Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/02—Generating seismic energy
  • G01V1/04—Details
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/02—Generating seismic energy
  • G01V1/143—Generating seismic energy using mechanical driving means, e.g. motor driven shaft
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/02—Generating seismic energy
  • G01V1/04—Details
  • G01V1/047—Arrangements for coupling the generator to the ground
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/18—Receiving elements, e.g. seismometer, geophone or torque detectors, for localised single point measurements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/20—Arrangements of receiving elements, e.g. geophone pattern
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/22—Transmitting seismic signals to recording or processing apparatus

Definitions

• the present invention relates in particular to a harmonic seismic wave generator which continuously generates seismic waves with a single precisely defined frequency and amplitude and with a special radiation characteristic.
• a signal generated by the generator propagates through the rock environment, is received by an array of seismic stations and is processed in a way that detects and analyzes nonlinear phenomena in a propagation of seismic waves.
• a use of the generator allows, in particular, a detection of critical stress before an earthquake.
• it is advantageous to use it for example, in a detection of groundwater level changes, magma movements, changes in hydrocarbon saturation in rocks during oil and natural gas extraction or the penetration of gases and liquids into the earth's crust.
• the present invention generally relates to an assembly for seismic prospecting, the so-called seismic beacon, which is the assembly comprising a harmonic seismic wave generator (also referred to as a transmitter) and an array of seismic stations (seismographs) which are specifically distributed.
• the seismic beacon makes it possible to detect the above-mentioned non-linear effects, i.e. splitting the original frequency into two close but different frequencies, rising higher harmonic frequencies that are an integer multiple of the fundamental frequency, changes in propagation of the seismic vawes, their attenuation and anisotropy, with great accuracy, and thus eliminating shortcomings of seismic prospecting performed by methods according to the prior art.
• the present invention relates in particular to the harmonic seismic wave generator, which is preferably used as part of the seismic beacon assembly according to the invention, or separately in another suitable configuration.
• the seismic beacon assembly includes the array of the seismic stations in a specific configuration, as will be described below.
• the harmonic seismic wave generator (briefly also generator) of the invention may be used with other or otherwise arranged seismic stations than those described herein.
• the generator continuously emits seismic waves with a special radiation characteristic and with a single precisely given frequency and amplitude, which are constant over time. The transmission takes place continuously and for a long time with great stability of the properties of the emitted waves.
• the seismic stations are preferably arranged in several groups and are placed around the transmitter so that non-linear effects can be efficiently analyzed.
• the processing of measurement records (seismograms) from these stations consists of monitoring the time changes of seismograms at individual stations and also of comparing recorded seismograms between individual seismic stations.
• the result is temporal changes in the nonlinear behavior of rocks. Based on them, it is possible to detect critical stress before the earthquake, but also, for example, to monitor changes in groundwater levels, changes in hydrocarbon saturation in oil and gas extraction, magma movements, penetration of gases and liquids into the earth's crust, etc.
• a generator according to the present invention is realized in the form of a rotor, which comprises two rolling bodies, moving during the rotation of the rotor along a closed circular path in a closed underground chamber.
• the bodies are fastened to the vertical rotor shaft with at least one arm and a shaft of the body so that they are arranged symmetrically (i.e. at the same distance) in opposite positions relative to the axis (shaft) and are in contact with the path (optionally pressed to the path) and can roll along the path as the rotor moves.
• Both the rotor and the path are located in a closed underground chamber, for example in a closed circular well excavated in solid and elastic rock.
• a diameter and a depth of the well are in the range of units of meters, for example the diameter is about 1 m to 3 m and the depth is 3 m to 5 m deep, preferably about 2 m and 4 m, respectively.
• the rotor is placed in the cylindrical well symmetrically around the rotation axis, the path is connected to the bottom of the well.
• the rolling bodies exert a force on the path, the force being a result of the gravity and centrifugal force, and optionally additional pressure force.
• the movement of the bodies along the circular path causes a small elastic deformation of the path and, consequently, also of the walls and the bottom of the well, and, consequently, also deformation of the surrounding rocks, and thus seismic waves are generated.
• Moving bodies are identical mainly in weight, but also in shape, which can be, for example, a ball, cylinder, cone or truncated cone.
• the truncated cone appears to be a preferred shape.
• the weight of each of the rolling bodies is, for example, about 500 kg to 4000 kg, preferably about 1000 to 3000 kg.
• the path is in a shape of a ring (the wall of the path is perpendicular to the horizontal) lying at the bottom of the well, or of a lateral surface of the truncated cone with a larger base at the top (the wall is deviated from the vertical by 1 to 89°, preferably 30 to 60°).
• the preferred shape of the path is the shape of the truncated cone lateral surface, preferably with a wall inclination of 45°.
• the rolling bodies are in contact with the path and are "pressed" against the path by their own weight and centrifugal force as the rotor rotates, and thus no additional mechanism is needed to ensure contact between the bodies and the path.
• the arms, which support the rolling bodies and attach them to the axis of the rotor can, for example, be composed of several parts, articulated or insertable, and additionally provided with an elastic element (e.g.
• the rotational frequency is chosen so that the resultant forces on the body are perpendicular to the tangential plane of the path.
• a movement of the rolling bodies along the path is mediated by the rotation of the rotor, which is ensured by a motor, preferably an electric motor, so that the speed of the moving bodies remains constant with great accuracy.
• the frequency, amplitude and radiating characteristics of the waves are thus stable for a long time. All factors that could affect the temporal changes in radiation rate are compensated at maximum extent.
• the moving bodies and the path along which they move are constructed of a very durable and elastic material that is not subject to corrosion and other time changes (for example, preferably stainless steel).
• the well is closed and preferably an air is exhausted from it to a low pressure, which is kept constant by means of air exhausting (for example by means of a vacuum pump).
• the temperature in the well is kept constant, e.g. by means of heating elements.
• the well is provided with at least one heating element.
• the temperature and pressure are kept constant by means of a control system using the pressure and temperature sensors and feedback to the heating element(s) and air exhausting pump.
• the speed of movement of the bodies is controlled by speed sensors and is kept constant by means of feedback to the motor system (electric motor) using a revolution counter (preferably for example laser revolution counter).
• the usual frequency range suitable for seismic prospecting is 0.1 to 100 Hz, the preferred frequency range is 0.5 to 30 Hz, most preferred range is between 1 and 5 Hz, for example about 1.5 Hz.
• a harmonic seismic wave generator is used as a transmitter and an array of the seismometers arranged in a specific way is used as a receiver.
• the choice of the location or rock for the location of the generator is important, because it is necessary to ensure that the rock is not damaged during the transmission of seismic waves and its mechanical properties remain the same for a long time.
• the generator should therefore be placed in a solid and well elastic rock.
• the receiver consists of an array of the seismic stations (seismometers), which are located on the surface or in boreholes around the generator. Their distance from the transmitter (generator) depends on the depth at which the area to be examined is located. It must be ensured that at least part of the waves propagating from the transmitter to the seismic station pass through this area. Furthermore, it is necessary to arrange the stations into groups, which will enable to effectively compare the mutual ratios of wave amplitudes at different stations. Usually the distances between stations within one group are less or comparable than the wavelength along the surface. A distance between groups of the stations is much greater than the wavelength. Station groups are usually located in different directions from the transmitter.
• An arrangement is preferred in which at least two groups of the stations located in two mutually perpendicular azimuths are used, both groups at the same distance from the generator, in the range of 25 to 75 km.
• Each group consists of 2 to 15 stations.
• a distance between the stations is preferably in the range of 250 to 1000 m.
• the stations are preferably arranged linearly in a radial direction from the generator, but other arrangements are also possible depending on the purpose of the measurement and the characteristics of the transmitted waves.
• the characteristics of the seismic stations must guarantee correct registration of the waves generated by the transmitter.
• the stations must be sufficiently sensitive to measure with at least the same sensitivity as natural seismic noise at the frequency considered.
• the frequency range of the stations must cover both the basic transmitted frequency and several times higher frequencies for the detection of higher harmonic frequencies.
• Suitable stations have been described e.g. in the patent CZ301217 ( Rotary seismic sensor system, seismic measuring kit comprising the system and method of seismic prospecting) or CZ306566 (Opto-mechanical sensor system for measuring seismic movements of the soil and a method of seismic measurements using this system).
• Data obtained from the seismic beacon system, i.e. from the seismic stations described above, in the form of seismograms are further processed and evaluated in the manner described below.
• the processing method focuses on finding four phenomena in the propagation of the seismic waves, which indicate the achievement of the critical state of the strain in the monitored area: a) Time changes in the speed of propagation of the seismic waves; b) Temporal changes in seismic wave attenuation; c) Formation of the higher harmonic frequencies; d) Splitting the frequency emitted by the transmitter into two close frequencies.
• the amplitudes of the seismic waves emitted by the transmitter are weak (compared to strong earthquakes, for example). Therefore, there is a risk that the searched phenomena will be overlaid by seismic noise. Finding nonlinear effects in seismic noise is a major problem in data processing. It helps a lot if the transmitted frequency is chosen so that the natural seismic noise at this frequency is as low as possible.
• Seismograms are processed in several steps:
• Step 1 Noise reduction at individual stations
• Step 2 Determination of the phase differences between the transmitter and the stations and their time changes
• a phase spectrum method is used to determine the phase difference.
• the complex Fourier spectrum of the seismograms is calculated and a phase at the fundamental frequency is determined.
• the transmitter phase at the same time and for the azimuth that corresponds to the geometric azimuth at the station is then subtracted from this phase.
• the premise of this method is that the time at the transmitter and at the station is very precisely synchronized. This can be achieved using a GPS satellite signal or other time service. With standard equipment, it is possible to achieve time synchronization better than 10’ 6 s. When using special procedures, even up to 10’ 9 s.
• the limiting factor in the accuracy of determining the phase difference is the noise of registered seismograms.
• phase determination accuracy is estimated to be 10’ 6 rad using a time window of one day.
• Step 3 Determination of the amplitude at stations and its time changes
• the procedure is similar to determining the phase difference, except that the amplitude spectrum is used instead of the phase one. Again, depending on the amount of noise, a compromise must be found between the accuracy of the amplitude determination and the accuracy of the determination of the moment of change.
• the amplitude determination accuracy estimate is 10’ 5 using a one-day time window.
• Step 4 Joint determination of the time changes of the amplitudes and phase differences
• Time changes can be searched separately on individual stations and for individual components. However, better results can be achieved if all stations in the group are processed together. In this case, it is assumed that the group of stations is hit by several different plane waves, which are mutually superposed. If the amplitudes, phases, angles of arrival and type of all these waves were known, it would be possible to calculate seismograms for all components at all stations in the group. This is a so-called direct problem. However, it is necessary to solve the so-called inverse problem, i.e. to determine the parameters of the incident waves on the basis of seismograms. This problem can be solved by various methods, such as the isometric method (see Malek and Brokesova, 2021, submitted). It determines the time changes in the parameters of the incident waves, which is more stable in terms of seismic noise.
• Step 5 Comparison of the time changes between individual groups of the stations
• the time changes in the parameters of the incident waves do not have to be caused solely by the increase of the stress to the critical limit. For example, a fall or rise of groundwater can also cause changes in the seismic field. Therefore, it is necessary to compare the results from different groups of the stations. If the changes in group A (along the fault) are statistically significantly larger than the changes in group B (perpendicular to the fault), this is evidence that the changes in the seismic field are caused by the changes in the fault.
• Step 6 Finding higher harmonic frequencies
• the procedure is the same as for processing the fundamental frequency, i.e. the amplitude and frequency spectra are analyzed, but at frequencies equal to integer multiples of the fundamental frequency.
• the analysis is performed only for group A, the occurrence of higher harmonic frequencies is not probable for group B.
• Step 7 Finding split frequencies
• step 6 The procedure is the same as in step 6, but a number of frequencies around the fundamental frequency are analyzed sequentially. Calculations are performed only for the group A.
• the invention relates to the harmonic seismic wave generator as described above and as defined in the appended claims 1 to 8.
• the invention further relate to the assembly useful for seismic prospecting and seismic prospecting method as described above and as defined in the appended claims 9 and 10.
• FIG. 1 Schematic drawing of a harmonic seismic wave generator according to the present invention.
• FIG. 1A shows a side section of a well in which a rotor with rolling bodies for movement on a circular path at the bottom of the well is located.
• FIG. IB shows a "cross" section - top view of the open well.
• FIG. 2 Schematic drawings of alternative embodiments of the harmonic seismic wave generator.
• FIG. 3 Schematic drawing of an exemplary arrangement of a seismic beacon - location of the generator (circle) and seismic stations (triangles).
• the thick line shows a trace of the tectonic fault on the surface.
• Group A of 12 stations is located on the other side of the fault than the transmitter in the vicinity of its trace.
• Group B stations are located in the direction perpendicular to the fracture. Both groups are linear in the direction away from the wave generator and are placed at equal distances.
• FIG. 4 Photograph of the laboratory model of the seismic beacon.
• the transmitter model is in 1:10 scale. An arrangement with cylindrical bodies and a perpendicular path wall along which the bodies move is used.
• FIG. 5 Result of a laboratory experiment with the seismic beacon model. An average amplitude spectrum of the seismogram over 1 hour of the measurement is shown. The fundamental frequency of 9.35 Hz is visible. As a result of nonlinear effects, higher harmonic frequencies 18.70 Hz, 28.05 Hz, 37.40 Hz, 46.75 Hz and 56.10 Hz are generated. Furthermore, the frequency 23.2 Hz, which corresponds to the inherent oscillations of the model, and the frequency 4.68 Hz, which corresponds to half of the base frequency, are visible.
• FIG. 6 Result of a laboratory experiment with a seismic beacon model. The detail from the amplitude spectrum in FIG. 4 in the interval 9.1 to 9.6 Hz is shown. The fundamental frequency of 9.35 Hz is visible. Due to the nonlinearity of the environment, this frequency splits, creating another significant maximum at 9.32 Hz and several other less significant maxima.
• a rotor 1 of the generator (FIG. 1) with two rolling bodies 1.2 in the shape of a truncated cone is placed in a closed cylindrical well 2 with a diameter of 2.2 m and a depth of 4 m, which is excavated in hard rock 3.
• Walls 2.1 and a bottom 2.2 of the well 2 are made of concrete and are insulated to prevent water penetration.
• the well 2 is covered by a concrete lid 2.3, which is placed airtight on the walls 2.1 of the well 2 and which is provided with insulation 2.4 of 1 m thick polystyrene foam on the underside.
• a constant temperature of 25 °C in the well 2 is ensured by heating elements 2.5 and thermometers 2.6, which are located at two depth levels 2 m and 3 m around the circumference of the well 2.
• a pressure is maintained at 1 kPa by means of a pump 2.7, which is located outside the well 2 and is connected to the well 2 with a tube 2.8 passing through the lid 2.3 with insulation 2.4.
• a path 2.9 along which the bodies 1.2 of the rotor 1 move is in the shape of the truncated cone lateral surface, where a diameter of the lower base is smaller than a diameter of the upper base, and is made of 1 cm thick stainless steel.
• the path 2.9 is placed around the bottom 2.2 of the well 2 so that it touches both the bottom 2.2 and the wall 2.1 of the well 2.
• the space, which is formed under the inclined path 2.9 is also filled with concrete.
• the path 2.9 is inclined at an angle of 45 0 to the vertical.
• the speed of movement of the bodies 1.2 along the path is adapted so that the centrifugal force during the movement of the bodies 1.2 is as large as the gravitational force, so that in this preferred arrangement the resultant force of each body 1.2 acting on the path 2.9 is perpendicular to the path 2.9.
• An elastic element is not used, the pressure on the path is realized only as a resultant of the centrifugal and gravitational force.
• the angle of the resultant of said forces will be different, but in order to minimize a load of the body suspension it is advantageous to choose these parameters so that the resultant forces are perpendicular to the path.
• the diameter of the path 2.9 on the underside is 1.4 m, the diameter on the top is 2.2 m.
• the width of the flat part of the path 2.9 is 0.5657 m.
• the path 2.9 can be provided with curved edges or raised edges to prevent the bodies 1.2 to leave the path 2.9.
• a vertical shaft 1.1 of the rotor 1 is provided with suspension arms 1.3, to which the rolling bodies 1.2 are rotatably connected by means of a shaft 1.5.
• the shaft 1.5 forms an axis of symmetry of each body 1.2 and is fixed by a bearing 1.4 in the axis of the body 1.2. This attachment allows the bodies 1.2 to be rolled along the path 2.9 when the vertical shaft 1.1 rotates.
• the vertical shaft 1.1 is driven by an electric motor 1.6 located above the lid 2.3 of the well 2.
• the lower end of the shaft 1.1 is seated in vertical shaft bearing 1.7 at the bottom 2.2 of the well 2.
• Rotational speed of the motor 1.6 is kept constant by accurate measurement with a laser revolutional counter 1.8 and a feedback that regulates the electric current with which the electric motor 1.6 is supplied.
• the vertical shaft 1.1 is provided with an airtight sleeve 1.9 which passes through the cover 2.3 and the insulation 2.4.
• the bodies 1.2 are made (as well as the path 2.9) of stainless steel. They have a shape of the truncated cone.
• the ratio of the diameter of the lower and upper base of the body 1.2 is the same as the ratio of the diameter of the lower and upper base of the path 2.9, the lower diameter is 0.7 m and the upper is 1.1 m.
• the wall of the truncated cone body 1.2 is as long as the width of the path 2.9, i.e. 0.5657 m.
• the weight of each of the pair of bodies 1.2 is
• the pair of the bodies 1.2 moves evenly along the path 2.9, wherein the bodies are always located at opposite points of the path 2.9.
• the bodies 1.2 move along the path 2.9 by rolling with a constant speed 2.389 m/s of the center of gravity, which is located 0.5818 m from the axis of rotation. At this speed, the bodies 1.2 make one revolution in 1,530 s.
• the bodies exert a force of 33.14 kN perpendicular to the path 2.9 and indirectly to the wall 2.1 of the well 2.
• the force acting perpendicular to the path 2.9 (resultant of gravity and centrifugal force) generates primarily longitudinal seismic P wave, which propagates in the direction of the applied force.
• the wave field from all moving bodies is composed together.
• the radiation characteristic is symmetrical according to the axis of rotation, but waves with different phases are emitted in different directions.
• a number of other waves are formed, including transverse S waves and surface waves. These further interact with each other and during their extension further reflections, conversion at interfaces, refraction in a velocity gradient medium, scattering, attenuation and other wave phenomena, including nonlinear effects, which are the subject of research, occur.
• the result is a complex wave field, with some waves passing only through the subsurface layers, others passing through deep parts of the Earth.
• FIG. 2 shows schematically alternative embodiments of the generator, in which the rolling bodies 1.2 have the shape of A) a sphere, B) a cone or C) a cylinder.
• the transmitter i.e. generator, essentially identical to the generator according to Example 1, was placed into a block of mass of rubber crumb connected by an adhesive, which simulates the rock mass.
• the generator differed from the generator according to Example 1 in the shape of the rolling bodies 1.2, since cylinders were used here, being mounted so that their longitudinal axis is vertical.
• the arms 1.3 carrying the rolling bodies 1.2 are formed by two parts which allow the bodies to move radially and thus ensure contact with the path 2.9.
• the arms 1.3 are provided with a spring which ensures the pressure of the rolling bodies 1.2 against the path 2.9.
• the path 2.9 in this model is a "hoop" - a ring with a vertical wall. A hole simulating a well was created in the mass, adapted so that the path 2.9 inserted into it was in perfect contact with the surrounding mass in its entire outer surface.
• FIG. 5 shows the average amplitude spectrum calculated from the measurement section with a length of 1 hour on a functional model.
• the fundamental frequency 9.35 Hz
• FIG. 6 we see a detail around the fundamental frequency. We see that the fundamental frequency has split, which is a non-linear phenomenon exploited in the present invention.
• a seismic beacon assembly i.e. the generator according to Example 1 as a harmonic seismic wave transmitter and two groups of the seismometers as a receiver, is used to detect a critical strain condition at a vertical tectonic fault at a depth of 5 km.
• Each group contains 12 six-component seismic stations (e.g. according to patent CZ306566).
• the transmitter is located on the fault track on the surface.
• Both groups of seismic stations are located on the surface at a distance of 50 to 56 km from the transmitter in two mutually perpendicular azimuths.
• Group A is located approximately on the fault track. To this group, therefore, the seismic rays propagate along the plane of the fault.
• Group B is positioned so that the link between the transmitter and the group is perpendicular to the fault.
• the groups are formed by stations arranged linearly in the radial direction from the transmitter. The first station is at 50 km, the next at regular intervals so that the twelfth station is at a distance of 55 km. These distances are chosen on the basis of a calculation so that the seismic rays of the refracted P waves, which are emitted at an angle of 45 degrees and reach a maximum depth of approximately 5 km, come to the surface at a distance within the group of stations. Distances and radiation angles need to be adjusted according to the specific seismic velocity model in the area, which is within the ordinary skills of a person skilled in tha art.
• Seismograms in group A are affected by changes at the fault to a depth of 5 km, while seismograms from stations in group B are not affected by these changes.
• the wavelength along the surface is approximately 5 km for P waves with a transmitted frequency of 1.307 Hz at this distance.
• the spacing between stations is chosen so that the profile of the stations covers this particular wavelength.
• the seismic beacon assembly can also be used for purposes other than detecting critical strain before an earthquake. In general, it can be used wherever the properties of the rock mass change over time. These can be changes in the groundwater level, magma movements, saturation the of rocks with hydrocarbons during oil and gas extraction, the penetration of the gases and liquids into the earth's crust, etc.
• Waves from all transmitters i.e. generators, can be measured by the same seismic stations. This results in more data, which usually gives more reliable results.
• Patent CZ301217 Brokesova, J., Malek J., Strunc J.: Rotary seismic sensor system, seismic measuring set containing this system and method of seismic survey.
• Patent CZ306566 Prokop Brokesova J., Malek J.: Optical-mechanical sensor system for measuring seismic soil movements and a method of seismic measurement using this system.

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• 2020
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