Application of integrated magnetic resonance sounding and resistivity methods for borehole implementation. A case study in Cambodia

Abstract

A geophysical survey was conducted in Cambodia to measure the contribution of geophysics to a running drilling programme. The geology of the area (the province of Siem Reap) mainly consists of recent heterogeneous sediments of sand, silt and clay. The thickness of this formation ranges from 20 to 100 m and lies on Jurassic to Tertiary rocks. Surveys were done with direct current methods [one-dimensional vertical electrical sounding (1D VES), and two-dimensional (2D) electrical imaging], time domain electromagnetic (TDEM) sounding and proton magnetic resonance sounding (MRS). To validate the geophysical results, boreholes were drilled and tested with electrical logs and pumping tests. We found that: (1) The resistivity methods (VES, 2D electrical imaging and TDEM) are very sensitive to the groundwater electrical conductivity which is highly heterogeneous within the province. A preliminary relationship between measured groundwater conductivity and aquifer resistivity is proposed. (2) The MRS gives accurate information on groundwater occurrence for the 5- to 60-m-deep layers. A preliminary relationship between MRS data (aquifer transmissivity estimated from MRS field measurements) and hydrodynamic parameters (aquifer local transmissivity and borehole relative specific capacity estimated from borehole pumping tests) is proposed. (3) The resistivity methods and MRS are complementary, and a joint use is recommended. (4) At the survey scale, the borehole success rate was improved from 56% to 90% by the use of geophysics. Crossing the technical and cost analyses, we propose a geophysical methodology to implement boreholes in the province of Siem Reap. This methodology could both increase the borehole success rate and save money at the programme scale. It consists of the use of: 1. MRS, TDEM and electrical methods jointly where the borehole success rate is less than 30%. 2. MRS and TDEM jointly where the borehole success rate ranges from 30% to 50%. 3. Electrical methods (VES and 2D electrical imaging) as standard methods.