The subject of the invention is the method of measuring vibrations, particularly seismic ones, and a device for measuring vibrations, particularly seismic ones, allowing for repetitive, cheap and safe assessment of especially the vibrations amplitude, while maintaining and even increasing the sensitivity of earthquake detection in relation to known solutions.

Vibration measurement is one of the basic methods for determining the risk or occurrence of structural hazard states, especially in the case of seismic vibrations. Such measurement helps to determine the nuisance of the neighbourhood, e.g. busy traffic junctions, and the impact of such neighbourhood on, for example, the construction of buildings or structures, or the assessment of hazards to the very traffic routes or bridges resulting from vibrations of transport-related origin, and the natural vibrations of the earth's crust. The evaluation of vibration amplitude and its variability in time also allows to determine the probability of occurrence of dangerous events such as earthquakes, landslides, etc.

Sensors are used for the measurement and evaluation of vibrations; most often these are mechanical sensors, which record or transmit displacement data, and sometimes also the speed or the acceleration in the measurement field. Displacements in the measurement area provide information about the magnitude of vibrations of the earth's crust. In particular, it is possible to observe the vibrations of buildings, machines and other objects using the invention.

Pendulum seismographs are among the most common sensors of vibrations, especially of tectonic origin; in these sensors the inert mass forming a vertical or a horizontal pendulum, oscillates depending on the design of the seismograph, as a result of which a recording is produced, corresponding to the "force of an earthquake". The recording takes place on both physical and digital media. The key feature of these solutions is mobility or the lack of mobility of the pendulum. In the latter case, the measurement does not concern the force with which the ground on which the seismograph is placed vibrates, but the force necessary to keep the pendulum in balance.

The oldest known vibration sensors recorded data in the sand, in which the end of the pendulum "carved" a groove as a result of displacements. Later, the pendulum was equipped with a stylus and the sand layer was replaced by a roller or a set of rollers with a paper tape on which the stylus drew a seismogram. Newer constructions process electrical data that are amplified and recorded, usually with a galvanometer on a photosensitive tape, or in newer solutions in computer memory. A prerequisite for the operation of the seismograph is that the period of oscillation of the pendulum should be long or very long in relation to the period of vibration of the observed surface, for example the earth's crust. The description of EP2906916 presents the vibration sensor, which has a base, inert mass, a light source and a detector; in that sensor the optical signal is used to measure the change in relative position of the inert mass and the base. In particular, interference between two light beams was applied during the measurement. In the described invention, a volumetric interferometer was used for the construction of the sensor. The description of EP2385357 presents a fibre-optic vibration sensor, which uses optical fibres as a horizontal pendulum. The free and unfastened end of the optical fibre is an inert pendulum and is placed in a bushing ending with a mirror or a glass plate. Thus, the vibrations of the optical fibre cause the light beam coming out of it to oscillate, and these oscillations are the measure of vibrations. The free movement of the inert fibre-optic pendulum may introduce harmonic vibrations which interfere with the reading of the measured vibrations.

In turn, the description of US5381492 reveals a vibration sensor in which two single optical fibres are used, which are coupled by couplers, so that the signal entered into them is divided preferably symmetrically. One of these arms is fixed to the ground or the housing of the device as the reference arm, and the other arm is freely "stretched" between the light source and the output coupler. The second optical fibre is a measuring arm that changes its position due to vibrations in the substrate or sensor environment.

The sensor according to CN205785495 has a similar principle of operation; in that sensor the arms are in the form of loops, one of which is stationary and the other reacts to vibrations of the environment, while the optical fibres on which both loops are structured are connected so that the signal from one loop passes to the other. This system requires a relatively complex data processing system in which overlapping signals from both fibres are eliminated.

There is also a vibration measurement system according to US4893930, which contains a cylinder with silicone rubber mandrels placed radially around the central core of the cylinder. Around each of the cores an optical fibre is wound, which changes its position as the device moves and the mandrels oscillate, while also changing the light propagation. Thus, the fibres used are similar to those used in Michelson's interferometer, allowing for the assessment of vibration amplitude based on the measurement of changes in the propagation of the optical signal.

The known devices, even very sensitive ones, are affected by a relatively large measurement error, which results mainly from the limitations of kinematic elements present in known vibration meters, for example seismographs. The devices with purely mechanical design require that the force exerting a vibrating effect on the measuring centre be relatively high in order to activate the electrical signal processing components; therefore, information about, for example, an impending earthquake or secondary quakes comes with a huge delay. Mechanical devices require additional power supply, which makes them susceptible to failures and does not allow for their easy application outside electrified areas. Additionally, electrically powered devices are susceptible to damage resulting from theft of cables or power supply elements and require frequent and accurate calibration resulting from changes in the electrical signal depending on the length of cables, even power supply ones. In addition, the signal transmitted electronically over long distances by copper cables is degraded, which results in a significant limitation of the maximum transmission range.

On the other hand, in devices where optical fibres are used and in which no electrical signal is propagated, as an element, inaccuracies can be observed due to mechanical limitations of the fibre itself, the elasticity and inertia of which is often insufficient to detect the beginning of vibrations, particularly seismic ones.

Therefore, it was advisable to develop a device and a method for measurement of vibrations, particularly seismic ones, which would combine the advantages of simple construction of mechanical devices and fibre-optic measurement accuracy, which at the same time could use the available and developing fibre-optic infrastructure. Due to the use of optical fibres in the measurement system, there is no need to supply the measurement site with electrical energy, which is avoided especially in places threatened by explosions. Additionally, with the use of this invention it is possible to obtain a high sensitivity regardless of the scope in which vibrations are measured, in particular seismic vibrations (earthquakes). The sensors within current technical scope measure relatively high vibrations, in particular earthquakes with low accuracy, while in the case of apparatuses dedicated to relatively small vibrations, in particular earthquakes, the sensors become saturated with high vibrations.

The measuring device for the measurement of vibrations, particularly seismic ones according to the invention incorporates the well-known base and inert mass assembly, which co-operate with the arms of at least one fibre-optic interferometer, the reference arm of which is firmly and indivisibly attached to the base and the measuring arm of which is attached at least in part to the inert mass, in particular in part to the base and in part to the inert mass, and shall have at least one frail, movable section. The reference arm is fixed to the base so that it is immobilised along its entire length and it is particularly mechanically fixed, for example by being glued, soldered, welded, connected by electrolysis, or passed through the structure of the base. The measuring arm is clamped to the inert mass in such a way that at least a part of the measuring arm is immobilised on the surface of or in the structure of the inert mass at least at one point. In particular, the measuring arm is mechanically fixed, for example by being glued, soldered, welded, connected by electrolysis, at least locally to the surface of the inert mass or otherwise passed through it.

In particular, the analogous effects can be achieved when the measurement arm is attached to the other movable element of the seismograph, relative to the base. The fact of fixing the measurement arm, in whole or in the part, to the other than the mass movable element of the seismograph, relative to the base, do not change the principle of the operation of the invention. The arms: the reference and measuring one are connected to at least one coupling element having at least one input arm and at least two output arms, preferably in the form of a coupler, in particular X coupler. The light source is connected to the input arm of the first coupling element, and the detector, depending on the design example, is connected, in the case of using an output coupling element, to the output arm of the second coupling element, or in the case of using only the input first coupling element, it is connected to the second output arm of the first coupling element.

In the special case where the reference and the measuring arms are coupled with only one coupling element, the detector is connected to the input arm of the coupler indirectly via a circulator and the coupler of the reference and the measuring arms is of the Y type.

Preferably, where a single input coupling element is used, the reference and the measuring arm ends are equipped with elements increasing the reflection of radiation, preferably in the form of mirrors applied to the front surfaces of the optical fibres or in the form of Bragg gratings. In particular, the fibre to be used as the reflecting surface shall be cut, preferably with its cut surface perpendicular to the fibre axis.

A light source is freely selected from a laser light source, narrow or broadband, SLED or other, for which the condition of interference in the fibre-optic interferometer system is fulfilled.

The fibre-optic interferometer, the arms of which are attached to the base and inert mass is, in particular, a Mach Zehnder interferometer or, in another example, a Michelson interferometer.

The method of measuring vibrations, particularly seismic ones according to the invention characterised in that the base and the inert mass to which the arms of at least one fibre-optic interferometer are attached, the reference arm of which is firmly and inseparably attached to the base and the measuring arm of which is at least partly attached to the inert mass, and in particular partly to the base and partly to an inert mass, and which has at least one least one frail, movable section is placed in the location where vibrations, particularly seismic ones, are observed, after which a light beam is introduced through at least one input coupling element, which is distributed in at least one input coupling element preferably at the ratio of 50:50 and is carried through the measuring and the reference arms up to the other coupling element, or to the ends of both optical fibre arms, preferably with elements increasing the reflection of radiation at their ends, preferably in the form of mirrors applied to the front surfaces of the fibre-optic cables or Bragg gratings, and the phase variations of the interference signal received by the detector resulting from the vibrations of the base in relation to the inert mass are measured on the basis of the signal received by the detector connected to the second coupling element or the second input arm of the first coupling element or via a circulator connected to the first input arm of the first coupling element; the result of the measurement is then interpreted and the variation in vibration amplitude over time, in particular the amplitude of the earth's crust vibrations, is determined.

Preferably the method of vibration measurement assumes is that it is possible to calibrate the device by putting the inert mass into motion by supplying electric current to the coil, which is a part of the vibration sensor and which, when powered by electric current, forces inert mass to vibrate. In this way, controlled vibrations of the inert mass in relation to the base are introduced, which enable the performance of the control measurement using a fibre optic interferometer. Such measurement will allow to verify the correct operation of the fibre-optic vibration sensor. The device for measuring vibrations, particularly seismic ones according to the invention is shown in the drawing, the figure 1 of which shows the vibration measuring device scheme, and fig. 2 shows another variant of the vibration measuring device scheme.

Example I

The vibration measuring device, particularly the seismic ones according to the invention contains a well-known base 1 and an inert mass 2 suspended above he base, which co-operate with the arms 3 and 4 of the fibre-optic interferometer, the reference arm 3 of which is permanently and inseparably attached to the base 1 and the reference arm 4 of which is attached to inert mass 2, so that it is immobilised on the surface of inert mass 2. The measuring arm shall be glued to least one point of the inert mass.

The arms: the measuring one 4 and the reference one 3 are connected to the coupling elements in the form of couplers 5 and 6. The light source 7 is connected to the input arm of the first coupling element 5 and the detector 8 is connected to the output arm of the second coupling element 6. The light source is a laser 7.

The method of measuring vibrations, particularly seismic ones according to the invention characterised in that the base and the inert mass on which the arms of the fibre-optic interferometer are attached, the reference arm 3 of which is firmly and inseparably attached to the base 1 and the measuring arm 4 of which is at partly attached to the base 1 and in partly to the inert mass 2, and which has frail, movable sections is placed in the location where vibrations are observed, particularly seismic ones, and then a light beam is introduced through the input coupling element, which is distributed in the coupling element at the ratio of 50:50 and is carried through the reference arm 3 and the measuring arm 4 up to the other coupling element, and the phase variations of the interference signal received by the detector resulting from the vibrations of the base in relation to the inert mass are measured on the basis of the signal received by the detector connected to the second coupling element; the result of the measurement is then interpreted and the variation in vibration amplitude over time, in particular the amplitude of the earth's crust vibrations, is determined.

Example II

The device for measuring vibrations, particularly the seismic ones according to the invention that contains a well-known base assembly 1 and the inert mass 2 suspended above the base, which cooperate with the arms 3 and 4 of the fibre-optic interferometer, the reference arm 3 of which is permanently and inseparably attached to the base 1 and the reference arm 4 of which is partly attached to the base and partly to the inert mass and has a frail, movable section located between the base 1 and the inert mass 2. The measuring arm shall be glued to least one point of the inert mass.

The arms: the measuring one 4 and the reference one 3 are connected to the coupling element in the form of coupler 5. The light source 7 is connected to the input arm of the coupling element, i.e. The coupling 5 and the detector 8 is connected to the second input arm of the coupling element, i.e. the coupling 5. At the end of arms 3 and 4 there are elements to increase the reflection of radiation in the form of mirrors applied to the front surfaces of the optical fibres. The light source is a SLED. The method of measuring vibrations, particularly seismic ones according to the invention characterised in that the base and the inert mass on which the arms of the fibre-optic interferometer are attached, the reference arm 3 of which is firmly and inseparably attached to the base 1 and the measuring arm 4 of which is at partly attached to the base and in partly to the inert mass, and which has a frail, movable section is placed in the location where vibrations are observed, particularly seismic ones, and then a light beam is introduced through the input coupling element, which is distributed in the input coupling element at the ratio of 50:50 and is carried through the reference arm 3 and the measuring arm 4 up to the mirrors 13 placed at the front of the optical fibres forming both arms, and the phase variation of the interference signal received by the detector 8 resulting from the vibrations of the base in relation to the inert mass is measured on the basis of the signal received by the detector 8 connected to the second input arm of the first coupling element; the result of the measurement is then interpreted and the variation in vibration amplitude over time, in particular the amplitude of the earth's crust vibrations, is determined.