Structural Architecture of the Hydrothermal System from Geophysical Data in Hammam Bouhadjar Area (Northwest of Algeria) BOUALEM BOUYAHIAOUI, 1 ABDESLAM ABTOUT, 1 LAMINE HAMAI, 1 HASSINA BOUKERBOUT, 1 HAMOU DJELLIT, 1 SAID SOFIANE BOUGCHICHE, 1 MOHAMED BENDALI, 1 and HAMZA BOUABDALLAH 1,2 Abstract—We determine the structural architecture of the hydrothermal system of Hammam Bouhadjar area (Northwest of Algeria) by the use of geophysical data. New gravity and electrical surveys covered an area of about 48 km 2 in 2009. There were 350 gravity measurements made with a sampling of 500 m and 45 electrical soundings (Schlumberger type, AB = 1000 m). The Bouguer anomaly map shows a regression of gravity field towards the NW and SE. All of the observed anomalies are elongated in NE–SW direction. The results obtained from different processing methods (gradients, upward continuation, Euler deconvolution, wavelet transform and modelling) of gravity data were used to generate structural map of the studied area. The vertical and hor- izontal variations of resistivity confirm the presence of superficial and deeper faults system. Following the geophysical (gravity and electrical) analysis and modelling, we propose a model to explain the origin of the Hammam Bouhadjar thermal waters. We suggest that the hot spring water comes from an aquifer located in sand- stones lenses in the Senono-Oligocene Tellian unit. Following the gravity modelling the aquifer is identified at about 800 m, the same depth where the geothermal gradient is insufficient to heat the water. In these circumstances, the aquifer is probably heated by volcanic processes connected with a hot compartment by faults and contacts affecting structures identified in depth. The presence of a conductor along of the horseshoe area suggests that the water percolates into this area and then is drained by the different acci- dents to invade the whole area. Key words: Gravity anomaly, electrical resistivity, thermal water, deep structure, Hammam Bouhadjar, Northwest of Algeria. 1. Introduction The Maghrebides part (external zones) of the peri- Mediterranean Alpine Belt includes about 1200 km- long EW–trending magmatic lineaments that extend from Galite Island off the northern coast of Tunisia to Ras Tarf in Morocco (Abbassene et al. 2016). The southern Mediterranean Sea is bounded in northern Algeria by an Alpine orogenic belt resulting from the collision of drifting of continental fragments from Europe with the North African margin (Durand- Delga and Fontbote ´ 1980; Bouillin 1986; Frizon de Lamotte et al. 2000; Brace `ne and Frizon de Lamotte 2002). Along this belt several hot springs were produced. There are more than 200 hot springs in the northern part of Algeria. The most important one is located in the Northeast part, with the temperature ranging between 20 °C and 97 °C (Belhai et al. 2014). At the Northwest, the western Algerian hydrothermal system belongs to the occidental Tell of the external zones of the Alpine-Magrebides belt (Belhai et al. 2015). The most important geothermal areas of western Algeria are Hammam Bouhadjar, Hammam Bouhnifia and Hammam Boughrara. In this work we focus on Hammam Bouhadjar’s hydrother- mal system, located at Ain Temouchent (Fig. 1). The tectonic characteristics, in Ain Temouchent area, correspond to thrust ruptures associated with NE–SW trending fold-related faults showing ‘‘en-echelon’’ right-stepping distribution (Meghraoui 1988; Benouar et al. 1994; Meghraoui and Doumaz 1996a). The Hammam Bouhadjar’s hydrothermal system is located in the narrow depression delineating the M’leta basin. It is known as a collapsed geological space, formed after the establishment of the Tellian napes, between the Oran coastal mountains in the North and the Tessala massif in the South (Fig. 1). Its history is related to tectonic structures post-napes, 1 Centre de Recherche en Astronomie, Astrophysique et Ge ´ophysique (CRAAG), BP 63, 16340 Bouzare ´ah, Alger, Alge ´rie. E-mail: b.bouyahiaoui@craag.dz; b.bouyahiaoui@yahoo.com 2 GeoExplo, Cite ´ 602 logements, Ba ˆt. SCB 12, N°06, Les Dunes, Che ´raga, Alger, Alge ´rie. Pure Appl. Geophys. Ó 2017 Springer International Publishing DOI 10.1007/s00024-017-1479-0 Pure and Applied Geophysics