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Squirt flow is a wave-induced fluid flow mechanism to account for the velocity dispersion and attenuation of fluid-saturated porous media. Theoretical models of squirt flow cannot deal with complex microcrack-pore networks and thus cannot give accurate dispersion and attenuation curves with respect to frequency. We adopt a numerical oscillatory compressibility test method, based on the quasi-static poroelastic equation of Biot, to model the squirt flow and calculate corresponding P-wave modulus dispersion and attenuation. We also designed nine porous medium models containing different crack-pore configurations and investigated the effects of microcrack porosity, orientation and connectivity on the elastic responses. Modelling results show that not only microcrack porosity but also their orientation has control on the magnitude of the P-wave modulus dispersion and attenuation. Preferentially aligned microcracks may cause frequency-dependent anisotropy in the P-wave modulus and attenuation. Microcrack connectivity has a negligible influence on the dispersion and attenuation magnitude but a large influence on the characteristic frequency of the squirt flow, which is not predicted by theoretical models based on simple representative element volume of crack-pore structure. Therefore, detailed geometry of crack-pore network, of which microcrack porosity, orientation, aspect ratio and connectivity are vital factors, dictates the unique variations in the elastic modulus and associated attenuation with frequency caused by the squirt flow. Pore structure obtained by CT scanning representative of that of the whole rock is needed to obtain more accurate poroelastic responses of the rock with the numerical oscillatory compressibility test method. In this regard, we provide a powerful tool for evaluating dispersion and attenuation of fluid-saturated rock media with complex crack-pore structure, which have potential applications in seismic exploration of hydrocarbon in the subsurface. [ABSTRACT FROM AUTHOR]

The severe rugged seabed interfaces bring great difficulties to seismic imaging in the marine environment. To accurately image submarine structures under the rugged seabed interface, an acoustic-elastic coupled curvilinear-coordinated least-squares reverse time migration (AE-CLSRTM) is proposed. This method is based on the coupled equation method, which uses the acoustic wave equations in seawater and the elastic wave equations in the underlying elastic medium. The pressure in the acoustic wave equations and the stresses in the elastic wave equations are transmitted steadily and continuously by using acoustic-elastic control equations at the seabed interface. To overcome the influence of the rugged seabed interface, the acoustic-elastic model is meshed into non-uniform curvilinear grids, and the corresponding mapping technique is used to transform the model with the rugged seabed interface to a horizontal one in the curvilinear coordinate system through the coordinate transformation. Based on the acoustic-elastic coupled equations in the curvilinear coordinate system, the acoustic-elastic coupled LSRTM algorithm in the rugged seafloor structure is realised. The numerical examples on a simple model and an actual area model show that the proposed LSRTM method can obtain the accurate imaging results of submarine structures in both P- and S-velocity components. [ABSTRACT FROM AUTHOR]

Staggered-grid finite-difference (SFD) method has higher accuracy and stability than conventional method, so it is widely used in reverse time migration and full waveform inversion. However, due to the high dominant frequency and large grid interval, numerical dispersion is an inevitable problem. To suppress numerical dispersion, we first propose a weighted convolution combination window function for SFD, which is based on traditional binomial window function. Accuracy error analysis shows that the spectral coverage of new method far exceeds that of the Taylor-expansion SFD method, and the new method also has better spectral coverage and stability than the previous optimisation methods. Finally, we perform numerical forward modelling, which denotes that our method is superior than other optimum methods. From an economic point of view, this method will effectively reduce the computation cost and improve efficiency. [ABSTRACT FROM AUTHOR]

The finite-difference (FD) scheme is extensively applied in seismic modelling, imaging and inversion due to its advantages of large-scale parallel computing and programming. However, numerical dispersion caused by using a difference operator in substitution for the differential operator is non-negligible, which reduces the accuracy of the modelling and can lead to some misinterpretations. In addition, the computing resources required by the FD scheme is highly demanding when dealing with large models, which limits its applicability. In this paper, a new optimised FD scheme is proposed, which is based on an improved particle swarm optimisation (PSO) algorithm. We improve the conventional PSO algorithm by introducing strategies related to local learning and global learning, which contribute to accelerating the convergence rate and effectively avoiding getting trapped in local extrema. Then, the improved PSO algorithm is used to improve the conventional FD scheme. Dispersion analysis and numerical modelling demonstrate that the low-order optimised FD scheme can achieve higher accuracy than a high-order conventional operator. Compared with the conventional FD scheme and a FD scheme based on the Remez exchange algorithm, the optimised FD scheme based on the improved PSO algorithm can more efficiently suppress numerical dispersion and increase computational efficiency. [ABSTRACT FROM AUTHOR]

Steeply dipping structures bring huge challenges in high-resolution imaging. The imaging of the steeply dipping structures can be improved by incorporating the prismatic waves, which has some steeply dipping structure information where primaries cannot contain. In this paper, an elastic reverse time migration of prismatic waves (ERTM-P) is proposed to produce high-resolution multi-component images of steeply dipping structures with complete compressional (P-) and shear (S-) wave illumination of the subsurface. To avoid severe spurious diffraction caused by dramatic surface topography, the velocities are meshed into curvilinear grids and migration is conducted in curvilinear coordinates. In addition, the imaging conditions of prismatic waves in P- and S-components are derived using the decoupled curvilinear-coordinated elastic equations to suppress the imaging artefact from P- and S-wave crosstalk. In numerical examples, we use an elastic horizontal- and vertical-layer model, topographic elastic sag model and salt dome model with surface topography to demonstrate that the proposed method can accurately handle with the dramatic surface topography and obtains high-quality multi-component images by improving the images of steeply dipping structures. [ABSTRACT FROM AUTHOR]

In seismic exploration, the dominant applications are still based on the acoustic wave assumption because of the simplicity and computational efficiency. Compared to the acoustic wave equation, the elastic wave simulation is more accurate, especially for land data, to characterise the elastic property of the real earth, but it is also far more expensive in terms of the computational cost. The traditional way to solve the first order velocity-stress elastic wave equation is based on a second order staggered grid time domain finite difference stencil, and this stencil is expensive because the time step is usually very small due to the limitation the stability condition and numerical dispersion relations. When the fourth order Lax-Wendroff stencil is applied, the time step could be larger but the computational cost triples that of the second order stencil for each time step. To improve the efficiency of the Lax-Wendroff stencil, we rewrite the first order velocity-stress equations into a combination of second order and zeroth order temporal derivative equations. When the second order stencil is applied to the new equation, there is no benefit compared to the original equation; when the fourth order Lax-Wendroff stencil is applied, the computational cost only doubles for each time step, which is 33% faster than the Lax-Wendroff stencil applied on the original equation. Both memory consumption and computational cost are compared among the four different numerical stencils, and a staggered grid Pseudo Spectral method is applied to prevent the numerical dispersion for computing the spatial derivatives. Numerical tests are performed on a 2D homogeneous media and a 2D line of the SEAM II Arid model. The results suggest that although the memory cost is increased, the Lax-Wendroff stencil applied to the second order temporal derivative equations is the optimal option in terms of the both accuracy and computational cost. [ABSTRACT FROM AUTHOR]

Seismic inversion and reservoir identification are complicated problems with integrated data sets and need a velocity or density model to ensure the stability of the results. However, an elastic parameter model, such as velocity or density, is difficult to obtain with high precision by conventional migration or interpolation, especially during the early exploration stage, which contains few available logging data. The kriging method has proved to be an effective technique for mineral exploration and petroleum geophysics, and has led to a series of expansive techniques in earth science. In this paper, we proposed a new matching method, base value compensation (BVC), for integrating multiscale data sets into model elastic parameters. Considering the variety of original information, we used multiple constrain cokriging for three types of data set at different scales. These three data sets come from logging, seismic attributes and sedimentary facies information. The P-wave velocity model is stable and more representative of the subsurface condition, but we found that this method produces few points with anomalous values in the density model. Those abnormal points cause substantial loss of geological detail at sedimentary boundaries. In most modelling methods, this shortcoming is due to the fact that these three input parameters have significantly different properties. The proposed method can effectively solve this problem by matching the input data sets at a similar observational scale before modelling. We demonstrated this method using a case study in the South China Sea. The decrease in abnormal points in the final modelling results verifies the effectiveness of BVC. [ABSTRACT FROM AUTHOR]

To date, studies of elastic reverse time migration (RTM) have undergone much improvement, but mainly focus on isotropic media. Because anisotropic media is widespread, it is necessary to explore the application of elastic RTM in anisotropic media. We extend the wavefield decomposition method, which is based on decoupled propagation, and three vector imaging conditions to transversely isotropic media with vertical symmetry isotropy (VTI). Two of the imaging conditions are based on the excitation amplitude (EA) and the third is based on source-normalized cross-correlation. First, the wavefield decomposition method is extended to VTI media. This is then tested in a two-layer model. The results show that this extension cannot decompose P- and S-waves perfectly in VTI media; some weak residuals remain. However, the results of a simple model test show that no obvious crosstalk is generated by these weak residuals. Finally, a Hess VTI model is adopted to test the adaptive use of this method in complex media. Many subsurface structures can be clearly recognized in the migrated result, for example, the high-velocity rock body, a fault and two low-velocity interlayers. Compared with PP images, converted PS images have many merits, such as clearer imaging of the anisotropic body, higher resolution and a wider migration aperture. We conclude that the vector decomposition method and three vector imaging conditions can be applied to prestack elastic RTM for VTI media and satisfactory results obtained. [ABSTRACT FROM AUTHOR]

The seismic velocity of rock material is sensitively affected by the pore geometry, and pore geometry is a key parameter related to the hydraulic properties of the rock. In this study, we propose a grid-search inversion method to estimate pore geometry and grain elastic moduli from observed velocity-porosity relationships. In our inversion, we compare laboratory-derived velocity-porosity relationships with the theoretical relationship calculated via the differential effective medium (DEM) model, assuming rock samples of the same lithology have the same crack aspect ratio. Compared with existing approaches to estimate elastic moduli and pore geometry, our approach is easy to apply because it can be applied to physical properties measured at atmospheric pressure without changing pressure. We tested our proposed inversion method using synthetic data, and successfully estimated the grain elastic moduli and crack aspect ratio. We also applied our inversion method to P-wave velocity and porosity measured in the laboratory on various rock samples acquired at the hydrothermal field and plate spreading centre, and found that the velocity-porosity relationship derived from DEM theory for the inverted model parameters agreed with the laboratory data. Using our proposed method, we estimate the average pore aspect ratio and grain bulk moduli for basaltic and hydrothermal ore deposit samples. Furthermore, the root mean square error (RMSE) distribution obtained in the grid-search inversion enables us to evaluate the uncertainty of the estimated values. [ABSTRACT FROM AUTHOR]

Ultrasonic measurements using laser Doppler interferometry (LDI) have been reported to provide robust estimates of elastic anisotropy of rock samples. In this approach, an ultrasonic wave is emitted by a piezo-electric source and detected by the LDI, which can be configured to measure three components of the particle velocity in a very small area (~1 mm2) of the sample. Repeating these measurements for a dense array of points on the sample's surface gives a distribution of traveltimes and polarisation fields on the surface. Anisotropy is then obtained by inverting these fields using analytical expressions or numerical algorithms for computing phase and group velocities. The existing implementation of this approach involves the inversion of direct compressional (P) and shear (S) wave arrivals only. A previous study showed that this approach produces stable results if only a small range of source–receiver offsets is included in the inversion. This limitation resulted in a relatively large uncertainty of the result. This uncertainty can be reduced by inverting the entire traveltime field. To this end, we numerically simulate the wavefield in the sample. Analysis of the computed wavefield reveals the presence of P- and S-waves as well as a critically refracted converted PS-wave. Hence, the inversion of the entire traveltime field must include these three waves. We implement this inversion using global minimisation of the traveltime misfit function, coupled with numerical computation of ray velocities. Application of this algorithm to laboratory LDI measurements on a transversely isotropic phenolic sample provides stable anisotropy estimates consistent with previous studies. Laser Doppler interferometers were previously employed to detect ultrasonic waves propagating in different directions and to estimate elastic anisotropy from these measurements. Our numerical simulations and laboratory measurements show that the recorded wavefield contains converted PS-waves, which need to be taken into consideration to obtain robust estimates of anisotropy. [ABSTRACT FROM AUTHOR]