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Understanding the mechanism of photoacoustic generation at the nanoscale is key to developing more efficient photoacoustic devices and agents. Unlike the far-field photoacoustic effect that has been well employed in imaging, the near-field profile leads to a complex wave-tissue interaction but is understudied. Here we show that the spatiotemporal profile of the near-field photoacoustic waves can be shaped by laser pulses, anisotropy, and the spatial arrangement of nanostructure(s). Using a gold nanorod as an example, we discovered that the near-field photoacoustic amplitude in the short axis is ∼75 % stronger than the long axis, and the anisotropic spatial distribution converges to an isotropic spherical wave at ∼50 nm away from the nanorod's surface. We further extend the model to asymmetric gold nanostructures by arranging isotropic nanoparticles anisotropically with broken symmetry to achieve a precisely controlled near-field photoacoustic ''focus'' largely within an acoustic wavelength.

This study discusses the properties and decompositions of the traveltime sensitivity kernels in elastic isotopic and transversely isotropic media. Analytical solutions in homogeneous isotropic elastic media are derived and help better understand their properties. One key component is the kernel phase function which determines the frequency components of the sensitivity kernels, the geometrical setting of the Fresnel zones, and the resolving ability for velocity inversion. For the purpose of exploration seismology, the sensitivity kernels for finite-frequency traveltime inversion in transversely isotropic elastic media are expressed in terms of Thomsen’s anisotropic parameters. The sensitivity kernels in anisotropic media can be decomposed into the summation of the isotropic terms constructed by the elastic properties in the symmetry direction and the correction terms due to the presence of anisotropy. Sensitivity kernels computed from different types of acquisitions contain information for different parameters. We compare the kernels computed using finite-difference with those by dynamic ray tracing. We conclude that the full wave numerical method is able to provide high-resolution kernels, which form the basis for 3D finite-frequency traveltime inversion for velocity and anisotropic parameters.

Seismic migration and inversion are closely related techniques to portray subsurface images and identify hydrocarbon reservoirs. Seismic migration aims at obtaining structural images of subsurface geologic discontinuities. More specifically, seismic migration estimates the reflectivity function (stacked average reflectivity or pre-stack angle-dependent reflectivity) from seismic reflection data. On the other hand, seismic inversion quantitatively estimates the intrinsic rock properties of subsurface formulations. Such seismic inversion methods are applicable to detect hydrocarbon reservoirs that may exhibit lateral variations in the inverted parameters. Although there exist many differences, pre-stack seismic migration is similar with the first iteration of the general linearized seismic inversion. Usually, seismic migration and inversion techniques assume an acoustic or isotropic elastic medium. Unconventional reservoirs such as shale and tight sand formation have notable anisotropic property. We present a linearized waveform inversion (LWI) scheme for weakly anisotropic elastic media with vertical transversely isotropic (VTI) symmetry. It is based on two-way anisotropic elastic wave equation and simultaneously inverts for the localized perturbations (ΔVp0 =Vp0;ΔVs0 =Vs0;Δe;Δδ) from the longwavelength reference model. Our proposed VTI-elastic LWI is an iterative method that requires a forward and an adjoint operator acting on vectors in each iteration. We derive the forward Born approximation operator by perturbation theory and adjoint operator via adjoint-state method. The inversion has improved the quality of the images and reduces the multi-parameter crosstalk comparing with the adjoint-based images. We have observed that the multi-parameter crosstalk problem is more prominent in the inversion images for Thomsen anisotropy parameters. Especially, the Thomsen parameter d is the most difficult to resolve. We also analyze the multi-parameter crosstalk using scattering radiation patterns. The linearized waveform inversion for VTI-elastic media presented in this article provides quantitative information of the rock properties that has the potential to help identify hydrocarbon reservoirs. [ABSTRACT FROM AUTHOR]

Common-image gathers are extensively used in amplitude versus angle (AVA) and migration velocity analysis (MVA). The current state of methods for anisotropic angle gathers extraction use slant-stack, local Fourier transform or low-rank approximation, which requires much computation. Based on an anisotropic-Helmholtz P/S wave-mode decomposition method, we propose a novel and efficient approach to produce angle-domain common-image gathers (ADCIGs) in the elastic reverse time migration (ERTM) of VTI media. To start with, we derive an anisotropic-Helmholtz decomposition operator from the Christoffel equation in VTI media, and use this operator to derive the decomposed formulations for anisotropic P/S waves. Second, we employ the first-order Taylor expansion to calculate the normalized term of decomposed formulations and obtain the anisotropic-Helmholtz decomposition method, which generates the separated P/S wavefields with correct amplitudes and phases. Third, we develop a novel way that uses the anisotropic-Helmholtz decomposition operator to define the polarization angles for anisotropic P/S waves and substitute these angles to decomposing formulations. The polarization angles are then calculated directly from the separated vector P- and S-wavefields and converted to the phase angles. The ADCIGs are thusly produced by applying the phase angles to VTI ERTM. In addition, we develop a concise approximate expression of residual moveout (RMO) for PPreflections of flat reflectors in VTI media, which avoids the complex transformations between the group angles and the phase angles. The approximate RMO curves show a good agreement with the exact solution and can be used as a tool to assess the migration velocity errors. As demonstrated by two selected examples, our ADCIGs not only produce the correct kinematic responses with regards to different velocity pertubatation, but also generate the reliable amplitude responses versus different angle. The final stacking images of ADCIGs data exhibit the identical imaging effect as that of VTI ERTM. [ABSTRACT FROM AUTHOR]

[Display omitted] • Shear-wave splitting in and around the NE Tibetan Plateau mainly reflects anisotropy in the lithospheric mantle. • Dominant NW-SE anisotropy in the lithospheric mantle indicates NE-SW compression. • Fossil anisotropies exist in the interior Alxa Block and the western Ordos Block. The northeastern Tibetan Plateau (NETP) is a key region to clarify the nature and mechanism of the Tibetan Plateau expansion. This region includes the eastern Qilian Orogenic Belt, the Alxa Block, the Helan-Yinchuan Tectonic Belt and the Ordos Block. We make shear-wave splitting measurements using teleseismic waveforms recorded at 17 portable broadband stations that we newly deployed in the study region. Our measurements provide new information on seismic anisotropy and deformation patterns in the lithospheric mantle, which can be used to constrain the extent of the NETP northeastward expansion. The dominant fast polarization direction (FPD) is NW-SE in the eastern Qilian Orogenic Belt, the southern Alxa Block and the Helan-Yinchuan Tectonic Belt, subparallel to the prominent orientation of tectonics in the NETP. The same NW-SE FPD in the three tectonic units indicates NE-SW compression in the lithospheric mantle there caused by the NETP northeastward expansion. NE-SW FPDs are revealed in the interior Alxa Block, which may reflect fossil anisotropy probably formed by the Paleo-Asian oceanic subduction and closure. ENE-WSW FPDs exist in the western Ordos Block, reflecting fossil anisotropy probably caused by convergence of the South China Craton and North China Craton. Combined with the NW-SE FPDs in the southwestern margin of the Ordos Block revealed by previous studies, our results suggest that the NETP northeastward expansion has obviously affected the southern Alxa Block and the southwestern margin of the Ordos Block. However, its recent northeastward expansion mainly occurred along the Helan-Yinchuan Tectonic Belt. [ABSTRACT FROM AUTHOR]