Relating Geophysical and Hydrologic Properties Using Field-scale Rock Physics

Understanding how geophysical and hydrologic properties are related is a key problem in hydrogeophysical data fusion. Traditionally this link has been made by rock physics investigations that define how pore-scale variations in properties like mineralogy, fluid content, and grain geometry affect the geophysical response of a rock or sediment. As the scale of investigation increases from the lab to the field, an individual geophysical measurement averages over different types of rocks and sediments in addition to local porescale variations. As a result, spatial heterogeneity and preferential sampling at the field-scale can cause a shift in the rock physics relationship away from that determined in the lab. In a typical field survey, multiple large-scale measurements are combined through inversion to estimate the geophysical properties of the subsurface. In this case, additional factors, e.g., the spatially varying resolution of geophysical surveys, may also affect the rock physics relationship. As a result, there is a need for methods that integrate variations in pore-scale rock properties with an understanding of geophysical sampling at the field-scale. In this paper we consider the role of rock physics in general hydrogeophysical data fusion problems and review the spectrum of approaches to rock physics currently available, which ranges from traditional pore-scale approaches to emerging field-scale methods.