The Miocene Roof Mapping Using Microtremor Recording and Electrical Survey Method in Blida City, Algeria ASSIA BOUCHELOUH, 1,2 RABAH BENSALEM, 1 NAIMA ZAOURAR, 2 DJAMEL MACHANE, 1 HAKIM MOULOUEL, 1 and EL HADI OUBAICHE 1 Abstract—Bedrock depths in the Mitidja basin in general and in the Blida region in particular are still poorly known despite, the existence of some relatively deep hydraulic boreholes that intersect only superficial alluvial formations. To assess the seismic risk of Blida town, knowledge of soil amplification requires the thickness and properties of sedimentary formations that cover the substratum. For the purposes of our study, the thicknesses obtained by the vertical electric soundings, carried out in the hydrogeological study of the basin, were combined with horizontal-to-vertical spectral ratio (HVSR) microtremor recordings. This combination made it possible to determine an empirical relationship between frequency and thickness specific to the Blida site area, which enabled the roof of the Miocene to be mapped and shows slight undulations with directions compatible with the tectonic constraints of the region. The boundaries between the low and high frequencies obtained by HVSR are well materialized, at south by Sidi El Kebir river, at west by Chiffa river and in the central part by a line of direction SE–NW corresponding to the old passage of Sidi El Kebir river. The presence of low frequencies attributed to the old alluvial deposits with significant thicknesses that originate just after Sidi El Kebir river confirms that the South Mitidjian contact is subvertical. Key words: Blida, sediment thickness, microtremors, HVSR, VES, amplification. 1. Introduction The main geological hazards threatening northern Algeria are related mainly to the seismic activity generated by the displacement of active faults (e.g., Meghraoui 1988; Yelles-Chaouche et al. 2006; Moulouel et al. 2017) and to a lesser extent landslides (e.g., Cheikh Lounis et al. 2017) and flooding (e.g., Machane et al. 2008). In this region, the seismogenic faults are most often located along the edge of the Miocene, Pliocene, and Quaternary sedimentary basins with a particular geology consisting of more or less loose formations which can amplify the seismic movement. It is very important to study in detail the local geological conditions and the geotechnical properties of the underground to design and dimen- sion earthquake resistant buildings. The local soil properties are essential in assessing site effects due to the local geology (Bard and Bouchon 1985; Singh et al. 1988; Gaffet and Bouchon 1989; Bard 1995; Bourdeau and Fleurisson 2003; Hellel et al. 2010). The site effects due to geological conditions are well quantified by the horizontal-to-vertical spectral-ratio (HVSR) method (e.g., Nogoshi and Igarashi 1970, 1971; Nakamura 1989; Bard et al. 1999; Gue ´guen et al. 2000; Bensalem et al. 2010, 2017) by determining the resonance frequency of the site. Several studies have been carried out on the com- position of ambient noise and have shown that the wave field consists of body and surface waves (Nakamura 1989; Bonnefoy-Claudet 2004; SESAME 2004). However, there are two theses concerning the type of wave that contributes to the peak of HVSR. Theoretical studies (Okada et al. 1990; Lachet and Bard 1994; Konno and Ohmachi 1998; Bard et al. 1999; Bonnefoy-Claudet 2004) suggest that HVSR peak is explained by the fundamental mode of Ray- leigh waves. While (Nakamura 1989, 2000, 2009); Bonnefoy-Claudet et al. 2008; Oubaiche et al. 2012, 2016) linked the HVSR peak to the vertical incidence SH transfer function. Bonnefoy-Claudet et al. (2008) showed that 68% of the cases on 136 1 CGS, Centre National de Recherche Applique ´e en Ge ´nie Parasismique, Rue Kaddour Rahim, Hussein Dey, 16040 Algiers, Algeria. E-mail: assiabouchelouh@yahoo.fr 2 De ´partement de Ge ´ophysique, USTHB, Universite ´ des Sci- ences et de la Technologie Houari Boumedienne, LGEOPH USTHB, BP32 El Alia, Bab Ezzouar, Algiers, Algeria. Pure Appl. Geophys. Ó 2017 Springer International Publishing AG DOI 10.1007/s00024-017-1684-x Pure and Applied Geophysics