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INVESTIGATING THE EFFECT OF MICROBIAL GROWTH AND BIOFILM FORMATION ON SEISMIC WAVE PROPAGATION IN SEDIMENT
Citation:
Davis, C., L. Pyrak-Nolte, E. Atekwana, M. Haugen, AND D. D. WERKEMA. INVESTIGATING THE EFFECT OF MICROBIAL GROWTH AND BIOFILM FORMATION ON SEISMIC WAVE PROPAGATION IN SEDIMENT. Presented at American Geophysical Union Chapman Conference on Biogeophysics, Portland, MA, October 13 - 16, 2008.
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Presentation
Description:
Previous laboratory investigations have demonstrated that the seismic methods are sensitive to microbially-induced changes in porous media through the generation of biogenic gases and biomineralization. The seismic signatures associated with microbial growth and biofilm formation in the absence of biomineralization, however, remain uncertain. Biofilm formation can result in significant changes to the hydraulic and mechanical properties of a porous medium. Here, we report on the results of a laboratory experiment aimed at assessing the spatial and temporal changes in acoustic wave propagation associated with microbial growth in porous media, while concurrently measuring the complex conductivity of the same system. Microbial growth was stimulated in silica sand-packed columns, and complex conductivity measurements and acoustic (compressional) wave data were collected over a two-dimensional region for 15 days. The imaginary component of the complex conductivity measured from the biostimulated column (nutrients and bacteria inocula) increased to peak values by Day 5, before decreasing to near background values by Day 15. The real component remained relatively steady through Day 7, decreased slightly to a minimum on Day 9, and then showed a gradual increasing trend through Day 15. In contrast, the complex conductivity results from the unstimulated column (nutrients, no bacteria) did not show any significant variations over time. The seismic signal from the biostimulated column shows a significant decrease in amplitude with time since biostimulation. The transmitted wave amplitude is relatively uniform over the scanned region for the standard sample with an average peak-to-peak amplitude of 0.56 +/- 0.02 Volt. However, transmitted amplitudes from the biostimulated column vary spatially with an average amplitude of 0.47 +/- 0.16 Volt. No significant change in velocity was observed. Visual examination of the biostimulated column showed biofilm growth. Planned microbiological and geochemical analyses upon destruction of the columns will help to constrain the measured geophysical results.