Recent advances in 3-D inversion of MT data for shallow crustal investigation: Application to Northern Turkana and Southern Omo sedimentary rift basins (East African Rift)

Abstract:

We developed a full tensor 3-D MT data inversion scheme that was tested on several real data sets collected in various regions of the East-African Rift (EAR) for the characterization of sedimentary basins. In the EAR, the basins are generally complicated geological structures because of active magmatism and control by inherited major tectonic directions. Since our 3-D inversion scheme do not require MT tensor decomposition, static distortion correction and heavy smoothing technique, it is possible to image upper crustal structures such as border faults or volcanic intrusion for example. Here, we present a new implementation of our inversion technique in order to increase the resolution of the models, particularly at shallow crustal scale. Our primary motivation was the observation that MT data generally contain more structural information than observed in usual regularized inverse models. For instance, high quality MT impedances exhibit variations within a short frequency band that may reflect resistivity variations within the sedimentary sequence itself. Furthermore in some regions of the EAR, the geoelectrical structure may present extremely low resistivity values in the uppermost sediments (1-2 Ohm-m in some regions). The challenge is then to be able to image very small resistivity changes reflecting lithological variations within the sediments together with large resistivity variations reflecting major contacts such as basalt/sediment or sediment/basement. The parametrization of the model and the strategy to finding a stable and accurate solution were developed for these critical situations. We discuss our new developments on results from a synthetic model of a typical rift basin, and with the new interpretation of MT data collected in two adjacent regions of the EAR well known for hydrocarbons exploration, Northern-Turkana and Southern Omo, in Kenya and Ethiopia respectively.