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A framework for joint petrophysically and geologically guided geophysical inversion Astic, Thibaut


Geophysical inversion is a mathematical process from which an image of the underground structures is recovered from geophysical data. Geophysical methods span a wide range of different physical phenomena, from measuring variations of the Earth’s magnetic field due to local changes in magnetic characteristics to injecting current in the ground through electrodes to observe electrical conductivity variations. However, this process suffers from two main flaws. First, the problem is highly non-unique as there are usually more unknowns than information in the data. To recover an informative image of the subsurface from geophysical data, a priori information about the area is necessary. The widely used minimum- structure approaches usually produce models that are smooth and lack edge resolution while presenting physical properties that are far from their expected values. Second, not all geophysical methods are sensitive to the same features. When multiple surveys with different physics are collected over the same area, combining them in a coherent picture can be complicated by apparent inconsistencies between the inversion results from each method, due to the lack of integration. iii To tackle these issues, I first propose a new framework for incorporating petrophysical and geological information into voxel-based geophysical inversion. The inverse problem is developed from a probabilistic perspective and redefined as three interlocked data fitting problems over the geophysical, petrophysical, and geological data, respectively. By quantitatively linking these data into a single framework, I recover a final inverted model that reproduces the observed, or desired, petrophysical and geological features while fitting the geophysical data. I then expand the framework to multiple physical properties to perform multi-physics joint inversions, using the prior information as a coupling term. By combining the data into a single inversion, I am able to uncover structures that are otherwise indistinguishable using single-physics inversions. The framework is finally applied to the DO-27 kimberlite pipe case study, in the Tli Kwi Cho (TKC) cluster. I jointly invert airborne and ground-based gravity data, along with airborne magnetic data, to recover a quasi-geology model that can distinguish between the diamondiferous and sterile kimberlite facies.

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