Prospecting Strategy for Water Borehole Implantation
in Discontinuous Media under Dry Climate: A Case Study from Moroccan Sahara

Mustapha Boualoul¹, Said Boutaleb², Rachid Ghandi³

¹ Equipe Géophysique & Géotechnique, Faculté des Sciences, Zitoune, Meknès, Morocco
e-mail: boualoul@yahoo.fr

² Centre Universitaire de Taza, Université Mohamed ben Abdallah, Taza, Morocco
³ DRHS, Laayoune, Morocco

Abstract

The objective of this study is to present a suitable methodology for groundwater prospecting in order to locate promising groundwater sites in fractured media under arid and semi-arid climate, to optimize water borehole implantation in such critical environment and thus reducing negative drills.
This approach consists of three stages: a satellite image analysis and a geophysical investigation, especially resistivity profiling and Proton Magnetic Resonance sounding. The proposed prospecting strategy was applied in Oued El Angua area. The region is located in south Moroccan Sahara, and characterized by low rainfall not exceeding 30 mm/y. The temperatures are generally above 32°C and beyond 40°C in summer.
From the geological point of view, the landmass is mainly made up of schists, shales and quartzites (Ordovician and Devonian), overlain mostly in flat zones by sand, clay and alluviums of Cretaceous or Quaternary age. The principal potentially aquifer consists in the discontinuous fractured medium represented by schists and quartzitic formations affected by an important fracturing.
The used remote sensed image for lineament detection is a Spot panchromatic type (60km x 60 km). The pixel size is 10 m x 10 m, the image centre UTM coordinates are X= 231 100, Y = 2964 000. The lineament analysis was performed after checking and removal of the questionable lineaments, the results were thus inserted into a Geographical Information System (GIS) using the ArcView software. The selection of the most interesting lineament intersections proposed from remote sensed image analysis is carried out after a geophysical investigation and verification.
The zones of connected lineaments, as good prospective aquifers, have been investigated by vertical electrical sounding, resistivity profiling, and by proton magnetic resonance sounding. The relaxation time of magnetic resonance signal is directly linked with groundwater content.
Among twelve (12) targets proposed from remote sensing data, ten (10) conductive anomalies have been confirmed by electrical prospecting. The PMR data has led to eliminate eight (8) targets from ten (10). In fact, the proton relaxation signal, performed with a NUMISplus equipment (Iris Instruments) and a square loop of 150 m side, reaching an investigation depth of about 150 m, was only successful in two (2) targets. The removed anomalies correspond probably to clay fillings or weathered materials. The PMR signal amplitude is directly related to water content.
The inversion for data measurements calculated with the standard software of NUMIS shows a water level situated between 35 and 75 m depth, which corresponds on a Vertical Electrical Sounding VES to a resistant substratum representing fractured quartzitic sandstones, the maximum of water content is situated between 44 and 60 m depth.
Although the low flow obtained (5 l/s) in a borehole test, the result can be considered as good for such environment.

Keywords: Hydrogeology, Fractured Aquifer, Remote Sensing Image, Sahara, Electrical Prospecting, Proton Magnetic Resonance