**1. Introduction**

Monitoring of vibrations from anthropogenic sources is becoming increasingly important, mainly due to the mass development of transport and construction–reconstruction in urban areas, where heavy construction equipment is used. Hence, effective low-cost, low-power, low-complexity methods for monitoring the dynamic response of buildings are intensively sought, which will transmit information about seismic or acoustic load continuously and in real time.

In the field of seismic engineering, when monitoring vibrations from anthropogenic sources, measuring technology based on sensors in the electrical domain has been used for several decades to measure the dynamic response of the rock environment as well as the dynamic response of the structure. Currently, standard seismic instrumentation

Martinek, R.; Kepak, S.; Hrubesova, E.; Pinka, M.; Kolarik, J. New Methods to Seismic Monitoring:Laboratory Comparative Study of Michelson Fiber-Optic Interferometerand Pneumatic Measurement Systems. *Photonics* **2021**, *8*, 147. https://doi.org/10.3390/ photonics8050147

Received: 24 March 2021 Accepted: 25 April 2021 Published: 28 April 2021

**Citation:** Stolarik, M.; Nedoma, J.;

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consisting of a digital station and a three-component piezoelectric velocity seismometer is used to monitor dynamic effects [1]. In the dynamically developing 21st century, however, new progressive solutions are being sought in this area as well; these solutions can be found, for example, in other physical principles than those that have been used so far. Sensory technologies based on other physical principles form a new alternative method of monitoring both seismic (dynamic) effects and acoustic (all-in-one) effects, with high added value. The main advantages of these technologies are small size, electrical passivity and immunity to electromagnetic interference, the simple internal structure, high mechanical resistance, the possibility of use in extreme conditions (humidity, extreme temperatures, explosive environments, oil industry, nuclear power plants, etc.) or also a significantly lower price. In recent years, there has also been considerable progress in the area of advanced methods of digital signal processing and work with Big Data. It is one of the fastest growing scientific disciplines and it is obvious that these progressive methods will penetrate the field of seismic monitoring. These new DSP (Digital Signal Processing) methods can make measurements more accurate, provide new measurement functions, etc. Modern digital signal processing methods have now been used in many areas and industries, and current practice suggests that the same trend will continue in the future. For the practical application of these progressive methods in real applications, among other things, both theoretical and applied research of the new as well existing methods are needed. Some application areas of these new approaches are not ye<sup>t</sup> very developed and, in some fields, including the area of seismic monitoring, they are completely missing. It can be expected that new methods of seismic monitoring will enable the development of progressive techniques that are not ye<sup>t</sup> very developed in this area, such as the use of artificial intelligence and machine learning techniques to predict, creation of seismic and acoustic maps of analyzed areas, linking with other smart cities entities, etc.

However, before the routine field deployment of similar equipment in common seismic engineering tasks, such as monitoring the dynamic response of the building structure and rock mass from various sources [2–7], it is first necessary to experimentally verify whether the equipment being developed is able to detect vibrations to the extent and accuracy required for engineering practice (Table 1). For this purpose, several successful experiments with different sources of vibration have been conducted in situ [8–10]. These experiments, when confronted with common instrumentation for seismic monitoring, proved the applicability of the devices tested and being developed for given tasks, specifically in a wide range of applications, with excellent agreemen<sup>t</sup> both in time (length of seismic events, detection of specific maxima) and the frequency domain (predominant frequency content, identical frequency peaks). The next logical step was to verify, at the experimental level, the possibility of recalculation of the measurements units in the time domain, so that new devices could be a direct alternative to commercially used systems. The paper is going to present such a laboratory experiment with finding of basic relationship for an interferometric sensor and a pneumatic sensor using a calibrated stroke in simplified conditions. A calibrated and certified standard seismic station with a velocity sensor installed was used as a reference standard.

The following sections will present State Of-the-Art , Methods, Experimental Setup, Results of Experiments, Discussion and Conclusions.


**Table 1.** Examples of anthropogenic vibrations [6].
