## 6 Conclusions  
Reliable measurement of surface vibrations propagated in a soil medium requires the use of such a coupling of measuring transducers to the ground that during the measurement there is no resonance vibration of the measuring bases with mounted to them transducers or other distortions of recorded signals in the range of amplitudes and frequencies. From the point of view of the predicted assessment of the impact of vibrations on buildings structures, we are generally interested in vibration frequencies in the band at least from about 0.5-1.0 Hz to 100 Hz [5; 19; 22]. Considering the above, and based on the results of modal analyses carried out using the Ansys Engineering Simulation and 3D Design Software, it should be stated that:  
* All three methods of coupling accelerometers to the ground by using so-called \"spikes\", theoretically provide a reliable measurement of surface vibration in terms of evaluating the predicted effects of recorded vibrations on the building structures (in each analyzed case the lowest natural frequencies of the measurement bases with attached accelerometers significantly exceeded 100 Hz);
* the weight of accelerometers mounted to measuring bases has the dominant influence on the natural frequencies of these measurement bases (the greater the total mass of transducers mounted--the lower the natural frequencies of the measuring bases embedded in the ground);
* the distribution of the natural frequencies of the measuring bases is affected by soil stiffness to a slightly smaller extent (the greater the soil stiffness modulus--the higher the identified natural frequencies of the measuring bases embedded in the ground);
* the optimal solution is a compromise in which the weight of the devices used to coupling transducers to the ground is minimized while using accelerometers with the highest possible sensitivity; and
* the widest frequency band in which there should be no resonance vibrations of the measuring bases with accelerometers mounted to them can be provided by the measuring base No. 3 (L50 \(\times\) 50 \(\times\) 5).  
The conclusions presented above are based on a simplified numerical model, which was developed for the needs of a linear modal analysis. In real conditions, the contact between the measuring bases/setups driven into the ground in the form of spikes and the ground is not linear. Also, the soil medium is neither homogeneous nor isotropic. Usually, when driving measurement bases in the soil, its surface layers loosen. For this reason, before measuring surface vibrations, the soil around the measuring bases must be well compacted. Unfortunately, under real conditions, much lower dominant natural frequencies of this type of measuring base with mounted transducers should be expected anyway.  
Bearing in mind the reliability of measurements of surface acceleration, it is advisable to perform analogous numerical simulations for other solutions commonly used for this purpose (e.g., accelerometers mounted on an aluminum cube fixed on a concrete block and partially buried into the ground, cross-measuring bases set on a four bolts driven into the ground, and plate bases set on a three bolts driven into the ground). For selected types of measurement bases it is planned to conduct experimental or operational modal analysis in laboratory conditions.  
This work: carried out at Bialystok University of Technology, was supported by Polish financial resources on science under project no. WZ/WB-III/2/2020.  
The research was carried out with the use of equipment purchased from EU funds as part of the \"INNO-EKO-TECH\" project and software made available to the Bialystok University of Technology under a partnership agreement with ANSYS, Inc. and MESco Sp. z o.o. The author gratefully acknowledges [PERSON] for help in numerical simulations.  
The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.