Biogeophysics is an emerging discipline aimed at understanding the impact of microbial activity and processes on geophysical properties of earth materials. Microbes modify the physical properties of their environment through growth, proliferation, and biofilm formation or due to their metabolic by-products. In shallow subsurface environments where microbial activity is greater than natural conditions (e.g. organic contamination), geophysical methods have been used to detect and characterize zones of microbial activity. The laboratory studies presented above suggest that changes in the geologic media resulting from direct and indirect microbial alteration of the petrophysical and chemical properties can be detected by different geophysical methods. Alterations of the geologic media occur when microbes directly colonize sediment surfaces forming biofilms that alter such properties as texture, surface area, pore size and pore geometry, tortousity, cementation, formation factor, and elastic moduli. Chemical changes in the microbial environment may result from the development of pH sensitive environment conducive to microbial survival or through secondary mineral-water reactions enhanced by byproducts of organic carbon metabolization. The mineral-water interaction driven by changing pH and redox conditions may result in mineral weathering and cause physical changes in the subsurface environment. Changing chemical conditions, some induced by microbial activity may result in mineral precipitation which changes the physical conditions of the subsurface environment and the groundwater chemistry. When the chemistry of the pore fluids change, the ionic strength, ionic charge density, and ionic mobility of the fluid phase also change, resulting in the alteration of the electrical conductivity. In addition, the interaction of the electrical properties of the pore fluids with that of the mineral surfaces may affect the condition of the electric double layer. The electric double layer may be altered directly by microbial colonization since microbes carry a net negative charge. The challenge therefore is to use geophysics and microbiology to shed light on these important microbial alterations of the subsurface environment.
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