RU2809469C1 - Method and system for seismoacoustic monitoring of rock mass - Google Patents

Abstract

FIELD: measurement instrumentation.

SUBSTANCE: seismoacoustic monitoring system for a rock mass consists of radiating well (15), at least one receiving well (4), control unit (1) and data processing unit (16). Electric spark emitter (14) is located in emitting well (15), connected by main electrical cable (13) to electrical energy storage device (11). Each of receiving wells (4) contains downhole fibre-optic cable (5) connected to interrogator (2). Fibre-optic cable (5) and interrogator (2) perform the function of seismic-acoustic sensors. At the lower end of fibre-optic cable (5), cable lug (9) is mounted, inside of which at least one pair of fibre-optic strands is welded together. Using electric spark emitter (14), elastic vibrations are periodically excited in emitting well (15). Elastic vibrations are recorded in receiving wells (4) using fibre-optic cable (5) and interrogator (2). In this case, direct, refracted head and reflected waves are received as a useful signal. For the direct and head waves, the cross-correlation function is calculated at predetermined time intervals. By changing the time and shape of the recorded signal, the presence of changes in the state of the array is determined. For reflected waves, a processing graph is performed to obtain a depth section of the acoustic contrast.

EFFECT: ensuring continuous monitoring of the state of geological processes in the soil and/or foundation of a building.

8 cl, 4 dwg

Description

The group of inventions relates to the field of geophysics, in particular to the field of seismic exploration and acoustics, and can be used to monitor the condition of soils in zones of hazardous natural or technogenic-geological processes, as well as monitoring the foundations of buildings and structures in undermined areas to identify and timely signal the presence of hazardous geological and technogenic processes (decompaction, cracking, subsidence, karst).

There is a known method for automatic seismoacoustic monitoring of the state of a rock mass, including registration of seismoacoustic pulses by geophones located on different sides of the studied massif, and in order to increase the reliability of monitoring, rectangular pulses are formed from the signals of opposing geophones, which are fed through a coincidence circuit to the indicator, and the duration of the rectangular pulses are chosen less than the travel time of the acoustic signal in the array between opposing geophones (SU 1057913 publ. November 30, 1983, IPC: G01V 1/24). The disadvantage of this method is the absence of an active source of elastic waves; therefore, there is no possibility of constant monitoring of the state of the massif.

There is a known method for geophysical monitoring of a rock mass, in which the average value and dispersion of received acoustic emission signals in each microzone at each selected level are assessed, and the physical changes that have occurred in the massif are judged using a calculation formula (RU 2090905 publ. 09/20/1997, IPC: E21C 39 /00). The disadvantage of this method is the low reliability of the system for determining and predicting the state of the rock mass, since control is carried out using indirect data on the magnitude of acoustic emission.

There is a known method for monitoring changes in discontinuity in a rock mass (RU 2498353 publ. November 10, 2013, IPC: G01V 1/28), which consists of sounding the rock mass with signals, receiving signals that have passed the controlled section of the rock mass, using a series of separate single signals as a probing signal rectangular acoustic pulses, additionally determine the energy of each received pulse in selected frequency intervals.

There is also a known method for monitoring changes in the physical and mechanical state of a rock mass (RU 2570824 publ. 12/10/2015, IPC: G01V 1/00, G01V 1/36), which consists in sounding the controlled section of rocks with acoustic pulses and receiving signals. This method is taken as a prototype. The disadvantage of these methods is that only direct transmitted waves are received as a useful signal, while subsequent arrivals (reflected and refracted waves) are ignored, despite the fact that they carry a large amount of information that can be used to assess the state of the rock mass .

A known system for monitoring the state of a rock mass during underground mining, including a module for recording the state of a rock mass, connected by communication lines to a data processing device connected to a danger signal indicator, containing seismic, seismoacoustic, strain gauge sensors for changes in the state of the rock mass. (RU 122119 published on November 20, 2012, IPC: E21C 39/00). The disadvantages of this system are the use of point sensors, which leads to high cost and labor-intensive work with little information content.

A device is known for monitoring changes in the stress state of a rock mass, including a pulse generator, receiving transducers, a spectrum analyzer, a memory unit, a comparison unit and an indicator. (SU 1645511 published 04/30/1991, IPC: E21C 39/00).

A device for monitoring changes in the physical and mechanical state of a rock mass is also known (RU 2539521 publ. 01/20/2015, IPC: E21C 39/00), taken as a prototype of the proposed system, including an elastic vibration generator, receiving transducers, a memory unit, a processing unit and indicator, and the generator is connected to the emitter. The device additionally contains a control unit, wherein the input of the control unit is connected to the synchronizing output of the generator, and the outputs are connected to the control unit and to the control inputs of the processing and comparison units.

The disadvantage of these devices, including the prototype, is the use of point seismic-acoustic sensors, the installation and connection of which is a very labor-intensive process; in addition, each sensor requires a separate block of analog-to-digital converters (ADC), which significantly limits the throughput of the system and the amount of information received .

The technical problem to be solved by the claimed invention is to ensure continuous monitoring of the state of the rock mass in areas prone to dangerous geological phenomena (subsidence, decompaction, cracking, karst) and timely notification of the presence of such phenomena.

The technical result consists in creating a system and method that provides continuous monitoring of the state of geological processes in the soil and/or foundation of a building, allowing timely detection of occurring dangerous deviations from normal values, and notification of the presence of dangerous geological phenomena.

The technical result is achieved through the use of an automatic system of periodic excitation and the associated registration of seismo-acoustic signals propagating in the controlled array by a fiber-optic distributed system consisting of an interrogator (interrogating device) and a fiber-optic cable mounted in receiving wells, performing the functions of a seismo-acoustic sensor. In the processing unit, the recorded wave pattern is compared with previous measurements, on the basis of which a conclusion is made about the presence of negative changes in the array, their localization is determined, and an alert is sent to the operator’s console.

The problem is solved using a set of features common to the prototype, such as: a method of seismoacoustic monitoring of rock masses, including excitation of elastic vibrations in a well and registration using seismoacoustic sensors in wells at certain intervals, as well as distinctive essential features, such as: a distributed fiber-optic system, which includes an interrogator and a fiber-optic cable, acts as a variety of seismic-acoustic sensors; excitation of the elastic wave and its registration are carried out in automatic mode, then in predetermined time windows of arrivals of the direct and head waves, the cross-correlation function is calculated, and by changing the time of the first arrivals of the direct and head waves, as well as the shape of the recorded signal, the presence of changes in the state of the array is determined, and localization of such changes is also carried out; reflected waves are processed according to a pre-generated graph to obtain a deep seismoacoustic section, the change in the wave pattern over time is assessed taking into account statistical criteria.

It is proposed to automate the process of excitation of a seismoacoustic signal by using an electric spark source of elastic vibrations. Automation of the operation of such a source is carried out by connecting an electric current to the network and using a control unit. A downhole electric spark emitter consists of an electrical energy storage device and a winch with a reel on which a main cable with an emitter lowered into the well is wound. At the lower end of the main cable there is an electric spark emitter in the form of a container with electrolyte, which contains discharge electrodes.

The winch is equipped with a depth counter and is connected to a control unit, which is programmed to set the excitation cycle, which includes moving the emitter along the wellbore with a certain step (S in Fig. 1) and excitation of oscillations.

The electric spark emitter is synchronized with the interrogator (interrogating device), which acts as a recorder of elastic vibrations, to start recording at the moment the vibrations are excited.

As a seismoacoustic vibration sensor, a distributed fiber-optic system is used, consisting of a cable containing an optical fiber and an interrogator (interrogating device) sending optical pulses along the fiber-optic line and recording back-reflected Rayleigh scattering. The relationship between the magnitude of the backscattered optical signal and the fiber stretching and compression makes it possible to construct the distribution of seismoacoustic strains along the entire fiber optic line, and to use such a system as a distributed seismoacoustic sensor. The number of recording channels in a distributed system can reach tens of thousands, which, with a sufficiently long line length, provides a significant increase in resolution and a reduction in labor costs for installation and maintenance of the system compared to point sensors. Several wells can be included in one optical line at once. To do this, a tip is mounted at the lower end of the fiber optic cable, inside which two fiber optic strands are welded to each other, which ensures that the optical signal rotates at the bottom of the well. The maximum length of an optical line is limited by the dynamic range of the recorder and the optical budget of the line.

The best seismic-acoustic signal is achieved when plugging wells in which the downhole fiber optic cable is located.

The optical fiber reacts to tension and compression, which determines a specific sensitivity directional pattern. Such a sensor responds to vibrations propagating along the fiber axis and is practically insensitive to vibrations normal to the fiber axis. To obtain sensitivity normal to the cable axis, the optical fiber is laid inside the cable in a spiral, which leads to the distribution of sensitivity along three spatial axes.

Measurements in a distributed fiber optic system involve averaging the measured parameter based on optical reception L, which leads to the effects observed when grouping seismic sensors: the appearance of signal rejection in the frequency domain depending on the angle of incidence of the wave. The use of downhole optical cable with spirally laid fiber leads to a decrease in the effective reception base of the rock mass associated with the cable; accordingly, the frequency response of such a system also changes, expanding the frequency spectrum and improving the resolution of the system.

Simultaneous recording with straight and spiral fiber allows you to cover the rejection areas and obtain a uniform response in the frequency domain.

In the process of working out one recording cycle, the emitter moves along the entire wellbore with a certain step S and at each point the seismoacoustic signal is excited and recorded, due to which a system of multiple overlaps is implemented.

The recorded seismoacoustic signal is compared with previous measurement cycles. The comparison is performed by calculating the cross-correlation function (CCF) in predefined windows of the first arrivals of the direct, head and reflected waves. A statistically significant shift of the FVC relative to zero means a change in the wave pattern and indicates a change in the physical and mechanical properties of the mass and its stability. A system of multiple overlaps makes it possible to identify areas where this change occurs.

In the case of a stable state of the rock mass, the acoustic signal should not change.

The presence of statistically significant deviations in the recording time of acoustic waves (more than 3 sigma) means a change in the physical and mechanical properties of the array. A decrease in registration time means a weakening of the strength properties of the massif and the presence of negative technogenic-geological phenomena in the controlled massif. In this case, the system sends a signal to the operator console.

Thus, the use of the proposed group of inventions for monitoring rock masses using periodic excitation and reception of seismoacoustic signals makes it possible to identify and promptly prevent the presence of dangerous geological phenomena (decompaction, cracking, subsidence, karst).

The invention is illustrated by the following diagrams:

FIG. 1 - Vertical section. Circuit for excitation and recording of oscillations.

FIG. 2 - Vertical section. Scheme of propagation of useful seismoacoustic signals.

FIG. 3 - Comparison of seismograms of two measurements at different points in time.

FIG. 4 - Cross-correlation function of signals at different times.

The following position names are indicated in the figures:

1 - control unit;

2 - interrogator (interrogating device);

3 - ground connecting fiber optic cable;

4 - receiving well - well for receiving seismoacoustic signal (recording);

5 - borehole fiber optic cable;

6 - straight optical fiber;

7 - spiral optical fiber;

8 - cement mortar;

9 - cable lug;

10 - fiber welding points;

11 - electrical energy storage;

12 - reel with power cable and winch;

13 - main power (electric) cable;

14 - emitter (electric spark);

15 - radiating well - well for exciting vibrations;

16 - data processing unit;

17 - seismoacoustic vibrations;

18 - straight wave;

19 - reflected wave;

20 - refracted head wave;

21 - salt roof;

22 - wave pattern of the current measurement;

23 - wave pattern of the previous measurement;

24 - intersection of previous and current measurements;

25 - hodograph of the refracted head wave;

26 - direct wave hodograph;

27 - curve of maximum FVC versus depth;

S - step of movement of the electric spark emitter.

The seismoacoustic monitoring system for rock mass includes

emitting well 15 (well for exciting vibrations) and one or more receiving wells 4 (well for receiving seismoacoustic signal),

in this case, in the radiating well 15, an electric spark emitter 14 is placed as a source of elastic vibrations, connected by a main power cable 13, with an electrical energy storage device 11, made with the ability to automatically move along the height of the well by adjusting the length of the main power cable 13 by means of a reel 12 with a winch, equipped with a depth counter and connected to control unit 1.

The electric spark emitter 14 is a container with an electrolyte in which discharge electrodes are placed (not shown in the drawing).

In the receiving well 4 there is a downhole fiber optic cable 5 containing straight 6 and/or spiral 7 optical fiber, at the lower end of the downhole fiber optic cable 5 a cable tip 9 is mounted, inside of which two fiber optic strands are welded 10 to each other, on the surface the fiber optic cable 5 is connected with interrogator 2 connected to control unit 1 and data processing unit 16.

In the case where the system contains more than one receiving well 4, sections of the downhole fiber optic cable 5 of each of them are connected on the surface in series by a ground connecting fiber optic cable 3 into a single fiber optic line.

The fiber optic cable 5 in the receiving well 4 can be placed in the well fluid 28, or cemented 8 for better adhesion.

The set of seismic-acoustic sensors is a distributed fiber-optic system, which includes an optical fiber in the section of the downhole fiber-optic cable 5, mounted in all receiving wells 4, and an interrogating device 2.

Interrogating device 2 performs the functions of an elastic vibration recorder, performs high-precision measurement of the length of the optical fiber along the entire fiber-optic line installed in all tested receiving wells, with subsequent conversion into the value of seismic-acoustic impact.

The method of seismic-acoustic monitoring of rock masses is carried out as follows.

Before starting work, in the control unit 1, which provides automation of the method, the parameters (movement step, frequency/strength of discharges) of the movement of the electric spark emitter 14 and the parameters for repeating the observation cycle, including the testing of all excitation points, are programmed.

In the process of implementing the method, block 1 transmits control signals to a reel 12 with a winch, which automatically moves the emitter 14 throughout the depth of the well 15 (Fig. 1) with a given step S.

The electric spark emitter 14, attached to the lower end of the main electrical cable 13, with the help of electrical energy supplied from the storage device 11, produces excitation of elastic oscillations 17.

The registration of seismoacoustic signals by interrogator 2 is started synchronously with the supply of a pulse to excite oscillations from control unit 1.

The control unit 1 supplies a pulse to the storage device 11, as a result, voltage is applied to the discharge electrodes of the electric spark emitter 14, and an electric spark discharge occurs in the emitter 14, which is converted into the energy of elastic waves 17. Elastic waves 17 are transmitted through the rock mass into the receiving well 4, testing the filtering influence of rock mass. As a result of interaction with the mass, in addition to the direct longitudinal wave 18, several classes of waves are formed, of which reflected 19 and refracted head waves 20 are used as a useful signal. Refracted head waves arise under conditions of a sharp jump in the velocities of elastic waves in a section, for example, on the roof of a salt formation 21. In the receiving well 4, elastic waves cause a change in the length of the optical fiber 6 and 7 in the downhole fiber optic cable 5. The interrogating device 2 recalculates the change in the length of individual sections of the optical fiber into the magnitude of the acoustic impact and transmits it to the data processing unit 16. If a statistically significant increasing the registration time of elastic waves, such changes are considered a sign of negative technogenic-geological phenomena and a report on these events is transmitted by block 16 to the operator’s console.

Reception of seismic-acoustic signals is carried out using a distributed fiber-optic system consisting of an interrogator 2 sending optical pulses along a fiber-optic line consisting of a ground trunk cable 3, a straight fiber 6 and a spiral fiber 7 in a downhole fiber optic cable 5.

Based on the analysis of backscattering occurring in a fiber optic line, the interrogator constructs the distribution of fiber deformation at a certain point in time. Sequential interrogation of the fiber optic line with a time quantization step dT makes it possible to register seismoacoustic vibrations affecting fiber optics 6 and 7 of the fiber optic cable 5. From the interrogator 2, the recorded seismoacoustic data enters the data processing unit 16.

In processing unit 16, the recorded wave pattern 22 is compared with the previous measurement 23 (24 is their intersection) by finding the cross-correlation function (CCF) in the time interval of useful seismoacoustic signals. The hodograph of the direct wave 26 and the refracted head wave 25 are shown in Fig. 3.

As a result of the path-by-path determination of the CCF for each source-receiver pair, the maximum value of the function 27 is determined, which is taken as a change in the signal propagation time. As a result, a matrix of such values is constructed for the entire set of a system of multiple overlaps and, based on the solution of a system of linear equations, the distribution of changes in speeds in the controlled array is calculated. The presence of statistically significant changes in the velocity characteristics in the massif means the presence of negative technogenic geological processes. If they are detected, a signal is sent to the operator's console indicating the location of such a process.

The invention is illustrated by the following example of practical implementation.

At the emergency site of the Verkhnekamsk potassium salt deposit, five wells were drilled for seismic-acoustic monitoring: four receiving wells and one to excite vibrations.

A fiber optic line is installed in the receiving wells 4. To do this, a fiber optic cable 5 is lowered into each well 4, containing both straight 6 and spiral 7 fiber. At the end of the cable in each well, a cable tip 9 is mounted, in which the optical signal is deployed by welding fiber pairs. On the surface, the fiber is welded in such a way as to connect the spiral and straight fiber. Thus, the optical signal passes through the well 4 times - straight fiber down, straight fiber up, spiral fiber down, spiral fiber up. The wells are water-filled, the cable is placed in the well freely, without fastening. A ground-based connecting fiber optic cable 3 will be stretched between the wells, connecting all the wells into a single fiber optic line. The fiber optic line is connected to interrogator 2.

The process of periodic monitoring of the elastic characteristics of the rock mass is carried out by control unit 1, which, with a periodicity of 30 minutes, sends a signal to the electrical energy storage device 11 to excite a series of elastic vibration pulses of 16 pulses each. Each pulse is an electrical discharge with an energy of 2500 J, which is supplied through the main electrical cable 13 to the emitter 14, lowered into the well 15. The main electrical cable is wound on a reel 12 with a winch and connected to the drive through a sliding contact.

Interrogator 2 is connected to the electrical energy storage device by a line for synchronizing the excitation time and recording oscillations. At the moment of discharge, the emitter converts electrical energy into seismoacoustic vibrations 17, which propagate from the emitting well to the receiving wells and represent a direct longitudinal wave 18. When a direct wave interacts with a rock mass, other classes of waves arise, of which reflected waves 19, formed, are considered useful at acoustically contrasting boundaries and refracted head waves 20, arising at the boundaries of a sharp increase in the speed of elastic waves, for example, on the roof of a salt deposit 21.

Seismoacoustic vibrations that reach the receiving wells cause a local change in the length of the fiber in cable 5 (tension or compression). Interrogator 2, sending optical pulses into the fiber optic line based on the magnitude of the backscattered signal, builds a distribution of changes in fiber length along the entire length of the line and converts the change in fiber length into the intensity of the seismoacoustic signal arriving at the fiber optic cable at each moment of recording time with a sampling step in time and along fiber optic line. The wave pattern recorded in this way (seismoacoustic signals) is transmitted to the data processing unit.

Based on a predetermined link between the fiber optic line and the cable location in the wells, headers are assigned to seismic traces corresponding to virtual channels along the fiber optic line. Traces related to one well make up one seismogram. For each well, within one observation cycle, 16 seismograms are generated, which are summed up to average and increase the signal-to-noise ratio.

On the summary seismograms, time intervals are preliminarily identified for each well in which the direct 18 and refracted 19 head wave is recorded. In the selected time intervals, separately for each well, the cross-correlation function (MCF) of the direct and head refracted waves is calculated. Based on the maximum value of the FVK, the deviation of the current value of the wave arrival time from the previous measurement is determined. A deviation exceeding the standard deviation of the registration time by more than 3 times with 99% probability indicates the presence of significant changes in the velocity of longitudinal waves on the selected transmission line. A decrease in the registration time of the direct wave (and, consequently, the speed of elastic waves in the massif) means a deterioration in the strength properties of the massif in the supra-salt stratum; a change in the registration time of the head refracted wave means a change in the state of the salt stratum. In the processing block, the presence or absence of changes compared to previous measurements is assessed and a report is generated containing the distribution of the registration time of the direct and head waves, as well as the amount of time deviation compared to the reference value, which is sent to the remote computer to the operator.

Claims (8)

1. A seismic-acoustic monitoring system for a rock mass, consisting of a radiating well (15), at least one receiving well (4), a control unit (1), a data processing unit (16), characterized in that in the radiating well (15) an electric spark emitter (14) is placed, connected by a main electrical cable (13) to an electrical energy storage device (11), which, in turn, is connected to the control unit (1); in at least one receiving well (4) there is a downhole fiber optic cable (5) connected to an interrogator (2), which together perform the function of seismic-acoustic sensors, the interrogator, in turn, is connected to a control unit (1) and a unit (16) data processing; At the lower end of the fiber optic cable (5), a cable lug (9) is mounted, inside of which at least one pair of fiber optic cores is welded together. 2. The system according to claim 1, characterized in that the electric spark emitter (14) is designed to automatically move in the emitting well (15) by adjusting the length of the main electrical cable (13) by winding it on a reel (12) with a winch equipped depth counter and connected to the control unit (1). 3. The system according to claim 1, characterized in that the installation of a fiber optic line in the receiving well (4) is carried out with subsequent plugging of the well. 4. The system according to claim 1, characterized in that the receiving well (4) uses a fiber optic cable containing a spirally laid optical fiber (7). 5. The system according to claim 1, characterized in that the receiving well (4) in the fiber-optic cable simultaneously contains straight (6) and spiral (7) optical fiber, and registration is carried out from both fiber strands by installing them in one optical line. 6. The system according to claim 1, characterized in that the control unit is programmed to set an excitation cycle, including moving the emitter along the wellbore with a certain step and excitation of oscillations. 7. The system according to claim 1, characterized in that sections of the downhole fiber optic cable (5), located in several receiving wells (4), are sequentially connected by a ground connecting fiber optic cable into a single fiber optic line. 8. A method for seismoacoustic monitoring of a rock mass, including periodic excitation of elastic vibrations in a radiating well and their registration using a downhole fiber optic cable and an interrogator, together performing the function of a plurality of seismoacoustic sensors in at least one receiving well in automatic mode, characterized in that direct, refracted head and reflected waves are received as a useful signal; for the direct and head waves in predetermined time intervals, the cross-correlation function is calculated, and by changing the time and shape of the recorded signal, the presence of changes in the state of the array is determined; and for reflected waves, a processing graph is performed to obtain a depth section of the acoustic contrast.