Application of the surface proton magnetic resonance method for the detection of fractured granite aquifers

The Surface Proton Magnetic Resonance (PMR) method was developed in Russia in early eighties for investigation of sedimentary aquifers. However, the magnetic resonance signal is also produced by water in any other types of rocks. It makes possible application of the PMR for water detection in fractured aquifers where traditional geophysica! methods are less effective. Results of the water prospecting project carried out in Saudi Arabia are presented. Investigated aquifers there are essentially composed of metasediments ("black slate"), granite, grandiorite and diorite. During the hydrogeological survey achieved by ANTEA in April of 1996 two promising areas for water supply were emphasized. In order to detect fractured zones these areas were then studied by BRGM using the Horizontal Loop Electromagnetic (HLEM) method. As HLEM anomalies nothing teil us about water in the fractures, exploration boreholes were also drilled and tested where these anomalies were observed. Totally 17 PMR soundings were carried out in April of 1997 using the NUMIS PMR instrument. Eight of them were performed just close to existing boreholes which have various yields ranging from 0 up to 22 m3/h. The aim of these measurements was to calibrate the PMR response. The other soundings were performed along profiles in order to investigate the lateral extends of the waterful fractured zones. Generally a good correlation between drilling and PMR results was observed. Thus, it was demonstrated that the PMR is a useful tooi for optimal location of productive boreholes in a fractured granite hydrogeological environment. For example, referring to the project in Saudi Arabia, if the PMR measurements would have been carried out at an intermediate stage between the HLEM survey and the drillings, the number of boreholes could be reduced at least twice without any loss of information.