While controlled-source electromagnetic geophysics has been often hailed as a potential breakthrough technology for subsurface hydrology, the current reality is that hydrogeologists often experience severe difficulties in interpreting electromagnetic data. In simple cases, where the electrical properties of the subsurface are reasonably approximated by a smooth or piecewise-constant distribution, classical geophysical methods of low-frequency electromagnetic induction are useful in constraining our knowledge of the hydrologic system. However, in cases where the setting is more complicated ? for instance, where the subsurface is characterized by length-scale-dependent heterogeneity ? the classical methods of electromagnetic induction must be modified accordingly. A promising analytic approach uses the historical methods of fractional calculus to accommodate scale-dependent lithologic complexity in the governing Maxwell Equations. Through a systematic study of the fractional Maxwell response (FMR) and corresponding classical electromagnetic simulations of appropriate complexity, we aim to illuminate geological settings where the FMR is present and to interpret the FMR in terms of hydrologic parameters such as porosity, clay content, pore water salinity, fracture density, position of a saline wedge or contaminant zone, etc. The technical innovation of our current proposal is based on extending traditional electromagnetic geophysics into new territory with the development of anomalous diffusion of electromagnetic fields into scale-dependent geological media. The broad sweep of electromagnetic geophysics, both traditional and innovative, must however be better communicated to the hydrological community. Accordingly we plan to utilize the newly developed virtual institute ?OpenEM.org? as a platform to better engage the hydrologic community in the application of electromagnetic geophysics.
Solving some of the most pressing worldwide societal concerns today, such as efficient water resource management and improved energy production from oil and gas reservoirs, requires much better capabilities to geophysically image subsurface fluid flow pathways in fractured and heterogeneous geological reservoirs. The electromagnetic geophysical technique is one of the few methods available for non-invasive remote sensing of subsurface fluids, owing to its great sensistivity to subsurface electrical conductivity. However, conventional deployments of electromagnetic geophysics have never achieved their full potential due to great oversimplifications that have been made in treating the true complexity of subsurface fluid reservoirs. For example, actual geological fluid reservoirs exhibit heterogeneities, structure, and fracture networks that can span a vast range of length scales. Such complexity can be represented by length-scale dependent heterogeneity similar to a fractal structure. However, conventional electromagnetic geophysical modeling and interpretation methods cannot handle such length-scale-dependent geology. A new formulation of the governing Maxwell equations for electromagnetic diffusion is required. In this project, we pushed the frontiers of electromagnetic imaging of subsurface fluid reservoirs by further developing our previously-derived fractional-diffusion extension of Maxwell's equations that properly treats subsurface geological structure as a length-scale dependent medium. We developed 2-D and 3-D simulations of fractional electromagnetic diffusion into subsurface fluid reservoir models, and thereby generated responses that could not be explained by the conventional approaches. We also acquired electromagnetic data in the field, over a fractured sandstone/granite fault zone that serves as an important aquifer in central Texas, and explained how the data can be interpreted in terms of fractional electromagnetic diffusion. The data showed that the upper levels of the aquifer allow subdiffusion of the penetrating electromagnetic signals, while superdiffusion prevails in the deeper levels. Our preferred hydrogeological interpretation is that the near-surface fractures are closed by conductive weathering products such as clay, while the deeper fractures are likely air-filled, although they could also be manifested as quartz veins.
