# The Contribution of Near-Surface Geophysics for the Site Characterization of Seismological Stations
## 3 Application of the Geophysical Surveys and Processing Results
### Seismological Station at Mandra, Attica (MDRA)  
Station _MDRA_, located at the monastery of Panagia Gorgoepikoou (Figure 3), in the vicinity of Mandra town, western Attica, was deployed on 22 September 2020. _MDRA_ station is currently equipped with a _GEObit GEOsix_, six-channel datalogger, and an R-sensors CME-6111, 60 s broadband seismometer. According to [68], the station (Figure 3) is based on dolostones, dolomitic limestones, and limestones of the Upper Triassic (_TRm_) of the Sub-Pelagonian Unit. Overlying them, limestones of the Upper Cretaceous (_Ks_) can be found, while in places, in the contact between the two carbonate formations, the development of lateritic bauxite ores can be observed.  
The HVSR for station _MDRA_ (Figure 4) exhibits a double peak at frequencies 2.3 and 2.7 Hz, with an amplification factor \(A\approx\) 1.7 \(\pm\) 0.3. However, a lower secondary frequency can be observed in some cases at \(f\approx\) 1.6 Hz, with \(A\approx\) 1.4 \(\pm\) 0.3. Additionally, one more peak, at almost 0.9 Hz, is present in some records, but it is not very prominent. At frequencies greater than 5.5 Hz, the base level of amplification is around unity.  
The results of the geophysical measurements carried out on the Triassic dolostones formation (_TRm_), at the _MDRA_ seismological station, are presented in Figure 5. The SRT technique revealed three seismic layers with velocities: \(V_{P1}\) = 1500 m/s, \(V_{P2}\) = 2000 m/s, and \(V_{P3}\) > 2700 m/s. The thicknesses of the first and second layers are relatively small, about 1.0 m each, while at depths greater than 2.0 m the seismic bedrock is located. Regarding the MASW technique results, the 1D model (Figure 5), due to the small thickness of the two first layers, revealed only one seismic layer with \(V_{S}\) = 1860 m/s, which is considered to be the velocity of the seismic bedrock. The average shear-wave velocity of the first thirty meters of the subsurface was determined to be 1877 m/s. In Figure 5, the resistivity model from the ERT technique is also presented. The resistivity value of the solid bedrock, which in this case is Triassic dolostone, is considered to be greater than 9000 Ohm.m and is present below 2.0 m depth at the middle to the end of the ERT section.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Seismological** & **Geological Formation** & **Number of Specimens** & **Dry Density (gr/cm\({}^{3}\))** & **Saturated Density (gr/cm\({}^{3}\))** & **Standard Deviation** \\ \hline \multirow{4}{*}{MDRA} & Dolostones, dolomitic & & & & \\  & limestones, and limestones (TRm) & 20 & 2.69 & 2.69 & 0.02 \\  & Dolostones, dolomitic & & & & \\  & limestones, and limestones (TRm) & 20 & 2.66 & 2.67 & 0.03 \\  & Talus and scree cones & - & 2.3 * [62, 63] & - & 0.10 \\  & Marine-brackish marks & - & 1.72 * [62, 63] & - & 0.07 \\  & (Pl-Pt.m) & 20 & 2.81 & 2.82 & 0.01 \\  & Tachytes and traclytic tuffs & - & 2.04 * [67] & - & - \\ \hline \end{tabular}
\end{table}
Table 2: Density values of the geological formations of the investigated areas. The density values marked with an asterisk were obtained from the indicated literature (brackets).  
Figure 4: Average HVSR curves (solid lines) determined for station _MDRA_ on different days, from 60 s long windows selected after the application of STA/LTA anti-triggering on 6 h long records. Circles denote the global frequency on each curve, while diamond symbols mark the average of the frequency band denoting the range of peak frequencies of the HVSR lines from the individually selected windows (light/dark gray band in Figure 2b). Squares mark the prominent global and secondary peak frequencies recognized from the HVSR spectra, with error bars indicating the uncertainty of the amplification factor (Table A1).  
Figure 3: Location of the seismological station _MDRA_ and the geophysical measurements along with the geology of the area.