(1) (n.) Geology. The ability, or measurement of a rock's ability, to transmit fluids, typically measured in darcies or millidarcies. The term was basically defined by Henry Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. Formations that transmit fluids readily, such as sandstones, are described aspermeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock. Effective permeability is the ability to preferentially flow or transmit a particular fluid through a rock when other immiscible fluids are present in the reservoir (for example, effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1. 0. Calculation of relative permeability allows for comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow. See: Absolute Permeability, Cap Rock, Effective Permeability, Fracture, Fracture Porosity, Impermeable, Pore, Pore Pressure, Porosity, Relative Permeability, Reservoir, Saturation, Seal. More Details: Characterizing Permeability with Formation Testers. (2) (n.) Geophysics. In magnetics, the ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically in units of H/m. See: Magnetics, Skin Depth. (3) (n.) Well Testing. The capability of a rock to allow passage of fluids through it. The term was basically defined by Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. More Details: Characterizing Permeability with Formation Testers