Experimental modal analysis of ground driven measurement bases for surface vibration monitoring

Accurate assessment of ground–borne vibrations is essential for evaluating their effects on buildings, infrastructure, and vibration–sensitive equipment. Despite this importance, the influence of measurement base dynamics on recorded vibration levels is rarely quantified in engineering practice. This study investigates the intrinsic modal properties of four ground–driven measurement bases widely used in surface vibration diagnostics: two L–shaped steel spikes, an X–shaped spike, and a ring–type base with three anchors. A fully non–contact experimental methodology was applied using a PSV–400-M4 scanning laser vibrometer and a broadband piezoelectric actuator, enabling high–resolution identification of natural frequencies without the mass–loading effects typical of accelerometer–based modal testing. The results demonstrate large differences in stiffness and usable frequency ranges among the tested bases. The X–shaped base provides the widest resonance–free range (above 316 Hz), while the ring–type base offers the narrowest (approximately 92 Hz), limiting its applicability for in situ measurements. The findings reveal a significant influence of sensor mass on natural frequencies (reductions of 24–58%) and substantial variations in usable frequency range depending on base design. Such reductions may lead to erroneous amplification or attenuation of recorded vibrations, particularly within frequency bands typical of road and rail traffic. The findings provide validated reference data for improving numerical models of measurement bases and offer a methodological basis for selecting suitable base geometries and sensor masses. The work supports the development of more reliable procedures for surface vibration diagnostics, especially in applications requiring high–fidelity measurements for sensitive or precision equipment.