## 2 Measurement Bases/Setups  
In the paper it has been assumed that three different ways of coupling transducers to the ground belonging to the first group of methods described in DIN 45669-2:2005-06 [20] (i.e., methods based on transducers mounted on spikes) will be subjected to numerical simulations related to the identification of natural frequencies and the corresponding mode shapes of vibrations.  
The first of the ways of coupling accelerometers to the ground analyzed in the work (the measuring base No. 1--in accordance with DIN 45669-2:2005-06 [20] and DIN EN 10056-1 [36]) uses steel spikes hammered into the ground of about 500 mm long of an X-shaped cross section formed of two angles of about \(30\times 30\times 4\) mm. On one side the spike ends with a pointed end about 150 mm long. A flat plate of \(60\times 60\times 10\) mm is attached to the second, upper end of the spike. This solution allows mounting miniature triaxial accelerometer or one uniaxial accelerometer in the vertical direction to the upper surface of a flat bar. Due to the insufficient thickness, the flat bar does not have a sufficiently large contact surface for stable fastening of uniaxial accelerometers in the horizontal plane. For this reason, it was necessary to design a suitable mounting device (so-called adapter or mounting head), which, when attached to the flat bar, would allow measurements of vibration accelerations also in the horizontal plane using not only miniature triaxial accelerometers, but also much more precise, and at the same time heavier and larger seismic accelerometers. The mounting head for attaching accelerometers to the measuring base was designed as a lightweight aluminum cube with a 60 mm side with four through holes. The mounting head is screwed to the upper surface of the measuring base with Allen screws type DIN-912 M6.  
The measuring base No. 2 was adopted in the form of a steel spike with an L-shaped cross-section formed from an angle iron of about \(30\times 30\times 4\) mm--of identical length (500 mm) and pointed end (150 mm long) as in the case of measuring base No. 1 (also in accordance with guidelines DIN 45669-2:2005-06 [20] and DIN EN 10056-1 [36]). As before, the cubic mounting head is required to mount a set of uniaxial accelerometers in all three measuring directions.  
The measuring base No. 3 was designed as a steel spike with L-shaped cross-section hammered into the ground (made out of angle iron of about \(50\times 50\times 5\) mm section and 500 mm long) with welded flat bar (5 mm thick)--as described and drawings contained in [28] as well as in UNI 9916:2004 [37] and UNI 5783-66 [38] standards. Location of the bottom of the flat bar was assumed at a height of 445 mm from the pointed end of the base (it is also the depth of base anchoring). Threaded holes (1/4-28 UNI) for direct fixing of accelerometers have been made in the flat bar and on the side surfaces of the angle slightly above the flat bar (in this solution it is not necessary to use any additional devices for mounting the accelerometers). L-shaped bar of 40 mm length was sharp-pint ended (detailed dimensions of the measurement base were adopted based on descriptions and the proportions of the drawings contained in the work [28]).  
Geometric models of the measurement bases analyzed in the work are shown in Figure 1.  
Figure 1: The measuring bases: (**a**) Spike with X-shaped cross section (\(2\times 30\times 30\times 4\) mm) with attached mounting head, (**b**) spike with L-shaped cross section (\(30\times 30\times 4\) mm) with mounting head, (**c**) L-shaped section spike (\(50\times 50\times 5\) mm).