Focused Geophysical Imaging of the Chiweta Geothermal Field (Malawi)

Geophysical surveys may image buried tectonic structures and variations of lithology, hydrothermal alteration and porosity/fluid content of geothermal fields and inverse models can be directly used for the assessment of the conceptual models. However, conventional interpretation is commonly based on minimum-structure inverse modelling that produces inherently smooth and blurred images of the investigated geological structures (e.g Zhdanov, 2002). Therefore, while smooth modelling helps inversion convergence and prevent artefacts in the solutions, some important features with sharp transitions can be missed or smoothed out. Typical examples are the lithological variations between the sedimentary infill and the bedrock in a basin or the bedrock steps due to faulting in graben/horst structures. Focused geophysical imaging (Portniaguine and Zhdanov, 1999) with different regularization methods can be more effective to detect sharp boundaries because they did not excessively penalize sharp physical property variations. The total modified variation (MTV, Acar and Vogel, 1994) and the minimum gradient support (MGS, Portniaguine and Zhdanov, 1999) stabilizers were applied to the inversion of gravimetric and magnetotelluric data collected at the Chiweta geothermal prospect in Malawi. The gravimetric data was used to map the interface between the Karoo formation and the underlying Precambrian gneiss basement complex, assuming a density contrast of 200 kg/m3. The resulting horst-graben structure imaged by the 2D and 3D MTV and MGS inversion allowed to identify major faults affecting the basement and possibly driving the up-flow of hydrothermal fluids. The 1D magnetotelluric MTV and MGS inversions with lateral constrain provided a focused pseudo-2D image of the resistivity distribution. The models showed the lithological contact between the Karoo formation and the basement complex in the central portion of the survey area. Geothermal alteration in the Karoo formation has been revealed by different conductive anomalies. Some of them are associated with low temperature clay alteration in the Chiweta hot springs area, while others may represent fossil geothermal zones. Good correspondence between higher density and resistivity values has been observed where geothermal alteration is plausibly weaker or absent. It was found that focused inversion is strongly dependent upon the initial model and the chosen inversion parameters, but if a proper choice is done, it can be an effective tool to get detailed geological images. It was concluded that it can be considered as a refinement of the classical maximum smoothness approach, to be used when abrupt physical property changes are expected. REFERENCES Acar, R., and Vogel, C. R. “Analysis of total variation penalty methods” Inverse Problems, 10, 1217-1 229 (1994). Portniaguine, O., and Zhdanov, M. S. “Focusing geophysical inversion images”. Geophysics, 64, 3, 874-887 (1999). Zhdanov, M. S. “Geophysical inverse theory and regularization problems”, Methods in Geochemistry and Geophysics, 36, Elsevier (2002).