Studies of in situ pore pressure fluctuations at various scales

Pore pressure fluctuations in fluid saturated geological formations, either of natural or anthropogenic origin, can be observed at different scales. N atural fluctations, e.g., due to tidal, barometric or seismogenic forcing, or man-made effects as through use of underground fluid reservoirs, or init ial filling and cyclic lending of lake reservoirs may have wavelengths from meters to kilometers. In situ monitoring of processes, in which both rock deformation and pore pr essure changes are significant improves our knowledge on the mechanical beh aviour and the role of pore pressure in porous rocks and sedimentary layers . Pressure transducers for continuous recording of fluid level variations i n wells, reflecting pore pressure changes at depth, or borehole tiltmeters that are sensitive to ground deformation caused by gradients of pore pressu re flrrctltntions are relatively simple means to trace the dynamics of such rock-fluid interactions. The obtained dam series are usually interpreted i n two ways: by application of analytical solutions-adopting homogeneous por oelastic conditions or single fracture models in a uniform, elastic medium- and by simulation through numerical calculations allowing for some hereroge neity in the model volume. Field cases presented in this article include tilt measurements in the vici nity of pumped wells (1 to 100 in scale), fluid let el monitoring in wells (borehole scale), curd studies of pore pressure effects induced by seismic events (1 to 100 km scale). Specific rock parameters that call be constrain ed are the Skempton ratio, the hydraulic diffusivity and the type of the ef fective rheology. In cases of tiltmeter studies, anisotropy, of pore fluid flow can also be detected.