Through this Funding Opportunity Announcement (FOA), the Geothermal Technologies Office?s (GTO) Enhanced Geothermal Systems (EGS) Subprogram seeks to advance subsurface characterization via an integrated technical approach to EGS R&D ? GTO encourages the community to integrate technologies to address the challenges associated with quantifying reservoir complexity. GTO is interested in increasing the precision and accuracy of directly measured parameters and improving the ability to constrain calculated or inferred reservoir properties from these direct measurements. Critical parameters that GTO is interested in constraining include, but are not limited to: matrix to fracture heat transfer area; fluid mean residence time; in-situ stresses; fracture spacing; fracture aperture distribution; porosity; and reservoir volume. GTO seeks research projects that will facilitate a better understanding of physicochemical conditions to optimize subsurface en gineering and stimulation methods, as well as assess and better understand the evolution and sustainability of a reservoir during long-term operations.