Porous Tuff Research Articles

Mechanical and poroelastic behavior of porous tuff under drained and undrained conditions

A series of drained and undrained water-saturated constant mean-stress tests were performed to investigate the strength, elasticity, and poroelastic response of a water-saturated high porosity nonwelded tuff. Drained strengths are found to increase with increasing effective confining pressures. Elastic moduli increase with increasing mean stress. Undrained strengths are small due to development of high pore pressures that generate low effective confining pressures. Skempton’s values are pressure dependent and appear to reflect the onset of inelastic deformation. Permeabilities decrease after deformation from ∼ 10–14 to ∼ 10–16 m2 and are a function of the applied confining pressure. Deformation is dominated by pore collapse, compaction, and intense microfracturing, with the undrained tests favoring microfracture-dominant deformation and the drained tests favoring compaction-dominant deformation. These property determinations and observations are used to develop/parameterize physics-based models for underground explosives testing.

Compaction and Permeability Evolution of Tuffs From Krafla Volcano (Iceland)

AbstractPressure and stress perturbations associated with volcanic activity and geothermal production can modify the porosity and permeability of volcanic rock, influencing hydrothermal convection, the distribution of pore fluids and pressures, and the ease of magma outgassing. However, porosity and permeability data for volcanic rock as a function of pressure and stress are rare. We focus here on three porous tuffs from Krafla volcano (Iceland). Triaxial deformation experiments showed that, despite their very similar porosities, the mechanical behavior of the three tuffs differs. Tuffs with a greater abundance of phyllosilicates and zeolites require lower stresses for inelastic behavior. Under hydrostatic conditions, porosity and permeability decrease as a function of increasing effective pressure, with larger decreases measured at pressures above that required for cataclastic pore collapse. During differential loading in the ductile regime, permeability evolution depends on initial microstructure, particularly the initial void space tortuosity. Cataclastic pore collapse can disrupt the low‐tortuosity porosity structure of high‐permeability tuffs, reducing permeability, but does not particularly influence the already tortuous porosity structure of low‐permeability tuffs, for which permeability can even increase. Increases in permeability during compaction, not observed for other porous rocks, are interpreted as a result of a decrease in void space tortuosity as microcracks surrounding collapsed pores connect adjacent pores. Our data underscore the importance of initial microstructure on permeability evolution in volcanic rock. Our data can be used to better understand and model fluid flow at geothermal reservoirs and volcanoes, important to optimize geothermal exploitation and understand and mitigate volcanic hazards.

Hydromechanical characterization of gas transport amidst uncertainty for underground nuclear explosion detection

Given the challenge of definitively discriminating between chemical and nuclear explosions using seismic methods alone, surface detection of signature noble gas radioisotopes is considered a positive identification of underground nuclear explosions (UNEs). However, the migration of signature radionuclide gases between the nuclear cavity and surface is not well understood because complex processes are involved, including the generation of complex fracture networks, reactivation of natural fractures and faults, and thermo-hydro-mechanical-chemical (THMC) coupling of radionuclide gas transport in the subsurface. In this study, we provide an experimental investigation of hydro-mechanical (HM) coupling among gas flow, stress states, rock deformation, and rock damage using a unique multi-physics triaxial direct shear rock testing system. The testing system also features redundant gas pressure and flow rate measurements, well suited for parameter uncertainty quantification. Using porous tuff and tight granite samples that are relevant to historic UNE tests, we measured the Biot effective stress coefficient, rock matrix gas permeability, and fracture gas permeability at a range of pore pressure and stress conditions. The Biot effective stress coefficient varies from 0.69 to 1 for the tuff, whose porosity averages 35.3% ± 0.7%, while this coefficient varies from 0.51 to 0.78 for the tight granite (porosity <1%, perhaps an underestimate). Matrix gas permeability is strongly correlated to effective stress for the granite, but not for the porous tuff. Our experiments reveal the following key engineering implications on transport of radionuclide gases post a UNE event: (1) The porous tuff shows apparent fracture dilation or compression upon stress changes, which does not necessarily change the gas permeability; (2) The granite fracture permeability shows strong stress sensitivity and is positively related to shear displacement; and (3) Hydromechanical coupling among stress states, rock damage, and gas flow appears to be stronger in tight granite than in porous tuff.

The Role of Tuffs in Sealing Volcanic Conduits

AbstractAcid‐sulphate alteration commonly changes the physicochemical properties of volcanic conduits. Although several conduit pressurization models suggest hydrothermal sealing, little experimental evidence exists from conduit‐filling rocks on the development of such a seal. Here we show that acid‐sulphate alteration affects conduit‐filling lavas and tuffs differently, with implications for their role in sealing the conduit. In lavas, alteration creates fluid pathways and decreases rock stiffness by dissolving primary minerals. In contrast, in the inherently porous tuffs, alteration reduces fluid pathways and increases rock stiffness by precipitating secondary minerals. Compaction of tuffs under subsurface pressures together with such alteration‐related sealing can form low porosity and low permeability zones within the conduit. Such zones could promote fluid‐pressure build‐up and predispose the volcano to explosive eruptions. We discuss our results in the context of observed seismicity and ground deformation and suggest using our elastic properties data to constrain geophysical inversions of acid‐sulphate altered volcanic conduits.

Frictional stability of porous tuff breccia under subsurface pressure conditions and implications for shallow seismicity

Thick sediment layers frequently cover the Japan islands’ surface, and their frictional properties significantly affect the shallow slip behavior that occurs during earthquakes. However, laboratory data on the properties of the shallow zone remain limited. We collected tuff breccia samples from deep borehole cores in the Miocene “Green Tuff” formation, a major surface cover, and performed velocity-stepping friction tests on these samples under in situ stresses of 2 to 20 MPa to assess the velocity dependence of their frictional strength. The samples exhibit predominantly frictionally stable, velocity-strengthening behavior over the range of normal stresses tested, which supports the hypothesis that shallow sediment layers are seismically quiescent. This result is consistent with the low seismicity and attenuation of coseismic slip occurring in the shallow zone that is observed during regional earthquakes. Microstructural observations of the postmortem samples using optical and scanning electron microscopes indicate a fabric transition from boundary shear localization to distributed cataclastic flow with increasing normal stress. Our laboratory investigation of the depth-variable distribution of the frictional velocity dependence of a shallow sediment layer would provide further insight into the mechanical role for earthquake rupture dynamics and shallow seismicity.

Observations on the structure of Surtsey

Comparison of investigations of the 1979 and 2017 cored boreholes coupled with continued observations of the dynamic surface of Surtsey has modified our concepts of the subsurface structure of the volcano. A geometrical analysis of the 2017 vertical and inclined cores indicates that near-surface layering dips westerly, indicating that the boreholes are located inside the Surtur crater. In subaerial deposits, as well as in deep deposits below sea level and below the pre-Surtsey seafloor, there are zones of porous tuff that contain abundant pyroclasts with narrow rims of fine ash. These features, typical of near-surface deposits, could have been carried down the vent by downslumping during fluctuating explosive activity. They support the hypothesis that a broad diatreme underlies the Surtur vent. No major intrusions were encountered in the 2017 drilling except for coherent basalt in deep sub-seafloor deposits below the center of Surtur crater. The 2017 borehole temperature measurements indicate that the peak temperature in the vertical boreholes was 124 °C at 105 meters below the surface (m.b.s.) and that in the inclined hole it was 127 °C at 115 m.b.s. immediately after drilling. These peak temperatures are 72 meters apart horizontally yet closely resemble each other in shape and magnitude, suggesting a broad heat source. In addition, measurements in the inclined hole from 200 to 290 m.b.s. indicate a temperature of 60±2 °C. This is apparently residual heat from the volcanic action that created the diatreme. These facts cast doubt on the previous concept that the heat anomaly in the 1979 borehole was due to a nearby intrusion. Instead they suggest that heat would have been conducted down from the 85-meter-thick hot lava shield within the Surtur crater into a warm diatreme substrate containing original volcanic heat. As the conducted heat moved down into the water-saturated substrate it would have elevated the temperature above the boiling point curve, baked out water, and created a vapor-dominated system below sea level. Eventually loss of heat by boiling and rise of steam caused the vapor-dominated system to retreat upward. The resulting steam rose and warmed the tephra adjacent to the lava shields where it produced broad areas of palagonitized tuff.

Multifunctional use of magnetite-coated tuff grains in water treatment: Removal of arsenates and phosphates

Natural filtration material tuff (T) was modified by coating with nano-sized magnetite. The grain fraction of 0.6–1.9 mm was submitted to hydrothermal synthesis of magnetite. Thus formed magnetite modified tuff (MMT) was characterized in terms of Fe-content, N2 adsorption- desorption isotherm, SEM, zeta potential-pH analyses and adsorption behavior towards phosphates/arsenates in batch and column conditions. Elemental analysis showed that 36.54 mg g−1 of magnetite was attached to the porous tuff grains. This modification changed pore structure and specific surface area. An increase of cca 35% in Sp value was obtained. Batch experiments proved that MMT was 4-5 times more efficient in removal of phosphates/arsenates than non-modified T. The maximum sorption capacities of phosphates calculated based on Langmuir equation were 0.45 and 1.91 mg g−1, while those for arsenate were 0.551 m g−1 and 2.36 mg g−1 for T and MMT, respectively.The intra-particle diffusion model was the most suited for describing the adsorption process of phosphate and arsenate onto MMT.Fixed-bed column data corroborated batch results, i.e. MMT was 6 times superior in contaminant adsorption than T. Modification with magnetite improved T potential for usage in water treatment applications: its filtration ability remained unchanged, while adsorption capacity for phosphates/arsenates removal was improved.

Tangent modulus method – An original method to measure in-situ rock stress

This paper proposed Tangent Modulus Method (TMM) which is an improved oriented core method to determine in-situ rock stresses. In this approach, the cylindrical specimens prepared along different directions from thick core samples were uniaxially compressed twice to a given stress level. The stress value of the bending point in the first loading cycle of the stress-tangent modulus curve is considered as the normal component of the in-situ rock stress along the drilled direction of the specimen. Four types of rocks from soft porous tuff and sandstone to hard crystalline granite was investigated to evaluate the potential of this method. The effects of changes in strain rate, temperature, water content, confining and pore pressure, and stresses larger than the preload on the stress value of the bending point were experimentally investigated on preload specimens to investigate their influence on TMM. Comparison of the stress measurement results by TMM and an overcoring method at AK tunnel in Hokkaido, Japan was also performed to validate the TMM.

Nanocomposite filter made from porous mineral tuff with absorbed silver nanoparticles and its application for disinfection of water

Nanocomposite filters were developed by sorption of silver (Ag) nanoparticles into a matrix of household filters consisting of porous mineral tuff and their subsequent chemical deposition. The antimicrobial effects of the nanocomposite filter with Ag nanoparticles on Escherichia coli and Enterococcus faecalis and on a mixture of soil microorganisms were shown in water. The level of influence of Ag directly depended on its quantity to cause an inhibitory effect on bacteria; for each type of bacteria the amount of Ag causing antibacterial effect varied. Moreover, the efficiency of the filter for large-volume disinfection of water (up to 1,000 L) was determined. It depended on Ag nanoparticles’ quantity and the amount of microbial load. Nanocomposite filters from porous mineral tuff with adsorbed Ag nanoparticles have antibacterial properties and can be recommended for disinfection of a large volume of drinking water.

Subsurface deformation in hypervelocity cratering experiments into high-porosity tuffs

Hypervelocity impact experiments on porous tuff targets were carried out to determine the effect of porosity on deformation mechanisms in the crater's subsurface. Blocks of Weibern Tuff with about 43% porosity were impacted by 2.5 mm and 12.0 mm diameter steel spheres with velocities between 4.8 km s−1 and 5.6 km s−1. The postimpact subsurface damage was quantified with computer tomography as well as with meso- and microscale analyses of the bisected crater subsurface. The intensity and style of deformation in mineral clasts and the tuff matrix were mapped and their decay with subsurface depth was determined. Subsurface deformation styles include pore space compaction, clast rotation, as well as microfracture formation. Evaluation of the deformation indicates near-surface energy coupling at a calculated depth of burial of ~2 projectile diameters (dp), which is in conflict with the crater shape, which displays a deep, central penetration tube. Subsurface damage extends to ~2 dp beneath the crater floor in the experiments with 2.5 mm projectiles and increases to ~3 dp for 12 mm projectiles. Based on overprinting relationships and the geometrical orientation of deformation features, a sequence of subsurface deformation events was derived (1) matrix compaction, (2) intragranular crack formation in clasts, (3) deformation band formation in the compacted matrix, (4) tensile fracturing.

Formation of shatter cones in MEMIN impact experiments

AbstractShatter cones are the only macroscopic feature considered as evidence for shock metamorphism. Their presence is diagnostic for the discovery and verification of impact structures. The occurrence of shatter cones is heterogeneous throughout the crater record and their geometry can diverge from the typical cone shape. The precise formation mechanism of shatter cones is still not resolved. In this study, we aim at better constraining the boundary conditions of shatter cone formation in impact experiments and test a novel approach to qualitatively and quantitatively describe shatter cone geometries by white light interferometry. We recovered several ejected fragments from MEMIN cratering experiments that show slightly curved, striated surfaces and conical geometries with apices of 36°–52°. These fragments fulfilling the morphological criteria of shatter cones were found in experiments with 20–80 cm sized target cubes of sandstone, quartzite and limestone, but not in highly porous tuff. Targets were impacted by aluminum, steel, and iron meteorite projectiles at velocities of 4.6–7.8 km s−1. The projectile sizes ranged from 2.5–12 mm in diameter and produced experimental peak pressures of up to 86 GPa. In experiments with lower impact velocities shatter cones could not be found. A thorough morphometric analysis of the experimentally generated shatter cones was made with 3D white light interferometry scans at micrometer accuracy. SEM analysis of the surfaces of recovered fragments showed vesicular melt films alternating with smoothly polished surfaces. We hypothesize that the vesicular melt films predominantly form at strain releasing steps and suggest that shatter cones are probably mixed mode fractures.

Lithostratigraphic, Hydrothermal, and Tectonic Setting of the Boundary Volcanogenic Massive Sulfide Deposit, Newfoundland Appalachians, Canada: Formation by Subseafloor Replacement in a Cambrian Rifted Arc

The Boundary volcanogenic massive sulfide (VMS) deposit (~ 0.5 Mt @ 3.5% Cu, 4% Zn, 1% Pb, 34.0 g/t Ag) in the Tally Pond Group, central Newfoundland, represents one of the best preserved, subseafloor-replacement style VMS deposits in the northern Appalachian orogen. The deposit is hosted within a Late Cambrian (~510 Ma) volcanic sequence consisting predominantly of rhyolitic flows and associated volcaniclastic rocks. Footwall strata are dominated by rhyolitic lapilli tuff, tuffs, lesser rhyolite flows, and in situ rhyolite breccias. The hanging wall consists of massive, quartz-bearing, flow-banded lobe and breccia facies rhyolite. The deposit occurs at and below the contact between these two units, and comprises pyrite, chalcopyrite, and lesser sphalerite (and other minerals). Massive mineralization contains abundant clasts of the surrounding host rocks, including chlorite-sericite-quartz–altered rhyolite lapilli and ash. Hydrothermally altered rocks consist of variably intense chlorite with lesser sericite and quartz. Chlorite alteration occurs in a discordant geometry, likely representing hydrothermal upflow zones, and chlorite-sericite-quartz occur as blankets that are parallel to the volcanic stratigraphy, likely representing alteration associated with replacement. The hanging-wall rhyolite flows also contain moderate to intense, pervasive quartz and sericite alteration. Both the hanging wall and footwall are characterized by strong Na2O-Sr depletions, K2O-MgO-Fe2O3-Ba enrichments, high alteration index values (e.g., Ba/Sr, chlorite-carbonate-pyrite index (CCPI), alteration index (AI)), and enrichments in base metals and volatile metals (e.g., Cu, Zn, Pb, Hg). The presence of abundant remnant wall-rock and host-rock clasts within the ore, intricate sulfide replacement of laminated porous tuff and sand dikes, replacement fronts in host lithofacies, and intense alteration in both the footwall and hanging wall (where the hanging wall is preserved) are all features consistent with formation of the bulk of the Boundary deposit via subseafloor replacement. The deposit likely formed as a result of cooling of metal-bearing hydrothermal fluids, mixing with ambient seawater and pore water-entrained seawater within the volcanic rocks at a permeability interface between young, unlithified, highly permeable footwall volcaniclastic rocks and relatively impermeable hanging-wall rhyolitic flows. In our model, this permeability boundary was an important feature that promoted subseafloor replacement within the deposit. Immobile element systematics of rhyolitic rocks from the Boundary deposit lack major differences in primary petrochemistry between hanging-wall and footwall strata. All rocks from the deposit area are subalkalic with transitional Zr/Y ratios (2.8–4.5), La/Sm ratios <1 (normalized to upper crust), and primitive mantle normalized signatures with slightly light rare earth element (LREE)-enriched patterns with flat heavy REE (HREE), and negative Nb, Ti, and Eu anomalies. These geochemical features, coupled with existing Nd-Pb isotope data, zircon inheritance patterns, and geologic information, are consistent with rhyolitic rocks at the Boundary deposit having formed by re-melting of arc basement, with continental crust (or recycled continental crust) present in the source region. It is likely that the deposit and its associated rhyolitic host rocks formed within a Cambrian continental(?) or pericontinental rifted arc along the margin of Ganderia, within the Iapetus Ocean.

Silicon isotope variations in Central Siberian rivers during basalt weathering in permafrost-dominated larch forests

This work is devoted to the characterization of natural mechanisms of silicon isotope fractionation within Siberian watersheds and predicting the climate warming effect on Si fluxes from the land to the Arctic Ocean. To unravel the different sources of silica generated by basalt weathering in Central Siberia under permafrost and larch deciduous forest conditions, we measured the Si isotopic composition of large and small rivers, surface flow, interstitial soil solutions, plant litter and soils. The average annual discharge-weighted δ30Si values of the second largest tributary of the Yenissei River, Nyzhnaya Tunguska and its main northern tributary (Kochechum) are equal to 1.08±0.10‰ and 1.67±0.15‰, respectively, while their average annual Si concentrations are very similar (3.46 and 3.50mg/L, respectively). During summer baseflow, the dissolved Si isotope composition of both large rivers and a small stream ranges between 1.5 and 2.5‰. This is much heavier compared to the source basaltic rocks but similar to the fresh litter of Larix gmelinii, the dominating tree species in this region. It could be consistent with litter degradation in the uppermost soil horizons being the dominant source of solutes annually exported by Central Siberian rivers. During spring flood, accounting for 60–80% of annual Si flux, the δ30Si of the large rivers' dissolved load decreases by 1–1.5‰, thus approaching the value of the bedrock and the silicate suspended matter of the rivers (RSM). This may reflect the dissolution of the silicate suspended load at high water/mineral ratio. The winter δ30Si values of the large river dissolved load range between 1.0 and 2.5‰. During this period, contributing to ≤10% of the annual Si chemical flux, the interaction between bedrock (porous tuffs) and deep ground waters occurs at a very high solid/solution ratio, leading to the precipitation of isotopically light secondary minerals and enrichment of 30Si in the fluids that feed the river through the unfrozen flowpaths. Results of this study imply that more than a half of the silica transported by Siberian rivers may transit through the biogenic pool and that, like in other stable basaltic regions, bedrock–water interactions account for a lesser fraction of the silica flux. As a result of projected future climate warming and weathering increases in boreal regions, the δ30Si isotopic composition of large Siberian rivers is likely to shift towards less positive values.

The effect of zeolit rate on the thermo-mechanical properties of concrete

Zeolite (clinoptilolite) is mainly natural volcanic porous tuff and it has strong absorption ability, large specific surface area and lightness. The concrete has been produced using in the rate of 0, 5, 10 and 15% zeolite as an aggregate in the concrete. In the mixing, water to cement ratio was kept at 0.50. The thermo-mechanical properties of the concrete have been measured. It has been found that thermal conductivity has decreased with the zeolite in concrete but compressive strength increased with the increasing zeolite rate in the concrete. Key words: Zeolite, thermo-mechanical properties, lightweight concrete.

Temperature dependence of large-scale water-retention curves: a case study

A local-scale model for temperature-dependence of water-retention curves may be applicable to large scales. Consideration of this temperature dependence is important for modeling unsaturated flow and transport in the subsurface in numerous cases. Although significant progress has been made in understanding and modeling this temperature effect, almost all the previous studies have been limited to small scales (on the order of several centimeters). Numerical experiments were used to investigate the possibility of extending a local-scale model for the temperature-dependence of water retention curves to large scales (on the order of meters). Temperature effects on large-scale hydraulic properties are of interest in many practical applications. Numerical experiment results indicate that the local-scale model can indeed be applicable to large-scale problems for special porous media with high air entry values. A typical porous medium of this kind is the porous tuff matrix in the unsaturated zone of Yucca Mountain, Nevada, the proposed geologic disposal site for national high-level nuclear wastes. Whether this finding can approximately hold for general cases needs to be investigated in future studies.

Estimating liquid fluxes in thermally perturbed fractured rock using measured temperature profiles

A new temperature-profile method was recently developed for analyzing perturbed flow conditions in superheated porous media. The method uses high-resolution temperature data to estimate the magnitude of the heat-driven liquid and gas fluxes that form as a result of boiling, condensation, and recirculation of pore water. In this paper, we evaluate the applicability of this new method to the more complex flow behavior in fractured formations with porous rock matrix. In such formations, with their intrinsic heterogeneity, the porous but low-permeable matrix provides most of the mass and heat storage capacity, and dominates conductive heat transfer, Fractures, on the other hand, offer highly effective conduits for gas and liquid flow, thereby generating significant convective heat transfer. After establishing the accuracy of the temperature-profile method for fractured porous formations, we apply the method in analyzing the perturbed flow conditions in a large-scale underground heater test conducted in unsaturated fractured porous tuff. The flux estimates for this test indicate a significant reflux of water near the heat source, on the order of a few hundred millimeter per year-much larger than the ambient percolation flux of only a few millimeter per year.

The use of tuff stone cladding in buildings for energy conservation

Heating the houses in cold climates requires large quantities of heat energy to be spent. The building sector consumes more energy in the form of heat than other sectors. Therefore, considerable energy savings can be obtained by using natural rocks with low thermal conductivity in insulating the buildings. In this study, the amount of energy conserved by using porous tuff stone in external walls of buildings is calculated. It was shown that considerable energy savings can be achieved by using tuff stone for facing buildings in cold climate zones such as Isparta region. The cost of installing tuff stone panels for facing buildings will be paid back in four years by savings in heat energy.

Characterizing Unsaturated Diffusion in Porous Tuff Gravel

Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high-level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using X-ray computed tomography (CT) in partially saturated gravel and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2- to 4-mm granular tuff at multiple water contents were calculated from electrical conductivity (EC) measurements using the Nernst–Einstein equation. In the second, we used laser ablation with inductively coupled plasma–mass spectrometry (LA/ICP-MS) to perform microscale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube–cube and cube–tetrahedron). The X-ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 × 10−14 m2 s−1 for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.

Characterizing Unsaturated Diffusion in Porous Tuff Gravel

Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high‐level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using X‐ray computed tomography (CT) in partially saturated gravel and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2‐ to 4‐mm granular tuff at multiple water contents were calculated from electrical conductivity (EC) measurements using the Nernst–Einstein equation. In the second, we used laser ablation with inductively coupled plasma–mass spectrometry (LA/ICP‐MS) to perform microscale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube–cube and cube–tetrahedron). The X‐ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 × 10−14 m2 s−1 for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.

Geophysical investigations for the location of a proposed dam in Al Bishriyya (Al Aritayn) area, northeast Badia of Jordan

Vertical electrical sounding and seismic refraction geophysical surveys were conducted on the northeast Badia of Jordan over the location of a proposed dam, as part of a national project to develop the Jordan Badia at the same national level. The objective of the study was twofold: (1) to evaluate the possibility of mapping of volcanic medium in areas where little is known about the subsurface geology and to infer a shallow geological structure from the geophysical data interpretation, (2) to identify formations that may be present, and (3) subsequently estimate the relationship between groundwater resources and geological structures. Geoelectrical data collected at 12 locations and refraction data collected along two profiles were interpreted utilizing both manual and automatic techniques. Vertical electrical sounding data were pieced together along the survey lines to make a geoelectrical cross section. From the surface down, the cross sections reveal a thin layer of alluvial and wadi sediments, alluvial fans, alluvial mudflats and soils of Holocene to Recent age with high resistivity values. A layer of dense basalts of medium resistivity values, 100–400 Ωm, was recognized beneath the upper unit. Porous and fracture volcanic tuff bodies that have high resistivity value signatures were resolved at different depths along the eastern and western flanks of Wadi Tahawi. The seismic refraction data reveal three different units. From the surface down, the upper unit was 20–35 m thick and the velocity was 3,710–3,800 m/s underlain by a 20-m-thick low velocity unit of ~1,560–2,000 m/s and lower dense basalt unit with a velocity ranging from 4,400 to 4,580 m/s.