Characterization of fluid-transport properties of rocks is one of the most important, yet one of most challenging, goals of reservoir geophysics. However, active seismic methods have low sensitivity to rock permeability and mobility of pore fluids. On the other hand, it would be very attractive to have the possibility of exploring hydraulic properties of rocks using seismic methods because of their large penetration range and their high resolution. Microseismic monitoring of borehole fluid injections is exactly the tool that can provide us with such a possibility. Borehole fluid injections are often applied for stimulation and development of hydrocarbon and geothermal reservoirs. Production of shale gas and heavy oil as well as CO2 sequestration are relatively recent technological areas that require broad applications of this technology. The fact that fluid injection causes seismicity has been well established for several decades (see, for example, Pearson, 1981, and Zoback and Harjes, 1997). Current ongoing research is aimed at quantifying and controlling this process. Understanding and monitoring of fluid-induced seismicity is necessary for hydraulic characterization of reservoirs and for assessments of reservoir stimulations.

Fluid-induced seismicity covers a wide range of processes between the two following limiting cases. In liquid-saturated hard rocks with low to moderate permeability (10−5 −10−2 darcy) and moderate bottom hole injection pressures (as a rule, less than the minimum absolute value of the principal compressive tectonic stress) the phenomenon of microseismicity triggering is often caused by the process of linear relaxation of pore-pressure perturbations (Shapiro et al., 2005a,b). Note that we speak here about the linearity in the sense of corresponding differential equations. In porodynamics this process corresponds to the Frenkel–Biot slow wave propagation (see Biot, 1962, and a history review by Lopatnikov and Cheng, 2005, as well as an English translation of Frenkel, 2005). In the porodynamic low-frequency range (hours or days of fluid-injection duration) this process reduces to a linear pore-pressure diffusion.