Impacts of magnetic permeability on electromagnetic data collected in settings with steel-cased wells

Electromagnetic methods are increasingly being applied in settings with steel infrastructure. These include applications such as monitoring of CO2 sequestration or even assessing the integrity of a wellbore. In this paper, we examine the impacts of the magnetic permeability of a steel-cased well on electromagnetic responses in grounded source experiments. We consider a vertical wellbore and simulate time and frequency domain data on 3D cylindrical meshes. Permeability slows the decay of surface electric fields in the time domain and contributes to a phase shift in the frequency domain. We develop our understanding of how permeability alters currents within, and external to, the casing by focussing first on the time domain response and translating insights to the frequency domain. Following others, we rewrite Maxwell's equations to separate the response into terms that describe the magnetization and induction effects. Magnetic permeability impacts the responses in two ways: (1) it enhances the inductive component of the response in the casing, and (2) it creates a magnetization current on the outer wall of the casing. The interaction of these two effects results in a poloidal current system within the casing. It also generates anomalous currents external to the casing that can alter the geometry and magnitude of currents in the surrounding geologic formation. This has the potential to be advantageous for enhancing responses in monitoring applications

Incorporating geological dip information into geophysical inversions

ABSTRACT Geological bodies are often linear structures that have well-defined strike direction and dip angle. We develop a new model objective function that allows this important information to be incorporated into geophysical inversions. A rotation matrix is applied to the horizontal and vertical derivatives of the model so that the derivative in an arbitrary direction is obtained. A model objective function that measures the flatness with respect to the rotated derivatives favors models that have elongated features with the specified strike and dip angle. Formulations for both 2-D and 3-D cases are presented, and they are illustrated using examples from dc resistivity and induced polarization (IP) problems. Synthetic and field examples show that an inversion carried out using known dip information produces a model that has higher resolution and provides a better representation of the true structure

Sensor Placement via Optimal Experiment Design in EMI Sensing of Metallic Objects

This work, under the optimal experimental design framework, investigates the sensor placement problem that aims to guide electromagnetic induction (EMI) sensing of multiple objects. We use the linearized model covariance matrix as a measure of estimation error to present a sequential experimental design (SED) technique. The technique recursively minimizes data misfit to update model parameters and maximizes an information gain function for a future survey relative to previous surveys. The fundamental process of the SED seeks to increase weighted sensitivities to targets when placing sensors. The synthetic and field experiments demonstrate that SED can be used to guide the sensing process for an effective interrogation. It also can serve as a theoretic basis to improve empirical survey operation. We further study the sensitivity of the SED to the number of objects within the sensing range. The tests suggest that an appropriately overrepresented model about expected anomalies might be a feasible choice