# The Contribution of Near-Surface Geophysics for the Site Characterization of Seismological Stations
## 2 Methodology
### SRT Technique  
The seismic refraction data processing was carried out using the _SeisImager/2D_ software package provided by _Geometrics Inc._ (San Jose, CA, USA). Using the _Pickavin_ module of the software, six group file lists were created, containing every seismic record (11 in total) that was acquired at each measurement site (Figure A1). For each file list, the shot locations, the first receiver location, and the spacing between the receivers were defined. After the assignment of the geometric characteristics of the arrays, the first break arrivals of the P-waves were picked for every seismic record. Afterwards, the picked arrival times of the P-waves were plotted as a function of the distance between the receivers, to construct the traveltime-distance curves for each measurement site. These curves were edited using the _Plotfera_ module of the software, where some arrival times were corrected and the traveltime reciprocity between opposite shots was verified. Two methods, provided by _Plotfera_, were used to analyze the seismic refraction data. First, the time-term inversion method was implemented, where the first break arrival times are inverted through linear least-squares and delay time analysis for the generation of a seismic velocity section [55]. To start the inversion process, each arrival time (direct or refracted) of the traveltime-distance curves was assigned to a specific layer, and the topography of the seismic line was also defined. After the inversion procedure, a 2D layered velocity model was created with distinct boundaries between the layers and a constant velocity value at each layer. The obtained velocity model was used as the starting initial model for the application of the SRT method. Therefore, the velocity model was converted into a grid model with a constant velocity value assigned at each cell. The software employs the wavefront propagation method for forward modelling, where the synthetic traveltime curves, corresponding to the initial model, are generated. Subsequently, the synthetic traveltime curves are compared with the observed traveltime curves, and through the application of the nonlinear least-squares approach [40; 55], the velocity model is updated in order to reduce the misfit error between the two traveltime curve sets. This procedure is repeated until the minimum possible RMS error is achieved within a predefined number of iterations. The outcome of the application of the SRT technique was a 2D P-wave velocity distribution to the subsurface in the direction of the seismic line. The RMS error for all the SRT sections was below 1.0 ms, providing high reliability of the results. MASW Processing  
The MASW technique is based on the propagation characteristics of surface waves, and especially the Rayleigh, which travel with different propagation velocities (phase velocities) at different frequencies throughout a heterogeneous medium. As a result, the low-frequency components of the surface waves penetrate deeper into the subsurface and are mainly affected by the elastic properties of the deeper layers [56]. On the other hand, the high-frequency components are constrained to the shallow subsurface zone and contain information about the shallow layers. This dispersive property is utilized to construct the dispersion curve, which is the phase velocity vs. frequency plot. By applying a mathematical inversion to the dispersion curve, the distribution of the S-wave velocity through the subsurface can be calculated. MASW data processing was carried out using the _SeisImager/SW_ software package provided by Geometrics Inc. The acquired seismic records were first processed using the _Pickwin_ module, to improve their quality and assign the geometry of the seismic experiment. For that reason, frequency filters were applied in order to reduce the influence of the recorded body waves and the unwanted noise, while the geophone spacing and the _near-offset_ distance were defined for each seismic record.