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Complex conductivity measurements (0.1–1000 Hz) were obtained from biostimulated sand‐packed columns to investigate the effect of microbial growth and biofilm formation on the electrical properties of porous media. Microbial growth was verified by direct microbial counts, pH measurements, and environmental scanning electron microscope imaging. Peaks in imaginary (interfacial) conductivity in the biostimulated columns were coincident with peaks in the microbial cell concentrations extracted from sands. However, the real conductivity component showed no discernible relationship to microbial cell concentration. We suggest that the observed dynamic changes in the imaginary conductivity (σ″) arise from the growth and attachment of microbial cells and biofilms to sand surfaces. We conclude that complex conductivity techniques, specifically imaginary conductivity measurements are a proxy indicator for microbial growth and biofilm formation in porous media. Our results have implications for microbial enhanced oil recovery, CO2 sequestration, bioremediation, and astrobiology studies.

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m À2 ) beneath a~60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m À2 ), corresponding to thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.

[1] The effect of microbial processes on electrical properties of unconsolidated sediments was investigated in a laboratory experiment consisting of biotic and abiotic sand columns. The biotic column (nutrient, diesel and bacteria) showed (a) temporal increase in the real, imaginary, and surface conductivity, and (b) temporal decrease in the formation factor. The abiotic columns (nutrient; and nutrient and diesel) showed no significant changes. Increase in microbial population numbers, decrease in organic carbon source, nitrate, and sulfate and increase in dissolved inorganic carbon and fluid conductivity were indicative of microbial activity in the biotic column. We also measure relative increase in the interfacial electrical properties that exceed relative increase in the electrolytic conductivity. Thus changes in the real and imaginary conductivity were induced by microbial processes. These results suggest that interpretation of geoelectrical data from near surface environments should consider effects of microbial processes.

Microorganisms participate in a variety of geologic processes that alter the chemical and physical properties of their environment. Understanding the geophysical signatures of microbial activity in the environment has resulted in the development of a new sub-discipline in geophysics called ''biogeophysics''. This review focuses primarily on literature pertaining to biogeophysical signatures of sites contaminated by light non-aqueous phase liquids (LNAPL), as these sites provide ideal laboratories for investigating microbialgeophysical relationships. We discuss the spatial distribution and partitioning of LNAPL into different phases because the physical, chemical, and biological alteration of LNAPL and the subsequent impact to the contaminated environment is in large part due to its distribution. We examine the geophysical responses at contaminated sites over short time frames of weeks to several years when the alteration of the LNAPL by microbial activity has not occurred to a significant extent, and over the long-term of several years to decades, when significant microbial degradation of the LNAPL has occurred. A review of the literature suggests that microbial processes profoundly alter the contaminated environment causing marked changes in the petrophysical properties, mineralogy, solute concentration of pore fluids, and temperature. A variety of geophysical techniques such as electrical resistivity, induced polarization, electromagnetic induction, ground penetrating radar, and self potential are capable of defining the contaminated zones because of the new physical properties imparted by microbial processes. The changes in the physical properties of the contaminated environment vary spatially because microbial processes are controlled by the spatial distribution of the contaminant. Geophysical studies must consider the spatial variations in the physical properties during survey design, data analysis, and interpretation. Geophysical data interpretation from surveys conducted at LNAPL-contaminated sites without a microbial and geochemical context may lead to ambiguous conclusions.