Impedance Contrast Research Articles

Robust AVO inversion for the fluid factor and shear modulus

Seismic estimation of the fluid factor and shear modulus plays an important role in reservoir fluid identification and characterization. Various amplitude variation with offset inversion methods have been used to estimate these two parameters, which are generally based on approximate formulations of the Zoeppritz equations. However, the accuracy of these methods is limited because the forward modeling ability of approximate equations is incorrect under the conditions of strong impedance contrast and large incidence angles. Therefore, to improve the estimation accuracy, we have used the Zoeppritz equations to directly invert for the fluid factor and shear modulus. Based on the poroelasticity theory, we derive the Zoeppritz equations in a new form containing the fluid factor, shear modulus, density, and dry-rock velocity ratio squared. The objective function is then constructed using these equations in a Bayesian framework with the addition of a differentiable Laplace distribution blockiness constraint term to the prior model to enhance the fluid boundaries. Finally, the nonlinear objective function is solved by combining the Taylor expansion and the iterative reweighted least-squares algorithm. Numerical experiments indicate that the inversion accuracy of our method may heavily depend on the parameter of the dry-rock velocity ratio square that is assumed to be static. However, tests on synthetic and field data show that our method can estimate the fluid factor and shear modulus with satisfactory accuracy in the case of choosing a reasonable static value of this parameter. In addition, we determine that the accuracy of this method is higher than that of the linearized formulation.

Design of a one-dimensional underwater acoustic leaky wave antenna using an elastic metamaterial waveguide

Acoustic imaging in water traditionally relies on phased arrays of active electro-acoustic transducers to steer acoustic energy in specific directions. One potential alternative approach to steer acoustic beams is to use a single transducer attached to a dispersive antenna that radiates or receives acoustic energy from different directions as the frequency of operation changes. This is known as a leaky wave antenna (LWA). While LWAs have been proven effective in beam steering for electromagnetic and air-borne acoustic waves, the design of an analog device in water presents a unique challenge due to the low contrast in acoustic impedance between elastic solids and water, which necessitates the consideration of fluid-elastic coupling in the design of the elastic LWA. This work presents an approach to design an elastic metamaterial waveguide coupled to an external fluid domain as one means to create an acoustic LWA for underwater operation. Forward-to-backward radiation is achieved through the design of mass-in-cavity structures that produce simultaneous negative effective mass and modulus by considering fluid-elastic coupling. The design is presented through finite element analysis of individual unit cells and a water-loaded elastic LWA. A design example is presented that steers through backfire to endfire as a function of input frequency.

Modeling the effects of fracture infill on frequency-dependent anisotropy and AVO response of a fractured porous layer

In a fractured porous hydrocarbon reservoir, wave velocities and reflections depend on frequency and incident angle. A proper description of the frequency dependence of amplitude variations with offset (AVO) signatures should allow effects of fracture infills and attenuation and dispersion of fractured media. The novelty of this study lies in the introduction of an improved approach for the investigation of incident-angle and frequency variations-associated reflection responses. The improved AVO modeling method, using a frequency-domain propagator matrix method, is feasible to accurately consider velocity dispersion predicted from frequency-dependent elasticities from a rock physics modeling. And hence, the method is suitable for use in the case of an anisotropic medium with aligned fractures. Additionally, the proposed modeling approach allows the combined contributions of layer thickness, interbedded structure, impedance contrast and interferences to frequency-dependent reflection coefficients and, hence, yielding seismograms of a layered model with a dispersive and attenuative reservoir. Our numerical results show bulk modulus of fracture fluid significantly affects anisotropic attenuation, hence causing frequency-dependent reflection abnormalities. These implications indicate the study of amplitude versus angle and frequency (AVAF) variations provides insights for better interpretation of reflection anomalies and hydrocarbon identification in a layered reservoir with vertical transverse isotropy (VTI) dispersive media.

Compact Wideband Microstrip Patch Antenna Design for Breast Cancer Detection

The current breast cancer detection techniques are mostly invasive and suffer from high cost, high false rate and inefficacy in early detection. These limitations can be subdued by development of non-invasive microwave detection system whose performance is predominantly dependent on the antenna used in the system. The designing of a compact wideband antenna and matching its impedance with breast phantom is a challenging task. In this paper, we have designed a compact antenna matched with the breast phantom operating in wideband frequency from 1 to 6 GHz capable to detect the dielectric (or impedance) contrast of the benign and malignant tissue. The impedance of the antenna is matched to a cubically shaped breast phantom and a very small tumor (volume=1 cm3). The antenna is tuned to the possible range of electrical properties of breast phantom and tumour (permittivity ranging from 10 to 20 and conductivity from 1.5 to 2.5 S/m). The return loss (S11), E-field distribution and specific absorption rate (SAR) are simulated. The operating band of antenna placed near the phantom without tumor was found to be (1.11-5.47)GHz and with tumor inside phantom is (1.29-5.50)GHz. Results also show that the SAR of the antenna is within the safety limit.

Gas hydrate identification and distribution in the Mexican Ridges, Mexico

We have mapped gas hydrates, free gas, and bottom-simulating-reflector (BSR) distributions in an area of the Mexican Ridges, central Gulf of Mexico, Mexico, revealing the relationship between these three elements and the tectonostratigraphy. The three elements are more visible when the host rock is a high-porosity sandstone because there is a large seismic impedance contrast between the solid gas hydrates above and the free gas below, which manifests itself on the seismic as a BSR. Gas hydrates are identified in the well as higher resistivity sandstone layers with a strong positive amplitude. When the host rock has a higher shale content with lower porosity, the impedance contrast is lower and the BSR is weak or not visible. The study area in Mexican Ridges is an anticline where gas hydrates and free gas are trapped on the crest after migrating through the dipping layers and faults from synclines that are generated in calcareous shale. The main seal is mass transport complex deposits from the Pliocene; when they are not deposited at the crest of the anticline, there is gas escape to the seafloor in the form of a gas chimney. In this way, we established a complete petroleum system for gas hydrates and free gas on the Mexican Ridges.

Near-surface seismic site characterization using Nakamura-based HVSR technique in the geological complex region of Kangra Valley, northwest Himalaya, India

The two major towns, Kangra and Dharamshala, located in the northwest Himalaya have been a subject of great interest for the first-level microzonation due to their high seismic hazard potential. Tectonically, the region is geologically complex and lying between two major thrusts, i.e., the Main Boundary Thrust (MBT) to the northeast and the Jawalamukhi Thrust (JMT) to the southwest. Therefore, the valley is highly deformed and fractured and has developed several small and major faults longitudinally and transversely. The Kangra town is underlain by the Upper Siwalik Boulder Conglomerates, whereas Dharamshala town by the sandstone, claystone, and mudstone with variable thickness of soft soil cover above the bedrock. These regions faced devastating damage from the 1905 Kangra earthquake (Ms 7.8). So, the ambient noise measurement has been performed using a dense network of 200 free-field stations to measure the fundamental frequency of each site. The analysis revealed a high variation in the typologies of HVSR peaks. The iso-frequency map of the study region indicated a wide variation within and on the peripheries of the basin in a frequency ranging from 0.26 to 12 Hz. Some isolated locations in the north-eastern and north-western parts of the study area also showed a dominance of the high resonance frequency component (12-20 Hz). The study further indicated the amplification factor varying from 1.67 to 7.68, reflecting the impact of impedance contrast between the soft soil and the bedrock.

Rock Physical Modeling and Seismic Dispersion Attribute Inversion for the Characterization of a Tight Gas Sandstone Reservoir

Gas identification using seismic data is challenging for tight gas reservoirs with low porosity and permeability due to the complicated poroelastic behaviors of tight sandstone. In this study, the Chapman theory was used to simulate the dispersion and attenuation caused by the squirt flow of fluids in the complex pore spaces, which are assumed to consist of high aspect-ratio pores (stiff pores) and low aspect-ratio microcracks (soft pores). The rock physics modeling revealed that as the gas saturation varies, P-wave velocity dispersion and attenuation occurs at seismic frequencies, and it tends to move to high frequencies as the gas saturation increases. The velocity dispersion of the tight gas sandstone causes a frequency-dependent contrast in the P-wave impedance between the tight sandstone and the overlying mudstone, which consequently leads to frequency-dependent incidence reflection coefficients across the interface. In the synthetic seismic AVO modeling conducted by integrating the rock physics model and the propagator matrix method, the variations in the amplitudes and phases of the PP reflections can be observed for various gas saturations. The tests of the frequency-dependent AVO inversion of these synthetic data revealed that the magnitude of the inverted P-wave dispersion attribute can be used to indicate gas saturation in tight sandstone reservoirs. The applications of the frequency-dependent AVO inversion to the field pre-stacked seismic data revealed that the obtained P-wave dispersion attribute is positively correlated with the gas production from the pay zone at the well locations. Thus, the methods of the rock physics modeling and the frequency-dependent AVO inversion conducted in this study have good potential for the evaluation of the gas saturation in tight gas sandstone reservoirs.

Fourier-Domain Ultrasonic Imaging of Cortical Bone Based on Velocity Distribution Inversion.

There is a significant acoustic impedance contrast between the cortical bone and the surrounding soft tissue, resulting in difficulty for ultrasound penetration into bone tissue with high frequency. It is challenging for the conventional pulse-echo modalities to give accurate cortical bone images using uniform sound velocity model. To overcome these limitations, an ultrasound imaging method called full-matrix Fourier-domain synthetic aperture based on velocity inversion (FM-FDSA-VI) was developed to provide accurate cortical bone images. The dual linear arrays were located on the upper and lower sides of the imaging region. After full-matrix acquisition with two identical linear array probes facing each other, travel-time inversion was used to estimate the velocity distribution in advance. Then, full-matrix Fourier-domain synthetic aperture (FM-FDSA) imaging based on the estimated velocity model was applied twice to image the cortical bone, utilizing the data acquired from top and bottom linear array, respectively. Finally, to further improve the image quality, the two images were merged to give the ultimate result. The performance of the method was verified by two simulated models and two bone phantoms (i.e., regular and irregular hollow bone phantom). The mean relative errors of estimated sound velocity in the region-of-interest (ROI) are all below 12%, and the mean errors of cortical section thickness are all less than 0.3 mm. Compared to the conventional synthetic aperture (SA) imaging, the FM-FDSA-VI method is able to accurately image cortical bone with respect to the structure. Moreover, the result of irregular bone phantom was close to the image scanned by microcomputed tomography ( μ CT) in terms of macro geometry and thickness. It is demonstrated that the proposed FM-FDSA-VI method is an efficient way for cortical bone ultrasonic imaging.

Evaluation of site amplification factors for shallow rock sites of Islamabad, Pakistan.

Current seismic provisions in Building Code of Pakistan (BCP-2007), seismic site classifications, and corresponding site amplification factors were determined similar to Uniform Building Code (UBC-1997), which were based on the local site conditions of Western United States with low impedance contrast, deep sites, and high seismicity conditions. The use of these deep sites based amplification factors to the shallow sites may not be appropriate to capture shallow sites' response. In this study, we performed a non-linear site-specific response analysis suite to compute the amplification factors for the six (06) representative shallow bedrock sites of Islamabad, Pakistan. The computed amplification factors are compared with BCP-2007 code-based design estimates; it is found that BCP-2007 code-based guidelines underestimate amplification factors at a short period whereas overestimating at an extended period. The study's findings highlight the potential implications of current code-based amplification factors for Islamabad's shallow sites and suggest improving the current seismic guidelines.

Broadband Q-Factor Imaging for Geofluid Detection in the Gulf of Trieste (Northern Adriatic Sea)

Seismic surveys allow estimating lithological parameters, as P-wave velocity and anelastic absorption, which can detect the presence of fracture and fluids in the geological formations. Recently, a new method has been proposed for high-resolution imaging of anelastic absorption, which combines a macro-model from seismic tomography with a micro-model obtained by the pre-stack depth migration of a seismic attribute, i.e., the instantaneous frequency. As a result, we can get a broadband image that provides clues about the presence of saturating fluids. When the saturation changes sharply, as for gas reservoirs with an impermeable caprock, the acoustic impedance contrast produces “bright spots” because of the resulting high reflectivity at its top. When the fluid content changes smoothly, the anelastic absorption becomes a good detector, as fluid-filled formations absorb more seismic energy than hard rocks. We apply this method for imaging the anelastic absorption in a regional seismic survey acquired by OGS in the Gulf of Trieste (northern Adriatic Sea, Italy).

Assessment of gas hydrate using prestack seismic inversion in the Mahanadi Basin, offshore eastern India

The Indian National Gas Hydrate Program Expedition-01 in 2006 has discovered gas hydrate in the Mahanadi offshore basin along the eastern Indian margin. However, well-log analysis, pressure core measurements, and infrared anomalies reveal that gas hydrates are distributed as disseminated within the fine-grained sediment, unlike massive gas hydrate deposits in the Krishna-Godavari Basin. The 2D multichannel seismic section, which crosses holes NGHP-01-9A and 19B located at approximately 24 km apart, indicates a continuous bottom-simulating reflector (BSR) along it. We aim to investigate the prospect of gas hydrate accumulation in this area by integrating well-log analysis and seismic methods with rock-physics modeling. First, we estimate gas hydrate saturation at these two holes from the observed impedance using the three-phase Biot-type equation. Then, we establish a linear relationship between the gas hydrate saturation and the impedance contrast with respect to the water-saturated sediment. Using this established relation and impedance obtained from prestack inversion of seismic data, we produce a 2D gas hydrate-distribution image over the entire seismic section. The gas hydrate saturation estimated from resistivity and sonic data at well locations varies within 0%–15%, which agrees well with the available pressure core measurements at hole 19. However, the 2D map of gas hydrate distribution obtained from our method indicates that the maximum gas hydrate saturation is approximately 40% just above the BSR between the common-depth points of 1450 and 2850. The presence of gas-charged sediments below the BSR is one of the reasons for the strong BSR observed in the seismic section, which is depicted as low impedance in the inverted impedance section. Closed sedimentary structures above the BSR are probably obstructing the movements of free-gas upslope, for which we do not see the presence of gas hydrate throughout the seismic section above the BSR.

Basin effects and limitations of 1D site response analysis from 2D numerical models of the Thorndon basin

Plane strain (2D) finite element models are used to examine factors contributing to basin effects observed for multiple seismic events at sites in the Thorndon basin of Wellington, New Zealand. The models consider linear elastic soil and rock response when subjected to vertically-propagating shear waves. Depth-dependent shear wave velocities are considered in the soil layers, and the effects of random variations of soil velocity within layers are modelled. Various rock shear wave velocity configurations are considered to evaluate their effect on the modelled surficial response. It is shown that these simple 2D models are able to capture basin reverberations and compare more favourably to observations from strong motion recordings than conventional 1D site response models. It is also shown that consideration of a horizontal impedance contrast across the Wellington Fault affects spectral response and amplification at longer periods, suggesting the importance of this feature in future ground motion modelling studies in the Wellington region.

Time–frequency analysis for delineating gas hydrates and free gas in the Mahanadi offshore, India

The present work demonstrates a case study in the Mahanadi offshore basin in the eastern margin of India, where gas hydrates have been delineated by seismic experiment, mainly by identifying a bottom simulating reflector or BSR on seismic section, based on its characteristic features. The BSR represents an acoustic impedance contrast between the high-velocity gas hydrates bearing sediments above and low-velocity gas-bearing or water-saturated sediments below. At some places along the continental margins of the world, the BSR has not been identified but gas hydrates were recovered by drilling, whereas at some other places BSR was identified but no gas hydrates were found. Thus, we need to find out an attribute that can be used to characterise gas hydrates and free gas reservoirs. The purpose of this work is to demonstrate this through spectral decomposition in the time–frequency domain. The study based on continuous wavelet transform (CWT) approach, which uses a mathematical mother wavelet, shows the illuminated zone with higher energy low-frequency anomaly, associated with free gas below the BSR. The study demonstrates that the CWT technique provides higher spectral-spatial resolution than that obtained by the short-term Fourier transform technique.

Ultrasonic Imaging of High-contrasted Objects Based on Full-waveform Inversion: Limits under Fluid Modeling.

Quantitative ultrasound techniques have been previously used to evaluate biological hard tissues, characterized by a large acoustic impedance contrast. Here, we are interested in the imaging of experimental data from different test-targets with high acoustic impedance contrast, using the Full Waveform Inversion (FWI) method to solve the inverse problem. This method is based on high-resolution numerical modeling of the forward problem of interaction between waves and medium, considering the full time series. To reduce the complexity of the numerical implementation, the model considers a fluid medium. Therefore, the aim is to evaluate the precision of the reconstruction under this assumption for materials with a different level of attenuation of shear waves, to study the limits of this hypothesis. Images of the sound speed obtained using the experimental data are presented, and the precision of the reconstruction is evaluated. Future work should include viscoelastic materials.

Influence of site conditions on seismic design parameters for foundations as determined via nonlinear site response analysis

Site conditions, including geotechnical properties and the geological setting, influence the near-surface response of strata subjected to seismic excitation. The geotechnical parameters required for the design of foundations include mass density (ρ), damping ratio (βs), shear wave velocity (Vs), and soil shear modulus (Gs). The values of the last three parameters are sensitive to the level of nonlinear strain induced in the strata due to seismic ground motion. In this study, the effect of variations in soil properties, such as plasticity index (PI), effective stress (σ′), over consolidation ratio (OCR), impedance contrast ratio (ICR) between the bedrock and the overlying strata, and depth of soil strata over bedrock (H), on seismic design parameters (βs, Vs, and Gs) was investigated for National Earthquake Hazards Reduction Program (NEHRP) site classes C and D, through 1D nonlinear seismic site response analysis. The Morris one-at-a-time (OAT) sensitivity analysis indicated that βs, Vs, and Gs were significantly influenced by variations in PI, while ICR affected βs more than it affected Vs and Gs. However, the influence of H on these parameters was less significant. It was also found that variations in soil properties influenced seismic design parameters in soil type D more significantly than in soil type C. Predictive relationships for βs, Vs, and Gs were derived based on the 1D seismic site response analysis and sensitivity analysis results. The βs, Vs, and Gs values obtained from the analysis were compared with the corresponding values in NEHRP to determine the similarities and differences between the two sets of values. The need to incorporate PI and ICR in the metrics for determining βs, Vs, and Gs for the seismic design of foundations was highlighted.

The Abra sedimentary-hosted Pb-Ag-Cu-Au deposit, Western Australia: A geophysical case study

Abra is a high-grade sedimentary-hosted Pb deposit located in the Paleoproterozoic Edmund Basin in Western Australia. Mineralization is blind, with the top of the deposit occurring 250 m beneath the land surface. The deposit consists of a stratiform apron of Pb-Ag-Ba mineralization in laminated iron-oxide- and barite-altered dolomite and siltstone, which overlies a feeder zone of chlorite-altered, brecciated, and veined carbonatic siltstone that contains Pb-Ag mineralization in the core that transitions to Pb-Cu and Cu-Au at depth. Abra is characterized by discrete geophysical anomaly responses in magnetic, gravity, time-domain electromagnetic (TDEM), and induced polarization survey data. A +450 nT magnetic anomaly is caused by magnetite in the lower stratiform zone. Dense galena, barite, dolomite, and iron-oxide mineralization in the apron and galena in the feeder zone is surrounded by lower-density sedimentary host rocks, which results in a +1 mGal gravity anomaly. Airborne, ground, and downhole TDEM surveying resolved known mineralization as weak electromagnetic conductor responses, and petrophysical testing on core samples shows that this is caused by galena. Pole-dipole-induced polarization surveying resolved a +20 ms chargeability anomaly on the southern flank of the deposit. This chargeable anomaly response is related to disseminated galena, pyrite, chalcopyrite, and alteration. Joint audiomagnetotelluric-magnetotelluric 2D inverted data sections resolved Abra as a broad weakly conductive anomaly. Weak conductor responses associated with Abra were also resolved in 2D and 3D inversion modeling of airborne Z-axis tipper electromagnetic data. 2D seismic reflection surveying resolved Abra as strong flat-lying seismic reflectors, which are bounded and offset by faults and surrounded by a seismically bland zone. The seismic reflections are related to significant density contrasts between high-density stratiform mineralization that is in contact with low-density sedimentary host rocks, as the mineralization and host rocks have similar seismic velocities. Passive seismic horizontal to vertical spectral ratio surveying resolved the top of the deposit as a subtle layer sitting below a flat impedance contrast horizon that is interpreted as weathered siltstone on top of diagenetically cemented siltstone.

Seismic Signature of Transition Zone (Wolf Ramp) in Shale Deposits with Application of Frequency Analysis

The concept of a transition zone, known as Wolf ramp, was incorporated into the seismic interpretation of a 3D seismic survey situated within the Baltic Basin (Northern Poland). Within the survey area, there exists one formation, the Pasłęk Formation, (Lower Silurian—Llandovery), that exhibits a linear change of velocity. This characteristic—linear change of velocity—causes a reflection coefficient (i.e., seismic amplitude) produced at such a boundary to be frequency dependent. The Pasłęk Formation was considered to be a potential shale gas reservoir and it was necessary to determine its structural position and thickness. The formation is challenging for robust seismic interpretation on the migrated seismic section—it does not manifest a stable reflection coefficient, and the amplitude contrast associated with the borders of the formation is low. There is no impedance contrast that would produce a reflection of high amplitude at the top or base of the formation which excludes determination of the formation thickness, hence the estimation of reservoir volume. Within a 3D dataset, there exists only one well with complete logs that were used for the analysis. The Pasłęk Formation is a flat-lying layer that continues itself far beyond the 3D survey. It is present in wells in the vicinity of the study area. These wells lay within other 3D or 2D datasets, but the quality of the seismic is poor, and similar seismic analysis is not possible. Nevertheless, these wells were incorporated in the research to reason about the possible link between the existence of transition zone and mineral content. The method used for recognition of transition zone is spectral decomposition and spectral analyses. The integrated studies enabled us to find a link between the Wolf ramp and mudstone-claystone interval of the Silurian age and give a new example of a transition zone which is present in shale plays. The transition zone concept might be applied for shale plays identification and analysis.

Practical guidelines for near-field mitigation on array-based active surface wave testing

SUMMARY This paper aims to develop practical guidelines for near-field mitigation for active source surface wave testing. To this end, extensive field measurements were performed considering different factors, including depth to impedance contrast, source offset, source type, surface wave type (Rayleigh or Love) and transformation technique used for data processing. According to the results, near-field effects are independent of surface wave type and depth to impedance contrast. For sites with a very shallow impedance contrast, the cylindrical frequency domain beamformer transformation technique outperforms others in terms of dispersion resolution by significantly mitigating near-field effects. On the other hand, for sites with a very deep impedance contrast, the four transformation techniques provide the similar dispersion resolution when only considering near-field effects. It is also revealed that the normalized array centre distance (NACD) criteria required to mitigate near-field effects is a function of source type. Using 10–15 per cent as the accepted error boundary, a NACD of 1.0 or greater is recommended for low-output impulsive sources such as a sledgehammer source, whereas, for high-output harmonic sources such as a vibroseis, a normalized array centre distance of 0.5 is recommended. A standard deviation on the NACD of approximately 0.3 was observed in much of the data indicating near-field effects are site-specific and can affect the surface dispersion velocity at wavelengths longer or shorter than the recommended NACD. These NACD criteria should not be violated when using a limited number of source offsets (1 or 2). However, if the multiple source offset approach (≥3 source offsets) is used where some of the source offsets meet the criteria, the near-field criteria can be violated for some of the source offsets, given that the near-field effects can be mitigated using the composite dispersion data generated from different source offsets if those offsets produce acceptable dispersion data.

Revealing the geological materials properties by a shallow seismic method for investigating slope site effects: a case study of Qiaozhuang town, Qingchuan County, China

This article presents a description of the lithology and weathered features of the subsurface under Qiaozhuang Town through a 2-D interpretation based on a shallow seismic reflection geophysical survey. To obtain the overburden, phyllite, and limestone geological parameters for the dynamic effect assessment of the slope site, twelve shallow seismic profiles were collected in the target area at slope sites where intensive ground damage was caused by the Wenchuan earthquake. To expose the geological features of rock masses with different lithologies, six shots field recordings for each profile were performed. The results showed reflection signals of a seismic wave that exhibited four to five weathered layers within a shallow depth range. The interpretation of velocity analysis of the geological imaging shows that the elastic velocities of the reflection layers increase with depth. The overburden, the first layer of slope surface, had velocities in the range of 724 m/s to 1024 m/s with the thickness changing in a large domain from 5 to 25 m, which was partially revealed in an exploration shaft and in a borehole. In deeper bedrock, the limestone was characterized by a mean velocity of 1470 m/s to 3760 m/s and the phyllite was characterized by a mean velocity of 1440 m/s to 3580 m/s. Moreover, the geological imaging of partial profiles showed that a weak and discontinuous reflection seismic wave may be interpreted as a fractured zone. The interpretation results reveal that the elastic velocity changing with lithology material is clear presence, leading to the impedance contrast varies from the slope inside to the surface. In particular, the presence of lower velocity layer, such as the overburden, which big material impedance contrast (> 1.0) likely formed strong slope site effects. Therefore, the geological material property of the study area was probably another factor causing the slope failure during the Wenchuan earthquake.

On the Simulation of the Influence of Defects on Immersed Plane Periodic Multilayer Viscoelastic Media

This work deals with the study of a plane periodic multilayer structure in which the elementary stack consists of two plates in contact: one in aluminum (AL) and the other one in polyethylene (PE). These isotropic materials, present a high acoustic impedance contrast. The attenuation of the longitudinal and transverse waves is taken into account in the polyethylene but neglected in the aluminum plate. The effect of different defects is analyzed. Firstly, we focus on the effect of the presence of grease inclusion in the polyethylene plate (considering the two plates of the elementary stack in perfect contact). Secondly, the effect of disbond simulated by the insertion of a thin Teflon layer between the interfaces of the two layers constituting the elementary stack of the multilayer structure is investigated. Finally, the effect of the stacking sequences of the multilayer is analyzed. In order to obtain the effective acoustic parameters of polyethylene layer, allowing to evaluate the reflection and transmission coefficients using the stiffness matrix method developed by Rokhlin et al., four homogenization models are analyzed, then the best one to our configuration is chosen. The comparison of the simulation results is carried out.