Ultrasonic wave propagation in heterogeneous elastic and poroelastic media

The influence of small- and large-scale heterogeneities on an incident wavefield was investigated in ultrasonic experiments. The field generated by conventional piezoelectric transducers was investigated separately. It was confirmed that the far-field diffraction was correctly predicted by Fraunhofer theory, but that the near-field diffraction exhibited significant deviations from the Fresnel theory. These deviations are attributed to the assumption that the source oscillates uniformly over its surface, which seems unrealistic for the actual transducers. Small rubber and teflon spheres with velocities in the range of the surrounding water acted as small-scale heterogeneities in an otherwise homogeneous medium. The influence on amplitude and traveltime was measured with a needle hydrophone, a focused transducer was utilized for wave generation. These experiments successfully validated first-order scattering theory where a Maslov correction is applied for non-linear effects resulting from a strong contrast between sphere and background medium. Ray theory was found to be unsuitable to properly predict the measured phenomena. Large-scale heterogeneities were also investigated. Ultrasonic reflection and transmission experiments at different angles of incidence were carried out on plane-parallel layers of aluminum. Here the signal was generated by a conventional plane piezoelectric transducer source and the recording was by means of an identical receiver. Unlike in the conventional bulk-wave transit-time method where successive reverberations within the sample are separated in time, we captured all direct and scattered energy by means of a special recording technique where the individual traces are added and compared with traces recorded in absence of the sample. We found that this recording technique gives much improvement over single-position measurements at normal incidence and is ideally suited for full wavefield analysis. Excellent agreement between experiment and Thomson-Haskell theory was obtained. Consequently, the same measurement technique was applied to plane-par-allel layers of poro-elastic materials. The samples were made out of sintered glass powder and had different grain sizes. The Thomson-Haskell theory was extended and new analytical expressions for the reflection and transmission coefficients were derived. These expressions were validated through comparison with the Kennett reflectivity method and stability regimes were determined. Effective medium parameters were obtained through inversion of transmission measurements. These effective parameters were found to be in good agreement with the conventional laboratory values although there was a systematic error in the permeabilities. Both the reflection and transmission experiments were generally found to be in good agreement with theoretical predictions although significant deviations were found for samples with larger grains and for larger angles of incidence. In the former case, deviations from continuum Biot theory are observed, whereas in the latter case the effect of reflections from the edges of the samples played an important role.Civil Engineering and Geoscience

Minimization of finite beam effects in the determination of reflection and transmission coefficients of an elastic layer

A special data acquisition technique was applied to determine the acoustic plane-wave reflection and transmission properties of a plane-parallel aluminum plate. In this technique, the reflected and transmitted wavefield along a plane or line normal to the reflected or transmitted wave vector is recorded at equidistant receiver positions. The obtained traces are subsequently added up in the temporal domain to satisfy plane-wave conditions, thus effectively removing the effect of the limited beam of commonly used transducers. The agreement between plane-wave theory and experiment was found to be excellent, both in the temporal and in the frequency domain

Ultrasonic measurements on poroelastic slabs : determination of reflection and transmission coefficients and processing for Biot input parameters

Ultrasonic reflection and transmission measurements on saturated, plane-parallel, poroelastic slabs are presented. A data processing technique is proposed to obtain poroelastic parameters from transmission measurements. A special experimental data acquisition and processing technique is applied to minimize the finite beam effects of the transducers. This technique yields results which can be compared with poroelastic plane-wave theory. Results of normal- and oblique-incidence measurements are presented, and transmission data are processed to yield wave speeds, sample thickness, angle of incidence, tortuosity, and permeability. The results show good agreement with independent measurements, and they are subsequently used as input for a forward modeling of the complete transmitted and reflected waveforms utilizing Biot theory. The agreement between recorded and modeled signals is good, both in time and frequency domain

Isoform Selectivity of Adenylyl Cyclase Inhibitors: Characterization of Known and Novel Compounds

Nine membrane-bound adenylyl cyclase (AC) isoforms catalyze the production of the second messenger cyclic AMP (cAMP) in response to various stimuli. Reduction of AC activity has well documented benefits, including benefits for heart disease and pain. These roles have inspired development of isoform-selective AC inhibitors, a lack of which currently limits exploration of functions and/or treatment of dysfunctions involving AC/cAMP signaling. However, inhibitors described as AC5- or AC1-selective have not been screened against the full panel of AC isoforms. We have measured pharmacological inhibitor profiles for all transmembrane AC isoforms. We found that 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22,536), 2-amino-7-(furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), and adenine 9-β-d-arabinofuranoside (Ara-A), described as supposedly AC5-selective, do not discriminate between AC5 and AC6, whereas the putative AC1-selective inhibitor 5-​[[2-​(6-​amino-​9H-​purin-​9-​yl)​ethyl]​amino]​-​1-​pentanol (NB001) does not directly target AC1 to reduce cAMP levels. A structure-based virtual screen targeting the ATP binding site of AC was used to identify novel chemical structures that show some preference for AC1 or AC2. Mutation of the AC2 forskolin binding pocket does not interfere with inhibition by SQ22,536 or the novel AC2 inhibitor, suggesting binding to the catalytic site. Thus, we show that compounds lacking the adenine chemical signature and targeting the ATP binding site can potentially be used to develop AC isoform–specific inhibitors, and discuss the need to reinterpret literature using AC5/6-selective molecules SQ22,536, NKY80, and Ara-A.Fil: Brand, Cameron S.. University Of Texas; Estados UnidosFil: Jocker, Harrison J.. University Of Texas; Estados UnidosFil: Gorfe, Alemayehu A.. University Of Texas; Estados UnidosFil: Cavasotto, Claudio Norberto. University Of Texas; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigación en Biomedicina de Buenos Aires; ArgentinaFil: Dessauer, Carmen W.. University Of Texas; Estados Unido

Quantitative comparison between simulations of seismic wave propagation in heterogeneous poro-elastic media and equivalent visco-elastic solids for marine-type environments

There is increasing evidence to suggest that the presence of mesoscopic heterogeneities constitutes an important seismic attenuation mechanism in porous rocks. As a consequence, centimetre-scale perturbations of the rock physical properties should be taken into account for seismic modelling whenever detailed and accurate responses of specific target structures are desired, which is, however, computationally prohibitive. A convenient way to circumvent this problem is to use an upscaling procedure to replace each of the heterogeneous porous media composing the geological model by corresponding equivalent visco-elastic solids and to solve the visco-elastic equations of motion for the inferred equivalent model. While the overall qualitative validity of this procedure is well established, there are as of yet no quantitative analyses regarding the equivalence of the seismograms resulting from the original poro-elastic and the corresponding upscaled visco-elastic models. To address this issue, we compare poro-elastic and visco-elastic solutions for a range of marine-type models of increasing complexity. We found that despite the identical dispersion and attenuation behaviour of the heterogeneous poro-elastic and the equivalent visco-elastic media, the seismograms may differ substantially due to diverging boundary conditions, where there exist additional options for the poro-elastic case. In particular, we observe that at the fluid/porous-solid interface, the poro- and visco-elastic seismograms agree for closed-pore boundary conditions, but differ significantly for open-pore boundary conditions. This is an important result which has potentially far-reaching implications for wave-equation-based algorithms in exploration geophysics involving fluid/porous-solid interfaces, such as, for example, wavefield decomposition