MSc defence by Fred Ssemuyaba

14 May 2024

Fred Ssemuyaba from Uganda, MSc Fellow in Geophysics at University of Iceland will present his MSc project on Thursday 16th of May at 14:15-14:27 at University of Iceland, Askja building, room 131.  The presentation will be a part of the UI series "Open seminar at Masters-day of Natural Sciences".  

The title of the project is:
Analysis, 2D and 1D Joint Inversion of Magnetotelluric and Transient Electromagnetic Data from Buranga Geothermal Prospect, Uganda

Fred's supervisors are:
Gylfi Páll Hersir, geophysicist and independent researcher
Ásdís Benediktsdóttir, geophysicist, Reykjavik Energy
Halldór Geirsson, Associate professor, University of Iceland

His external examiner is Kristján Óttar Klausen, geophysicist and innovation specialist, Reykjavík Energy 

Resistivity surveying is considered one of the most important methods when it comes to geothermal exploration due to its direct relationship with reservoir temperature and other physical properties. However, the choice of an appropriate interpretation technique is not always straightforward. In this study, results obtained through different interpretation techniques of electromagnetic data from the Buranga geothermal prospect in Uganda are compared to identify the most suitable interpretation technique. It is then used to assess the presence and understand the resistivity structure that characterizes the nature of the resource at Buranga.  Resistivity data from closely located Transient Electromagnetic (TEM) sites (88 stations) and Magnetotelluric (MT) sites (165 stations) is used in a 1D joint inversion to correct for static shifts caused mainly by near-surface inhomogeneities. Results of the 1D inversion from the rotationally invariant determinant and average as well as the rotationally variant XY and YX apparent resistivity and phase presented as depth slices and cross-sections are compared. Static shift factors from the 1D joint inversion for both the Transverse Electric (TE) and Transverse Magnetic (TM) modes are used in the 2D inversion of the MT data. Convergence and robustness of the 2D models are explored by using 100 Ωm and 30 Ωm homogeneous half-space initial models which yielded similar results and are presented as cross-sections with an RMS between 1.0 - 1.9 for all the TE, TM, TE+TM and TE+TM+Tzy modes.  Results from the 1D inversion show similarities for the average and the determinant apparent resistivity while for the XY and YX apparent resistivity and phase, they differ immensely as the YX is more influenced by the conductive structures while the XY strikes a balance between near-surface conductive structures and deep resistive structures. 2D resistivity models compare well with the 1D results of the XY apparent resistivity and phase and thus the general subsurface resistivity interpretations are based on the XY inversion results and the 2D TE+TM mode inversion. Both 1D and 2D models highlight a surface layer which corresponds to fresh unaltered rocks. It is underlain by a very conductive zone that could represent sediments that have been overly infiltrated with geothermal fluids. Underneath these sediments is a layer of relatively high resistivity which can be thought of as basement rocks. Below 5 km depth a high conductivity structure emerges which has been left out of the interpretation here due to poor data quality and hence resolution constraints of the data set at hand. The preliminary resistivity conceptual model characterizes Buranga as a low to medium-temperature, deep-circulation amagmatic fault-controlled system extracting heat from the crustal heat flow. Up-flow is due to buoyancy-driven convection within the permeable damage zones and fault splays along multiple fault segments linked to the N to NE striking west dipping faults.