Maria Zamora

Seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite: Abundance of serpentine in slow spreading ridge

The effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P- and S- wave velocities are linearly correlated with serpentine content; ii) anisotropy of P- and S-waves decreases with increasing serpentinization (while in fresh peridotites anisotropy is of the order of 6%, for 70% of serpentinization it drops to 1% for P-waves, and to 2%, for S-waves). Comparison of published laboratory velocity data in oceanic lithologies, like gabbros and dolerites, with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40% by seismic waves velocities only. This comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites (up to 40% of serpentinization) in the oceanic crust formed at slow-spreading ridges in accordance with geological observation.

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.

Transport properties of pyroclastic rocks from Montagne Pelée volcano (Martinique, Lesser Antilles)

The hydraulic and electrical properties of pyroclastic rocks have been investigated in laboratory on a representative sampling of Montagne Pelee (Martinique, France) deposits with renewed interest in geophysical applications. This sampling covers all the lithologic units of this volcano: lava dome and lava flows, pumices from ash-and-pumice fall and flow deposits, lava blocks from block-and-ash flow and Peleean “nuees ardentes” deposits, scoriae from scoria flow deposits. The connected porosity varies over a wide range from 3 to 62%. The unconnected porosity is important only on pumices where it can reach 15%. The permeability covers more than 5 orders of magnitude, ranging from 10−16 to 35 × 10−12 m2. The higher values are obtained on lava blocks and the scoriae, even if these rocks are less porous than the pumices. The formation factor ranges from 7 to 1139. The transport properties of these rocks are slightly correlated with porosity. This indicates that these properties are not only controlled by the connected porosity. To connect the transport properties to the textural characteristics of the pore network of pyroclastic rocks, different models, based on geometrical considerations or percolation theory, were tested. The pore access radius distribution and the tortuosity control the transport properties of pyroclastic rocks. Consequently, the models (electric and hydraulic) based on the concept of percolation (e.g., the models of Katz and Thompson), apply better than the equivalent channel model of Kozeny-Carman. In addition, the difference in transport properties observed on lava blocks and pumices confirms that the mechanisms of degassing and vesiculation are different for these two types of rock.

An empirical relationship between thermal conductivity and elastic wave velocities in sandstone

Measurements in three samples of very clean quartz sandstone in the porosity range 4–16 %, under dry and 100 % water-saturated conditions, show that P- and S-wave velocities are linearly correlated with thermal conductivity. The experimental results agree with the theoretical relation between seismic velocities (predicted by the Kuster and Toksoz model (1974)) and thermal conductivity (predicted by weighted geometric mean).

Percolation of CO2-rich fluids in a limestone sample: Evolution of hydraulic, electrical, chemical, and structural properties

Percolation of CO2-rich fluids in limestones causes the dissolution (and eventual reprecipitation) of calcium carbonate minerals, which affect the rock microstructure and change the rock petrophysical properties (i.e., hydraulic, electrical, and elastic properties). In addition, microstructural changes further feed back to affect the chemical reactions. To better understand this coupled problem and to assess the possibility of geophysical monitoring, we performed reactive percolation laboratory experiments on a well-characterized carbonate sample 35 cm in length and 10 cm in diameter. In a comprehensive study, we present integrated measurements of aqueous chemistry (pH, calcium concentration, and total alkalinity), petrophysical properties (permeability, electrical formation factor, and acoustic velocities), and X-ray tomography imaging. The measured chemical and electrical parameters allowed rapid detection of the dissolution of calcite in the downstream fluid. After circulating fluids of various salinities at 5mL min−1 for 32 days (about 290 pore sample volumes) at a pCO2 of 1 atm (pH = 4), porosity increased by 7% (from 0.29 to 0.31), permeability increased by 1 order of magnitude (from 0.12 D to 0.97 D), and the electrical formation factor decreased by 15% (from 15.7 to 13.3). X-ray microtomography revealed the creation of wormholes; these, along with the convex curvature of the permeability-porosity relationship, are consistent with a transport-controlled dissolution regime for which advection processes are greater than diffusion processes, confirming results from previous numerical studies. This study shows that nonseismic geophysical techniques (i.e., electrical measurements) are promising for monitoring geochemical changes within the subsurface due to fluid-rock interactions.

A physical model of the low‐frequency electrical polarization of clay rocks

Low-frequency (0.18 Hz to 1.5 kHz) effective dielectric spectra have been measured on a set of near-saturated samples of argillite. The measured spectra of the real part of the complex apparent permittivity did not show significant correlation with cation exchange capacity (CEC) per unit mass of rock values. They satisfied a power law relationship with the frequency, at least for samples with CEC values lower than 10 cmol/kg. The Maxwell-Wagner-Hanai-Bruggeman formulation used for a two-phase mixture has been modified to account for mutual polarization between the pockets of water located in the micropores and those located in the macropores. The results of the modeling calculations illustrate (1) the ability of this new formulation to reproduce the power law relationships of the measured spectra of the real and imaginary components of the complex permittivity and (2) the strong impact of the pore electrical conductivity.

Effect of the local clay distribution on the effective electrical conductivity of clay rocks

The “local porosity theory” proposed by Hilfer was revisited to develop a “local clay theory” (LCT) that establishes a quantitative relationship between the effective electrical conductivity and clay distribution in clay rocks. This theory is primarily based on a “local simplicity” assumption; under this assumption, the complexity of spatial clay distribution can be captured by two local functions, namely, the local clay distribution and the local percolation probability, which are calculated from a partitioning of a mineral map. The local clay distribution provides information about spatial clay fluctuations, and the local percolation probability describes the spatial fluctuations in the clay connectivity. This LCT was applied to (a) a mineral map made from a Callovo-Oxfordian mudstone sample and (b) (macroscopic) electrical conductivity measurements performed on the same sample. The direct and inverse modeling shows two results. First, the textural and classical model assuming that the electrical anisotropy of clay rock is mainly controlled by the anisotropy of the sole clay matrix provides inconsistent inverted values. Another textural effect, the anisotropy induced by elongated and oriented nonclayey grains, should be considered. Second, the effective conductivity values depend primarily on the choice of the inclusion-based models used in the LCT. The impact of local fluctuations of clay content and connectivity on the calculated effective conductivity is lower.

Electrokinetic in rocks: Laboratory measurements in sandstone and volcanic samples

Abstract The streaming potential due to fluid flow in rocks was measured on saturated sandstones. During triaxial failure test, the electrokinetic coupling coefficient was increased at about 72–86% of the failure stress. This increase is thought to be due to an increase of the macroscopic zeta potential in the shear zone where new surfaces are created and connected, allowing fluid to flow through these new cracks. The streaming potential variations could therefore be used as a precursor of the rupture. Measurements were also performed on volcanic samples, with the aim of using electrokinetic phenomena to monitor volcanic activity. Streaming potential was found to depend on the transport properties i. e., permeability and formation factor. Theoretical consideration suggest that such a dependence should be caused by surface conductivity, but this is not the case here.

On the Interest of Bulk Conductivity Measurements for Hydraulic Dispersivity Estimation from Miscible Displacement Experiments in Rock Samples

The determination of the hydraulic dispersivity and effective fraction of porous medium contributing to transport on soil and rock sample in the laboratory is important to understand and model the evolution of miscible contaminant plumes in groundwater. Classical methods are based on the interpretation of the breakthrough curve, i.e., the evolution of the concentration in contaminant at the downstream end-face of a sample into which a front of contaminant is advected. Here we present an experimental device aimed at performing such measurements, but also allowing the bulk electrical conductivity of the sample to be measured. We show that the dispersivity and effective fraction can be inferred from this electrical measurement, and that the combined use of both out-flowing fluid conductivity and bulk conductivity allows the incertitude on the dispersivity and effective fraction to be significantly enhanced.