Some contributions to the self-potential method

Résumé: 

The self-potential (SP) method is a passive geophysical method based on the monitoring of electrical currents naturally occuring in the subsurface. It is sensitive to fluid flows, to the existence of concentration or redox potential gradients, and to temperature gradients. As it is low-cost, easily implemented in the field and also allowing the field to be densely covered in space and time, it is of great interest for hydrogeophysical field studies. However, the signals are weak, and often different sources are acting together. It is thus important to understand them and develop methodology to discriminate them.
Here I will present some contributions that I and my coauthors have made during the past decade. First, I will report on laboratory experiments aimed at studying the response of salt fronts advected through natural sand. It will be shown that the fluid velocity and the dispersion coefficient can be estimated from such experiments. Secondly, I will present some laboratory results obtained during the injection of CO2 in sand. It will be shown that in the zone where gas circulates, the SP response varies, but the variations depends on the kind of electrodes used. Thirdly, I will focus on a field experiment aimed at recording the self-potential signals during oscillatory hydraulic pumping test. It will be evidenced that the response between the fluid flow and the SP signal is not linear, contrary to what predicts the classical theory. Then I will show the results and conclusions of a laboratory experiment designed to produce harmonic variations of the water level in a vertical sand column while recording the SP. The conclusions are similar to what was observed in the field study, and allowed us to propose a semi-empirical model for the SP signals. Afterwards, I will present a study carried out in a horizontal borehole where water infiltrations occurs: the SP method permits the hydraulically active fractures to be identified. I will also show a purely numerical study concerning the estimation of the distribution of the electrical potential along metallic well casing from a single surface SP profile. Finally, I will present the future project involving SP, called STREAM, aimed at studying the response to watertable oscillations in the laboratory.

Auteur: 
Alexis Maineult
Affiliation: 
METIS
Vendredi, 10 juin, 2016 - 13:00