# Characterization of Shallow Sedimentary Layers in the Oran Region Using Ambient Vibration Data
## 5 Conclusions  
In this study, the shallow subsurface structure of the Oran region has been investigated. The aim has been to characterize the first 30 m of the soil column and provide soil classifications for the study area. For this purpose, a measurement campaign of ambient noise was carried out at 15 sites in the study area using array configurations. Three different methods have been applied (HVSR, f-k, and SPAC techniques) in order to calculate the HVSR curves and Rayleigh wave dispersion curves. The soil parameters that have been provided in this work are the fundamental frequencies and the corresponding amplitudes, the shear-wave velocity model, and the \(V_{s30}\) values for each site. Additionally, the directionality of the ambient vibration sources has been investigated, and a \(V_{s30}\) predictive equation for the Oran region has also been provided. All these parameters aim to contribute to a better knowledge of the subsoil structure of the study area. The main frequency peaks shift between 0.3 and 1.6 Hz, and the corresponding amplitudes between 2 and 4. The high frequency values are found to the west of the study area at the foot of the Murdjado mountain, where the sedimentary layers are thin, and the low frequency values are located to the east and in the center of the Oran region. Ambient vibration array recordings have been used to characterize the shear-wave velocity in the first 30 m of the soil (\(V_{s30}\)) in the Oran region. The SPAC and f-k techniques have allowed for retrieving Rayleigh wave dispersion curves at each of the 15 sites. Both methods produced approximately similar results. The inversion of each curve, along with the introduction of the required input parameters (Vp, Vs, and density ranges, as well as the expected number of layers), resulted in a final Vs model. In a major part of the study area, the first 30 m of the soil are composed of three sedimentary layers. The topmost layer is composed of Quaternary alluviums, where the Vs value varies between 200 and 380 m/s. The alluvium layer reaches a maximum thickness of 20 m, for example, in borehole F03 in the SW of the study area, and P08 in the north. The second stratum is composed of Pliocene sands and sandstones. The Vs value for this layer changes between 400 and 800 m/s. It can reach a thickness of 24 m in the eastern part of the  
Figure 11: Spatial distribution of the array sites (blue circles) and directionality of the registered ambient noise (black arrows). The direction where the HVSR curve reaches the maximum amplitude for the resonant frequency is also indicated for each site. Oran region. The third layer is made up of Miocene limestones and marls. The Vs value for these formations varies between 850 and 1380 m/s. The Vs values are in accordance with the ones obtained previously using the HVSR technique. Furthermore, the soils have been classified using the NEHRP criteria for site classification based on the \(V_{s30}\) variation map. Measurements of \(V_{s30}\) in the urban environment of the Oran region vary between 400 and 670 m/s. The soil is categorized as very dense soil and soft rock (C). A \(V_{s30}\) predictive equation has been proposed to estimate the \(V_{s30}\) values directly without performing the inversion process, just using the Rayleigh wave phase velocity corresponding to a wavelength of 47 m.  
The results obtained in this study provide important information for evaluating the seismic risk in the Oran region, and they could be used in order to update the Algerian seismic code. [PERSON] acquired and processed the data and wrote the first draft of the paper. [PERSON] proposed the methodology and reviewed the paper. [PERSON] reviewed and supervised the paper. [PERSON] provided the acquisition materials and supervised the paper. [PERSON] helped with array processing and reviewed the paper. All authors have read and agreed to the published version of the manuscript.