**7. Discussion**

This paper describes the first basic laboratory experiment with the finding of the mathematical relationship between the measured values of the alternative sensory devices being developed working on a different physical principle than it is still common in the field of vibration measurement. The comparative measurements were performed in simplified conditions. In these conditions, all measuring instrumentation was placed as well as the source of vibrations, which was placed on a homogeneous base at a relatively small mutual distance. However, in conventional in situ measurements, the distances are significantly greater, in the order of tens to hundreds of meters, and the environment in which the vibrations propagate is always fairly heterogeneous. Likewise, a calibrated stroke is the simplest form of a possible source of vibrations, where commonly monitored vibrations have a much more complex character. Climate conditions will certainly be another, not insignificant factor influencing the measurement results. In contrast, it is also possible to perform the entire comparative process in a much more sophisticated manner, using a vibrating table in a vacuum chamber, which will be the next step in the research. Similarly, the next step of the research will include verification of the currently obtained correlation equations outside the laboratory, in situ, on seismic phenomena caused by, for example, transport, construction equipment or blasting work.

The team of author has already started this experimental verification of laboratory-obtained correlation equations by means of pilot measurements of vibrations from tram transport.

One of the main benefits of alternative systems may be, among other things, the higher explanatory power of measurement without human access to the monitored site, i.e., shortening the necessary downtime or unavailability time only for the period necessary for instrumentation installation. The indisputable advantage of the alternative fiber-optic solution is the possibility of control monitoring in areas with interfering electromagnetic fields (increased safety), the possibility of measuring without access to the monitored site, data acquisition to streamline the design of monitored structures, minimizing risks in construction and operation. The users may also benefit from the access to the measurement data evaluated in an almost online mode. The synergy of the aforementioned aspects may increase the ability of the contractor to accurately and operatively respond to the direct consequences of the work being carried out on the environment and the built-up area—it will be possible to optimize blasting, the material used, to implement maximum procedures, thus achieving greater efficiency, safety and reduction of the impact of construction on the environment in contrast to the contractors offering their work in conjunction with the currently used conventional measuring systems.

The degree of sensitivity of the sensors being developed was an interesting item of verification resulting from this comparative measurement, which is demonstrated by capturing even the secondary manifestation of the impact of the weight. Capturing the reflection of the plate from stiffer subsoil by an interferometric, or pneumatic, sensory device is enabled due to a significantly higher sampling frequency. The original sampling

rate for interferometric signal was 100 kHz to make the demodulation with maximum precision. The original pre-demodulation sampling rate is always much higher. After the signal processing (demodulation) the phase response was down sampled to 1 kHz for the comparison and display. Pneumatic sensor has sampling frequency 10.24 kHz. For the BRS32 seismic station, the sampling frequency is 250 Hz, which is a commonly used sampling frequency for monitoring and subsequent analysis of seismic loading in engineering practice. The pneumatic and interferometric sensor being developed offers a higher frequency range and, as a result, it is sampled at higher frequencies, which can enable a much more detailed analysis of the seismic recording.

Regarding common engineering practice, the results are very interesting, because technical standards (e.g., [38]) state the limit values of effective velocity in tenths, while we achieved the average deviation from the measured values of the amplitude of the oscillation velocity of 0.022 and even 0.002 for the optical interferometer and the pneumatic sensor respectively.
