Analysis of Self-Potential signals due to cable bacteria over different conductivity structures

Abstract

<p>Cable bacteria are multicellular microorganisms that are capable of long distance electron transport (LDET) along their length. This electron transport is the result of oxidation of hydrogen Sulfide (H2S) in the sulfidic sediment layer where electrons are conducted up through cable filament aided by cell-to-cell transfer in the oxic layer thus reducing oxygen by gaining electrons. Cable bacteria behave as dipoles where anaerobic zones interfere with oxic zones for example oil/tar pollution site and can generate enough natural SP fields as a function of redox mechanism that can be measured on the surface. This study focuses on the theoretical analysis of Self-Potential (SP) signals resulting due to the presence of dipole current source under different conductivity structures in the subsurface. To investigate the behavior of SP signals, four different types of forward models are synthesized by varying resistivity of subsurface layers and changing the depth of the dipole beneath the surface. The dipole has a default current density of 20 mA/m<sup>2</sup>. In the first model, a rectangular pollution patch carrying a dipole of the same shape is placed between two homogeneous layers where the top layer resistivity is swept from 10-1000 ohm-m while keeping the resistivity of bottom layer constant. In the second model, the pollution patch is placed between an inhomogeneous layer with low, intermediate, and high resistivity contrasts and a homogeneous layer. In this model, half of the patch lies in lower conducting region whereas the other part is in the high conductivity region. The third model is an extension of the second one, where the inhomogeneous layer is sandwiched between two homogeneous layers. In the last model, the pollution patch was moved beneath the surface to a depth where the SP signal cannot be observed at the surface. In this model, the depth is observed for three different pollution sources with current density values equal to 2, 20 and 200 mA/m<sup>2</sup> respectively. The results showed that SP anomaly caused by the patch when the conductivity of upper layer is high is smaller as compared to the anomaly due to the less conducting upper layer. Next two models with inhomogeneous layer, correlate well with the first model showing high SP anomaly caused by dipole when it is present in the lower conducting region and low values when in high conducting region. Fourth model demonstrates when the depth of pollution patch is increased beneath the surface, SP signal decreases and is not observed beneath a depth of around 10 m, even when the source has current density value as high as 200 mA/m<sup>2</sup>. This study explicitly demonstrates the behavior of SP anomaly and will help in improved interpretation of SP technique where inhomogeneity will be present beneath the surface.</p>