HR: 14:35h
AN: H13J-04    [Abstracts]
TI: A New Approach for Very Large Broadband Geophysical Monitoring of rock Deformations Into Deep Boreholes: The "High-Pulse Poroelasticity Protocol" (HPPP)
AU: * Guglielmi, Y
EM: guglielmi@geoazur.unice.fr
AF: geosciences azur, 250 Albert Einstein street, valbonne, 06560, France, Metropolitan
AU: Cappa, F
EM: cappa@geoazur.unice.fr
AF: geosciences azur, 250 Albert Einstein street, valbonne, 06560, France, Metropolitan
AU: Virieux, J
EM: viri@geoazur.unice.fr
AF: LGIT, BP53 Grenoble cedex9, Grenoble, 38041, France, Metropolitan
AU: Rutqvist, J
EM: JRutqvist@lbl.gov
AF: LBNL-ESD, 1 Cyclotron road, Berkeley, CA 94720, United States
AU: Tsang, C
EM: CFTsang@lbl.gov
AF: LBNL-ESD, 1 Cyclotron road, Berkeley, CA 94720, United States
AB: We present a new approach, called the "High-Pulse Poroelasticity Protocol" (HPPP), for a very large broadband geophysical monitoring of rock deformations into deep boreholes (from 200 m to 1 km depth). The HPPP consists in developing an innovative probe that allows the hydromechanical loading of rocks with synchronous fluid pressure – 3D deformations (translational components along and in the orthogonal plan of the borehole axis, and rotational components along the longitudinal axis) – seismic wave measurements over a broadband of frequencies (from static to dynamic [1-1,000 Hz]). In this protocol, the rock is subjected to a controlled source corresponding to a fast (few seconds) hydraulic pressure pulse (pressure wave) localized into a short injection chamber (from 1 to 3 m) which is isolated between two inflatable packers in a borehole. In the chamber, measurements are done with fibre-optic and acoustic sensors that makes possible to use a wide range of frequencies (1-1,000 Hz) and high accuracy (10-7) sampling of fluid pressure and 3D deformations. When the pressure wave is applied, several poroelastic effects are measured: (i) a static poroelastic response that is linked to the fluid diffusion in phase with mechanical deformation of the porous rock; (ii) a low-frequency slow Biot wave (P2 wave) associated with the motion out of phase of solid and fluid phases; (iii) a high-frequency pressure wave that is generated and converted to seismic waves (P1 and S waves) at the borehole wall. This new approach aims at determining the infinitesimal shear and axial components of the strain tensor within the rock crossed by a borehole. The HPPP also allows studying the relationships between elastic waves propagation and rock hydromechanical properties and state at an intermediate scale (mesoscopic scale), between the laboratory and crustal scales, in a volume of one to a few tens of meters around the borehole. This new approach was designed from previous pulse testing done in a fault zone with a first prototype of the HPPP probe capable of simultaneously measuring changes (with a high frequency [120 Hz] and high accuracy) in fluid pressure (± 1 kPa) and displacement normal to the fault (± 10-7 m). This prototype consisted of a fibre-optic fluid pressure and a fibre-optic normal displacement sensor fixed to the borehole walls by two anchors located on both sides of the fault which was isolated with two packers to create a 0.4 m injection chamber. Results indicated that fiber-optic measurements allow good capturing of all the high-frequency changes during the hydraulic pulse. The method appears useful for accurately measuring time discrepancies between pressure and deformation signals as small as a few milliseconds. Moreover, high-frequency measurement of the fault "pressure-deformation" poroelastic response allows highlighting of a loop-shaped evolution that is not observed in conventional laboratory or in situ experiments. Consequently, the HPPP approach will provide new data with axial and shear components of the strain tensor which will give us additional information for determination of the rock seismic and hydromechanical properties at various depths in the crust. Moreover, the HPPP will be adapted to study seismic and mechanical instability of fault zones under controlled hydraulic loading and localized in a point source.
DE: 1829 Groundwater hydrology
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
SC: Hydrology [H]
MN: 2007 Fall Meeting