Low Acoustic Velocity Research Articles

Directional and lattice engineering growth of sapphire fibers for ultrasonic ultra-high-temperature sensors

High-temperature sensors for extreme environments are in urgent demand with the rapid development of aerospace, nuclear energy, and advanced manufacturing. Ultrasonic temperature sensors (UTS) are widely used in high-temperature sensing due to their high working temperature that approaches the melting point of the acoustic waveguide. Al2O3 single-crystal fiber (SCF), also called sapphire fiber, is a promising candidate for ultrasonic thermometry due to its ultra-high melting point (∼2050 °C), high strength and oxidation resistance. Here, we report for the first time, to the best of our knowledge, large length-diameter ratio sapphire fibers with specific orientations (a-axis, m-axis, and c-axis) have been grown by laser-heated pedestal growth method and applied for ultrasonic temperature sensing. Sapphire fibers exhibit significant acoustic anisotropy, and a-oriented sapphire fiber has a lower acoustic velocity and a greater velocity-temperature variation, leading to higher sensitivity. Moreover, the acoustic velocity of sapphire fiber could be further decreased by doping with Cr ions owing to the enhanced lattice disorder and crystal density. As a result, an improved unit sensitivity of 28.98–52.88 ns°C−1m−1 and a superior resolution of 1.73–0.95 °C have been achieved in the range of 20–1200 °C via a-oriented Cr:Al2O3 SCF-UTS. More importantly, the sensor performance is positively correlated with ambient temperature, accompanied with the high working temperature (∼1800 °C) and excellent stability (∼20 h), demonstrating promising prospects for applications in high-temperature thermometry. This work provides a feasible approach for designing a high-performance UTS based on acoustic anisotropy and lattice engineering.

Phase transition, optical, and elastic properties of a new hybrid organic–inorganic perovskite: [(R)-(+)-3-aminoquinuclidine]KI3

A new hybrid organic–inorganic perovskite, (R-3AQ)KI3 [R-3AQ2+ = (R)-(+)-3-aminoquinuclidine], has been synthesized and comprehensively characterized by experimental approaches and density functional theory calculations. Our experimental results demonstrate that (R-3AQ)KI3 has a typical perovskite structure and exhibits a reversible order–disorder phase transition at temperatures of 457 and 443 K on heating and cooling, respectively. Under ultraviolet irradiation, a clear yellowish-green emission peaked at 556 nm was observed for (R-3AQ)KI3. The calculated electronic structure shows that (R-3AQ)KI3 possesses a typical direct bandgap with a value of 3.74 eV and its valence band maximum and conduction band minimum primarily arise from the I-5p and I-5s orbitals, respectively. In addition, the elastic calculations indicate that (R-3AQ)KI3 displays a relatively large structure stiffness, relatively small elastic anisotropy, and fairly low acoustic velocity, owing to the rigid K–I bonds and the strong hydrogen bond interactions between the [KI3]2− perovskite framework and R-3AQ2+ cations. These results suggest that the mechanical robustness of this multifunctional (R-3AQ)KI3 makes it a good candidate material for sensing applications.

Geoscience Poster G15: Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

Poster G15 The Northern Carnarvon Basin (NCB) hosts an extensive record of Jurassic–Cretaceous rift-related igneous activity, manifested by a >45 000 km2 intrusive complex and series of volcanic centres constrained by seismic mapping. However, there are relatively few well penetrations of these igneous rocks (<1% of ~1500 exploration wells) in comparison to other basins that witness extensive magmatism, and thus, their lithological and petrophysical characteristics are poorly understood. Here, we describe the properties of igneous rocks encountered in nine petroleum exploration wells and scientific boreholes in the NCB and evaluate their impacts on exploration and development issues. Igneous rocks in the NCB are characterised by pervasive alteration, with ramifications for seismic imaging and drilling. For example, low acoustic velocities in mafic lavas altered to clays in Toro-1 were mistaken for overpressure, whilst intrusive rocks in Palta-1 were initially unrecorded and only recognised due to subsequent post-drilling thermal history analysis. The alteration of mafic igneous rocks to clays reduces acoustic impedance contrasts relative to sedimentary host rocks, making their identification prior to drilling more challenging. Whilst the preferential emplacement of intrusive rocks in Triassic strata deeper than reservoir targets is primarily responsible for the paucity of well penetrations, our findings of extensive alteration of igneous rocks in the NCB suggests that additional wells may intersect as yet unrecognised intrusive or extrusive sequences. To access the poster click the link on the right. To read the full paper click here

Seismic, petrophysical and petrological constraints on the alteration of igneous rocks in the Northern Carnarvon Basin, Western Australia: implications for petroleum exploration and drilling operations

The Northern Carnarvon Basin (NCB) hosts an extensive record of Jurassic–Cretaceous rift-related igneous activity, manifested by a >45 000 km2 intrusive complex and series of volcanic centres constrained by seismic mapping. However, there are relatively few well penetrations of these igneous rocks (<1% of ~1500 exploration wells) in comparison to other basins that witness extensive magmatism, and thus, their lithological and petrophysical characteristics are poorly understood. Here, we describe the properties of igneous rocks encountered in nine petroleum exploration wells and scientific boreholes in the NCB and evaluate their impacts on exploration and development issues. Igneous rocks in the NCB are characterised by pervasive alteration, with ramifications for seismic imaging and drilling. For example, low acoustic velocities in mafic lavas altered to clays in Toro-1 were mistaken for overpressure, whilst intrusive rocks in Palta-1 were initially unrecorded and only recognised due to subsequent post-drilling thermal history analysis. The alteration of mafic igneous rocks to clays reduces acoustic impedance contrasts relative to sedimentary host rocks, making their identification prior to drilling more challenging. Whilst the preferential emplacement of intrusive rocks in Triassic strata deeper than reservoir targets is primarily responsible for the paucity of well penetrations, our findings of extensive alteration of igneous rocks in the NCB suggests that additional wells may intersect as yet unrecognised intrusive or extrusive sequences.

Phonon transport in Zintl Ba2ZnAs2 and Ba2ZnSb2: A first-principles study

Layered Zintl Ba2ZnX2 (X=As, Sb, and Bi) with intrinsically ultralow lattice thermal conductivity (κ) have attracted continuous attention because of their potential applications in thermoelectric devices. Understanding the low-κ mechanism is favorable for preserving the temperature gradient between the two ends of a material and achieving high thermoelectric performance. Herein, based on first-principles calculations, we systematically report the phonon-limited thermal conductivities of Ba2ZnX2. In the framework of the harmonic approximation, the absence of unstable vibrational modes indicates Ba2ZnAs2 and Ba2ZnSb2 are dynamically stable, while Ba2ZnBi2 is unstable with imaginary frequencies in the phonon dispersion. Remarkably, Ba2ZnSb2 exhibits low acoustic phonon group velocities (<4.5 km/s), large Grüneisen parameters, short phonon relaxation time (<5.5 ps), low-lying optical modes (∼1.5 THz), and strong optical-acoustic phonon couplings. These inherent phonon features can greatly hinder the heat transport ability and therefore give rise to an ultralow thermal conductivity of ∼ 0.11 W/mK at 300 K for Ba2ZnSb2. Furthermore, elastic properties calculated based on the density functional theory helped to gain insight into the origin of the low κ. Our study provides fundamental physical insights into the reason of low thermal conductivity of Ba2ZnX2, which is valuable toward the search for efficient thermoelectric materials based on layered Zintl phase compounds.

Enhanced Coupling Coefficient in Dual-Mode ZnO/SiC Surface Acoustic Wave Devices with Partially Etched Piezoelectric Layer

Surface acoustic wave (SAW) devices based on multi-layer structures have been widely used in filters and sensors. The electromechanical coupling factor (K2), which reflects energy-conversion efficiency, directly determines the bandwidth of the filter and the sensitivity of sensor. In this work, a new configuration of dual-mode (quasi-Rayleigh and quasi-Sezawa) SAW devices on a ZnO/SiC layered structure exhibiting significantly enhanced K2 was studied using the finite element method (FEM), which features in the partial etching of the piezoelectric film between the adjacent interdigitated electrodes (IDTs). The influences of piezoelectric film thickness, etching ratio, top electrodes, bottom electrodes, and the metallization ratio on the K2 were systematically investigated. The optimum K2 for the quasi-Rayleigh mode and quasi-Sezawa mode can exceed 12% and 8%, respectively, which increases by nearly 12 times and 2 times that of the conventional ZnO/SiC structure. Such significantly promoted K2 is of great benefit for better comprehensive performance of SAW devices. More specifically, a quasi-Rayleigh mode with relatively low acoustic velocity (Vp) can be applied into the miniaturization of SAW devices, while a quasi-Sezawa mode exhibiting a Vp value higher than 5000 m/s is suitable for fabricating SAW devices requiring high frequency and large bandwidth. This novel structure has proposed a viable route for fabricating SAW devices with excellent overall performance.

Brillouin light scattering study of microscopic structure and dynamics in pyrrolidinium salt based ionic liquids

Pyrrolidinium salts based ionic liquids are known to be good ionic conductors even in solid-state around room temperature, which is attributed to the highly disordered plastic crystalline phase. Moreover, these salts are characterized by multiple phase transitions which include plastic, structural glass, and glassy crystal phases with varying levels of molecular disorder. Temperature-dependent Brillouin light scattering is used to investigate the phase transitions in a series of alkylmethylpyrrolidinium Bis(trifluoromethanesulfonyl) imides (P1nTFSI, n = 1,2,4). Brillouin spectral features such as the number of acoustic modes, their shape, and linewidths provide the picture of different disordered phases resultant of dynamics at the microscopic scale. The longitudinal and transverse acoustic velocities in different phases are determined from the corresponding acoustic mode frequencies (Brillouin shift). Extremely low acoustic velocities in the solid phase of P11TFSI and P12TFSI are a consequence of a high degree of disorder and plasticity present in these systems. Anomalous temperature-dependent behavior of linewidths and asymmetric (Fano) line shape of acoustic modes observed in certain phases of P1nTFSI could be due to the strong coupling between the Brillouin central peak and the acoustic phonons. The present results establish that the Brillouin light scattering technique can be efficiently used to understand the complex phase behavior, microscopic structure, and dynamics of ionic liquids.

Nonlinear effects in locally resonant nanostrip phononic metasurface at GHz frequencies

In this paper, we report on the observation of nonlinear effects in a nanostrip phononic metasurface (NPM) that enable the tuning of resonance frequencies at 1.42 GHz. The NPM resonator made of a periodic nanostrip array is fabricated on a lithium niobate substrate. Each of the nanostrips is 250 nm wide and is made of a 680-nm-thick SiO2 layer stacking on 50-nm Al metal electrodes. Finite element analysis reveals that the device operates in a vertically polarized (compression) mode with substantial acoustic energy confined in the nanostrips, leading to a local resonance at low acoustic velocity. Due to the nonlinearity, the resonance frequency of the device decreases with the increase in stimulation power. The underlying mechanism of the nonlinearity is found to be the power-dependent coupling of the adjacent nanostrips. This coupling induces softening of the substrate surface region, which reduces the acoustic velocity and, hence, the bulk radiation. As a result, the quality factor of the NPM resonator is found to improve with the increase in stimulation power. The power-dependent coupling of nanostrips in the NPM resonator demonstrates a reliable method for the realization of nonlinearity in phononic metasurfaces, which would significantly enrich the mechanisms for the manipulation of surface acoustic waves at high frequencies.

Monolayer square-Ag2X (X = S, Se): Excellent n-type thermoelectric materials with high power factors

Thermoelectric devices based on thermoelectric materials, which can be utilized to transform heat to electric energy, are regarded as the clean and reliable energy-harvestings that reduce greenhouse gas and noxious gas missions. In this work, thermoelectric properties of square-Ag2X (X = S, Se) monolayers have been systematically studied through uniting Boltzmann transport theory and first-principles. The results show that square-Ag2X (X = S, Se) monolayers possess ultrahigh electrical conductivities due to their ultralight electron effective masses, together with high Seebeck coefficients, resulting in ultrahigh power factors (0.014 and 0.018 W/m K2). Moreover, phonon transport calculations unveil that square-Ag2X (X = S, Se) monolayers exhibit low lattice thermal conductivities (3.46 and 2.33 W/m K) at 300 K, which can be traced back to their small Debye temperature, low acoustic group velocities, short phonon relaxation times and large Grüneisen parameters. Accordingly, the obtained figure of merit of monolayer square-Ag2Se for optimal n-type doping at 700 K can approach 2.81, which is larger than the well-known n-type thermoelectric monolayer SnSe (2.32) and Mg3Sb2 (2.21). Therefore, the excellent thermoelectric properties of square-Ag2Se monolayer along with its all-round stability verified by the phonon dispersion, ab initio molecular dynamic simulation and accordance of Born–Huang criteria highlight its prominent potential for thermoelectric applications.

Enhanced Performance of ZnO/SiO2/Al2O3 Surface Acoustic Wave Devices with Embedded Electrodes.

With the advent of the 5G era, surface acoustic wave (SAW) devices with a larger bandwidth and better temperature stability are strongly required, meanwhile the dimensions of devices are continuously scaling down. In this work, a new layout of ZnO/SiO2/Al2O3 SAW devices with embedded electrodes was developed, and with the help of the finite element method (FEM), the propagation characteristics were simulated. Through adopting embedded electrodes, a large electromechanical coupling coefficient (K2) of 6.6% for the Rayleigh mode can be achieved (5 times larger than that of the conventional ZnO/Al2O3 structure), feasible for wideband SAW devices, and a low acoustic velocity (Vp) of 2960 m/s is exhibited simultaneously, which benefits the miniaturization of SAW devices. The dramatic enhancement of K2 is mainly attributed to the more efficient excitation of SAW in piezoelectric films. Furthermore, a SiO2 overlay is added on the top of the structure to gain an excellent zero temperature coefficient of frequency (TCF). Experimentally, we successfully fabricated SAW one-port resonators based on the proposed structure and good characteristics of high K2, low Vp, and small TCF as simulated were confirmed. Our results show that the proposed structure provides a viable route to design SAW devices with a large bandwidth, small size, and robust temperature compensation for practical use.

First-principles calculations of phonon transport in two-dimensional penta-X2C family

Two-dimensional (2D) materials exhibit enhanced thermoelectric (TE) performance compared to bulk materials, which relies heavily on lattice thermal conductivity. Penta-X2C (X = P, As, and Sb) is a newly predicted 2D material family with promising potential applications in photocatalytic water splitting and photovoltaic and optoelectronic devices. To achieve a combination of photovoltaic and TE technologies and further boost the energy utilization rate, in this paper, we systematically investigate the thermal transport of the penta-X2C family. Density functional theory combined with semiclassical Boltzmann transport approach was used to evaluate the thermal transport. Interestingly, the calculated lattice thermal conductivities (kl) of penta-Sb2C are two orders of magnitude smaller than that of penta-P2C, despite that they share similar atomic structure. The calculated kl of penta-P2C, penta-As2C, and penta-Sb2C are 75.27 W m−1 K−1, 19.11 W m−1 K−1, and 0.72 W m−1 K−1, respectively. Penta-Sb2C also exhibits low average acoustic group velocity, large Grüneisen parameters, strong optical–acoustic phonon coupling, and short phonon mean free path. These results qualify penta-Sb2C as a promising candidate for building outstanding TE devices.

Exposure to Nanoscience and Nanotechnology Using Guided-Inquiry-Based Activities with Silica Aerogel To Promote High School Students’ Motivation

Nanoscience and nanotechnology are the study and application of materials on a nanometer scale. These fields of study aid the developments of innovative technologies that play an important role in our everyday lives. Enabling students to learn these topics makes learning more meaningful and helps prepare them for future technologies. This study aimed to investigate the effects of learning activities about nanoscience and nanotechnology on students’ motivation. The designed activities were based on guided-inquiry learning using silica aerogel as the nanomaterial. This material has many interesting properties such as high thermal insulation, low acoustic velocity, very low density, very high specific surface area, etc., which are influenced by structures on the nanometer scale. A variety of teaching methods, including learning with models, multimedia, hands-on activities, and guided-inquiry learning, were implemented to promote the students’ motivation. There were 28 grade 12 science students sampled for this purpose. Three actual methods were adopted to determine the effectiveness of the learning activities, including questionnaires implemented before and after the activities, students’ self-assessments, and semistructured interviews at the end of the activities. The results from the questionnaires showed that the students were highly motivated by the nanoscience and nanotechnology learning activities. The students’ self-assessment and interviews revealed that the learning activities encouraged the students to learn by allowing them to experience various teaching materials. The students became more interested in finding more information about silica aerogel and other nanomaterials used in daily life.

Design and performance analysis of an ORC transonic centrifugal turbine

The centrifugal turbine is a new type of turbine that has lots of benefits including high efficiency, simple structure, small volume, which can be used as small-to-medium turbine in industry and waste heat utilization. The transonic ORC turbine has quite larger size compares to the steam and gas turbine for the low acoustic velocity of organic fluids. As requirements for smaller and lighter weight components push turbines to more compact, closely coupled designs, a transonic nozzle is adopted to enhance the single-stage enthalpy drop of centrifugal turbine and reduce the stage number and size of turbine. By consulting aerodynamic design method, parameters of axial turbine and using R141b as working fluid, a transonic nozzle and the corresponding rotor cascade of centrifugal turbine are designed, in addition, the three-dimensional numerical simulation and performance analysis are conducted in this paper. The aerodynamic performance of the transonic centrifugal turbine stage was subsequently assessed by three-dimensional numerical simulations. Likewise, the flow characteristics analysis was carried out under design and off-design conditions. Results of numerical simulation indicate that the aerodynamic performance of the transonic centrifugal turbine stage is in conformity to the design requirement with reasonable flow field distribution, smooth streamline. The enthalpy drop of designed transonic nozzle is at least twice more than that a subsonic nozzle can achieve, which can greatly enhance turbines’ load capacity and reduce the size of turbine. Meanwhile the off-design performance analysis shows that the transonic centrifugal turbine has wide stable operation range and strong load adaptability.

Evaluating petrophysical properties of volcano-sedimentary sequences: A case study in the Paraná-Etendeka Large Igneous Province

Abstract The Parana-Etendeka represents a major magmatic province associated with the rifting of West Gondwana and the formation of the South Atlantic Ocean. The area represents a direct analogue for similar aged volcanic rocks buried within hydrocarbon-rich basins offshore the South Atlantic margin. We present here a detailed integration of outcrop data with laboratory measurements of porosity, permeability, and ultrasonic acoustic velocities (P and S-waves) for volcanic and interbedded sedimentary rocks of the Parana-Etendeka Province in southern Brazil. The lava pile is formed of compound pahoehoe basaltic lavas at the base (Torres Formation) followed by thick tabular rubbly pahoehoe basaltic andesites (Vale do Sol Formation) and the upper stratigraphy is characterized by locally fed domes and extensive tabular acidic units. Sedimentary interbeds occur throughout the entire lava pile. For the volcanic rocks petrophysical properties have a cyclic variation controlled by the lava internal structure. Lava upper and lower crusts have relatively high porosity (>10%) and low acoustic velocities, whilst lava flow cores are characterized by porosities of less than 5% and velocities typically 0.5–1.0 kms−1 higher than lava crust facies. The highest porosities are found in the upper crust of both rubbly pahoehoe (c. 28.3%) and pahoehoe lavas (c. 26.6%) where vesicles account for most of the pore space. Permeability is relatively low in the volcanic rocks ( 15%) and permeability (avg. 450 mD) and represent the best reservoir rocks within the Parana-Etendeka Province. Nevertheless, where diagenesis is intense porosity and permeability are significantly diminished. The petrophysical properties of volcanic rocks are controlled primarily by lava emplacement mechanisms (e.g. inflation, degassing and flow fragmentation), and secondarily by bulk mineral composition. Petrophysical properties can be further modified by diagenetic and hydrothermal alteration processes. Understanding the interplay between primary and secondary processes on the final petrophysical characteristics of the rocks is key for defining reservoir properties in offshore areas, such as the North and South Atlantic margins, where volcanic rocks are intrinsically associated with prolific hydrocarbon-bearing sedimentary basins.

Large Brillouin gain in Germania-doped core optical fibers up to a 98 mol% doping level.

Germanosilicate glasses are substantial materials in fiber optic technology that have allowed the control of optical properties such as numerical aperture, photosensitivity, dispersion, nonlinearity, and transparency toward mid-infrared. Here, we investigate stimulated Brillouin scattering in single-mode germanosilicate core fibers with increasing GeO2 content from 3.6mol% up to 98mol%. Our results reveal a wide Brillouin frequency shift tunability over more than 3GHz with a strong decrease down to 7.7GHz at high GeO2 content owing to the low acoustic velocity, while the Brillouin linewidth significantly broadens up to 100MHz beyond 50mol% of GeO2 content. In addition, large Brillouin gain up to 6.5 times larger than in standard silica fibers is also reported by means of a pump-probe experiment.

Effective thermal and mechanical properties of polycrystalline diamond films

Polycrystalline diamond films were demonstrated as good candidates for electron field emitters, and their mechanical/thermal properties should thus be considered for real devices. We utilized ultrafast optical techniques to investigate the phonon dynamics of several polycrystalline diamond films, prepared by microwave plasma enhanced chemical vapor deposition. The mechanical properties (longitudinal acoustic velocity) and thermal conductivities of diamond films were evaluated from the coherent and incoherent phonon dynamics, respectively. Ultrananocrystalline diamond films were grown using a CH4 (2%)/Ar plasma, while microcrystalline diamond films were grown using a CH4 (2%)/H2 plasma. The ultrananocrystalline diamond film (with a grain size of several nanometers) possesses low acoustic velocity (14.5 nm/ps) and low thermal conductivity (3.17 W/m K) compared with other kinds of diamond films. The acoustic velocity of diamond films increased abruptly to nearly the same as that of natural diamond and remained there when the rod-shaped diamond grains were induced due to the incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivities of the materials increased monotonously with increasing incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivity of 25.6 W/m K was attained for nanocrystalline diamond films containing spherical diamond grains (with a size of several tens of nanometers). Compared with single crystalline diamond, the low thermal conductivity of polycrystalline films results from phonon scattering at the interfaces of grains and amorphous carbon in the boundary phases.

A novel five-wire micro anemometer with 3D directionality for low speed air flow detection and acoustic particle velocity detecting capability

In this paper, a novel five-wire micro-fabricated anemometer with 3D directionality based on calorimetric principle is proposed, which is capable of measuring low speed airflow. This structure is realized by vertically bonding two different dies, which can be fabricated on the same wafer resulting in a simple fabrication process. Experiments on speed lower than 200 mm s−1 are conducted, showing good repeatability and directionality. The speed of airflow is controlled by the volumetric flow rate. The measured velocity sensitivity is 9.4 mV · s m−1, with relative direction sensitivity of 37.1 dB. The deviation between the expected and the measured directivity is analyzed by both theories and simulations. A correction procedure is proposed and turns out to be useful to eliminate this deviation. To further explore the potential of our device, we expose it to acoustic plane waves in a standing wave tube, showing consistent planar directivity of figure of eight. The measured velocity sensitivity at 1 kHz and 120 dBC is 4.4 mV · s m−1, with relative direction sensitivity of 27.0 dB. By using the correction method proposed above, the maximum angle error is about ±2°, showing its good directionality accuracy.

Solidly mounted resonators fabricated for GHz frequency applications based on MgxZn1-xO piezoelectric film

Solidly mounted resonator is a type of film bulk acoustic resonator for microwave operation applications. ZnO piezoelectric films provide a choice to produce solidly mounted resonator, but show a low longitudinal acoustic velocity and a relatively low response. In this work, a ternary compound magnesium doped zinc oxide (MgxZn1-xO) piezoelectric film is introduced and investigated, which has a flexible frequency response and a higher acoustic velocity. X-ray diffraction (XRD) θ-2θ coupled scans of the MgxZn1-xO films are investigated versus different RF magnetron sputtering deposition conditions. Residual stresses are studied to ensure a reliable fabrication of solidly mounted resonators. A typical device fabricated based on MgxZn1-xO piezoelectric film show a resonant frequency at 2.046 GHz with a return loss of −38.9 dB, a quality factor of 965 and a coupling coefficient of 2.5%. The calculated longitudinal acoustic velocity of a MgxZn1-xO film is 6854.1 m/s, faster than that of ZnO.

Interpreting direct hydrocarbon indicators of low-API biodegraded oils — A case study from a deepwater South Atlantic Basin

Strong seismic flat spots are typically associated with the oil/water contact (OWC) in many amplitude-supported deepwater South Atlantic Basin (SAB) discoveries, particularly in relatively shallow (∼2000 mbml), high-porosity unconsolidated Eocene-age reservoirs. These reservoirs contain biodegraded oils that have low API gravity, low gas-oil ratios, high viscosities, and fluid acoustic velocities between 4500 and 5000 ft/s. Often, a mismatch in amplitude is noted when comparing an observed seismic-amplitude response with a synthetic forward model generated using sonic logging data. These observations can be attributed to the high viscosity (low API gravity) of biodegraded oil that can lead to wave-induced heterogeneous pore pressure at logging frequencies, thus deviating from a key assumption in Gassmann's fluid-substitution theory. These dispersion effects can lead to significantly slower acoustic velocities of these oil-bearing reservoirs at seismic frequencies than those measured at logging frequencies. Using a generalized fluid/solid-substitution theory, we model this change in acoustic velocity and find that the theory can explain the observed bright-spot response.

A Void Fraction Characterisation by Low Frequency Acoustic Velocity Measurements in Microbubble Clouds

Low frequency acoustic velocity measurements have been applied for the characterization of microbubble clouds generated in water. This method, based on the Wood's model (1941) links the acoustic velocity throughout a two-phase medium to its void fraction value. Low frequency means below resonance frequencies of the bubbles inside the cloud. An original bench was developed to allow the qualification of this method. The experiments conducted allowed us to characterize void fraction values between 10-3 and 10-7. The radii of the studied microbubbles are between a few micrometers and a hundred micrometers.