Surface Energy Effects on low Frequency seismic Attenuation at partial Saturation; Lab and Field.
Abstract
In the laboratory we have shown that dissipative meniscus motion can cause substantial attenuation of low frequency strain waves (seismic waves) in partially saturated porous media. This observation offers the possibility for geophysical applications in the field, such as waste site monitoring, groundwater contamination assessment and secondary oil recovery. The laboratory results were obtained at frequencies from 0.001 Hz to 100Hz, i.e. at frequencies where viscous dissipation was not important. Results from measurements of capillary flow, dynamic, as well as static surface tension on clean and contaminated surfaces were combined to construct a frequency dependent attenuation model. An attenuation spectrometer working over the above mentioned frequency range was constructed and used to measure attenuation in single cracks as well as partially saturated rocks. The single crack data agreed quantitatively with the attenuation expected from the flow and surface tension measurements. The rock data agreed qualitatively. Based on the laboratory results we have initiated a pilot field program to observe near surface seismic attenuation changes. Our laboratory results suggest that the attenuation due to meniscus hysteresis is largest at frequencies below 1 Hz. Rather than using artificial sources for the generation of strain waves, we are exploring the use of natural phenomenon. In our pilot study we are using tidal strains and microseisms as sources. We are comparing the temporal changes in station corrections between stations that are close to and far from the site that is being altered by the introduction of liquid. We have installed three seismometers and 3 tilt-meters at a field site of the University of Arizona near Maricopa Arizona. The first data are now arriving and we are analyzing them for temporal changes between the recorded signals.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....2726S