Geophysical prospecting in the Doukkala area (Swalah commune) in Morocco

The collapse of the subsoil creates a risk for the population whether it is urban or rural. Each year, the damage caused by these collapses has considerable socio-economic consequences, and the damage costs are very high. Thus, the detection of these areas of collapse in urban and rural areas is important to prevent and avoid socio-economic consequences, and to establish a preventive risk planning to have a better protection of people and goods. The commune of Swalah, study area, belongs to the province of El Jadida which is part of those areas of Doukkala exposed to the risk of collapse due to the presence of underground cavities. These cavities are potentially dangerous for humans, especially in urban areas. They have different extensions that can be caused by natural or anthropic origin. Their size, as well as the physical properties of the external environment in which they are located, allow the use of different geophysical methods. The use of these geophysical methods is the best to detect and delineate cavities in this region. The present study was based on a geophysical compaign of vertical electrical soundings. Indeed, 50 electrical soundings were modeled and reinterpreted and allowed to detect and delineate any potential cavities in the region.


Introduction
In many areas, the existence of ancient natural or artificial cavities constitutes a permanent major risk for the population and is, in most cases, the cause of collapse of buildings [1,4]. These underground cavities have been abandoned for centuries without any security measures.
The Swalah region is one of the regions of Doukkala exposed to the risk of collapse of underground cavities. The electric resistivity measurements by the electric geophysical method make it possible to determine a strong contrast of the resistivity between the geological host and the cavity itself [2]. Under these conditions, the use of the vertical electrical sounding method based on resistivity becomes necessary. In this context, a compaign of 50 electrical soundings was conducted in the study area. Its main objective is to study the stability of the ground and to establish maps defining the zones at risk, by identifying the extension of dissolved gypsiferous formations in the first 200 meters. This geophysical method has already been used successfully for subsoil exploration in other contexts [3].These studies showed that the presence of a cavity could be detected by the contrast of resistivity. The studied area, Sidi Smail province of El Jadida is part of Abda-Doukkala located in the East (Fig.1).From a geological point of view, the study area is related to the Moroccan Meseta and more particularly to the central zone. The stratigraphic series is represented by a Paleozoic basement on which rests, a sedimentary series ranging from Permo-Trias to silty quaternary [5]. The regional geological section (Fig. 2

Geoelectric prospecting
The vertical electric sounding method used in this study is based on the injection of an electric current between two current injection electrodes A and and the measurement of a potential difference V between two other electrodes of potential M and N [9].
We deduce the apparent resistivity of the subsoil: K : The geometric coefficient of the device only function of the distances between electrodes.
age. These sediments are followed by a serie of gray and green clays. Borehole 4 located in the center of the study area, crosses a Jurassic Berriasian formation composed of gypsum limestones, with a power of up to 200 m where gypsum occupies an important place [6]. Dridrat limestones are probably not deposited in the studied or eroded zone [7]. In this carbonate medium (limestone) and sulphate medium (gypsum), chemical alteration is very frequent and causes the appearance of cavernous zones in the study area [8].
The studied area ( Fig.4) was prospected during a geophysical survey of 50 electric survey spaced 10 m, of which 33 SEV were made with AB = 2000m and 17 SEV for AB = 1500m. The Schlumberger configuration used in this work requires the alignment and the symmetry of the electrodes referring the centre O with MN / 2 and AB / 2 [10]. The raw data are first represented as curves ρa = f (AB / 2) in a bilogarithmic coordinate system. These curves were interpreted automatically by mathematical inversion using the IPI2WIN software [11]. The solution obtained is not unique, but the calibration of this interpretation with nearby drilling helps to remove some of these uncertainties. By collecting the results of this interpretation of each of the soundings, we made

Geoelectric Sounding interpretation
The inverse modeling of the 50 soundings leads to remarkable curves, according to the shape of these curves, one distinguishes 3 families of electrical soundings (1, 2, 3) (

Geoelectric Cross section
Several geo-electric sections based on the inversion of the electrical soundings were realized, Only the most representative sections will be presented: m1m6, c1c6, a1m1 ( fig.6,7,8). In the region of Abda-Doukkala the tertiary tectonics gave its structure to this region, the Paleozoic formations under the secondary cover, are subjected to Hercynian deformations between the Oulad Ziane fault and the Drabla-Sidi Smaïl fault, and the NS SW-NE to EW folds are often exposed to these Hercynian tectonic elements [6].

Map resistivity
Four maps of apparent resistivity iso-contour were created using the Krigeage spatial interpolation method, which predicts the apparent resistivity value studied at a non-sampled site by a linear combination of adjacent point data [12]. Figure 9 shows the resistivity maps, one for each AB =

Conclusion
The results of the inversion of the 50 SEVs made in the city of Sidi Smail enabled us to establish geo-electric sections characterized by a highly conductive level (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) Ohm.m), identifying with a sequence of gypsiferous formations of Upper Jurassic spread over lengths of 1696m. This level contains cavities up to 110 m thick at depths between 40 and 110 m. The solubility of the gypsums favored the dissolution and thus assisted the appearance of these empty spaces filled with air. The results of this study show that this method is well adapted to the recognition of the subsoil, they also show the interest of the SEV method in the detection of underground cavities. Of course, the lack of resolution does not allow this method to distinguish the precise dimensions of the cavities. These very interesting results should be the basis of any future prospection in the city of Sidi Smail.