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Electrical conduction in oceanic dikes, Hole 504B

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... (1), we get (e.g. Waxman & Smits 1968;Revil et al. 1996;Revil 2013a,b), ...
... Surface conductivity σ S (in S m −1 ) versus cation exchange capacity (in C kg −1 ). We compare here the results from sedimentary rocks and volcanic rocks The volcanic rocks include those from this study (with a corrected tortuosity F φ of 3.0 for all the samples), the volcaniclastic materials from Revil et al. (2002) (with the exception of samples BU 96-8A and BU 96-8B, which are less porous than the other samples, see Table 4) and the oceanic dike samples studied by Revil et al. (1996) (normalized by the tortuosity). The data from the literature are from Bolève et al. (2007, glass beads, NaCl), Vinegar & Waxman (1984, shaly sands, NaCl), Churcher et al. (1991) (CEC for the Berea sandstone), Lorne et al. (1999, Fontainebleau sand KCl), Kurniawan (2005, clean Table 6. ...
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We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu'ula Groundwater Research Project (Hawai'i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m⁻¹ prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1 °C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5 per cent wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... eq. (2). Revil et al., (1996) reported that the rate in which water conductivity (σ F ) i n c r e a s e s is nearly constantand independent of the fluid salinity, with  F =0.023 K -1 . At temperature higher than 250°C, the rate of viscosity decrease is reduced and so is the change in ionic mobility. ...
... Similar rates were found in previous works for magmatic rocks (e.g. Flovenz et al., 1985, Kristindottir et al., 2010, Revil et al., 1996. These rates are explained by an increase in surface conduction due to an increase in ionic mobility in the electrical double layer of minerals surfaces. ...
Article
Electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth. Up to this point, technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs. In this study, we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus. This allows us to perform resistivity measurement at temperatures up to 700 °C and at effective pressures up to 100 MPa (i.e. a confining pressure of 130 MPa and an equilibrium pore pressure of 30 MPa) using cm-scale plugs. Rock samples originate from five boreholes located in the Icelandic geothermal fields of Reykjanes (RN-17B/Hyaloclastite, RN-19/RN-30/dolerites) and Hengill (NJ-17/basalt and NJ-17B/Hyaloclastite). These samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole facies (i.e. temperature of 250 °C and 400 °C respectively), and their wide range of porosities (from 3% to 20%). To determine the effects of surface, mineral and electrolytic conductions on bulk electrical conduction, experiments were performed under dry and saturated conditions using three different fluid salinities. At temperatures ranging from 25 to ~350 °C, electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction. Then, under supercritical conditions, i.e. temperature from 374 °C to 600 °C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. At higher temperatures (500 °C–700 °C), the rock conductivities lie within the range of dry rock electrical conductivity values, suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction. Amphibole-rich samples show an irreversible increase in conductivity at temperature above 500 °C–600 °C, which can be attributed to amphibole dehydration. Comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large-scale surveys. Our results provide a general trend that helps interpreting electrical conductivity surveys in the Icelandic crust.
... (1), we get (e.g. Waxman & Smits 1968;Revil et al. 1996;Revil 2013a,b), ...
... Surface conductivity σ S (in S m −1 ) versus cation exchange capacity (in C kg −1 ). We compare here the results from sedimentary rocks and volcanic rocks The volcanic rocks include those from this study (with a corrected tortuosity F φ of 3.0 for all the samples), the volcaniclastic materials from Revil et al. (2002) (with the exception of samples BU 96-8A and BU 96-8B, which are less porous than the other samples, see Table 4) and the oceanic dike samples studied by Revil et al. (1996) (normalized by the tortuosity). The data from the literature are from Bolève et al. (2007, glass beads, NaCl), Vinegar & Waxman (1984, shaly sands, NaCl), Churcher et al. (1991) (CEC for the Berea sandstone), Lorne et al. (1999, Fontainebleau sand KCl), Kurniawan (2005, clean Table 6. ...
Article
We performed complex conductivity measurements on 28 core samples from the hole drilled for the Humu'ula Groundwater Research Project (Hawai'i Island, HI, USA). The complex conductivity measurements were performed at 4 different pore water conductivities (0.07, 0.5, 1.0 or 2.0, and 10 S m⁻¹ prepared with NaCl) over the frequency range 1 mHz to 45 kHz at 22 ± 1 °C. The in-phase conductivity data are plotted against the pore water conductivity to determine, sample by sample, the intrinsic formation factor and the surface conductivity. The intrinsic formation factor is related to porosity by Archie's law with an average value of the cementation exponent m of 2.45, indicating that only a small fraction of the connected pore space controls the transport properties. Both the surface and quadrature conductivities are found to be linearly related to the cation exchange capacity of the material, which was measured with the cobalt hexamine chloride method. Surface and quadrature conductivities are found to be proportional to each other like for sedimentary siliclastic rocks. A Stern layer polarization model is used to explain these experimental results. Despite the fact that the samples contain some magnetite (up to 5 per cent wt.), we were not able to identify the effect of this mineral on the complex conductivity spectra. These results are very encouraging in showing that galvanometric induced polarization measurements can be used in volcanic areas to separate the bulk from the surface conductivity and therefore to define some alteration attributes. Such a goal cannot be achieved with resistivity alone. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... Several other papers have recently been published on this subject e.g. Revil et al. (1996Revil et al. ( , 1997Revil et al. ( , 2002. ...
... Values between 1 and 2, that frequently are reported, often result from the disregard of the surface conductivity. Similarly the value of 0.44 for the constant k is abnormally low compared to published values from ocean basalts and dikes (Pezard 1990, Revil et al 1996. Figure 10 show clearly how the data from the different alteration zones group together when the data are normalized. ...
Article
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In order to investigate the interface conduction in the basaltic rocks of the high temperature fields in Iceland we measured the electrical conductivity for the frequency range 0.1 – 100000 Hz versus pore fluid salinity of 12 selected samples of basaltic material from Iceland. These included 2 fresh and completely unaltered samples of recent basaltic lava, 5 samples of basalt and hyaloclastite from the smectite alteration zone and 5 from the chlorite zone. About 2-5% reduction in conductivity is observed per decade in frequency. For the unaltered samples a linear relationship is found between the bulk conductivity and the pore fluid conductivity over almost the whole range of salinities, showing that the pore fluid conduction is always dominant and the interface conduction is negligible. The samples from the smectite zone show almost no dependence on the pore fluid salinity but considerable interface conduction as predicted, the value being from 20-300 µS/cm with the isoelectrical point at fluid conductivity in the range of 4000 – 6000 µS/cm. In contrast to previous hypothesis, the samples from the chlorite zone show also significant interface conduction, 4 out of 5 samples show value in the range of 10-30 µS/cm but the isoelectrical point is lower than in the smectite zone, usually at fluid conductivity in the range 1000-3000 µS/cm. Since the temperature dependence of conductivity is at least twice as high for the interface conduction as for the pore fluid conduction, our results imply that interface conduction is the dominant conduction mechanism for most high temperature geothermal fields regardless of their pore fluid salinity. Furthermore, the observed change in conductivity at the top of the chlorite zone is not due to change in dominant conduction mechanism, i.e. from interface conduction to pore fluid conduction, as has previously suggested, but probably rather due to reduced degree of interface conduction in the chlorite zone associated with the lower cation exchange capacity of chlorite compared to smectite. As a consequence of this we present a revised version of the model for the electrical resistivity of the basaltic upper crust in Iceland.
... The second additive term in this last equation corresponds to the surface conductivity σ S . Equation 6 is also very close to the equation proposed by Revil et al. (1996) (their equation 15) for watersaturated doleritic core samples from the oceanic crust. A simple generalization of the above expression to anisotropic media yields the following expression for the in-phase conductivity tensor ...
... Tortuosity in this case can also be related to the aspect ratio of grains, as discussed in Mandelson and Cohen (1982). Other published experimental works have report very high tortuosities; for instance Revil et al. (1996) reported tortuosities in the range of 2-46 for doleritic core samples. Zhang and Scherer (2012, their Table 6) have report electrical tortuosities in the range 2-118 for tight shales. ...
Article
A model was recently introduced to describe the complex electrical conductivity and high-frequency dielectric constant of isotropic clayey porous materials. We generalized that approach to the case of anisotropic and tight hydrocarbon-bearing shales and mudrocks by introducing tensorial versions of formation factor and tortuosity. In-phase and quadrature conductivity tensors have common eigenvectors, but the eigenvectors of the dielectric tensor may be different due to influence of the solid phase at high frequencies. In-phase and quadrature contributions to complex electrical conductivity depend on saturation, salinity, porosity, temperature, and cation exchange capacity (alternatively, specific surface area) of the porous material. Kerogen is likely to have a negligible contribution to the cation exchange capacity of the material because all exchangeable sites in the functional groups of organic matter may have been polymerized during diagenesis. An anisotropic experiment is performed to validate some of the properties described by the proposed model, especially to verify that the electrical anisotropy factor is the same for in-phase and quadrature conductivities. We used two samples from the Bakken formation. Experimental data confirm the validity of the model. Also, the range of values for cation exchange capacity determined when implementing the new model with experimental data agree with the known range of cation exchange capacity for the Bakken shale. Measurements indicate that the bulk-space tortuosity in the direction normal to bedding plane can be higher than 100.
... If the normalized chargeability would be determined over 6 orders of magnitude, we would expect to have α = 8.8. Revil et al. (1996) and Revil et al. (2002). The overall trend confirms the linear dependence (r 2 = 0.94 in a log-log space) between the surface conductivity and the CEC for high porosity core samples. ...
Article
Estimates of soil properties such as Cation Exchange Capacity (CEC), water content, grain size characteristics, and permeability are important in geotechnical engineering, water resources, and agriculture. We develop a non-intrusive approach to estimate these properties in the field using spectral induced polarization (SIP) tomography. This geophysical method provides information about the frequency dependence of the complex electrical conductivity of porous media. Using 18 soil samples collected from a Bordeaux vineyard, we first conducted a laboratory study using SIP over the frequency range 10 mHz-45 kHz. The laboratory data were used to confirm the accuracy of a recently developed dynamic Stern layer petrophysical model. The results are consistent with published values from previous works using soils. A comparison was made by comparing the field complex conductivity spectra and the experimental data at two locations where core samples were obtained. The model was then used in concert with field data to image the spatial distribution of CEC, water content, permeability, and mean grain size along a vineyard transect. For clay and sandy textures found in the field, measured and estimated CEC agree rather well (from 6 to 40% discrepancy). Our approach provides an efficient way to estimate important soil properties in a non-invasive manner, in high resolution, and over field-relevant scales of the critical zone of the Earth.
... Fluids are key features of conventional and supercritical geothermal systems (Tsuchiya et al., 2016). Magnetotelluric (MT) methods with resistivity imaging are useful geophysical tools to explore subsurface fluids (Wannamaker et al., 2009) because rocks with fluids show lower resistivity than the surrounding dry rock (Ogawa et al., 2014;Revil et al., 1996;Shimojuku et al., 2014). The MT method can image shallow conductors of fluids (< 2 km depth) in geothermal areas (Amatyakul et al., 2016;Asaue et al., 2006;Erdogan and Candansayar, 2017;Maithya and Fujimitsu, 2019;Newman et al., 2008;Patro, 2017;Rosenkjaer et al., 2015;Uchida and Sasaki, 2006;Zhang et al., 2015) and volcanic areas (Aizawa et al., 2005;Hata et al., 2015;Piña-Varas et al., 2018;Tseng et al., 2020). ...
Article
Fluids trapped under supercritical conditions are potential geothermal resources yielding high well-productivity. These supercritical fluids may be imaged as deep conductors (> 2km depth) using the magnetotelluric (MT) method. However, MT imaging of a deep conductor is strongly dependent on the conductor geometries and the overlying clay layers. The ability of the MT method to image a deep conductor needs clarification to understand the usefulness of MT for exploring supercritical fluids and accurately relate the imaged conductor to supercritical fluids. In this study, we investigated MT imaging of a deep conductor of supercritical fluids using numerical tests, based on a conceptual resistivity model for a supercritical geothermal system in the Kakkonda area, northeast Japan. The test result showed that the MT method was able to image the deep conductor of supercritical fluids. However, we found that the shape and resistivity value of the imaged deep conductor might significantly differ from those of the true conductor. Specifically, if a deep vertically elongated conductor is imaged by MT inversion, it is necessary to be aware that the actual bottom part may be much wider in the horizontal directions than the imaged size.
... These measurements are used to determine the cementation exponent m of Archie's law and the dependence of the surface conductivity, the normalized chargeability, and the quadrature conductivity versus the cation exchange capacity. The measurements are all consistent with the Revil et al. (1996) and Revil et al. (2002). Fig. 11. ...
Article
Searching for recyclable materials of construction, in the objective of building sobriety and resilience, is a major issue of our current societies. Mudbricks of compacted rammed earth represent an ancient construction material with many advantages associated with its availability, cost of production, potential reuse, and with a very low carbon footprint. Moisture content affects the mechanical resistance of such materials, which could become mechanically weak above a critical value. Therefore, non-intrusive characterization techniques able to image the water content distribution of these materials is highly in demand. We apply a recently developed theory of complex electrical conductivity (alias induced polarization) to characterize core samples of rammed earth materials in the laboratory. Complex conductivity describes both the ability of a porous material to conduct an electrical current (characterized by the in-phase conductivity) and its ability to store reversibly electrical charges (characterized by two interconnected properties namely the quadrature conductivity and the normalized chargeability). Samples of rammed earth and clayey soils with different pore water salinities, saturations, and compaction states are measured with the complex conductivity method in the frequency range 100 mHz–45 kHz. The in-phase and quadrature conductivities of the complex conductivity of rammed earth are connected to the water content offering therefore a new non-intrusive tomographic technique to study the water content distribution in walls made of rammed earth. The data are all consistent with the so-called dynamic Stern layer model of complex conductivity for clayey materials. This new approach provides a general method to image the change in the water content of walls made of rammed earth, a task that electrical conductivity imaging cannot perform as a stand-alone technique.
... We compare here the results from sedimentary rocks and volcanic rocks. The volcanic rocks include those from , the volcaniclastic materials from Revil et al. (2002), and the oceanic dike samples studied by Revil et al. (1996) (normalized by the tortuosity). The data from the literature are from Bolève et al. (2007, glass beads, NaCl), Vinegar and Waxman (1984, shaly sands, NaCl), Churcher et al. (1991) Fig. 3. Relationship between the surface conductivity and the specific surface area taken as a proxy for the alteration of the volcanic rocks (expressed here in m 2 g −1 and measured with the BET method, see Brunauer et al., 1938) for volcanic and sedimentary rocks (no carbonates). ...
Article
Electrical conductivity tomography is a well-established galvanometric method for imaging the subsurface electrical conductivity distribution. We characterize the conductivity distribution of a set of volcanic structures that are different in terms of activity and morphology. For that purpose, we developed a large-scale inversion code named ECT-3D aimed at handling complex topographical effects like those encountered in volcanic areas. In addition, ECT-3D offers the possibility of using as input data the two components of the electrical field recorded at independent stations. Without prior information, a Gauss-Newton method with roughness constraints is used to solve the inverse problem. The roughening operator used to impose constraints is computed on unstructured tetrahedral elements to map complex geometries. We first benchmark ECT-3D on two synthetic tests. A first test using the topography of Mt. St Helens volcano (Washington, USA) demonstrates that we can successfully reconstruct the electrical conductivity field of an edifice marked by a strong topography and strong variations in the resistivity distribution. A second case study is used to demonstrate the versatility of the code in using the two components of the electrical field recorded on independent stations along the ground surface. Then, we apply our code to real data sets recorded at (i) a thermally active area of Yellowstone caldera (Wyoming, USA), (ii) a monogenetic dome on Furnas volcano (the Azores, Portugal), and (iii) the upper portion of the caldera of Kīlauea (Hawai'i, USA). The tomographies reveal some of the major structures of these volcanoes as well as identifying alteration associated with high surface conductivities. We also review the petrophysics underlying the interpretation of the electrical conductivity of fresh and altered volcanic rocks and molten rocks to show that electrical conductivity tomography cannot be used as a stand-alone technique due to the non-uniqueness in interpreting electrical conductivity tomograms. That said, new experimental data provide evidence regarding the strong role of alteration in the vicinity of preferential fluid flow paths including magmatic conduits and hydrothermal vents.
... Once saturated, the samples were immersed in their electrolyte for several weeks prior to acquiring any set of measurements at a given salinity to be certain that chemical equilibrium was reached. The electrical conductivity measurements were performed using a two-electrode system with a 1260 Solartron impedance meter at a frequency of 4 kHz to avoid contamination by the polarization of the electrodes (see Revil et al., 1996). The instrumental error (related to the voltage measurement) for the measured resistivities was 2%. Figure 3a shows the conductivity data for four samples characterized by distinct porosities, and Figure 3b shows the same trend for Fontainebleau sandstone Core F3 from the database of Börner (1992). ...
Article
The electrical conductivity of clay-free sandstones is customarily assumed to have negligible surface conductivity contribution. The Fontainebleau sandstone, a clean sandstone with relatively coarse (~250 μm) and well-rounded silica grains and silica cement, exhibits surface conductivity along the electrical double layer coating the surface of the grains. A recently developed volume-averaging model for the electrical conductivity was used to determine intrinsic formation factor and surface conductivity from electrical conductivity measurements performed at seven salinities with NaCl solutions. The bulk tortuosity of the pore space influenced the surface conductivity in a predictable way. Formation factor and permeability can be determined as a function of the porosity using the equations developed by Archie for the formation factor, and Revil and Cathles for the permeability. In both the cases, the data emphasize the existence of a percolation threshold of about 0.02 (2%) in porosity. Once corrected for the effect of this percolation threshold, the porosity exponent of Archie's equation was approximately equal to 1.5 as predicted from the differential effective medium theory for a pack of spherical grains suspended in an electrolyte. We illustrated that permeability can be predicted, within one order of magnitude, from surface conductivity, porosity, and formation factor. Spectral-induced polarization data indicated that the in-phase conductivity was nearly frequencyindependent (in the frequency range from 1 to 10 kHz) whereas the quadrature conductivity displayed a relationship between the surface conductivity and a peak frequency likely related to the pore throat size determined from mercury porosimetry measurements. © The Authors. Published by the Society of Exploration Geophysicists.
... Furthermore, model surface conductance is derived from measurements in NaCl solutions, and the influence of having a higher divalent cation content (40% of the total charge) in the boundary layers and/or interlayers is not known. An alternative explanation is to assume a high tortuosity for the network of conductive surfaces (Revil et al., 1996) ...
Article
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Measurements of grain densities in a shore-based laboratory show that the high dispersion of measurements obtained on board the JOIDES Resolution is probably related to insufficiently precise volumetric measurements, but that the effect of this imprecision on porosity determinations is generally less than 5%. A correlation is found between the total water content of the samples and their cation exchange capacity (CEC). This correlation confirms that smectites strongly influence the ability of th e sediment to retain both adsorbed and pore water. Chemical analysis show that (1) interlayer cations are dominantly Na, (2) divalent cations (Mg, Ca) take up to 30%-40% of the surface charge, and (3) to more than of the water present in the sample is chloride free. This chloride-free water corresponds to smectite interlayer water and to water adsorbed on external su r- faces. Electrical resistivity logs as well as measurements on samples indicate a sharp decrease in resistivity in the transitio n from lithologic Units II to III. Lithologies with the highest smectite content (and highest CEC) have the lowest electrical con - ductivities at a given porosity. This result may in part be explained by water and ion adsorption in the smectite interlayer sp aces but also leads to an unsolved question: Do 0.6-nm smectite interlayer spaces have the same conductance as the external sur- faces of the particles? 1 5 ∕ 13 ∕
... The results of Flóvenz et al. (1985) for core samples from Icelandic geothermal wells, and of Pezard (1990) for basaltic cores from the Ocean Drilling Project support this conclusion. A thorough theoretical and experimental study on this subject has been undertaken by Revil et al. (1996 Revil et al. ( , 2002) and Revil and Glover (1997). Pore fluid conductivity, σ f , at temperatures below 150 °C can be described by the linear model of: ...
Article
Measurements of electrical conductivity and P-wave velocity of seven rock samples were made in the laboratory under inferred in situ conditions. The samples were collected from smectite and chlorite alteration zones in boreholes from the Krafla and Hengill, Iceland, geothermal areas. The measurements were done in the 25-250 °C range, with pore pressure and confining pressure equal to inferred in situ hydrostatic and lithostatic pressures, respectively. Conductivity increases linearly with temperature over the 30-170 °C range; that rise is considerably smaller above 170 °C. Time-dependent effects on conductivity occur above approximately 100 °C. These effects may be related to ion exchange between the clay minerals or the Stern layer, and the pore fluid. The temperature coefficient of conductivity is found to be considerably higher than attributed to pore fluid conduction alone, indicating interface conduction in an electrical double layer on the mineral-water interface in the pores. The results also show that there is no distinction in electrical conduction mechanism in the smectite and chlorite alteration zones; both are dominated by interface conductivity under in situ conditions. The sharp decrease in conductivity at the top of the chlorite alteration zone, commonly observed in resistivity surveys in high-temperature geothermal systems, is most likely due to the lower cation exchange capacity of chlorite compared to that of smectite.
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Over the past 30 years, the boundary between seismic layers 2 and 3 in modern oceanic crust has been successively explained by changes in porosity, fracturing, lithology, and metamorphic grade. This transition was most recently proposed to lie within the sheeted-dike complex of a deep oceanic borehole (Hole 504B of the Deep Sea Drilling Project and the Ocean Drilling Program). Both models that promote this interpretation rule out the lithologic hypothesis, which suggests that the boundary between seismic layers 2 and 3 corresponds to the downward transition from sheeted dikes to gabbro. Downhole measurements in the same hole reveal a gradual change in stress regime within the sheeted-dike complex (1.3 to 1.7 km into basement), confirming the previous findings and allowing the proposal of a common cause to explain some of the earlier hypotheses. From compressional above to strike slip below the seismic transition, this stress change is proposed to induce a depth-dependent reopening of cracks and fractures. With a nearly constant porosity throughout the sheeted-dike complex, this variable reopening of cracks can have a significant impact on acoustic and hydraulic properties of the crust and hence on the mode and nature of fluid circulation and crustal alteration. Horizontal fluid movement and heat transfer are favored in the upper part of the crust under a compressional regime, whereas vertical mining of hot fluids and associated fluid-rock interactions are enhanced at the base of the dolerites under a strike-slip regime. As the crust ages and subsides, the transition between these two regimes (where the vertical load equals the minimum horizontal stress) migrates upward, which provides an explanation for the upward migration of the alteration front in dikes with time. More generally, this result indicates that seismic methods might allow one to map subsurface changes of the stress field with depth in the upper oceanic crust.
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We used a database derived from the integration of core material and geophysical downhole measurements in order to investigate the relationships between fracturing and alteration in the volcanic section of DSDP/ODP Hole 504B. The studied crustal section (from top of the basement to 1000 mbsf (metres below sea floor)) consists of low resistivity/high porosity pillow lavas associated with breccias and rubble material, alternating with high resistivity/low porosity massive basalt flows. A positive correlation between DLL (Dual Laterolog)-derived porosity and occurrence of breccias in the core suggests that breccias more than fractures contribute to the electrical resistivity signal. A structural analysis performed from core suggests that most fractures and veins are steeply dipping, and may represent tectonic features or cracks due to contractional cooling of the crust, the latter being more abundant in pillows. Fractures and veins recorded on core tend to be clustered in massive units or thin flows. This result may derive from criteria adopted during structural measurements and must be taken with care. The natural radioactivity (GR) profile delineates two main alteration zones in the volcanic section: an oxidizing zone with increased potassium above, and a reducing one without K gain below. Most of the GR maxima are found to be correlated with celadonite-bearing alteration halos. GR minima are frequently located at the boundaries between domains of contrasting fracture orientation, where metasomatic reactions may have occurred due to contrasting permeability.
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The electrical resistivity, porosity, and cation exchange capacity (CEC) of mid-ocean ridge basalt (MORB) samples from Deep Sea Drilling Project hole 504B have been measured in the laboratory. During Leg 111 of the Ocean Drilling Program, the Joides Resolution D.V. returned in the equatorial Pacific to deepen hole 504B and to perform a series of downhole experiments. A continuous electrical resistivity profile permitted to discriminate the large-scale seismic layers of the upper oceanic crust and to isolate individual lithologic units. In the extensive part of the crust, the massive flows (10-m thick or more) are found to constitute permeability barriers and, subsequently, to constrain fluid circulation. The relationship between morpohology, hydrological regime, and therefore alteration of the basaltic basement is proposed to be related to the accretion process of the upper oceanic crust. -from Author
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The first attempts to sample formation waters from basaltic basement in the oceanic crust were made at Sites 501, 504, and 505 of the Deep Sea Drilling Project. Two methods were used. In the first, the water that occupied the hole was sampled some time after pumping had stopped. In the second, water from both the hole and the surrounding rocks was sampled by sealing off a 3-meter section at the bottom of the hole and opening a large-volume sampler, thereby creating negative pressure. Neither method produced a sample that contained an appreciable and unambiguous component of true formation water, although the samples generally showed large compositional differences from seawater. Samples from Holes 501 and 5O5B were mainly mixtures of the surface seawater used as drilling fluid and pore water from sedi- ment that fell down the hole. Samples from Hole 504B contained a large fraction of seawater that displayed large chemical changes due to reaction with basement basalts. Tritium analyses revealed the samples to be surface seawater that had been pumped into the formation a few days earlier and had reacted rapidly with basalt at the in situ temper- ature of 80°C. The solutions had gained Ca and lost Mg on a mole-for-mole basis, lost K and possibly SO4, and gained Si. The Si/Ca ratio increased with depth. These results are consistent with those of laboratory experiments in which seawater reacted with basalt at 70 °C and indicate that at 80 °C in situ reaction rates in the crust are sufficiently rapid to produce large changes in solution chemistry almost instantaneously. The absence of an 18O shift in the solutions in- dicates that the amount of rock that reacted with the solution during its brief residence in the crust was negligible.
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The resistivity, porosity, and cation exchange capacity of 36 basaltic samples have been measured in the laboratory at room temperature and atmospheric pressure. The presence of chlorites and particularly smectites as alteration products of basalt phases is reflected by high values of cation exchange capacity (CEC). The massive units of Layers 2A and 2B are defined by high and uniform CEC values, the more fractured and altered pillows are characterized in the entire hole by even higher values of CEC and a large variability. The lowest CEC values, measured for the massive units of Layer 2C, are due to changes of basalt alteration facies with depth and the related decreasing abundance of smectites with increasing depth in the oceanic crust. The porosity and the apparent formation factor are related by an inverse power law similar to Archie's formula. The presence of microstructures reflects the extensional regime under which the rock formed at the ridge axis, and they are conserved by precipitation of clay minerals attributable to intense hydrothermal circulation. Values are similar to those found for sedimentary rocks and probably reflect an increased tortuosity of the conducting pore space with increasing age. An inverse relationship also relates CEC and apparent formation factor, indicating that surface conduction of clay minerals plays an important role during downhole electrical experiments. This provides a plausible key to the paradox of low permeability and high apparent porosity obtained from comparing the in situ experiments conducted in Hole 504B. -from Authors
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The theory of charge transport, conduction and polarisation mechanisms are briefly presented, followed by a review of the DC resistivity, relative dielectric permittivity and loss tangent for rocks and minerals. The importance of pore fluid conductivity is highlighted with a full summary of the electrical properties of water and aqueous solutions, and appropriate mixing laws for water in rocks. Water-rock chemical interactions may also be important. These effects are illustrated by a discussion of the electrical properties of basalt and sandstone. Extensive tables of results for the room temperature electrical properties of minerals, chemical compounds and rocks are given. The temperature dependence of electrical conductivity for a range of minerals and rocks are presented graphically as Arrhenius diagrams. -P.Denniss
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Two simplified microstructural models that account for permeability and conductivity of low-porosity rocks are compared. Both models result from statistics and percolation theory. The first model assumes that transport results from the connection of 1D objects or pipes; the second model assumes that transport results from the connection of 2D objects or cracks. In both cases, statistical methods permit calculation of permeability k and conductivity , which are dependent on three independent microvariables: average pipe (crack) length, average pipe radius (crack aperture), and average pipe (crack) spacing. The degree of connection is one aspect of percolation theory. Results show that use of the mathematical concept of percolation and use of the rock physics concept of tortuosity are equivalent. Percolation is used to discuss k and near the threshold where these parameters vanish. Relations between bulk parameters (permeability, conductivity, porosity) are calculated and discussed in terms of microvariables.
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Deformational properties, P-wave and S-wave velocities, and electrical resistivity were measured for three North Sea Malm shales in the laboratory under pressures to 800 bars and temperatures to 100 degree C. These data were used to evaluate how factors such as mineralogy, microstructure, compaction, and pore-fluid conductivity affect a shale's seismic and electrical responses. Deformation in the shales is dominated by inelastic processes which cause time-dependent changes in velocity, resistivity, and pore pressure. Overall, shales are less sensitive to pressure changes as compared to sandstones of similar porosity. However, changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands. P-wave and S-wave velocities may decrease by as much as 10 percent over the temperature range studied, and calculated activation energies for surface conduction are nearly twice those observed in shaly sands.
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A new viscosity‐temperature relationship for liquids has been derived, and its application has been studied. The derivation is based upon the theory of viscosity by Eyring et al., rectifying the ``activation energy'' term. The ``activation energy'' term has been shown to be equivalent to the free energy of formation of a surface and the error committed by previous workers regarding the evaluation of ``activation energy'' has been discussed. The proposed equation is free from any arbitrary or empirical terms, and since, in particular, it does not involve any quantities derived from viscosity measurements, it makes a direct calculation of viscosity possible.
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Black shale containing about 5 per cent organic matter has very low electrical resistivity in comparison to shale which contains no organic matter. The low resistivity of the black shale is attributed to carbon, produced by pyrolysis reactions associated with diagensis, located at grain boundaries in the black shale. Some conductivity anomalies in the Earth's crust and upper mantle may be caused by carbon produced in this manner.
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A simple physical model was used to develop an equation that relates the electrical conductivity of a water-saturated shaly sand to the water conductivity and the cation-exchange capacity per unit pore volume of the rock. This equation fits both the experimental data of Hill and Milburn and data obtained recently on selected shaly sands with a wide range of cation-exchange capacities. This model was extended to cases where both oil and water are present in the shaly sand. This results in an additional expression, relating the resistivity ratio to water saturation, water conductivity and cation-exchange capacity per unit pore volume. The effect of shale content on the resistivity index-water saturation function is demonstrated by several numerical examples.
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In boreholes, temperatures vary and to extract hydrocarbon saturation from conductivity measurements, the influence of temperature on water and rock conductivities must be accounted for. The mobility (mu(DL)) of the counter-ions due to clays and the electrical conductivity of pore-filling brine show large changes with variation in temperature, whereas the microgeometry of the pore space exhibits negligible change. Using this idea, the temperature dependence of mu(DL) is extracted using data on dc electrical conductivity of shaly sands (sigma) containing varying amounts of clay. The mobility of Na+ counter-ions is found to vary approximately linearly with temperature. This explicit relationship is tested by comparing the predicted temperature dependence against the measured temperature dependence of conductivity of a set of rocks with high and low clay content. While the rock conductivity shows a large temperature dependence, the resistivity index is less sensitive to temperature. An approximate formula, which is superior to Arps's formula, for water conductivity as a function of temperature is obtained.
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Thesis--University of Southern California. Includes bibliographical references (leaves 110-117). Order no.: 2693A.
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For an arbitrary geometry of insulating, but charged, objects immersed in an electrolyte for which diffusion currents are important, the mathematical problem of the dc electrical conductivity can be mapped onto that of an ordinary conduction problem without diffusion currents but with a conductive surface layer. As a result, using variational arguments we can prove two general theorems which hold irrespective of the geometry of the porous medium: (a) At high salinities, so that the conductivity of the pore fluid, σf, is large, the conductivity of the system as a whole, σeff, is a linear function of σf, with a slope of 1/F and with an offset proportional to 1/Λ. (b) For lower values of salinity, σeff as a function of σf is convex-up as long as the conductivity within the double-layer region is independent of the salinity of the pore fluid. The parameters F and Λ introduced previously [D. L. Johnson, J. Koplik, and L. M. Schwartz, Phys. Rev. Lett. 57, 2564 (1986); D. L. Johnson, J. Koplik, and R. Dashen, J. Fluid Mech. 176, 379 (1987)] are hereby shown to be relevant to the electrolyte problem. An illustration of an ordered suspension is given to show how to implement these ideas.
Electrical resistivity of basalts, Leg 34
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Relationship of resistivity, velocity, and porosity for basalts from downhole well-logging measurements in Hole 418A
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College Station, TX (Ocean Drilling Program
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