Preliminary Investigation for Groundwater Exploration using Electromagnetic Method of Terrain Conductivity Variation in Papalanto, South-West Nigeria

Abstract

Papalanto District is a typical sedimentary formation within Dahomey basin where ground conductivity measurements were undertaking with Geonics EM 34-3 along 5 traverses with profile lengths varying between 160 and 200m. Comprehensive geophysical fieldworks were done adopting Frequency Domain Electromagnetic Method (FDEM) to ascertain both the vertical and lateral variations of subsurface conductivity probing depths of 20 m, 40 m and 60 m seeking different investigated depth. The EM data were acquired at 500 m intervals along 10 profiles. The Vertical Dipole Moment (VDM) in the first layer exhibited the highest and lowest true conductivity of 134.31 and 78.9 mmho/m for EMIPAP1 and EMPAP6 respectively and the corresponding Horizontal Dipole Moment (HDM) exhibited the highest and lowest true conductivity values of 1141.92 and 118.0 mmho/m for EMPAP5 (2^nd layer) with depth of 9m in HDM and 14m in VDM and EMPAP1 (1^st layer) respectively.  Lowest true conductivity values of 84.55 and 122.0 mmho/m were recorded in the second layer for VDM and HDM respectively.  The highest recorded true conductivity values in the first layer were 133.33 mmho/m (EMPAP1) and 167.48 mmho/m in EMPAP7 (depth 7.8m in HDM and 9.2m in VDM) respectively.  The highest depth of penetration respectively recorded for HDM and VDM in the first layer were 33m in EMPAP10 and 14m in EMPAP1 and EMPAP2 while the lowest depth of penetration respectively recorded for HDM and VDM in the first layer were 2m in EMPAP3 alongside EMPAP6 and 6.5m in EMPAP8 as the depths of penetration remained indeterminate in the second layer due to current termination. The appreciable variation in conductivity with recognizable positive peaks and broad bowl shaped anomalies observed in the high conductivities in both orientations is a resultant effects of weathering of the subsurface geological horizon and are possible indications of the vulnerability of its subsurface hydrogeological environment to invasion of contaminant seepages and consequent pollution of the investigated locations of the study area. The qualitative interpretation of EM results identified areas of hydrogeologic importance and forms a predictive and suggestive basis for Vertical Electrical Sounding (VES) investigation; points of positive EM anomalies were considered as priority area for prospective groundwater development necessitating more advanced groundwater exploration techniques.