Assessement of Integrated Geophysical Groundwater Prospecting Methods ON Basement Comiplex Aqutfersin NE Nuba Mountains - Sudan

Abstract

.r.;- I r .e | ~ Abstract A _,.~~ Integrated geophysical methods were applied to map the groundwater aquifers on complex geological settings, in the crystalline basement terrain in northeast Nuba Mountains, which are structurally controlled. The water flow is controlled by the northwest-southeast extensional faults as one of several in-situ deformational patterns that are attributed to the collisioniof the Pan-African oceanic assemblage dfthe Nubian shield against the pre-Pan African continental crust to the west. The used assessments are the electrical resistivity tomography (ERT), very low frequency-resistivity (VLF-R), and the audio magneto telluric (AMT) soundings, vertical electrical soundings (VES) in addition to water quality analysis and petrophysical measurements. These measurements were designed to be overlapped in order to prove the reproducibility of the geophysical data and to provide better interpretation of the hydrogeological setting in the aquifer complex structure. Different inversion schemes, accomplished by means of a cluster analysis, were attempted for the synthetic and observed ERT data to study their reliability to map the different geometries in the complex subsurface such as fault and graben structures. The sedimentary sequence is not symmetrical due the seasonal variations of the flow direction. Variations of the basement types were also recorded by their resistivity values and consequently their weathered products influence the water quality. The VLF-R data with Transverse Electric (TE) mode has been collected using the transmission stations GBR, GBZ and RHA of frequencies 16.0, l9.6 and 23.4 kHz respectively and it was two dimension (ZD) laterally-constrained inverted in a smoothly 2D resistivity distribution model after manual incorporation of a priori data . The inverted ERT and VLF-R lines confirmed the fracture zones and the deeper basins. Eight AMT soundings were acquired. True resistivity values and their relevant thickness were calculated and viewed as three dimension (3D) surface images. The VES data was conducted, where ERT survey was not accessible, and inverted smoothly and merged with the ERT in the 3D resistivity grid. Synthetic magnetotellurics MT data enabled us to test the inconsistencies between die interpreted geophysical results and the geologic expectations across AlBetira fold. Hydrochemical analysis (analyses) was applied to 42 water samples collected from the (hand) dug wells in the study area. Extremely high saline zones due to different reasons were recorded. Petrophysical measurements of an artificial saturated aquifer, using different soil types and various water salinities, provided a range for the porosity, formation factor and resistivity of the bulk formation and the later can be represented in terms of grain size distribution. A combination between the electric conductivity (EC) data and iso-resistivity horizons provides a better resolution of the target aquifer location, type and grain size distribution. The formation factor I

was estimated from the EC map and the corresponded bulk resistivity from the depth slices (W) and it varies from 1 to 6.7, except of Abu Gebiha which reaches 18. This result is confirmed by the previous petrophysical measurements and in addition to the water quality analysis, drainage pattems, and faults locations derived from VLF-R and ERT data. New target aquifers were suggested in the north eastem part of Al Terter area and along both Khor Baggara and Khor BanGadid in Al Betira area with respect to the water quality in both localities, and the southem pans of AbuGebiha town is considered as reasonable targets for groundwater supply. ii