Microseismic Event Spectrum Control and Strain Energy Release in Stressed Rocks
Rozanov, Alexander O.*1
(1) Nondestructive control, Ioffe Physical Technical Institute, St. Petersburg, Russian Federation.
Studying of passive seismicity concerns the physical processes of sound generation by cracks, propagation and recording of elastic waves. One of the main questions remains open - how do radiated elastic waves carry information about physical parameters of their sources (cracks) through a complex geomedium?
To develop this problem the lab experiments have been carried out on different kinds of rock put under different load conditions. Acoustic emission (AE) method has been used to study fracture process in rocks. Two types of deformation process in rocks have been investigated: brittle deformation of granites, and strain hardening in high porous sandstone. To analyze AE signals the author has developed a computer program based on spectral density calculations and correlation analysis over continuous AE data.
To analyze AE spectral data the author suggests a normal mode and median frequency approach. It is based on the following assumptions: 1. an output AE spectrum is strongly influenced by the normal mode spectra of discrete geomedium and transducer; 2. any AE spectrum can be quantified by the mode, the median, and the mean; 3. the mode indicates most intensive normal mode of a medium, the median and the mean characterize the source time function of AE event, square root of spectral density integral is a measure of AE event energy.
Using this technique we observed a good correlation between AE median/mean frequency trend and the time change of load in brittle fracture experiments. To interpret this observation the author makes a following assumption: the median/mean frequency of AE signal is proportional to the critical stress for a certain material contained in rock mass and for an initial crack length contained in this material. As rock mass yields, a smaller amount of elastic energy flux from intact area is possible to initiate a crack. Only those areas characterized by small critical stresses are involved in further fracture process. This process of rock weakening is indicated by the clear median/mean frequency decrease.
AE frequency trends for strain hardening in sandstone are less obvious because the fracturing is going on at micro-scales within the bands of localized deformation. Nevertheless, the above considerations for granites can also be useful for sandstones.
This method needs to be developed and could be used as a remote seismic control of stress in situ.
Note: The lab experiments on rocks are carried out in GFZ Potsdam, Germany.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain