Geoelectrical monitoring of dissolution and precipitation reactions in a saturated calcareous porous medium

<p>Precipitation and dissolution are prime processes in carbonate rocks and being able to monitor them is a major deal of reservoir exploitation for geo-resources (water, gas) or geological storage (CO<sub>2</sub>, H<sub>2</sub>, waste). Geophysics can be used to monitor these processes non-intrusively and at low cost. Among the existing techniques, we used two electrical methods to monitor the reactivity of a synthetic calcareous porous medium: self-potential (SP) and spectral induced polarization (SIP). SP is a passive technique that consists in measuring the electrical field as it is affected by water fluxes and concentration gradients through electrokinetic and electrochemical couplings. SIP is an active method that provides the electrical conductivity and the chargeability of a porous medium in a low frequency range (mHz to kHz). We carried out a two months laboratory experiment to monitor the geoelectrical signals generated by chemical variations in a synthetic medium composed of pure calcite grains. Three different solutions were injected to alternatively dissolve or precipitate calcite in the sample. The sample is equipped with four aligned non-polarizable Ag/AgCl electrodes in order to geoelectrically monitor the fluid percolation and the ionic concentration gradients changes through the medium. Moreover, we conducted chemical analyses of the downstream fluid to monitor its ionic composition. We made a 1D reactive-transport simulation with the software CrunchFlow to get the concentration gradients of all dissolved ions along the column. Following a theoretical framework, we used a physically based analytical model to relate our electrical signals to ionic concentrations of a multicomponent electrolyte. We find that dissolution and precipitation generate measurable geoelectrical signals because of chemical reactions and ions substitutions. These findings open the possibility to better understand geoelectrical signals in natural media and possibly use them to monitor in situ reactivity.</p>

The method of the phase control of the electrical installation during geodynamic monitoring

The paper substantiates the application of the phase control of the multipolar equipotential electrical installation to improve the geodynamic sensitivity of the geoelectric control method in geodynamic and geotechnical monitoring systems. The principle of the control of the source of sounding signals and the detection of geodynamics of the geological environment is shown in article. In this work is proposed the device on base of the phase modulator to eliminate the shortcomings of existing devices of the controlling of the phase of sound geoelectrical signals. The developed device contains two amplitude limiters, the integrating unit, the adder, the master oscillator and the amplitude modulator. In practice the application of the proposed device allow to obtain the technical and the economic efficiency by simplifying of the circuit of the generation of the signal with the phase-modulation, by increasing the linearity of the modulation characteristic, and by generating of the signal with the phase-modulate on output of the device.

Multifractal variability in geoelectrical signals and correlations with seismicity: a study case in southern Italy

Multifractal fluctuations in the time dynamics of geoelectrical data, recorded in a seismic area of southern Italy, have been revealed using the Multifractal Detrended Fluctuation Analysis (MF-DFA), which allows to detect multifractality in nonstationary signals. Our findings show that the geoelectrical time series, recorded in the seismic area of southern Apennine Chain (Italy), is multifractal. The time evolution of the multifractality suggests that the multifractal degree increases prior the occurrence of earthquakes. This study aims to propose another approach to investigate the complex dynamics of earthquake-related geoelectrical signals.