Imaging highly complex subsurface structures is a challenging problem because it ultimately necessitates dealing with nonlinear multiple-scattering effects (e.g., migration of multiples, amplitude corrections for transmission effects) to overcome the liminations of linear imaging. Most of the current migration techniques rely on the linear single-scattering assumption, and therefore, fail to handle these complex scattering effects. Recently, seismic imaging has been related to scattering-based image-domain interferometry in order to address the fully nonlinear imaging problem. Building on this connection between imaging and interferometry, we define the seimic image as a locally scattered wavefield and introduce a new imaging condition that is both suitable and practical for nonlinear imaging. A previous formulation of nonlinear scatteringbased imaging requires the evaluation of volume integrals that cannot easily be incorporated in current imaging algorithms. Our method consists of adapting the conventional crosscorrelation imaging condition to account for the interference mechanisms that ensure power conservation in the scattering of wavefields. To do so, we add the zero-lag autocorrelation of scattered wavefields to the zero-lag crosscorrelation of reference and scattered wavefields. In our development, we show that this autocorrelation of scattered fields fully replaces the volume scattering term required by the previous formulation. We also show that this replacement follows from the application of the generalized optical theorem. The resulting imaging condition accounts for nonlinear multiple-scattering effects, reduces imaging artifacts and improves both amplitude preservation and illumination in the images. We address the principles of our nonlinear imaging condition and demonstrate its importance in ideal nonlinear imaging experiments, i.e., we present synthetic data examples assuming ideal scattered wavefield extrapolation and study the influence of different scattering regimes and aperture limitation.
We provide theoretical and simulation analysis of the small signal response of SiO 2 /AlGaN/GaN metal insulator semiconductor (MIS) capacitors from depletion to spill over region, where the AlGaN/SiO 2 interface is accumulated with free electrons. A lumped element model of the gate stack, including the response of traps at the III-N/dielectric interface, is proposed and represented in terms of equivalent parallel capacitance, C p , and conductance, G p . C p -voltage and G p -voltage dependences are modelled taking into account bias dependent AlGaN barrier dynamic resistance R br and the effective channel resistance. In particular, in the spill-over region, the drop of C p with the frequency increase can be explained even without taking into account the response of interface traps, solely by considering the intrinsic response of the gate stack (i.e., no trap effects) and the decrease of R br with the applied forward bias. Furthermore, we show the limitations of the conductance method for the evaluation of the density of interface traps, D it , from the G p /x vs. angular frequency x curves. A peak in G p /x vs. x occurs even without traps, merely due to the intrinsic frequency response of gate stack. Moreover, the amplitude of the G p /x vs. x peak saturates at high D it , which can lead to underestimation of D it . Understanding the complex interplay between the intrinsic gate stack response and the effect of interface traps is relevant for the development of normally on and normally off MIS high electron mobility transistors with stable threshold voltage. V C 2015 AIP Publishing LLC. [http://dx.
The Green's function for wave propagation can be extracted by cross-correlating field fluctuations excited on a closed surface that surrounds the employed receivers. This study treats an acoustic multiple scattering medium with discrete scatterers and shows that for a given source the cross-correlation of waves propagating along most combinations of scattering paths gives unphysical arrivals. Because theory predicts that the true Green's function is retrieved, such unphysical arrivals must cancel after integration over all sources. This cancellation occurs because the scattering amplitude of each scatterer satisfies the generalized optical theorem. The cross-correlation of scattered waves with themselves does not lead to the correct retrieval of scattered waves, because the cross-terms between the direct and scattered waves is essential.
Using a generalized extraction method, the fixed charge density Nint at the interface between in situ deposited SiN and 5 nm thick AlGaN barrier is evaluated by measurements of threshold voltage Vth of an AlGaN/GaN metal insulator semiconductor high electron mobility transistor as a function of SiN thickness. The thickness of the originally deposited 50 nm thick SiN layer is reduced by dry etching. The extracted Nint is in the order of the AlGaN polarization charge density. The total removal of the in situ SiN cap leads to a complete depletion of the channel region resulting in Vth = +1 V. Fabrication of a gate stack with Al2O3 as a second cap layer, deposited on top of the in situ SiN, is not introducing additional fixed charges at the SiN/Al2O3 interface.
Recent advances in marine seismic acquisition allow for the recording of vector-acoustic ([VA] pressure and particle velocity) seismic data from dual-source configurations, i.e., using monopole as well as dipole sources. VA reverse time migration (RTM) can be custom designed to accurately handle amplitude and directivity information from 4C seismic data. We present a method for multicomponent RTM that is based on an adjoint-state formulation using the full VA wave equations for pressure and corresponding displacement fields. This method takes advantage of the directional finite-frequency information contained in the 4C acoustic fields by using source and receiver weighting operators in the adjoint-state imaging scheme. With this adjoint-state method, the source and receiver radiation properties are tailored by choosing specific weighting operators. Weighting operators were chosen so that source- and receiver-side ghost arrivals are jointly migrated with primary energy. Because the dipole field components (e.g., components of particle displacement or acceleration) are proportional to the spatial gradient components of the pressure field, our method is in fact a formulation for reverse-time map migration that images pressure fields while jointly using the directional information contained in its full 3C gradients. As a result, our reverse time 4C map migration method yields less aperture- and sampling-related artifacts when compared to imaging of the pressure-only or 2C seismic data. In addition, our method sets a framework for full-waveform inversion using dual-source 4C seismic data. We demonstrated our findings with synthetic data, including a subsalt imaging example.
Reverse‐time migration is a two‐way time‐domain finite‐frequency technique that accurately handles the propagation of complex scattered waves and produces a band‐limited representation of the subsurface structure that is conventionally assumed to be linear in the contrasts in model parameters. Because of this underlying linear single‐scattering assumption, most implementations of this method do not satisfy the energy conservation principle and do not optimally use illumination and model sensitivity of multiply scattered waves. Migrating multiply scattered waves requires preserving the non‐linear relation between the image and perturbation of model parameters. I modify the extrapolation of source and receiver wavefields to more accurately handle multiply scattered waves. I extend the concept of the imaging condition in order to map into the subsurface structurally coherent seismic events that correspond to the interaction of both singly and multiply scattered waves. This results in an imaging process referred to here as non‐linear reverse‐time migration. It includes a strategy that analyses separated contributions of singly and multiply scattered waves to a final non‐linear image. The goal is to provide a tool suitable for seismic interpretation and potentially migration velocity analysis that benefits from increased illumination and sensitivity from multiply scattered seismic waves. It is noteworthy that this method can migrate internal multiples, a clear advantage for imaging challenging complex subsurface features, e.g., in salt and basalt environments. The results of synthetic seismic imaging experiments, including a subsalt imaging example, illustrate the technique.
S U M M A R YGreen's function reconstruction relies on representation theorems. For acoustic waves, it has been shown theoretically and observationally that a representation theorem of the correlationtype leads to the retrieval of the Green's function by cross-correlating fluctuations recorded at two locations and excited by uncorrelated sources. We extend the theory to any system that satisfies a linear partial differential equation and define an 'interferometric operation' that is more general than cross-correlation for the reconstruction. We analyse Green's function reconstruction for perturbed media and establish a representation theorem specifically for field perturbations. That representation is then applied to the general treatment of scattering problems, enabling interpretation of the contributions to Green's function reconstruction in terms of direct and scattered waves. Perhaps surprising, Green's functions that account for scattered waves cannot be reconstructed from scattered waves alone. For acoustic waves, retrieval of scattered waves also requires cross-correlating direct and scattered waves at receiver locations. The addition of cross-correlated scattered waves with themselves is necessary to cancel the spurious events that contaminate the retrieval of scattered waves from the crosscorrelation of direct with scattered waves. We illustrate these concepts with numerical examples for the case of an open scattering medium. The same reasoning holds for the retrieval of any type of perturbations and can be applied to perturbation problems such as electromagnetic waves in conductive media and elastic waves in heterogeneous media.
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