RESEARCH - seismics |
Discontinuity stiffness determination from normal incidence in-situ seismic transmission measurements.
Author: Eric-Jan Luijk, M.Sc.
Centre for Technical Geosciences, Delft, The Netherlands
CTG research report / M.Sc. thesis
November 1998
Address: Centre for Technical Geosciences, Technical University Delft, Applied Earth Sciences, Mijnbouwstraat 120, 2628 RX Delft, The Netherlands
This research has been done in the context of the research project "Seismic wave behavior in discontinuous rock masses" of the research school "Centre for Technical Geosciences" (CTG). The Technical University Delft and the International Institute for Aerospace Survey and Earth Sciences (ITC) in Delft co-operate in the research school.
The research has been supervised by Dr. H.R.G.K. Hack, International Institute for Aerospace Survey and Earth Sciences, Section Engineering Geology, and Prof. Dr. Ir. J.T. Fokkema, Technical University Delft, Sub-faculty of Applied Earth Sciences, Section Technical Geophysics.
In-situ seismic transmission measurements recorded in the Tarragona Province of Spain have been used to study discontinuity stiffness determined with seismic waves. A coarse grained sandstone containing calcite filled discontinuities was investigated by analysis of normal incidence direct waves. A hammer source was used to generate the signals. The signal was recorded with 3-component geophones and a Bison 24-bit delta sigma stacking seismograph. The in-situ stresses in the sandstone are thought to be very low (vertical in the order of 25 kN and just above 0 kN horizontal) because the measurements have been done on a small flat top of a hill side.
The research showed that the absorption coefficient of the intact rock is not linear related with frequency. Secondly, based on the seismic field data the discontinuity stiffness was calculated with the aid of a model developed by Pyrak-Nolte for laboratory experiments. A scale effect is observed such that the in-situ discontinuity stiffness calculated from field seismic waves is an order 1000 lower than the values found in the laboratory experiments from Pyrak-Nolte. The stiffness of the discontinuity linear increases with increasing frequency, hence with increasing pressure. This may indicate that it is possible to relate the stiffness of a discontinuity in-situ determined with seismic field waves to the stiffness defined in the low-stress region of a shearbox test.