The highly heterogeneous permeability field of the Earth’s brittle crust has long been a challenge to those seeking to extract its fluid resources. We discuss the geologic and geophysical evidence that the ambient seismic method of permeability field imaging (PFI) solves this problem not by modeling, but by directly mapping the permeability field in its quantitative manifestation and temporal evolution. Further, we describe a permeability-producing mechanism that operates across the full range of micro- to macroscales. These results are placed in the context of the principles of critical state rock physics as they apply to the PFI method.
The PFI signal is due to minute elastic vibrations of these fluid-filled voids activated by the near-continuous movement of episodic stress waves through the brittle crust. Passive seismic observation of both ambient and induced seismic events can be recorded, for example, using standard three-dimensional seismic reflection receiver technology. These signals, whose source is the permeability itself and is processed with PFI methods, can produce a five-dimensional (space, time, energy) permeability field map. Whereas sedimentary basins have a prominent role in our discussion, our current knowledge is that PFI permeability field maps can be made for all lower-temperature (<250°C), brittle rock, nonmetamorphic environments.