of the measurement lines 2.

In Figure 3, the device comprises eight cubes arranged in two layers of four cubes each. Each of the cubes of a layer with two common faces with cubes of the same layer. Each of the cubes of the upper layer has a common lower face with an upper face of a cube of the lower layer. The eight cubes thus arranged define 1 8 vertical edges, some of which are common to 2 or 4 cubes. A measurement line is arranged along each of these 18 edges, so that each of the measurement lines of the upper layer is arranged in the extension of a single measurement line of the lower layer. This arrangement makes it possible to define for the same sample of earth's crust eight zones corresponding to each of the eight cubes 1 9 and to model the overall deformation of the sample with greater precision while retaining the possibility of modeling for each of the zones its own deformation.

In Figures 4 to 6, in addition to a cube 1 9, the device 1 also includes two cubes 1 91, 1 92. The cube 1 91 is inside the cube 1 9 and the cube 1 92 is inside the cube 1 92 so that the three cubes 1 9, 1 91, 1 92 have a common geometric center G (see Figure 6). That is to say that any of the cubes can be deduced from any other of the other cubes by a homothety of center G. The cube 1 9 is larger than the cube 1 91 which is itself larger than the cube 1 92. Measuring lines 2,21, 22 are disposed respectively on the vertical edges of each cube 1 9, 1 91, 1 92. Figure 5 is a top view of the three nested cubes. Each of the measurement lines is represented by a point. The measurement lines are included in one or the other of the diagonal planes (VI-VI) common to the three cubes. FIG. 6 is a representation of the arrangement of the measurement lines included in one of these plans. As in the example in FIG. 3, this arrangement allows greater precision in the measurement of the deformations of the sample, particularly in the vicinity of point G.

In Figure 7, the device 1 comprises two polyhedra 1 9, 1 93 which are two tetrahedrons of which the second 1 93 is smaller than the first 1 9, the second is included in the first. The two tetrahedra have a horizontal base 30 and a point 31 opposite the base oriented downwards. The base of the largest tetrahedron 1 9 is close to the surface. A measurement line is arranged on each of the edges of the tetrahedra. Drilling for lines 2, going from a base 30 to a point 31 are made inclined. The measurement lines 23 at the periphery of the base of the first tetrahedron 1 9, therefore close to the surface, are arranged in trenches. The drillings for the measurement lines 24, at the periphery of the base of the second tetrahedron, are horizontal in their part comprising the measurement lines, and they are produced by the technique of directed drilling.

In FIG. 8, a first two-dimensional device 1 A comprises two lines of concurrent measurements 2 arranged in a first vertical plane. A second two-dimensional device 1 B is the distant image of the first device 1 A by a substantially horizontal translation. These two two-dimensional devices are associated to form a device 1 capable of simultaneously processing information transmitted by the first and second two-dimensional devices 1 A, 1 B. FIGS. 9 and 10 are illustrations of two possible deformations for a sample initially defined by a cube whose upper face is horizontal. The edges of the cube, that is to say of the sample before deformation, are represented in dotted lines.

A first deformation, represented in FIG. 9, is due to a horizontal sliding in a direction F of the layers of the earth's crust traversed by the sample, one with respect to the other. The lower face considered immobile does not deform. The upper side moves horizontally along D without deforming. For a measurement line 2 arranged along a vertical edge of the cube, the first deformation is perceived as a first elongation.

A second deformation, shown in Figure 1 0, is due to a progressive horizontal crushing of the sample as one is deeper in the earth's crust. The upper face of the cube remains stationary and does not deform. The underside is deformed in a horizontal plane while its corners approach according to D. The cube takes the form of a regular tetrahedron trunk. For a measurement line 2 arranged along a vertical edge of the cube, the second deformation is perceived as a second elongation.

The first deformation is done with little or no energy accumulation, that is to say that the deformed sample is substantially stable. The second deformation is done by accumulating energy within the deformed sample, that is to say that it is unstable and that the accumulated energy tends to give it back its initial cubic shape. However, for a distance traveled along D identical for the two deformations, the first and second elongations are perceived to be identical for the two deformations. If it is not possible to determine, before installing a device according to the invention, privileged deformations for the sample which one chooses to instrument, it is advisable to have certain measurement lines in order to be able to remove an uncertainty on the type of deformation. For example, we will have a measurement line along a diagonal of the cube or a diagonal of certain sides of the cube. Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to these examples without departing from the scope of the invention.

Thus, polyhedra can be of any type and not just limited to cubes and tetrahedra. All or only some of their edges can be equipped and certain measurements can only be made on part of the equipped edges. Similarly, the measurement lines can be arranged, not according to the surface of the earth's crust but, for example, according to

he fracture lines of this bark, known or predicted. A configuration and a use of a device according to the invention can therefore be very different depending on the place of its installation, the knowledge that one has of this place and according to the desired results. Thus, in the case of a device comprising several polyhedra, each polyhedron can be assigned to a part of the sample having its own elastic modulus.

Similarly, it is also possible to use, for a measurement line, any means making it possible, such as optical fibers, to measure a length or a variation in length between two ends of a line. It is also possible to use a decentralized unit, that is to say close to a measurement line, comprising a signal source and receiver and communication means, for example radio means, for communications with a central processing unit. . These means can be combined with each other provided that they make it possible to give, within the meaning of the invention, simultaneous measurements. The method and the device according to the invention are particularly suitable for measuring deformations or displacements of infinite wave period, that is to say deformations or continuous displacements, of substantially zero frequency, at least for a certain duration, and are not only suitable for vibration measurements.

Claims

1. Seismic monitoring method in which deformation measurement signals emitted by at least one measurement line (2.21 -24) mechanically coupled to the earth's crust (1 3) are analyzed, characterized in that signals are collected for measuring variable deformation, emitted by at least two measurement lines (2,21 -24) coupled with the earth's crust (13) being sufficiently spaced from one another to define an at least two-dimensional sample of said Earth's crust, and in that a correspondence is established between substantially instantaneous measurement signals produced substantially simultaneously by the measurement lines.

2. Method according to claim 1, characterized in that the measurement lines define a three-dimensional sample.

3. Method according to claim 1 or 2, characterized in that a local modeling of the instantaneous deformations of the earth's crust is deduced therefrom.

4. Method according to one of claims 1 to 3, characterized in that one mainly monitors deformations of the earth's crust of frequency less than 0.1 Hz.

5. Method according to one of claims 1 to 4, characterized in that one deduces from two measurements made at two different times, the variation, between these two times, of the energy accumulated in the sample.

6. Method according to one of claims 1 to 5, characterized in that the measurement signals are collected at a common site to which all the measurement lines are connected by means of substantially instantaneous transmission.

7. Method according to one of claims 1 to 6 characterized in that the measurement lines are arranged along at least one substantially closed contour.

8. Device (1) for implementing a method according to one of claims 1 to 7, comprising strain measurement lines (2,21 -24) and means (4) for processing transmitted signals by measurement lines, characterized in that the processing means are means for establishing a correspondence between instantaneous and simultaneous values of the variable signals of at least two measurement lines coupled to the Earth's crust at spaced sites one of the other.

9. Device according to claim 8, characterized in that the processing means are arranged in a processing site to which the measurement lines are all connected by transmission means (3) ensuring an almost simultaneous transmission between the deformations undergone by each measurement line and the corresponding signals received by the processing means.

10. Device according to claim 8 or 9, characterized in that some of the measurement lines are arranged in boreholes (1 2) made in the earth's crust (1 3).

1 1. Device according to one of claims 8 to 9, characterized in that measuring lines are coupled in movement with the earth's crust by a matrix, deformable with the earth's crust.

1 2. Device according to one of claims 8 to 1 1, characterized in that it comprises at least three non-coplanar measurement lines.

1 3. Device according to one of claims 8 to 1 2, characterized in that it comprises at least three lines of convergent measurements.

1 4. Device according to one of claims 8 to 1 3, characterized in that it comprises measurement lines arranged along parallel edges of a parallelogram (1 9, 1 91, 1 92).

1 5. Device according to one of claims 8 to 1 3, characterized in that it comprises measurement lines arranged along the edges of a tetrahedron (1 9, 1 93) whose base (30) is substantially parallel on the surface of the ground (1 7) and a point (31) opposite said base is oriented downwards.

1 6. Device according to one of claims 8 to 1 5, characterized in that it comprises measurement lines along edges of several polyhedra having a common edge equipped with one of the measurement lines.

1 7. Device according to one of claims 8 to 1 5, characterized in that it comprises measurement lines along edges of several polyhedra substantially proportional to each other.

1 8. Device according to one of claims 8 to 1 7, characterized in that it comprises measurement lines along edges of at least two polyhedra, one of which is included in the other.

1 9. Device according to one of claims 8 to 1 8, characterized in that it comprises measurement lines along the edges of at least two polyhedrons deduced from each other by a homothety.

20. Device according to one of claims 8 to 1 9, characterized in that it comprises at least one measurement line (23), close to the surface and substantially parallel to said surface.

21. Device according to one of claims 8 to 20, characterized in that at least one of the measurement lines is formed with an optical fiber.