Acoustic and electrical property changes due to microbial growth and biofilm formation in porous media

A laboratory study was conducted to investigate the effect of microbial growth and biofilm formation on compressional waves, and complex conductivity during stimulated microbial growth. Over the 29 day duration of the experiment, compressional wave amplitudes and arrival times for the control (nonbiostimulated) sample were observed to be relatively uniform over the scanned 2-D region. However, the biostimulated sample exhibited a high degree of spatial variability in both the amplitude and arrival times, with portions of the sample exhibiting increased attenuation (similar to 80%) concurrent with an increase in the arrival times, while other portions exhibited decreased attenuation (similar to 45%) and decreased arrival times. The acoustic amplitude and arrival times changed significantly in the biostimulated column between days 5 and 7 of the experiment, consistent with a peak in the imaginary conductivity (sigma '') values. The sigma '' response is interpreted as recording the different stages of biofilm development with peak sigma '' representing maximum biofilm thickness and decreasing sigma '' representing cell death or detachment. Environmental scanning electron microscope imaging confirmed microbial cell attachment to sand surfaces and showed apparent differences in the morphology of attached biomass between regions of increased and decreased attenuation. The heterogeneity in the elastic properties arises from the differences in the morphology and structure of attached biofilms. These results suggest that combining acoustic imaging and complex conductivity techniques can provide a powerful tool for assessing microbial growth or biofilm formation and the associated changes in porous media, such as those that occur during bioremediation and microbial enhanced oil recovery.