Three-component ambient noise beamforming in the Parkfield area

ACKNOWLEDGEMENTS Many thanks to Andrew Curtis and Ian Main for their critical and perceptive comments and to Sjoerd de Ridder for fruitful discussions. We are particularly grateful to Elmer Ruigrok for his detailed and constructive review, which has substantially improved our manuscript. We acknowledge the facilities of the IRIS Data Management System for providing access to the seismic data. This work is funded within the DFG project ‘SynPaTh’ and the EU project ‘GEMex’.Peer reviewedPublisher PD

Digital rock physics: numerical prediction of pressure-dependent ultrasonic velocities using micro-CT imaging

Digital rock physics combines modern microscopic imaging with advanced numerical simulations to analyse the physical properties of rocks. Elastic-wave propagation modelling based on the microstructure images is used to estimate the effective elastic properties of the rock. The goal of this paper is to describe and understand how laboratory experiments compare with digital rock physics results using Berea sandstone. We experimentally measure pressure-dependent ultrasonic velocities and the pore size distribution. The effective elastic properties resulting from numerical simulations are based on microcomputed tomography (micro-CT) images, which are systematically stiffer than the laboratory measures. Because the tomographic images do not resolve the small-scale pore and crack network of the sample, we hypothesize that the numerical overprediction is attributable to the smallest pores and grain-to-grain contacts that are missing in the images. To reconcile the difference between numerical and experimental data, we suggest to use a grain boundary reconstruction algorithm. This allows to implement and approximate so far unresolved features in the virtual rock model. As a result, we can predict pressure-dependent effective velocity using micro-CT image

A statistical strategy for ambient seismic wavefield analysis: investigating correlations to a hydrocarbon reservoir

Theoretical work and modelling studies have led to the hypothesis that the ambient seismic wave field on the surface can be affected by hydrocarbon reservoirs (>800 m depth). Several field studies have linked spectral features on the vertical component between 1 and 10Hz to reservoir locations. However, such evidence has been criticized due to concerns that surface recordings typically contain a large amount of surface wave noise and correlations to hydrocarbon targets could be caused by non-hydrocarbon variables such as topography or weathering layer thickness. In this paper, we suggest a two-step analysis strategy to address such issues. First, spectral power is only averaged over time periods and frequencies where the distribution of polarization attributes show no obvious dominance of a few surface wave sources. An interferometric test reveals differences in the wave field composition between the filtered and unfiltered data. Second, the residual seismic power is correlated to hydrocarbon as well as non-hydrocarbon targets. The correlations are quantitatively compared using rank correlation and bootstrap confidence intervals. The method is illustrated on a passive seismic data set acquired with three-component, broad-band seismometers at the tight-gas Jonah field in Wyoming, USA. We find evidence that the wave field was dominated by a small number of surface sources in all of the data except for the quietest time periods in the low-frequency range 1.5-3.0Hz. Seismic power within this subset significantly correlates to a published reservoir map but not with a digital elevation model and less so with an infrastructure density map. The investigated hypothesis can thus not be rejected with this dat

Temperature-dependent poroelastic and viscoelastic effects on microscale—modelling of seismic reflections in heavy oil reservoirs

We develop a new model for elastic properties of rocks saturated with heavy oil. The heavy oil is represented by a viscoelastic material, which at low frequencies and/or high temperatures behaves as a Newtonian fluid, and at high frequencies and/or low temperatures as a nearly elastic solid. The bulk and shear moduli of a porous rock saturated with such viscoelastic material are then computed using approximate extended Gassmann equations of Ciz and Shapiro by replacing the elastic moduli of the pore filling material with complex and frequency-dependent moduli of the viscoelastic pore fill. We test the proposed model by comparing its predictions with numerical simulations based on a direct finite-difference solution of equations of dynamic viscoelasticity. The simulations are performed for the reflection coefficient from an interface between a homogeneous fluid and a porous medium. The numerical tests are performed both for an idealized porous medium consisting of alternating solid and viscoelastic layers, and for a more realistic 3-D geometry of the pore space. Both sets of numerical tests show a good agreement between the predictions of the proposed viscoelastic workflow and numerical simulations for relatively high viscosities where viscoelastic effects are important. The results confirm that application of extended Gassmann equations in conjunction with the complex and frequency-dependent moduli of viscoelastic pore filling material, such as heavy oil, provides a good approximation for the elastic moduli of rocks saturated with such material. By construction, this approximation is exactly consistent with the classical Gassmann's equation for sufficiently low frequencies or high temperature when heavy oil behaves like a fluid. For higher frequencies and/or lower temperatures, the predictions are in good agreement with the direct numerical solution of equations of dynamic viscoelasticity on the microscale. This demonstrates that the proposed methodology provides realistic estimates of elastic properties of heavy oil rock

Fracture Unclogging: A Numerical Study of Seismically Induced Viscous Shear Stresses in Fluid‐Saturated Fractured Rocks

Dynamic shaking imposed by passing seismic waves is able to promote various hydrological processes in fractured reservoirs. This is often associated with seismically‐induced fracture unclogging due to mobilization of deposited colloids in the fracture network which, in turn, affects permeability at the reservoir scale. Numerous laboratory and field studies pointed out that fracture unclogging can be initiated when viscous shear stresses in the fracture fluid are in the range of 0.1‐1 Pascals. In this numerical study, we compute viscous shear stress in a fluid‐saturated fractured medium due to the action of passing P‐ and S‐waves. We perform a sensitivity analysis in terms of fluid, fracture, and host rock physical properties as well as seismic wave characteristics. Our results show that seismically‐induced viscous shearing increases with frequency and seismic strain and can be in the order of those initiating fracture unclogging for typical seismic strains and frequencies. S‐waves tend to produce viscous shearing approximately two times larger than P‐waves and, for anisotropic distribution of fractures, it is extremely dependent on the direction of wave propagation. Moreover, larger viscous shearing is expected for more viscous fluids and stiffer host rocks. Regarding the fracture network distribution, for the same fracture density, the presence of longer fractures drastically increases the potential of fracture unclogging at seismic frequencies. The fracture aperture distribution, on the other hand, can also affect the development of viscous shearing. Fractures with correlated distributions of contact areas exhibit an order of magnitude larger viscous shearing than uncorrelated ones

Low-Frequency Elastic Properties of a Polymineralic Carbonate: Laboratory Measurement and Digital Rock Physics

We demonstrate that the static elastic properties of a carbonate sample, comprised of dolomite and calcite, could be accurately predicted by Digital Rock Physics (DRP), a non-invasive testing method for simulating laboratory measurements. We present a state-of-the-art algorithm that uses X-ray Computed Tomography (CT) imagery to compute the elastic properties of a lacustrine rudstone sample. The high-resolution CT-images provide a digital sample that is used for analyzing microstructures and performing quasi-static compression numerical simulations. Here, we present the modified Segmentation-Less method withOut Targets method: a combination of segmentation-based and segmentation-less DRP. This new method assigns the spatial distribution of elastic properties of the sample based on homogenization theory and overcomes the monomineralic limitation of the previous work, allowing the algorithm to be used on polymineralic rocks. The method starts by partitioning CT-images of the sample into smaller sub-images, each of which contains only two phases: a mineral (calcite or dolomite) and air. Then, each sub-image is converted into elastic property arrays. Finally, the elastic property arrays from the sub-images are combined and fed into a finite element algorithm to compute the effective elastic properties of the sample. We compared the numerical results to the laboratory measurements of low-frequency elastic properties. We find that the Young’s moduli of both the dry and the fully saturated sample fall within 10% of the laboratory measurements. Our analysis also shows that segmentation-based DRP should be used cautiously to compute elastic properties of carbonate rocks similar to our sample.ISSN:2296-646

Frequency-dependent seismic attenuation in shales: experimental results and theoretical analysis

Samples of shales from the Ordovician Bongabinni and Goldwyer source rock formations were recovered from the Canning Basin (Western Australia). Attenuation was experimentally measured on preserved plugs from these formations in the frequency range between 10−2 and 102 Hz. Samples cored with different orientations with respect to the sedimentary bedding were prepared and tested in their native saturated state and after drying in the oven at 105 °C for 24 hr to assess the effect of fluids and of the sediment anisotropy on attenuation. To aid the interpretation of the experimental results, the clay-rich samples were characterized in terms of mineralogy, water content, porosity, permeability and microstructure. The two shales have significantly different quality factors; and this is seen to be dependent on both the saturation state of the samples and the propagation direction of the oscillatory signal. The attenuation coefficient for compression/extension parallel to bedding is less than that vertical to bedding in both the preserved and partially dehydrated situations. No frequency dependency is observed in the preserved samples within the range of frequencies explored in this study. On the other hand partially saturated samples show peaks in attenuation at around 40 Hz when the stress perturbation is transmitted normal to the macroscopic bedding. The interpretation of the attenuation measurements in terms of well-established theoretical models is discussed in view of the physical characteristics and microstructure of the tested rock

Numerical analysis of wave propagation in fluid-filled deformable tubes

The theory of Biot describing wave propagation in fluid saturated porous media is a good effective approximation of a wave induced in a fluid-filled deformable tube -- Nonetheless, it has been found that Biot’s theory has shortcomings in predicting the fast P-wave velocities and the amount of intrinsic attenuation -- These problems arises when complex mechanical interactions of the solid phase and the fluid phase in the micro-scale are not taken into account -- In contrast, the approach proposed by Bernabe does take into account micro-scopic interaction between phases and therefore poses an interesting alternative to Biot’s theory -- A Wave propagating in a deformable tube saturated with a viscous fluid is a simplified model of a porous material, and therefore the study of this geometry is of great interest -- By using this geometry, the results of analytical and numerical results have an easier interpretation and therefore can be compared straightforward -- Using a Finite Difference viscoelastic wave propagation code, the transient response was simulated -- The wave source was modified with different characteristic frequencies in order to gain information of the dispersion relation -- It was found that the P-wave velocities of the simulations at sub-critical frequencies closely match those of Bernabe’s solution, but at over-critical frequencies they come closer to Biot’s solutio

31st Annual Meeting and Associated Programs of the Society for Immunotherapy of Cancer (SITC 2016) : part two

Background The immunological escape of tumors represents one of the main ob- stacles to the treatment of malignancies. The blockade of PD-1 or CTLA-4 receptors represented a milestone in the history of immunotherapy. However, immune checkpoint inhibitors seem to be effective in specific cohorts of patients. It has been proposed that their efficacy relies on the presence of an immunological response. Thus, we hypothesized that disruption of the PD-L1/PD-1 axis would synergize with our oncolytic vaccine platform PeptiCRAd. Methods We used murine B16OVA in vivo tumor models and flow cytometry analysis to investigate the immunological background. Results First, we found that high-burden B16OVA tumors were refractory to combination immunotherapy. However, with a more aggressive schedule, tumors with a lower burden were more susceptible to the combination of PeptiCRAd and PD-L1 blockade. The therapy signifi- cantly increased the median survival of mice (Fig. 7). Interestingly, the reduced growth of contralaterally injected B16F10 cells sug- gested the presence of a long lasting immunological memory also against non-targeted antigens. Concerning the functional state of tumor infiltrating lymphocytes (TILs), we found that all the immune therapies would enhance the percentage of activated (PD-1pos TIM- 3neg) T lymphocytes and reduce the amount of exhausted (PD-1pos TIM-3pos) cells compared to placebo. As expected, we found that PeptiCRAd monotherapy could increase the number of antigen spe- cific CD8+ T cells compared to other treatments. However, only the combination with PD-L1 blockade could significantly increase the ra- tio between activated and exhausted pentamer positive cells (p= 0.0058), suggesting that by disrupting the PD-1/PD-L1 axis we could decrease the amount of dysfunctional antigen specific T cells. We ob- served that the anatomical location deeply influenced the state of CD4+ and CD8+ T lymphocytes. In fact, TIM-3 expression was in- creased by 2 fold on TILs compared to splenic and lymphoid T cells. In the CD8+ compartment, the expression of PD-1 on the surface seemed to be restricted to the tumor micro-environment, while CD4 + T cells had a high expression of PD-1 also in lymphoid organs. Interestingly, we found that the levels of PD-1 were significantly higher on CD8+ T cells than on CD4+ T cells into the tumor micro- environment (p < 0.0001). Conclusions In conclusion, we demonstrated that the efficacy of immune check- point inhibitors might be strongly enhanced by their combination with cancer vaccines. PeptiCRAd was able to increase the number of antigen-specific T cells and PD-L1 blockade prevented their exhaus- tion, resulting in long-lasting immunological memory and increased median survival