Horizontal Shear Wave Velocity Research Articles

VTI Anisotropy in the Jamieson and Echuca Shoals Formations in the Browse Basin

Overburden shales that overlie and seal hydrocarbon reservoirs usually exhibit polar anisotropy, also called Vertical Transverse Isotropy (VTI). This anisotropy is important for correct seismic inversion, seismic-to-well ties as well as having geomechanical implications. P-wave anisotropy cannot usually be determined from a vertical well unless a walkaway vertical seismic profile (VSP) has been obtained, however, such measurements are still rare. S-wave anisotropy though can be estimated from logs if the speed of sound in mud and the Stoneley wave velocity in the shale are known. Then, the P-wave anisotropy can be computed using theoretical models or empirical trends. The Stoneley wave velocity is nowadays routinely measured by sonic tools and, if a reliable mud velocity is known, the horizontal shear wave velocity (parallel to and polarised in the bedding plane) can be estimated. Thomsen’s gamma parameter for S-wave anisotropy can then be calculated. If mud velocity is not known, the horizontal shear wave velocity can be obtained using calibration in an isotropic interval. Using this method, we analyse the VTI anisotropy in the Torosa-6 well in the Caswell Sub-basin of the Browse Basin, Australia. Torosa-6 drilled through the Jamieson and Echuca Shoals shaly formations where Vclay reaches ~75%. Elastic anisotropy of the shaly Jamieson and Echuca Shoals Formations has been analysed. Thomsen’s gamma shows a good correlation with the clay fraction in each of these formations. However for the same clay fraction, anisotropy is about 20% higher in the Jamieson Formation compared to the Echuca Shoals. This Jamieson Formation contains up to 15% of smectite, and we are investigating how this may lead to higher levels of VTI anisotropy compared with illitic clays predominant in the Echuca Shoals Formation.

Evaluation method of the least horizontal principal stress by logging data in anisotropic fast formations

Abstract For transverse isotropic fast formations, the evaluation method of the least horizontal principal stress by using logging data is an important unresolved issue. An innovative method is proposed to solve this problem by derivation of five independent stiffness coefficients (C11, C33, C44, C66 and C13) in this kind of formation. Based on the functional relations between acoustic anisotropy coefficients and clay volume, and that between different stiffness coefficients, which are all approved by the assorted experiment data, an effective method is built to calculate the stiffness coefficients and the least horizontal stress of anisotropic fast formations. Successful applications in the Ordos Basin illustrate that the method is complementary to that based on the horizontal shear wave velocity which is only fit for slow formations, and is applicable to evaluating rock mechanical parameters of tight oil and gas reservoirs and selecting intervals for fracturing and testing oil.

Crustal structure beneath Liaoning province and the Bohai Sea and its adjacent region in China based on ambient noise tomography

The velocity structure of the crust beneath Liaoning province and the Bohai sea in China was imaged using ambient seismic noise recorded by 73 regional broadband stations. All available three-component time series from the 12-month span between January and December 2013 were cross-correlated to yield empirical Green’s functions for Rayleigh and Love waves. Phase-velocity dispersion curves for the Rayleigh waves and the Love waves were measured by applying the frequency-time analysis method. Dispersion measurements of the Rayleigh wave and the Love wave were then utilized to construct 2D phase-velocity maps for the Rayleigh wave at 8–35 s periods and the Love wave at 9–32 s periods, respectively. Both Rayleigh and Love phase-velocity maps show significant lateral variations that are correlated well with known geological features and tectonics units in the study region. Next, phase dispersion curves of the Rayleigh wave and the Love wave extracted from each cell of the 2D Rayleigh wave and Love wave phase-velocity maps, respectively, were inverted simultaneously to determine the 3D shear wave velocity structures. The horizontal shear wave velocity images clearly and intuitively exhibit that the earthquake swarms in the Haicheng region and the Tangshan region are mainly clustered in the transition zone between the low- and high-velocity zones in the upper crust, coinciding with fault zones, and their distribution is very closely associated with these faults. The vertical shear wave velocity image reveals that the lower crust downward to the uppermost mantle is featured by distinctly high velocities, with even a high-velocity thinner layer existing at the bottom of the lower crust near Moho in central and northern the Bohai sea along the Tanlu fault, and these phenomena could be caused by the intrusion of mantle material, indicating the Tanlu fault could be just as the uprising channel of deep materials.

Assessing the coefficient of the earth pressure at rest from shear wave velocity and electrical resistivity measurements

Determination of earth pressure coefficient K0 in natural soil deposits is a significantly important issue in geotechnical engineering. This paper evaluates the feasibility of predicting K0 values from measurements of vertical and horizontal shear wave velocities [Vs(VH), Vs(HH), and Vs(HV)], and vertical and horizontal electrical resistivities (SV, SH). A series of seismic piezocone tests (SCPTU), improved crosshole (CH) tests, and soil resistivity tests using a modified laboratory apparatus were carried out at two research sites in Jiangsu Province of China and results were used to develop two existing K0-shear wave velocity correlations for prediction of K0. The seismic and electrical resistivity test results showed that both the shear wave velocity ratios, Vs(HH)/Vs(HV) or Vs(HV)/Vs(VH), and the horizontal to vertical electrical resistivity ratio, SH/SV, may provide a good indicator of the inherent soil anisotropy at these sites. The existing Vs–K0 prediction framework largely requires determining Cs(HV)/Cs(HH) ratio and a simple procedure for evaluating this ratio based on the measured horizontal and vertical electrical resistivities. The K0 values predicted by shear wave velocity at both sites are also compared with values determined by Jaky's formula and Mayne & Kulhwy's formula. In normally consolidated (NC) soils, the K0 values predicted from these methods showed good agreement. However, for over-consolidated (OC) soils at shallow depths, certain variations between different methods are noted. Further investigations of the effects of overconsolidation ratio on K0 predictions will lead to the correction factors for better prediction of K0 for OC soils. This paper has confirmed that reasonable predictions of K0 for natural soil deposits can be made using the proposed methodology.

Elasticity of (K,Na)AlSi 3O 8 hollandite from lattice dynamics calculations

We compute the elastic constants tensor and the seismic properties of KAlSi 3O 8 and K 0.8Na 0.2AlSi 3O 8 up to the ferroelastic transition using density-functional theory and density-functional perturbation theory in the ABINIT implementation. We observe a softening of the tetragonal shear with pressure that precedes the ferroelastic transition. The Reuss shear moduli become negative at respectively 23 GPa and 13 GPa for the two compositions considered in here. The ferroelastic transition is associated with a strong decrease of the horizontal shear wave velocities and a corresponding increase of the seismic anisotropy. The presence of Na enhances these features.

Surface-mounted bender elements for measuring horizontal shear wave velocity of soils

The bender element testing features its in-plane directivity, which allows using bender elements to measure the shear wave velocities in a wider range of in-plane configurations besides the standard tip-to-tip alignment. This paper proposed a novel bender element testing technique for measuring the horizontal shear wave velocity of soils, where the bender elements are surfacemounted and the axes of the source and receiver elements are parallel to each other. The preliminary tests performed on model ground of silica sand showed that, by properly determining the travel distance and time of the shear waves, the surface-mounted bender elements can perform as accurately as the conventional “tip-to-tip” configuration. Potentially, the present system provides a promising nondestructive tool for characterizing geomaterials and site conditions both in laboratory and in the fields.

Stress Induced and Inherent Anisotropy on Elastic Stiffness of Soft Clays

The influence of anisotropy on the elastic shear modulus of clays was addressed using test results obtained from a series of laboratory tests; i.e., triaxial test and Square oedometer. Under various stress stages imposing under triaxial condition, the deviator stress had an indiscernible influence on the elastic shear modulus. The path of vertically logged elastic shear modulus of clay could reasonably be expressed in terms of mean effective stress. Two pairs of bender elements were installed in the square oedometer so that shear wave velocities of samples which were trimmed parallel or perpendicular to their bedding direction can be measured in the vertical and horizontal directions. For sample trimmed parallel to the bedding direction (conventional one), the horizontal shear wave velocity was larger than the vertical one. The opposite observation, however, was observed for sample trimmed 90 degrees apart. The extents of different in vertically and horizontally logged shear wave velocities from both types of samples were almost similar. This clearly indicated the influence of inherent anisotropy rather than the stress induced. The mean effective stress can well represent the stage variable of elastic shear modulus.

Stoneley-wave speed modeling in general anisotropic formations

For determining anisotropy using acoustic logging data, Stoneley waves are the only wave modes that are sensitive to formation properties transverse to the borehole axis. We derive Stoneley-wave speeds in deviated and horizontal wells penetrating anisotropic formations. We first apply the elastic theory of an anisotropic body to find the radial displacement of a borehole cross section under uniform pressure. We then derive the effective formation modulus based on quasistatic analysis. Finally, we derive a two-dimensional analytical solution for low-frequency Stoneley-wave speed in a horizontal well penetrating a transversely isotropic formation with a vertical symmetry axis. For deviated wells, we derive a 3D analytical solution that is also valid for general anisotropic formations. Our analytic solutions agree with the finite-difference modeling results and are particularly suited for strongly anisotropic formations. With increasing well deviation the sensitivity of Stoneley-wave speed to horizontal shear-wave velocity decreases and the sensitivity to vertical shear-wave velocity increases.