Basic Research Needs for Geosciences - Energetics Meetings and ...

PANEL REPORT: SUBSURFACE CHARACTERIZATIONthe reaction rates, as well as the spatial and temporal pattern of reactivity. Tracers that respond toexternal fields (e.g., pH or pressure) by changing their response to probing fields, or by emittingan electric or acoustic signal, may open up new avenues for understanding processes. Thesetracers could be chemical or electro-mechanical, possibly in combination with biologicalcomponents. Subsurface relay stations in some form are needed for the required transmission ofinformation. Recent developments in micro/nanotechnology are promising and may play animportant role in distributed sensors networks in the subsurface.New tracers of groundwater age are needed that can be used to estimate the full spectrum of fluidresidence times within individual water samples representative of large subsurface controlvolumes tapped by production wells or of local points within the subsurface. Our currentinability to estimate water ages in the 50 to ~3000 year range is particularly limiting.Lastly, methods of measuring state variables such as pressure, temperature, saturation andconcentration continuously in space rather than mainly at points or over finite volumes areneeded. Linear measurement technologies such as fiber optics may provide the means ofaccomplishing this (e.g., Selker et al. 2006).Improved geophysical imaging techniquesGeophysical techniques which remotely probe the properties of the subsurface at their criticallength and time scales are still generally unavailable. Despite spectacular advances in 3D seismicimaging, current imaging techniques are still limited. The current resolution is too low to identifymany of the smaller-scale structures and processes believed to be relevant to the criticalproperties of the subsurface and their response to anthropogenic perturbations. The sensitivity ofcurrent monitoring methods is not sufficiently high to match the detection threshold desired foreffectively managing the subsurface. Seismic waves are primarily sensitive to seismic velocitiesand density. From measurements of these properties other quantities, such as porosity, cansometimes be inferred. In practice, many important properties of the subsurface cannot beestimated from seismic data, although combining different receiver configurations at the surfaceand down boreholes, or through vertical seismic profiling, can improve results. For someproperties, such as hydraulic conductivity, the combination of seismic data with electromagneticdata is promising. Many (bio)geochemical properties, such as the pH or the redox potential,cannot be probed remotely with current imaging techniques.Imaging may be more effective by introducing contrast agents into the subsurface in such a waythat it is more sensitive to probing fields. This approach is akin to the use of tracers in medicalimaging. An example is the CO 2 injection shown in Figure 15 that can be used to determine theflow pathways in a reservoir. More advanced applications may include, for example, the use oforganometallic tracers. The organic part of the tracers makes it possible to track (bio)chemicalproperties, while the metallic part of the tracers creates the opportunity to detect the presence andconcentration of the tracers with electromagnetic fields. Well-designed methods to manipulatethe subsurface may increase the effectiveness of geophysical imaging techniques to monitorstructures and processes in the subsurface.46 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems