DE102022108730A1 - Method and device for determining the properties of an acoustic signal source in solid or loose rock - Google Patents

Abstract

The invention relates to a method for determining properties of an acoustic signal source in solid or loose rock, in particular for determining the position or type of an acoustic signal source in solid or loose rock, in particular for inferring mass movements in solid or loose rock and predicting this following failure conditions such as landslides, fractures, rock falls or landslides, as well as a suitable device for this.

Description

The present invention relates to a method for determining properties of an acoustic signal source in solid or loose rock, in particular for determining the position or type of an acoustic signal source in solid or loose rock, in particular for inferring mass movements in solid or loose rock and predicting subsequent failure conditions such as landslides, fractures, rock falls or landslides, as well as a suitable device for this.

As part of global climate change, the melting permafrost in the Alpine countries will be a new challenge in the coming years in direct connection with the idea of security for the population. Due to the decline in permafrost, individual structures such as cable cars, but also entire towns and villages, are threatened by possible rock falls. This will inevitably mean that the existing rock in these regions will have to be permanently monitored. A similar risk potential arises with the so-called tipping problem in old open-cast mines (slides) or in areas of mountain subsidence (fractures). Monitoring systems based on single-point monitoring (hotspot monitoring) are currently predominantly used to monitor these risk areas. However, such single-point monitoring requires that it be installed directly at the location of the possible failure. In order to enable large-scale monitoring, a large number of individual points would have to be monitored, or specific risk areas would have to be known in advance. However, site-unspecific soil mechanical or rock mechanical events are difficult to detect in this way.

What all possible failure states have in common are leading mass movements and the associated acoustic anomalies in the solid or loose rock. If the acoustic anomalies associated with the ongoing mass movements can be detected in advance, a specific failure state can be determined in advance in good time. In recent years, experiments have been carried out here using Distributed Strain Sensing (DSS) and Distributed Temperature Sensing (DTS). These methods are suitable for clearly defined structures for detecting all hotspots without prior specification of the location, but they are only partially suitable for detecting hotspots for undefined structures. This is due to the fact that laying fiber-optic cables over the entire surface to detect deformations is in principle possible, but is also associated with significant costs for fixing and protecting the fiber-optic cables, especially in Alpine regions.

It is therefore the object of the present invention to provide a method and a device for determining properties of an acoustic signal source in solid or loose rock, in which soil mechanical or rock mechanical events are both site-specific and non-site-specific in a simple, cost-effective manner and even in undefined rock structures can be reliably detected.

1. (a) ein faseroptisches Kabel im Fest- oder Lockergestein in einer im Wesentlichen horizontalen Richtung verlegt wird;
2. (b) ein Lichtsendesignal in das faseroptische Kabel eingespeist wird;
3. (c) durch das Auftreffen eines akustischen Signals auf das faseroptische Kabel Licht des Lichtsendesignals rückgestreut und von einer Lichtempfangseinheit als Lichtempfangssignal detektiert wird; und
4. (d) wobei mittels der Auswertung des Lichtempfangssignals eine oder mehrere Eigenschaften der akustischen Signalquelle bestimmt werden.

For this purpose, according to the invention, a method for determining one or more properties of an acoustic signal source in solid or loose rock, in particular for determining the position or type of an acoustic signal source in solid or loose rock, in particular for inferring mass movements in solid or loose rock and the prediction from subsequent failure conditions such as landslides, fractures, rock falls or landslides, in which

1. (a) a fiber optic cable is laid in solid or loose rock in a substantially horizontal direction;
2. (b) a light transmission signal is injected into the fiber optic cable;
3. (c) when an acoustic signal hits the fiber-optic cable, light from the light transmission signal is backscattered and detected by a light reception unit as a light reception signal; and
4. (d) one or more properties of the acoustic signal source being determined by evaluating the light reception signal.

The expression “laid in a substantially horizontal direction” is intended herein to mean that the theoretical straight line connecting the two ends of the fiber optic cable has a slope or slope of less than 45°.

With the method according to the invention it is possible to detect acoustic signals at long distances of at least 30 km, more preferably at least 40 km, even more preferably at least 50 km, even more preferably at least 60 km and most preferably at least 70 km. The upper limit of the length of the fiber optic cable in the method according to the invention is 100 km, more preferably 90 km and most preferably 80 km. In other words, in the method according to the invention, the fiber-optic cable therefore preferably has these minimum and maximum lengths.

In one embodiment of the method according to the invention, the light transmission signal is preferably a laser pulse. A substantially narrow-band laser is preferably used.

In one embodiment of the method according to the invention, the light reception signal is preferably temporally correlated with the light transmission signal for evaluation in step (d). In this way, knowing the time at which the light transmission signal is emitted and the time at which the backscattered light reception signal is detected, the distance to the location where an acoustic signal hits the fiber-optic cable can be calculated, taking the speed of light into account. This is the distance from the end of the fiber optic cable where the light transmission signal is fed to the location of the fiber optic cable where the acoustic signal arrives. In other words, with the method according to the invention, information about the distance of the acoustic signal can be obtained by evaluating the light reception signal. Likewise, the method according to the invention also makes it possible to obtain information about the intensity and/or the frequency spectrum of the acoustic signal and/or the distance of the acoustic signal source from the fiber optic cable and/or the nature of the medium through which the sound propagates.

The method according to the invention therefore uses the principle known as Distributed Acoustic Sensing (DAS), which is based on the analysis of optical signals that arise from the coherent elastic scattering (Rayleigh scattering) of laser light in fiber-optic cables or the glass fibers contained therein. Using the backscattered light signals, the smallest dynamic changes in length in the fiber optic can be analyzed in a spatially resolved manner. Such dynamic changes in length in fiber-optic cables occur in particular when acoustic waves hit the fiber optic, causing it to dynamically alternately compress and stretch on microscopic scales. A significant advantage of this method over geophones is that a single standard fiber optic, such as that used in telecommunications, is sufficient to analyze acoustic signals at thousands of measurement points. Another advantage of this method is that no sensors or electrical connections need to be installed in the area to be measured, only the fiber optic cable. This method is schematic in 1 shown.

In one embodiment of the method according to the invention, it is particularly preferred that conclusions about the type of acoustic signal source are drawn by comparing the light reception signal with a predetermined and stored light signal. For this purpose, for example, the frequency spectrum and/or the time-resolved intensity spectrum of the light reception signal can be compared with a large number of known frequency spectra and/or intensity spectra stored in a database. If the database already contains numerous frequency spectra and/or intensity spectra of seismological events such as mass movements that precede possible landslides, fractures, rock falls or landslides, a positive comparison of the respective spectrum can be used to conclude that the associated seismological event is occurring .

In the method according to the invention, the acoustic signal source is therefore preferably a mass movement in the solid or loose rock. The method according to the invention is therefore preferably used to draw conclusions about mass movements in solid or loose rock and to predict subsequent failure states such as landslides, fractures, rock falls or landslides.

According to the invention, a fiber-optic cable comprises an optical waveguide, which is preferably installed in the fiber-optic cable for its mechanical protection. The optical waveguide is preferably a glass fiber. According to the invention, any glass fibers can be used, such as a single-mode glass fiber or a multi-mode glass fiber, but preferably a single-mode glass fiber such as a glass fiber of the SM 9/125 µm type. The latter are now available in large quantities at low cost. In principle, it is also possible to use reserve fibers in existing cables (railway/road support cables). For protection, the glass fiber in the fiber-optic cable is preferably installed in a surrounding cable jacket, although different cable designs can be used depending on the demands of the application. The fiber optic cables can essentially be installed like electrical cables, although minimum bending radii and maximum transverse forces must be taken into account.

To lay the fiber optic cable in the loose rock, it is preferably placed loosely in the loose rock, either by simply placing it on the loose rock, or by being surrounded by the loose rock on all sides.

To lay the fiber-optic cable in solid rock, a horizontal hole is first drilled into the rock and then the fiber-optic cable is either laid loosely in it or with the cement suspension Rocks or mountains connected. In the method according to the invention, two parallel horizontal bores can also be carried out, into each of which a fiber-optic cable is inserted. It is preferred that one fiber optic cable is laid loosely and the other is connected to the rock or mountain using a cement suspension. The method according to the invention is carried out with both cables and the recorded light reception signals are evaluated separately with regard to their frequency spectrum. This makes it possible to distinguish between important seismological events and so-called civilization noise.

With the method according to the invention, a fiber-optic cable can be laid, for example, over a length of 70 km and acoustic signal sources in solid or loose rock can be detected over this distance. For example, to cover areas up to 140 km in length, the same light receiving unit can also detect light receiving signals from two separate fiber optic cables laid in different directions.

1. (a) mit einem faseroptischen Kabel, das im Fest- oder Lockergestein in einer im Wesentlichen horizontalen Richtung verlegt ist;
2. (b) mit einer Lichtsendeeinheit zum Erzeugen eines Lichtsendesignals zum Einspeisen des Lichtsendesignals in das faseroptische Kabel;
3. (c) mit einer Lichtempfangseinheit zum Detektieren eines durch das faseroptische Kabel rückgestreuten Lichtempfangssignals; und
4. (d) mit einer Steuer- und Auswerteeinheit zur Ansteuerung der Lichtsendeeinheit und zur Auswertung des von der Lichtempfangseinheit detektierten rückgestreuten Lichtempfangssignals, wobei die Steuer- und Auswerteeinheit das Lichtempfangssignal in Bezug auf das Lichtsendesignal hinsichtlich des Vorhandenseins von Reflexionsorten entlang des faseroptischen Kabels auswertet, die das Lichtempfangssignal verursachen.

In a further embodiment, the present invention also relates to a device for determining one or more properties of an acoustic signal source in solid or loose rock

1. (a) with a fiber optic cable laid in solid or loose rock in a substantially horizontal direction;
2. (b) a light transmission unit for generating a light transmission signal for feeding the light transmission signal into the fiber optic cable;
3. (c) a light receiving unit for detecting a light receiving signal backscattered by the fiber optic cable; and
4. (d) with a control and evaluation unit for controlling the light transmission unit and for evaluating the backscattered light reception signal detected by the light reception unit, the control and evaluation unit evaluating the light reception signal in relation to the light transmission signal with regard to the presence of reflection locations along the fiber-optic cable, which cause light reception signal.

All refinements and preferred embodiments that are described herein in connection with the method according to the invention can also be transferred to the device according to the invention.

The control and evaluation unit present in the device according to the invention can in particular carry out the method steps mentioned above, which are related to the control of the laser and the evaluation of the light reception signal.

The control and evaluation unit is preferably designed in such a way that it compares the light reception signal with a predetermined and stored light signal and conclusions are drawn about the type of acoustic signal source. The control and evaluation unit is preferably a digital data processing unit that can control the light transmission unit to send a light transmission signal and can evaluate the light reception signal.

The device according to the invention preferably also has optics for coupling the light transmission signal into the optical waveguide of the fiber-optic cable. Furthermore, the device according to the invention preferably also has optics for coupling out the light reception signal from the optical waveguide of the optical cable.

• 1 den Aufbau einer DAS-Installation zur ortsverteilten Messung von akustischen Signalen entlang eines faseroptischen Kabels;
• 2 auf der linken Seite die momentane DAS-Signalstärke über 73 km eines faseroptischen Kabels hinweg aufgetragen in Bezug zur maximalen Signalstärke bei 70 km Entfernung; das dort detektierte akustische Ereignis hat eine relative Stärke von nur 10 Nanostrain; 2 auf der rechten Seite das Frequenzspektrum im Wasserfalldiagramm im Bereich um 70 km; die ursprüngliche Frequenz der Signalquelle (173 Hz) wird korrekt wiedergegeben;
• 3 das Frequenzspektrum eines akustischen Signals entlang eines faseroptischen Kabels; zwei verschiedene Typen von Ereignissen können anhand ihrer spektralen Signatur unterschieden werden;
• 4A ein Ort-Zeit-Signalstärke-Diagramm eines direkten akustischen Signals, erzeugt von einer laufenden Person; die Person bewegt sich fast rechtwinklig zum verlegten faseroptischen Kabel;
• 4B ein Ort-Zeit-Signalstärke-Diagramm eines indirekten akustischen Signals, erzeugt von einem Geschoss; die Flugbahn erstreckt sich entlang des faseroptischen Kabels;
• 5 oben eine schematische Darstellung der Schallausbreitung; 5 in der Mitte simulierte Ort-Zeit-Signalverläufe für verschiedene Entfernungen der Signalquelle zum faseroptischen Kabel (Schallgeschwindigkeit 4 km/s); 5 unten simulierte Ort-Zeit-Signalverläufe bei fixer Entfernung (250 m) für verschiedene Schallgeschwindigkeiten im Fels.

The present invention will be explained by way of example using the following figures. Show in the drawing

• 1 the construction of a DAS installation for the spatially distributed measurement of acoustic signals along a fiber-optic cable;
• 2 on the left, the instantaneous DAS signal strength over 73 km of fiber optic cable plotted against the maximum signal strength at 70 km distance; the acoustic event detected there has a relative strength of only 10 nanostrain; 2 on the right the frequency spectrum in the waterfall diagram in the area around 70 km; the original frequency of the signal source (173 Hz) is reproduced correctly;
• 3 the frequency spectrum of an acoustic signal along a fiber optic cable; two different types of events can be distinguished based on their spectral signature;
• 4A a space-time signal strength diagram of a direct acoustic signal generated by a running person; the person moves almost perpendicular to the laid fiber optic cable;
• 4B a space-time signal strength diagram of an indirect acoustic signal generated by a projectile; the trajectory extends along the fiber optic cable;
• 5 above a schematic representation of the sound propagation; 5 in the middle, simulated place-time signal curves for different distances from the signal source to the fiber-optic cable (sound speed 4 km/s); 5 Below simulated place-time signal curves at a fixed distance (250 m) for different speeds of sound in the rock.

The in 1 The structure of a DAS installation shown contains a fiber-optic cable 1, which is laid in solid or loose rock in the method according to the invention. The fiber optic cable 1 can be laid in solid or loose rock over a distance of up to 70 km. Furthermore, the DAS installation has a light transmitting/receiving unit 5, which is shown here as a combined unit. It can couple a light transmission signal 2 into the fiber optic cable 1, which propagates along the fiber to the other end. If an acoustic signal 4 hits the fiber-optic cable 1, so-called Rayleigh scattering of the light transmission signal 2 occurs at this point in the form of backscattered light as the light reception signal 3, which propagates in the fiber-optic cable 1 in the opposite direction to the light transmission signal 2. The unit 5 can detect the light reception signal 3. The control/evaluation unit 6 can evaluate the light reception signal 3 in terms of time in relation to the light transmission signal 2 in order to draw conclusions about the location of the acoustic signal 4.

Furthermore, the control/evaluation unit 6 can detect and evaluate the intensity and the frequency spectrum of the light reception signal 3. The control/evaluation unit can also control the light transmission signal 2, which is preferably present as a laser pulse. The time interval between the time of the laser pulse sent as a light transmission signal 2 and the detection of the light reception signal 3 correlates with the distance to the point in the fiber optic cable 1 at which the acoustic signal 4 strikes. A display unit 7 gives a user the opportunity to view the information received and evaluated.

The spectra in 2 are recorded using a standardized signal source with a DAS meter. A section (here approx. the last 15 m) of an approximately 73 m long glass fiber is wound around a piezoelectric cylinder, which changes the length of the glass fiber depending on the electrical voltage applied and thus simulates the influence of sound waves. Here, a dynamic change in length with a strength of 10 nanostrain is generated at a distance of 70 km from the DAS measuring device, which corresponds to a change in length from 10 nm to 1 m. The time course of the applied voltage is sinusoidal with a frequency of 173 Hz. As in 2 shown, this tiny change in length can be detected at a distance of 70 km with a signal-to-noise ratio of almost 30 dB. Furthermore, the frequency of the dynamic change in length can be determined to be exactly 173 Hz through a spectral analysis of the measurement data. This shows the high sensitivity of the measurement even at large distances. The measurement time required to record data is only 200 ms, which enables real-time measurement.

In the 3 The waterfall diagrams shown are created using a device similar to that used when recording the ones in 2 shown spectra is used. The 3 shows the spectral analysis of DAS data to perform categorical discrimination of different types of signal sources. Basically, abrupt, short-term events show a spectrum that extends over a wide frequency range. In contrast, long-lasting, monotonous noises produce a spectral signature in which individual frequency peaks dominate. The 3 shows the acoustic frequency spectra measured along a fiber optic cable at a distance of approximately 30 km to 34 km. The event at 30.8 km is a persistent signal generated by the piezoelectric cylinder mentioned above. A signal caused by a short-term event is measured at 33.6 km. The fiber optic cable is laid underneath coarse gravel and a person landed on the gravel immediately before the measurement after jumping about 20 cm high. These results show the enormous potential of DAS to distinguish different types of signal sources based on their spectral signature, even over long distances.

To include the location-time signal strength diagrams in the 4A and 4B A fiber optic cable is laid over 180 m under gravel and a DAS measurement is carried out. This is how it shows 4A the recording of a direct source of sound, namely the footsteps of a person moving away perpendicular to the path of the fiber optic cable. The 4B shows the sound propagation of an indirect sound source, namely a projectile in the air that moves parallel to the fiber optic cable. It can be shown here that, based on the place-time signal strength diagram, it is possible to conclude whether the sound source is a direct or indirect one. It is also possible to obtain information about so-called civilization noise, which can therefore be distinguished from desired acoustic signals from mass movements, for example.

5 Above shows a schematic representation of the sound propagation. From an acoustic Signal source 8, a sound wave front 9 propagates in the direction of the fiber optic cable 1. The acoustic signal source 8 and the fiber optic cable 1 have a distance of 10. The place-time signal curve for different distances of the acoustic signal source 8 (20 m, 250 small, 1000 m and 2500 small) is in the middle of the 5 shown. Here you can see that a short distance from the acoustic signal source 8 to the fiber-optic cable 1 results in the sound wave front propagating mainly longitudinally along the fiber-optic cable 1 (20 m), while at a very large distance almost simultaneous impact of a widely extended wave front is observed (2500m). In this way, for example, the position and distance of the acoustic signal source 8 to the fiber-optic cable 1 can be determined.

Down in the 5 a place-time signal curve of sound waves is shown, both of which are emitted by acoustic signal sources which have the same distance of 250 m from the fiber optic cable 1. However, the sound waves travel at different speeds (3 km/s or 6 km/s) due to different rock materials. It can be seen that the different speeds of sound in the rock have a strong impact on the sound profile. This makes it possible, for example, to distinguish between different types of rock or seismic S and P waves.

This data is intended to demonstrate the potential of the DAS for monitoring rock fractures and to provide a geotechnical assessment of the situation in the rock after construction work has been completed. The frequency-resolved measurement of the acoustic signals offers the possibility for a more precise source analysis and assignment of events.

List of reference symbols:

1

fiber optic cable

2

Light transmission signal

3

Light receiving signal

4

acoustic signal

5

Light transmitting/receiving unit

6

Control/evaluation unit

7

Display unit

8th

acoustic signal source

9

Sound wave front of the acoustic signal source

10

Distance of the acoustic signal source to the fiber optic cable

Claims (9)

Method for determining one or more properties of an acoustic signal source (8) in solid or loose rock (a) a fiber optic cable (1) is laid in solid or loose rock in a substantially horizontal direction; (b) a light transmission signal (2) is fed into the fiber optic cable (1); (c) when an acoustic signal (2) hits the fiber-optic cable (1), light from the light transmission signal (2) is backscattered and detected by a light reception unit (5) as a light reception signal (3); and (d) one or more properties of the acoustic signal source (8) being determined by evaluating the light reception signal (3). Procedure according to Claim 1 , wherein the light transmission signal (2) is a laser pulse, preferably from a substantially narrow-band laser. Procedure according to Claim 1 or 2 , in which the light reception signal (3) is temporally correlated with the light transmission signal (2) for evaluation in step (d). Procedure according to one of the Claims 1 until 3 , in which information about the distance and/or the intensity and/or the frequency spectrum of the acoustic signal (4) is obtained by evaluating the light reception signal (3). Procedure according to one of the Claims 1 until 4 , in which conclusions about the type of acoustic signal source (8) are drawn by comparing the light reception signal (3) with a predetermined and stored light signal. Procedure according to one of the Claims 1 until 5 , in which the acoustic signal source (8) represents a mass movement in solid or loose rock, and the method is used to infer mass movements in solid or loose rock and to predict subsequent failure states such as landslides, fractures, rock falls or landslides. Device for determining one or more properties of an acoustic signal source (8) in solid or loose rock (a) with a fiber optic cable (1) which is laid in the solid or loose rock in a substantially horizontal direction; (b) with a light transmission unit (5) for generating a light transmission signal (2) for feeding the light transmission signal (2) into the fiber optic cable (1); (c) with a light receiving unit (5) for detecting a light returned by the fiber optic cable (1). scattered light reception signal (3); and (d) with a control and evaluation unit (6) for controlling the light transmission unit (5) and for evaluating the backscattered light reception signal (3) detected by the light reception unit (5), the control and evaluation unit (6) receiving the light reception signal (3 ) in relation to the light transmission signal (2) with regard to the presence of reflection locations along the fiber optic cable (1) which cause the light reception signal (3). Device according to Claim 7 , in which the control and evaluation unit (6) carries out the method according to one of the Claims 3 until 5 carries out. Device according to Claim 7 or 8th , in which the control and evaluation unit (6) is designed such that it compares the light reception signal (3) with a predetermined and stored light signal and conclusions are drawn about the type of acoustic signal source (8).

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