**2. State-of-the-Art**

In this part, the issue of vibration measurement with standard seismic instrumentation will be introduced, as well as alternative approaches to vibration measurement using developed sensors.

#### *2.1. Commercially Used Seismic Instrumentation*

Many manufacturers around the world produce seismic vibration monitoring stations, which are used for seismic monitoring purposes in engineering practice. These stations are manufactured as compact, or are supplied separately with an evaluation unit and a seismic sensor.

As a standard, these stations are used to measure vibrations to dynamically assess the response of a building structure due to anthropogenic sources. These sources include both wheeled and rail transport (tram transport especially in the urban area) as well as all construction technology generating vibrations, which are mostly inextricably connected with construction processes, especially with foundation of buildings and subsoil modification. Blasting work carried out in the vicinity of urban areas, whether it is large-scale blasting work in mining processes on the surface and underground, as well as blasting work as one of the work cycles in tunnel excavation, is a stand-alone issue. Here, these stations are then deployed as a necessary part of geotechnical monitoring, where, based on the measurement results, excavation and tunnelling procedures are optimized, thus reducing negative impacts on the environment. In general, vibrations from all these sources are called technical seismicity. The character of the recording in the time domain is subsequently dependent on the source of the dynamic load, which is a rapidly dampened seismic impulse or a longer-lasting shock.

Seismic sensors detect and measure ground vibration by means of the movement of a magne<sup>t</sup> suspended in a surrounding by a coil of wire. According to the Lenz Law of physics, a current is induced in the surrounding coil in a proportion to the velocity of the magne<sup>t</sup> movement with respect to the coil. The electronics in the monitor then measures this current, converts it to ground motion velocities and stores the raw data in the memory. Each of the three directions perpendicular to one another (longitudinal or radial, transverse, and vertical) has its own separate measuring coil in the transducer head, since the vibrations often differ significantly along the different measurement directions.

Standard seismic stations used for monitoring vibrations from anthropogenic sources have a common frequency range in the range from 2 to 200 Hz with respect to the sensor type. The mechanical principle of the measurement itself is the main disadvantage of these devices, since they are very sensitive to manipulation. The sensors as such are not immune to electromagnetic interference and, unlike the newly designed sensory technologies, do not withstand the long-term effects of extreme climatic conditions.

#### *2.2. Experimentally Developed Sensor Systems*

Sensory technologies based on optical fibers are an alternative method of monitoring seismic (dynamic) effects with a high added value. The fundamental advantages of fiber-optic sensors include small size, electrical passivity, resistance to electromagnetic interference (EMI) and low optical attenuation (measuring point can be separated from the evaluation optoelectronics). For high-precision measurements fiber-optic interferometers (such as Mach–Zehnder, Sagnac, Michelson, or Fabry–Perot) are the best choice. Interferometers are well known for their ability of making precision measurements of optical path difference between two fiber arms caused by a refractive index change in the interferometer arm or physical displacement [11]. Other technologies such as fiber gratings have limited frequency range, so the acoustic measurements are easier to implement in the case of interferometers. Below is a summary of current research in the field of optical sensing technology applicable in seismic measurements and with emphasis on interferometers.

The current state of knowledge mentions the use of optical interferometers in the area of seismic measuring about two decades ago [12]. However, these interferometers used interference in free space, not in optical fibers. Optical fiber configurations did not begin to appear until later. As one of the first fiber-optic approach within interferometric seismic measuring Sagnac interferometers [13] were described. Sagnac interferometer enables measuring even with the use of less coherent radiation sources contrary to other types of interferometers. The ability to measure rotation during seismic activity proved interesting but without further application, their main domain is still gyroscopes. An example of possible sensor design is [14]. The sensor is in the form of a cylinder on which a coil of optical fiber is wound. The cylinder is then immersed in a second larger cylinder full of water. The fiber length used in the Sagnac interferometer was only about 30 m long, ye<sup>t</sup> the authors were able to sense the vibrations generated within the experiment.

Seismic stations can be substituted by another fiber-optic sensors operating on the principle of acceleration detection [15] such as Fabry–Perot, Mach–Zehnder or Michelson interferometers. Sensors with these types of interferometers can be constructed as triaxial [16]. They can be characterized with the output intensity modulation measurable with regular optical power detectors. Design issues of all-fiber interferometric seismometers are discussed in [17].

The authors of [18] describe a simple seismometer based on a Mach–Zehnder interferometer. The designers were facing a phase drift of the sensor, which they successfully solved by actively controlling the operating point using a piezoelement in the arm of the interferometer. However, this solution is no longer completely optically passive and requires active electronic control, which is a considerable disadvantage. The resonant frequency of the resultant sensor is relatively low (60 Hz), which is also not practical in terms of calibrating the sensor frequency response. These issues were overcome in our setup using passive optical demodulation as described in [19] and a sensor design with short optical fibers in the interferometer arms.

The demonstration of in-fiber Fabry–Perot interferometer with fiber Bragg grating mirrors (FBG-FPI) is described in [20]. The sensor can monitor a wide range of vibration frequencies and therefore can be applicable in monitoring of seismic responses.

A distributed fiber-optic sensing technology called DAS or DVS (Distributed Acoustic/Vibration Sensing) was also presented as an alternate approach [21]. The principle uses Rayleigh's backscattering for scanning interferences over the entire length of the optical fiber acting as the sensor. Resulting acoustic and vibration signals at any point of the monitored fiber length have no parallel in the conventional measuring technology. The downside is the impossibility of measuring of wave direction at the measuring point and very high price of the evaluation units. For these reasons, it has not been put into practice ye<sup>t</sup> and is used rather on an experimental basis. In [22] the authors deal with the use of DAS for the measurement of underground propagation of acoustic and vibration waves, is an example of a successful application in practice, but, for surface measurements, this method is not entirely suitable.

In case of non-fiber interferometric approach of measuring, several interesting studies can be mentioned. The authors of [23] provided results describing the monitoring system of bridge statics and dynamic vibrations. Two different camera types were used to monitor the response of a bridge to a passing train. Image processing techniques (pattern matching, edge detection, and digital image correlation) were used for the analysis of the acquired images. Results were compared to reference measurements obtained by single point measurements using the laser interferometer. The laser interferometer sensors can provide a method for observing low-frequency ground motion on seismic, geodetic, and intermediate time scales, as described by the authors of [24]. Comparison of individual interferometers with respect to the description of the sensor design, and the advantages and disadvantages are shown in Table 2.

**Table 2.** Table of comparison of fiber-optic methods of vibration measurement.


A unique pneumatic sensory device was also employed in the experiments using other physical approaches to vibration measurement. The first instruments used to measure vibrations historically consisted mainly of weights suspended on a cable or a spring, and these, using their inertia, were able to detect slight movements of the earth due to seismic waves. The device, which is historically close in principle to the presented solution, is called a geophone. However, the pneumatic sensor does not detect movement by direct contact of the armature from the permanent magne<sup>t</sup> inserted in the coil, but uses a pressure sensor to measure the change in sound pressure inside the closed tube. This innovative method was a by-product of experiments in which dilatation was measured using a similar pneumatic system in a strong magnetic field, where standard sensors designed for this type of measurement could not be used. This device is still unparalleled in its research field and thus has not a non-commonly used and defined name. The device is, by its principle, similar to closed-hose presence detectors [26–28]. In contrast, this application does not depend on deformation, and its subsequent pressure changes, under the weight of the

measured object. The presented device measures vibrations, which are propagated by material surrounding the pressure tube.

A partly similar topic can be found in publications examining the manifestation of changes in closed tube-based vibration measurement [29–31]. The authors analyze vibrations produced by changes in flow, pressure, and density of the medium propagated by the tubes. This gives them a better understanding of the process that takes place inside the pressure tube, in contrast to our measurement, which focuses on the detection of external excitations passing into the tube.
