The influence of dry and water saturated cracks on seismic velocities of crustal rocks - A comparison of experimental data with theoretical model

The pressure dependence of P- and S-wave velocities, velocity anisotropy, shear wave splitting and crack-porosity has been investigated in a number of samples from different crustal rock types for dry and wet (water saturated) conditions. At atmospheric pressure, P-wave velocities of the saturated, low-porosity rocks (< 1%) are significantly higher than in dry rocks, whereas the differences for S-wave velocities are less pronounced. The effect of intercrystalline fluids on seismic properties at increased pressure conditions is particularly reflected by the variation of the Poisson's ratio because P-wave velocities are more sensitive to fluids than S-wave velocities in the low-porosity rocks. Based on the experimental data, the respective crack-density parameter (), which is a measure of the number of flat cracks per volume unit contained within the background medium (crack-free matrix), has been calculated for dry and saturated conditions. There is a good correlation between the calculated crack-densities and crack-porosities derived from the experimentally determined volumetric strain curves. The shear wave velocity data, along with the shear wave polarisation referred to a orthogonal reference system, have been used to derive the spatial orientation of effective oriented cracks within a foliated biotite gneiss. The experimental data are in reasonable agreement with the self consistent model of O'Connell and Budiansky (1974). Taking the various lithologies into account, it is clear from the present study, that combined seismic measurements ofV _p andV _s , using theV _p V _s -ratio, may give evidence for fluids on grain boundaries and, in addition, may provide an estimate on the in-situ crack-densities.     

Upwelling by a wave pump

A rigid open-ended pipe is submerged in the ocean below the troughs of the surface waves and held fixed in the vertical position, the lower end being at or below the depth of wave influence. When surface gravity waves propagate past the pipe, water flows up as long as waves are present. The steady upward vertical velocity in the center of the pipe is calculated to be proportional to the square of both the average wave steepness and the pipe’s radius. An application is to bring nutrient rich waters up into the sunlit surface layers of the open oceans.     

Shoaling waves: A discussion

Surface gravity waves are commonly observed to slow down and to stop at a beach without any noticeable reflection taking place. We assume that as a consequence the waves are continuously giving up their linear and angular momenta, which they carry with them, along with energy, as they propagate into gradually decreasing mean depths of water. It takes a force to cause a time rate of decrease in the linear momentum and a torque to produce a time rate of decrease in the angular momentum. Both a force and a torque operate on the shoaling waves, due to the presence of the sloping bottom, to cause the diminution of their linear and angular momenta. By Newton’s third law, action equals reaction, an equal but opposite force and torque are exerted on the bottom. No other mechanisms for transferring linear and angular momenta are included in the model. Since the force on the waves acts over a horizontal distance during shoaling, work is done on the waves and energy flux is not conserved. Bottom friction, wave interaction with a mean flow, scattering from small-scale bottom irregularities and set-up are neglected. Mass flux is conserved, which leads to a shoreward monotonic decrease in amplitude consistent with available swell data. The formula for the time-independent force on the bottom agrees qualitatively with observations in seven different ways: four for swell attenuation and three for sediment transport on beaches. Ardhuin (2006) argues against a mean force on the bottom that is not hydrostatic, mainly by using conservation of energy flux. He also applies the action balance equation to shoaling waves. Action is a difficult concept to grasp for motion in a continuum; it cannot be easily visualized, and it is not really necessary for solving the shoaling wave problem. We prefer angular momentum because it is clearly related to the observed orbital motion of the fluid particles in progressive surface waves. The physical significance of wave action for surface waves has been described recently by showing that in deep water action is equivalent to the magnitude of the wave’s orbital angular momentum (Kenyon and Sheres, 1996). Finally, Ardhuin requires that there be a significant exchange of linear momentum between shoaling waves and an unspecified mean flow, although the magnitude and direction of the exchange are not predicted. No mention is made of what happens to the orbital angular momentum during shoaling. Mass flux conservation is not stated.     

Calcite texture development in experimentally induced ductile shear zones

Specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear, a combination of simple and pure shear, and in simple shear. Temperatures were 400° C and 500° C, confining pressure was 100 MPa. In the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2–3 mm wide shear zone. Shear displacement is documented by marker lines and circles. Shear strain γ varies between 0.84 and 1.56. Strain is recorded by flattening of individual grains, defining a foliation normal to the axis of principal finite shortening ε ₁. No twinning is observed on a macroscopic scale. X-ray and neutron diffraction techniques were used to characterize texture before and after deformation. All specimens display strong preferred orientation as documented by 0006, 10¯14 and 11¯220 pole-figures, c axes pole-figures display three maxima in the ε₁–ε₃ plane. If the axes of the strain ellipsoids are used as a coordinate system textures in pure and simple shear are similar but there is considerable monoclinic distortion in simple shear which is attributed to the noncoaxial strain path.     

Über die Synthese von Albiten bei Temperaturen von 250 bis 700° C

A glass of composition NaAlSi₃O₈ with small contents of Fe has been crystallized in the presence of water vapor under 1000 bars, at temperatures in the range of 250 to 700° C for varying periods of time. The synthetic crystals were investigated microscopically and by x-ray powder diffraction methods. Most of the crystals are twinned according to the albite law, but other twins are found too. Size and form of the crystals seem to depend on the temperature of crystallization. The optic axial angles (2V _x ) of the albites being measurable on the universal stage are in the range of 50 to 100 degrees. There seems to exist a linear relation between the 2₁₃₁–2_1¯31 values (used as an indicator for the degree of ordering) and the 2V _x values. The change of the lattice parameters from albite (D) to albite (O) is suggested to deviate slightly from linearity.In contrast to previous investigations the steady value of 2₁₃₁–2_1¯31 which is characteristic of the temperature of crystallization was reached more rapidly. These results suggest that small content of Fe in the glass had a catalytic effect on the Al/Si ordering.

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Herrn Dr. F. Seifert (Ruhruniversität Bochum) danken wir herzlich für die Einweisung in die experimentellen Untersuchungsmethoden sowie für manchen kritischen Hinweis. Danken möchten wir auch Herrn Dr. P. K. Hörmann für die Kontrolle der chemischen Zusammensetzung des Albitglases sowie Herrn Dr. C. W. Burnham (Harvard University) für die Bereitstellung des Rechenprogramms. Nicht zuletzt danken wir Herrn Prof. Dr. F. Karl für die kritische Durchsicht des Manuskriptes.     

Machine Learning Based Predictive Modeling of Debris Flow Probability Following Wildfire in the Intermountain Western United States

It has been recognized that wildfire, followed by large precipitation events, triggers both flooding and debris flows in mountainous regions. The ability to predict and mitigate these hazards is crucial in protecting public safety and infrastructure. A need for advanced modeling techniques was highlighted by re-evaluating existing prediction models from the literature. Data from 15 individual burn basins in the intermountain western United States, which contained 388 instances and 26 variables, were obtained from the United States Geological Survey (USGS). After randomly selecting a subset of the data to serve as a validation set, advanced predictive modeling techniques, using machine learning, were implemented using the remaining training data. Tenfold cross-validation was applied to the training data to ensure nearly unbiased error estimation and also to avoid model over-fitting. Linear, nonlinear, and rule-based predictive models including naïve Bayes, mixture discriminant analysis, classification trees, and logistic regression models were developed and tested on the validation dataset. Results for the new non-linear approaches were nearly twice as successful as those for the linear models, previously published in debris flow prediction literature. The new prediction models advance the current state-of-the-art of debris flow prediction and improve the ability to accurately predict debris flow events in wildfire-prone intermountain western United States.     

Methane Occurrences in Aquifers Overlying the Barnett Shale Play with a Focus on Parker County,Texas

Clusters of elevated methane concentrations in aquifers overlying the Barnett Shale play have been the focus of recent national attention as they relate to impacts of hydraulic fracturing. The objective of this study was to assess the spatial extent of high dissolved methane previously observed on the western edge of the play (Parker County) and to evaluate its most likely source. A total of 509 well water samples from 12 counties (14,500 km²) were analyzed for methane, major ions, and carbon isotopes. Most samples were collected from the regional Trinity Aquifer and show only low levels of dissolved methane (85% of 457 unique locations <0.1 mg/L). Methane, when present is primarily thermogenic (δ¹³C 10th and 90th percentiles of ?57.54 and ?39.00‰ and C1/C2+C3 ratio 10th, 50th, and 90th percentiles of 5, 15, and 42). High methane concentrations (>20 mg/L) are limited to a few spatial clusters. The Parker County cluster area includes historical vertical oil and gas wells producing from relatively shallow formations and recent horizontal wells producing from the Barnett Shale (depth of ～1500 m). Lack of correlation with distance to Barnett Shale horizontal wells, with distance to conventional wells, and with well density suggests a natural origin of the dissolved methane. Known commercial very shallow gas accumulations (<200 m in places) and historical instances of water wells reaching gas pockets point to the underlying Strawn Group of Paleozoic age as the main natural source of the dissolved gas.     

Halogen oxide measurements at Masaya Volcano,Nicaragua using active long path differential optical absorption spectroscopy

Active Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) measurements of halogen oxides were conducted at Masaya Volcano, in Nicaragua from April 14 to 26, 2007. The active LP-DOAS system allowed night-time halogen measurements and reduced the ClO detection limit by an order of magnitude when compared to previous passive DOAS measurements, as wavelengths below 300 nm could be used for the DOAS retrievals. BrO was detected with an average BrO/SO₂ molecular ratio of approximately 3 × 10⁻⁵ during the day. However, BrO values were below the detection limit of the instrument for all night-time measurements, a strong indication that BrO is not directly emitted, but rather the result of photochemical formation in the plume itself according to the autocatalytic “bromine explosion” mechanism. Despite the increased sensitivity, both ClO and OClO could not be detected. The achieved upper limits for the X/SO₂ ratios were 5 × 10⁻³ and 7 × 10⁻⁶, respectively. A rough calculation suggests that ClO and OClO should be present at similar abundances in volcanic plumes. Since the DOAS technique is orders of magnitude more sensitive for OClO than for ClO, this indicates that OClO should always be detectable in plumes in which ClO is found. However, further LP-DOAS studies are needed to conclusively clarify the role of chlorine oxides in volcanic plumes.