# The Contribution of Near-Surface Geophysics for the Site Characterization of Seismological Stations
## 2 Methodology
### Seismic Method  
The HVSR technique [24] is a widely used method to assess the effects of local ground conditions on seismic wave amplification. It is based on the ratio between horizontal (\(H\)) to vertical (\(V\)) components of ground motion. By calculating the HVSR, the fundamental frequency and the corresponding amplification factor of the ground motion can be estimated. To apply the HVSR technique, ground ambient noise recordings, or microtremors, need to be collected. This method enables the rapid and efficient evaluation of the impact of local ground conditions on seismic wave amplification, making it an important tool in earthquake engineering. The HVSR technique has been previously applied successfully in many studies worldwide [25; 26; 27; 28; 29; 30; 31] as well as in Greece [32; 33; 34; 35; 36].  
The HVSR technique is based on the fundamental assumption that on hard ground (rock) both vertical and horizontal components of ground motion exhibit similar amplification during seismic excitation, whereas on soft soil the horizontal components are strongly influenced by the response of the surface geological formations. Thus, the H/V ratio of the microtremors can be used to determine the fundamental frequency (\(f_{0}\)) and the corresponding soil amplification factor (\(A_{0}\)) of the surface layers at each location. For this technique, it is necessary to calculate the H/V spectral ratio from the continuous recording of the background ground noise.  
The Shallow Seismic Refraction (SSR) technique is commonly used in similar investigations [37; 38; 39]. It is considered to be one of the primary geophysical methods for the investigation of the subsurface lithological structure. Through the propagation of the elastic waves, their velocities in soil deposits and rocks can be determined and the corresponding elastic moduli can also be calculated [15; 16; 17; 18]. Depending on the applied technique used for the data processing, the subsurface structure can be represented either as a discrete layered model, with constant seismic velocities, or as a gridded model with gradient velocity variations, capable of adumbrating lateral heterogeneities. The discrete layered model can be obtained by the application of conventional techniques (e.g., time-term inversion, GRM), while the gridded model can be obtained by a tomographic inversion approach, widely known as SRT. The SRT technique has the benefit of producing more accurate and high-resolution velocity models, whereas the extensive amount of fieldwork and the increased processing time needed are some of the downsides [38]. Through this technique, the recorded seismic refraction data can be inverted, and the distribution of the seismic wave velocities can be obtained across two-dimensional (2D) sections, which are widely affected by the physical properties of the subsurface [40; 41; 42; 43; 44].  
The MASW technique is a non-invasive seismic method which has been widely used for soil characterization purposes [45; 46; 47]. It is implemented to obtain shear-wave velocities in shallow geophysical investigations, derived by analyzing the surface wave (Rayleigh)components on the seismic recordings. The MASW technique has the advantages of lower fieldwork costs and easier recording and processing. It is known for its effectiveness and reliability in providing shear-wave velocity profiles within the first 30 m of the subsurface [45; 46; 48; 49; 50; 51]. It is acknowledged that P- and S-wave velocities provide useful context regarding the liquefaction potential, ground motion, and natural frequency function in case of an earthquake. Hence, considerable knowledge of soil stiffness is a key parameter for more accurate prediction of ground response and amplification estimation.