S-wave anisotropy from two dipole sonic data processing methods, confronted with fracture permeability, logs and cores

Shear-wave splitting, also known as shear-wave birefringence, has been widely used to evaluate subsurface anisotropy. Yet, the effects of stress and fluid saturation on S-wave anisotropy are still not well understood. A laboratory study was conducted on a Bentheim sandstone sample subjected to different stress and saturation conditions. The S-wave stress-induced anisotropy of the sample was measured using a laser Doppler interferometer (LDI), which allows estimating the particle displacement vector at the central point on the surface of the sample. Based on the derived hodograms, the polarization of the fast S-wave and the degree of the S-wave anisotropy were inferred. In the dry sample, a shift in the polarization direction toward the direction of the applied stress and an increase in the anisotropy degree in some cases were observed, most likely due to the preferential opening of cracks in the direction along the applied stress. For the water-saturated case, no significant change in the S-wave anisotropy was observed.

Due to overburden and tectonic stresses, many deep formation rocks contain vertically oriented fractures, resulting in significant seismic S-wave anisotropy and attenuation, as commonly observed from borehole acoustic anisotropy measurements in deep wells. Understanding the fracture-induced phenomena is of theoretical interest and practical importance. By applying a fracture scattering theory to a fluid-saturated rock containing vertical fractures, we investigate the fracture-induced effects on dynamic S-wave moduli, anisotropy, and attenuation anisotropy in the rock medium. This includes the effects of fracture-background wave-induced fluid flow and elastic scattering on the fractured rock’s stiffness. The results indicate that fracture elastic scattering has a significant influence on the S-wave moduli and attenuation anisotropy. Compared with the velocity response of elastic scattering, Rayleigh scattering increases the S-wave anisotropy, whereas Mie scattering decreases it. The elastic scattering-dominated S-wave anisotropy and attenuation anisotropy are significantly affected by fracture size (diameter) and density. We use the theoretical results to interpret field acoustic anisotropy and attenuation anisotropy data acquired in a deep well, wherein the theory and measured data are in good agreement.

Summary The extent and seismic properties of magma bodies beneath the subsurface are normally ascertained via seismic methods. The seismic properties of such bodies depend on the composition and microstructure of both melt and crystals. Using effective medium theory, we assess the effect of microstructure, geometry and orientation of melt and crystals on the seismic properties, namely P- and S-wave velocities and anisotropy. We find that S-wave velocity and anisotropy are key observables in determining the composition and microstructure of magma bodies.

Crossed dipole sonic logging data has traditionally been used in the openhole environment to:identify natural fracture systems and their orientation,evaluate far field stress data for 3D stimulation modeling, andseismic correlations and synthetic modeling. Materials presented inthis paper will show open hole and cased hole comparisons of flexural/shearwave slowness data and the anisotropy analysis in both fast and slowformations. The paper will show through-casing fracture identification examplesfrom the Cretaceous-Jurassic formations in Southern Mexico that have beenconfirmed with inflow analysis. The paper will also show applications ofthrough-casing anisotropy analysis combined with oriented perforating in theBurgos Basin in North Eastern Mexico. These techniques have resulted in lowerbreakdown pressures on stimulation treatments due to minimizing near wellboretortuosity. 2D and 3D analytical and finite-difference wave propagationsimulators have been developed to verify the open-hole and cased-holescenarios. The finite-difference simulations are used as a predictive tool toidentify cased-hole conditions that are either favorable, or unfavorable forcased-hole dipole acquisition. Introduction Dipole logging requires the propagation of a low frequency out-of-phaseflexural wave that is detected by the acoustic receivers on opposite sides ofthe logging tool. To accomplish this symmetry is required. The wireline dipolesonic tool must be centralized in a near round borehole. For through casingdipole logging, the casing should also be centralized, and the material in thecasing-formation annulus must be radially uniform. The effects of boreholeovality, tool centralization, or casing centralization on waveform propagationare illustrated in Fig. 1 for a 1.5 kHz center frequency flexural wave. Lowerfrequency flexural/dipole transmitted pulses have longer wave lengths, thanhigher frequency flexural wave propagation, thereby slightly reducing the phaseshift due to borehole geometry effects. An elliptical borehole is common indeviated wells due to eccentralization of the drill string and bottom holeassembly. Acquiring crossed dipole sonic data at low frequency and in two axes(X and Y) often minimize these environmentally induced effects. The crossed dipole sonic logging tool is frequently run open hole in what isreferred to as a "quad-combo" (natural gamma ray, eccentered dual spacedneutron, density, dual induction, or dual lateralog with appropriate standoffs, centralized dipole sonic, and navigation tool). One such logging toolstring is configured such that the X-dipole measurement is aligned with thedensity pad, which due to gravity effects will seek the low side of theborehole in deviated well conditions. Therefore, the X dipole measurement maybe affected by tool positioning effects in the angle build sections of deviatedwellbore. The Y-dipole measurement is orthogonal to the density pad and in poorborehole conditions often provides quality shear wave slowness data. Aspreviously discussed, centralization is required, but in certain bore holeconditions this may not be operationally possible. Acquiring crossed dipoledata through casing is a viable option, provided the borehole conditions arefavorable. Unfavorable conditions for through casing dipole logging areexcessive washouts, poor radial placement of cement, lack of casingcentralization, and poorly consolidated formations. Advanced waveformprocessing, such as slowness anisotropy analysis, which is used to determinethe fast and slow shear wave travel times and their corresponding orientationrequires that a navigation device be run simultaneously with the dipole sonictool.1 In open hole logging conditions magnetometers andaccelerometers are utilized. In deviated cased hole logging conditionsmulti-axes accelerometers can be used. In near vertical cased hole loggingconditions a gyroscope is required. Anisotropy analysis can be performedwithout tool navigation data, to determine the magnitude of the anisotropy, butthe orientation of the fast and slow shear waves is referenced to the toolface, which can rotate while logging. This is frequently done in geologicconditions where the anisotropy is know to be a result of natural fracturing, and fracture orientation is not desired.

MgSiO 3 perovskite is a principal mineral in the upper part of the lower mantle, but its stability and possible phase transition at greater depths have long been uncertain. Recently, a phase transition to post-perovskite was discovered through a significant change in the X-ray diffraction (XRD) pattern at high-pressure and high-temperature conditions corresponding to the core-mantle boundary (CMB) region. Experiments on natural pyrolitic mantle and mid-oceanic ridge basalt (MORB) compositions also show that Al-bearing (Mg,Fe)SiO 3 post-perovskite is a predominant mineral in the lowermost mantle called D layer. Many characteristics of the D layer, such as D seismic discontinuity, S-wave anisotropy, and anti-correlation between the anomalies in S-wave and bulk-sound velocities, may be explained by this new phase without the need for chemical heterogeneities. However, simply by its location, the D layer likely has very complex chemical structures. Dense subducted MORE crust may have accumulated into chemically-distinct piles underneath upwellings. Partial melting at ultra-low velocity zone (ULVZ) could cause significant chemical differentiation. The bottom of the mantle is likely depleted in iron by the consequence of chemical reaction with the outer core.

Summary Despite continued improvements in acoustic-logging technology, sonic logs processed with industry-standard methods often remain affected by formation damage and mud-filtrate invasion. Quantitative understanding of the process of mud-filtrate invasion is necessary to identity and assess biases in the standard estimates of in-situ compressional- and shear-wave (P- and S-wave) velocities. We describe a systematic approach to quantify the effects of mud-filtrate invasion on borehole acoustic logs and introduce a new algorithm to estimate radial distributions of elastic properties away from the borehole wall. Radial saturation distributions of mud filtrate and connate formation fluids are obtained by simulating the process of mud-filtrate invasion. Subsequently, we calculate radial distributions of the elastic properties using the Biot-Gassmann fluid-substitution model. The calculated radial distributions of formation elastic properties are used to simulate array sonic waveforms. Finally, estimated P- and S-wave velocities for homogeneous, stepwise, and multilayered formation models are compared to quantify mud-filtrate-invasion effects on sonic measurements. We use a nonlinear Gauss-Newton inversion algorithm to estimate radial distributions of formation elastic parameters in the presence of invaded zones using normalized spectral ratios of array waveform data. Inversion examples using synthetic and field data indicate that physically consistent distributions of formation elastic properties can be reconstructed from array waveform data. In turn, radial distributions of formation elastic properties can be used to construct more-realistic near-wellbore petrophysical models for applications in reservoir simulation and production. Introduction During and after drilling, the near-wellbore formation is often altered by stress buildup and release, mud-filtrate invasion, chemical reactions, and many other factors. These alterations cause the physical properties in the near-wellbore region to be different from those of the virgin rock formation. Stress concentration around a wellbore may cause near-wellbore damage and induce formation anisotropy on P- and S-wave velocities. The stress-induced anisotropy can be identified by dispersion analysis (Plona et al. 2002). Positive radial velocity gradients focus the elastic waves propagating away from the wellbore back toward the borehole wall. Such a phenomenon can be identified easily from high-amplitude acoustic arrivals. In this paper, we focus our attention on mud-filtrate-invasion effects only. It is well known that formation properties inferred from wireline logging measurements may not reflect true properties of virgin formations. A realistic description of the invaded zone is important for the processing and interpretation of sonic logs. A common model used in the open literature assumes that a sharp interface exists between the altered zone and the undisturbed formation (Baker 1984). The term "stepwise" is used to describe this type of mud-filtrate-invasion model. Linear-gradient models have been described for syntheses of acoustic waveforms as well (Stephen et al. 1985). Actual radial distributions of elastic wave properties resulting from invasion can be complex and are dependent on the specific petrophysical properties of the rock as well as on the static and dynamic properties of the fluids involved. We divide the invaded zone into a set of concentric radial layers to represent an actual invasion profile and call it a "multilayered" model. Subsequently, we describe a procedure for calculating the radial distributions of formation elastic properties with the Biot-Gassmann fluid-substitution model starting from numerically simulated radial distributions of flow saturation.

Laboratory measurements of P-wave and S-wave velocities across a surface analog of the continental crust–mantle boundary: Cabo Ortegal, Spain

Experimentally deformed lawsonite at high pressure and high temperature: Implication for low velocity layers in subduction zones

Pierre shale and its stratigraphic equivalents represent a potentially favorable geologic environment for underground storage of hazardous waste products. These rock formations cover great areal and vertical extents, and represent some of the least permeable rock formations within the continental United States. There are, however, several engineering problems associated with constructing underground openings in Pierre shale. This formation is relatively weak and tends to deteriorate rather rapidly if not protected from the mine environment. It will be necessary to place all underground openings below the surficially weathered upper 50 to 70 feet of Pierre shale which contains groundwater moving on fracture permeability. The optimum site for disposal of hazardous waste in Pierre shale, or its stratigraphic equivalents, would be a seismically stable platform bounded on all sides by faults. The optimum size of individual openings would be the minimum necessary for access, storage, and retrieval of waste components. Underground excavations in Pierre shale must be made with care, must be of limited dimensions, must be widely spaced, must be protected from prolonged contact with the mine environment, must be supported immediately after excavation, and must be sited to avoid areas of faulting and(or) intense jointing. Underground openings constructed with boring machines and supported with wet shotcrete are recommended.

This chapter shows that the principles and the sensitivity of the ''High-Pulse Poroelasticity Protocol'' (HPPP) test, provides an example of an in situ estimation of joint properties in a natural slope, and the results in terms of joint strength estimation. A long-term injection induces a large slope deformation, measured at the injection point with the HPPP probe and in the near field with tiltmeters. The HPPP is a new in situ approach developed for a very large broadband geophysical monitoring of rock hydromechanical deformations within boreholes. For a given slope geometry, the HPPP test enables an in situ estimation of the hydraulic and mechanical properties of discontinuities at the single fracture scale, as well as the coupling relationship between those key properties. The initial normal stiffness Kni and initial hydraulic aperture bhi can be estimated using the hyperbolic normal stiffness-hydraulic aperture relashionship.

A thorough understanding of a rock formation’s structure and sedimentology is crucial before engaging in hydrological exploration or any development project. Islamabad is going through rapid development, so it’s crucial to update the subsurface studies for the benefit of the citizens. This research aims to study the Sedimentological characteristics of the Murree Formation through lithofacies analysis and to know the hydrological characteristics through Fracture analysis. Fracture analysis was conducted using the circle inventory method to calculate fractured porosity and permeability within the Murree Formation in the Islamabad region, and the lithofacies analysis was performed by observing the Sedimentological characteristics of each rock bed. Fracture analysis indicates that the rock formation in the Shah Alla Ditta region is not suitable enough to be classified as an aquifer. At the same time, the lithofacies study shows that the Murree Formation was deposited in a fluvial environment.

<p>We present a new S-wave velocity tomographic model of the Australian Plate, Aus21.  It is constrained by waveforms of 0.9 million seismograms with both the corresponding sources and stations located within the half-hemisphere centred at the Australian continent. Waveform inversion extracts structural information from surface, S- and multiple S-waves on the seismograms in the form of a set of linear equations. These equations are then combined in a large linear system and inverted jointly to obtain a tomographic model of S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle. The model has been validated by resolution tests and, for particular locations in Australia with notable differences with previous models, by independent inter-station measurements of surface-wave phase velocities, which we performed using available array data. </p><p> </p><p>Aus21 offers new insights into the structure and evolution of the Australian Plate and its boundaries. The Australian cratonic lithosphere occupies nearly all of the western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it is colliding with island arcs, without subducting. The rugged eastern boundary of the cratonic lithosphere provides a lithospheric definition of the Tasman Line. The thin, warm lithosphere below the eastern part of the continent, east of the Tasman Line, underlies the Cenozoic volcanism locations in the area. The lithosphere is also thin and warm below much of the Tasman Sea, underlying the Lord Howe hotspot and the submerged part of western Zealandia. A low velocity anomaly that may indicate the single source of the Lord Howe and Tasmanid hotspots is observed in the transition zone offshore the Australian continent, possibly also sourcing the East Australia hotspot. Another potential hotspot source is identified below the Kermadec Trench, causing an apparent slab gap in the overlying slab and possibly related to the Samoa Hotspot to the north. Below a portion of the South East Indian Ridge (the southern boundary of the Australian Plate) a pronounced high velocity anomaly is present in the 200-400 km depth range just east of the Australian-Antarctic Discordance (AAD), probably linked to the evolution of this chaotic ridge system.</p>

SUMMARY Data from four three-component ocean bottom seismographs (OBS) situated on a 145 km long profile on the Rost High on the continental shelf off Lofoten, northern Norway, have been modelled using 2-D seismic ray tracing, and a P- and S-wave velocity model along the profile is presented. The S-wave model has been obtained from a study of the high quality, horizontal component data. A Vp/Vs ratio of 1.95–2.15 is found in the sedimentary layers on the Rost High. These high values are consistent with shaly sediments, which are identified in dredged bedrock samples. South of the Rost High a considerably lower Vp/Vs ratio (ca. 1.6) is estimated for the sediments, which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology. In the upper and middle crystalline crust a Vp/Vs ratio of 1.75 is obtained. Strong S reflections and P-to-S conversions at the Moho are observed on all four instruments, and the modelling of these arrivals indicates that the S-wave velocity in the lower crust varies with angle of incidence, from 3.5 km s−1 (Vp/Vs= 1.95) at vertical incidence to 4.0 km s−1 (Vp/Vs= 1.7) at 60–70d. This angle of incidence dependent S-wave anisotropy (the order of 10 per cent) is consistent with some horizontal or subhorizontal layering in the lower crust.

Anisotropic tomography of the Atlantic Ocean

Compressional (P-wave) and shear (S-wave) velocities are used to estimate the dynamic geomechanical properties including: Poisson’s ratio, Young’s modulus, and Lame parameters. These parameters are mainly used in estimating the static properties of the formation rocks as well as the in situ stresses. The sonic logs are not always available, epically for old wellbores. Also, in several occasions when the sonic logs are available, missing sections found in the well logs might affect the analysis results. To the authors’ knowledge, there is no single straightforward correlation that can be used to accurately estimate both P- and S-wave travel times directly from the well log data. Most of the existing correlations use the P-wave velocity to measure the S-wave velocity. The main purpose of this study is to develop accurate and simple empirical models using wireline log data (bulk density, gamma ray, and neutron porosity) to predict the sonic travel times (P-wave and S-wave). These wireline logs are slandered wireline log data that are commonly recorded in most of the wells. Three robust artificial intelligence techniques, namely: support vector machine (SVM), artificial neural network (ANN), and adaptive neurofuzzy interference systems (ANFIS), were employed and compared based on their prediction performance. Ultimately, using the weights and biases of optimized ANN model, a simple generalized empirical correlation is derived that can be used without the need of costly commercial software’s to run the AI models. The obtained results showed that ANN, ANFIS, and SVM can be used to estimate P-wave and S-wave travel times. ANN outperformed the ANFIS and SVM by yielding the lowest average absolute percentage error (AAPE) and the highest coefficient of determination $$\left( R^{2}\right) $$ for predicting P-wave and S-wave travel times. ANN model could predict the P-wave and S-wave travel times from wireline log data with high accuracy giving $$R^{2}$$ of 0.98 when compared to actual field data. In addition, the developed empirical correlations prediction completely matched the ANNs prediction. The AAPE of the predicted P and S-waves travel times was less than 5%. The developed correlations are very accurate and can help geomechanical engineers to determine the dynamic geomechanical properties (Poisson’s ratio and Young’s modulus) and propose any operation in case where sonic logs are missing.