A 3D GEOLOGICAL MODEL OF THE NASIA SUB-BASIN, NORTHERN GHANA-INTERPRETATIONS FROM THE INVERSION RESULTS OF REPROCESSED GEOTEM DATA

Elikplim Dzikunoo;Flemming Jørgensen;Giulio Vignoli;Bruce Banoeng-Yakubu;Sandow Mark Yidana

2018-01-01

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Abstract

Inversion of regional-scale airborne electromagnetic (AEM) data was used in building a 3D geologic model of the Nasia Sub-Basin, Northern Ghana. Geological interpretation of the AEM data was guided by a priori knowledge obtained from previous research in the area, geological maps and reports. A key requirement of the modelling process was to define the subsurface stratigraphy of the area, chiefly to provide 1) a detailed stratigraphical context for hydrogeologic use, 2) a repository of comprehensive stratigraphic knowledge of the study area which will be easily accessible. The AEM measurements, consisting of GEOTEM data, was originally collected for mineral exploration purposes. The B-field data have been (re)processed to enhance the resolution and inverted by using spatial constraints to preserve the lateral and vertical coherence. During the processing and inversion phases, the regularization strategies and associated parameters have been tuned by following an iterative approach characterized by a tight collaboration between geologists and geophysicists to retrieve the geophysical model that is in the best agreement, not only with the geophysical data, but also with the geological expectations. The new (pseudo-)3D inversion showed very different features with respect to the previous Conductivity-Depth Images (CDI). The new geophysical model led to new interpretations of the geological settings and to the construction of a comprehensive 3D geomodel of the basin based on the integration of AEM and borehole information. Nevertheless, in order to have a model, suitable for hydrogeological characterization, it will be necessary to include more details regarding the upper, weathered zone as the AEM survey was optimized to have a very deep penetration and not an extremely high shallow resolution. These aspects will be remedied by the inclusion, for example, of the interpretations from Electrical Resistivity Tomography (ERT) data, which will provide the necessary resolution in the upper sections.