HR: 1340h
AN: T13A-1347    [Abstracts]
TI: Accurate measurement of P and S wave travel times under controlled laboratory conditions over long time scales
AU: Loggia, D
EM: loggia@msem.univ-montp2.fr
AF: Laboratoire de Tectonophysique, Universite Montpellier II/CNRS, Montpellier, 34095 France
AU: * Mainprice, D
EM: david@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, Universite Montpellier II/CNRS, Montpellier, 34095 France
AU: Silver, P
EM: silver@dtm.ciw.edu
AF: Carnegie Institution, 5241 Broad Branch Road NW, Washington, DC 20015 United States
AU: Bokelmann, G
EM: bokelmann@dstu.univ-montp2.fr
AF: Laboratoire de Tectonophysique, Universite Montpellier II/CNRS, Montpellier, 34095 France
AB: The measurement of small changes in travel time associated with changes in stress, crack, or fluid distribution has been the goal of many seismological studies in regions of seismic risk. Several recent studies have shown that properties of the seismic wavefield recorded in the same region over a period of years can reveal temporal changes, probably associated with transient deformation in the fault region. Other studies, however, have shown that seasonal climate changes can also influence seismic properties. We seek to develop a laboratory-based quantitative relationship between changes in stress and wavefield properties that also accounts for the possible influence of environmental effects. For this purpose we are utilizing a small triaxial rig with pore fluid connections, P and two perpendicularly polarised S wave transducers in each piston to study the effect of small controlled changes in deviatoric stress, confining pressure and pore fluid pressure on ultrasonic traces under isothermal conditions. Our objective is to obtain velocity precision (fractional change in travel time) of order 10-6, which is the precision needed to monitor stress changes of order 1KPa. At this level it is possible detect stress changes due to tidal stresses and atmospheric loading (two useful calibrating signals in field applications), as well as tectonic activity. Achieving this level of precision requires a variety of procedures that maximize both signal-to-noise ratio and resolution. For example, we are developing a procedure for the massive stacking of waveforms on a digital oscilloscope with a high sampling rate of 2.5GS/s (0.4ns resolution). We are also experimenting with the characteristic frequency and waveform characteristics of the source. We have begun a series of experiments on thermally cracked Westerly granite cores (38mm diameter and 80mm length). The samples are being subjected to temporal variations in deviatoric stress, confining pressure, and pore fluid pressure over a variety of timescales to establish the stress sensitivity of seismic wavefield properties.
DE: 5102 Acoustic properties
DE: 5144 Wave attenuation
DE: 5194 Instruments and techniques
DE: 7203 Body wave propagation
DE: 3909 Elasticity and anelasticity
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting