Laurence Jouniaux

Streaming potential and permeability of saturated sandstones under triaxial stress: Consequences for electrotelluric anomalies prior to earthquakes

The streaming potential, due to fluid circulation in rock, was measured on saturated sediments (Fontainebleau sandstones). The electrokinetic coupling coefficient, which is the ratio of the streaming potential and the excess pore pressure, is proportional to the fluid resistivity. Additionally, for a fluid conductivity of 10−3 S/m, the electrokinetic coupling coefficient varies from 10 to 6642 mV/0.1 MPa for sample permeability in the range of permeabilities from 0.15 × 10−15 to 1220 × 10−15 m2. The different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes nonnegligible compared to fluid conductivity for low permeability. When the sample is deformed under triaxial stress up to failure, the vertical permeability (along the principal stress) drops by about 0.20%/0.1 MPa when failure occurs. The typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75% of the yield stress and stopping at the failure. This increase of the electrokinetic coupling coefficient is due to an increase of ζ potential in the shear zone when new surfaces are created and connected. Possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake. It is shown that in some cases, self-potential anomalies reported in the deformed zone preceding an earthquake occurrence could be due to an increase of the electrokinetic coupling coefficient from 75% of the yield stress to rupture in the vicinity of one of the electrodes. Any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient, inducing a surface electrokinetic potential anomaly. In regard to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter, a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow, for instance, in a shallow borehole. An electrokinetic potential anomaly up to 30 mV, for a fluid conductivity of 0.01 S/m and a rock permeability of 10−12 m2, could be observed with a change of the underground water table level as slight as 50 cm (50 mbar). Moreover, if the permeability between the electrodes is increased by a factor of 8 × 103, the electrokinetic coupling coefficient could be enhanced by a factor up to 650.

Detailed analysis of acoustic emission activity during catastrophic fracture of faults in rock

Abstract The detailed time-space distribution of acoustic emission (AE) events during the catastrophic fracture of rock samples containing a pre-existing joint or potential fracture plane is obtained under triaxial compression using a high-speed 32-channel waveform recording system, and the results are discussed with respect to the prediction and characterization of catastrophic fault failure. AE activity is modeled quantitatively in terms of the seismic b -value of the magnitude–frequency relation, the self-excitation strength of the AE time series, and the fractal dimension of AE hypocenters. Consistent with previous studies on rock samples containing a fracture plane with several asperities, the present analyses reveal three long-term phases of AE activity associated with damage creation on heterogeneous faults, each clearly identifiable based on the above parameters. A long-term decreasing trend and short-term fluctuation of the b -value in the phase immediately preceding dynamic fracture are identified as characteristic features of the failure of heterogeneous faults. Based on the experimental results it is suggested that precursory anomalies related to earthquakes and other events associated with rock failure are strongly dependent on the heterogeneity of the fault or rock mass. A homogeneous fault or rock mass appears to fracture unpredictably without a consistent trend in precursory statistics, while inhomogeneous faults fracture with clear precursors related to the nature of the heterogeneity.

Streaming potential in volcanic rocks from Mount Pelée

Streaming potential and electric conductivity have been measured in a laboratory on 11 consolidated samples coming from five deposits of the different evolutionary stages of Mount Pelde volcano. The streaming potential coupling coefficient ranges from-35 to-4905 mV MPa-1 and increases with increasing permeability. This increase is mainly due to the dependency of rock effective conductivity with permeability. The permeability of the samples varies from 0.146x10-12 to 34x10-12 m 2. The zeta potential, at pH-7 and water conductivity of 2.1x10-4 S m-l• is relatively small for the majority of the samples. It ranges from-4 to-19 reV. According to water conductivity analysis on Mount Pel•e, streaming potential coupling coetficients of-25 to-406 mV MPa-1 can be expected for this volcano.

Permeability dependence of streaming potential in rocks for various fluid conductivities

Streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities. The electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity. A model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity. When fluid resistivity is below 1 Ω.m permeability and streaming potential are no more related.

Modification of streaming potential by precipitation of calcite in a sand–water system: laboratory measurements in the pH range from 4 to 12

Spontaneous Potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential, which is usually positive in the flow direction (i.e. Zeta potential of the rock is negative). The water-rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals. This work addresses the study of calcite precipitation in a sand composed of 98% quartz and 2% calcite using streaming potential measurements. The precipitation of calcite as a secondary mineral phase, inferred by high calcite saturation indices and by a fall in permeability, has a significant effect on the electrokinetic behaviour, leading to a significant reduction in the Zeta potential (in absolute value) and even a change in sign. The measured decrease in Zeta potential from -16 mV to -27±4 mV takes place as the pH rises from 4 to 7, while it remains constant at -25±1 mV as the pH increases from 8 to 10.5. For pH higher than 10.5, calcite precipitates and is expected to coat the quartz surface. The measured Zeta potential vary from -17 to +8 mV for pH ranging from 10.6 to 11.7 depending on the amount of precipitated calcite indicated by the decrease in permeability. The observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of SP anomalies in order to determine fluid circulations, even at pH lower than 9 if calcite is widely present as a secondary mineral phase, since the electrical surface potential of calcite depends also on CO2 partial pressure and [Ca2+]. Therefore SP anomalies as measured in hydrothermal field, without mineralogical analyses of hydrothermal deposits, and without geochemical fluid survey, should be interpreted with caution.

Laboratory measurements anomalous 0.1–0.5 Hz streaming potential under geochemical changes: Implications for electrotelluric precursors to earthquakes

Streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments (Fontainebleau sandstones). The streaming potential ΔV was found to be proportional to the driving pore pressure ΔP. Pulses of amplitude 15–40 mV in the frequency range of 0.1 to 0.5 Hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low, corresponding to a Darcian velocity of 17 to 30 cm/h. The amplitudes of these pulses are 47% to 133% of the corresponding steady components of the ΔV values. Such geochemically induced effects may possibly be responsible for the frequency signals from 0.1 to 0.5 Hz that were sometimes observed before an earthquake.

Electrokinetics in Earth Sciences: A Tutorial

We describe in this paper the theoretical background for the electrokinetics in rocks and in porous media, to be included in the special issue “Electrokinetics in Earth Sciences” of International Journal of Geophysics. We describe the methodology used for self-potential (SP) and for seismoelectromagnetic measurements, for both field and laboratory experiments and for modelling. We give a large bibliography on the studies performed in hydrology to detect at distance the water flow, to deduce the thickness of the aquifer and to predict the hydraulic conductivity. The observation of SP has also been proposed to detect fractures in boreholes, to follow the hydraulic fracturing, and to predict the earthquakes. Moreover, we detail the studies on geothermal applications.

Anisotropy of electrical conductivity record of initial strain at the toe of the Nankai accretionary wedge

An approach based on Marchs theory is applied to measurements of the anisotropy of electrical conductivity on samples and is used to quantify initial strain at the toe of the Nankai accretionary wedge. A quantitative determination of strain is possible from simple assumptions: passive reorientation offlat pores forming the porous network and existence of a linear relationship between fabric tensor and electrical conductivity tensor. We show that this simple model correctly accounts for the increase of anisotropy with compaction at a reference site located in the trench (Ocean Drilling Program drill Site 1173). At the toe of the accretionary wedge (Site1174), development of anisotropy in the horizontal plane and concurrent reduction of vertical plane anisotropy are observed. This can be explained by 12%horizontal ductile shortening, occurring after decollement initiation but before slip on imbricate thrust faults. Anisotropy in the underthrust sequence is correctly described by vertical compaction, consistent with decoupled stress states across the decollement. At Site 1174 the magnitude of ductile strain implies at least 75m slip on the decollement. Ductile shortening is associated with porosity loss, implying partly drained conditions above the decollement.

Changes in the permeability, streaming potential and resistivity of a claystone from the Nankai prism under stress

Permeability is the critical factor governing fluid flow in accretionary prisms. Accretionary wedges are highly deformed, so permeability changes in an indurated claystone sample from the Nankai accretionary prism were measured during a triaxial stress experiment by the pulse decay method. Three zones were identified from the loading test. In zone I, the sample has deformed less than 1.3% and its permeability was 5.10−20 m². In zone II, the deformation reached 1.5% and the permeability was unstable with time. In zone III, the sample deformation reached 2% and its permeability reached 3.8 10−18 m². The permeability strongly decreased (from 3.2 10−18 m² to 1.7 10−19 m²) with increasing effective pressure (from 2.3 to 10.8 MPa) after the sample was well deformed (zone III). The streaming potential was not measurable when the sample had a low permeability (zone I), but clearly occured when the permeability increased (zone III) : the streaming potential measured was 6 mV when the pore pressure pulse applied was about 1.17 MPa and the permeability 3.8 10−18 m². Our experiments suggest that shear deformation under low effective pressure increased the vertical permeability of sediments above the decollement. This increase in permeability may be detected by measuring the streaming potential. Variations of flow rate of expelled fluid in accretionary wedges may be detected by monitoring changes of electrokinetic potential, giving new insights on the state of stress related to the seismic cycles.

Detection of fluid flow variations at the Nankai Trough by electric and magnetic measurements in boreholes or at the seafloor

Detection of changes in the flow rate of expelled fluids in accretionary prisms by monitoring of electric and magnetic fields is discussed. A numerical model of the electric and magnetic fields associated with fluid flow variations at the Nankai Trough is presented which gives a numerical solution of the coupled system of electric convection currents and conduction currents that directly determines the magnetic anomaly itself. Measurements in a borehole located between two vents are shown to be well adapted to detection of fluid flow variations using the vertical gradient of the electric potential and the horizontal magnetic field. The vertical electric field is about 10 mV/km up to 500 m depth where there is a lithologic reflector and about 50 mV/km below this reflector. The horizontal gradient of the magnetic field is 2 nT/km at the seafloor. Modelization with a lower fault conductivity and a larger decollement thickness has also been modeled. The vertical gradient of the horizontal magnetic field is ∼5 to 15 nT/km. A variation of 3 mV and 1.5 to 3 nT at 600 m depth in a borehole could reveal a fluid flow rate variation of 20%, which is a reasonable fluid flow change based on some observations at short-scale time. Since a 1.5 to 3 nT anomaly seems easier to detect than a 3 mV anomaly, it is likely that the variation of the magnetic field would more sensitively reveal fluid flow variations. When monitoring the magnetic field at the seafloor, a change of 0.4 nT/km in the horizontal gradient could reveal a fluid flow rate variation of 20%.