Chaiwoot Boonyasiriwat

An efficient multiscale method for time-domain waveform tomography

This efficient multiscale method for time-domain waveform tomography incorporates filters that are more efficient than Hamming-window filters. A strategy for choosing optimal frequency bands is proposed to achieve computational efficiency in the time domain. A staggered-grid, explicit finite-difference method with fourth-order accuracy in space and second-order accuracy in time is used for forward modeling and the adjoint calculation. The adjoint method is utilized in inverting for an efficient computation of the gradient directions. In the multiscale approach, multifrequency data and multiple grid sizes are used to overcome somewhat the severe local minima problem of waveform tomography. The method is applied successfully to 1D and 2D heterogeneous models; it can accurately recover low- and high-wavenumber components of the velocity models. The inversion result for the 2D model demonstrates that the multiscale method is computationally efficient and converges faster than a conventional, single-scale method.

Applications of multiscale waveform inversion to marine data using a flooding technique and dynamic early-arrival windows

A recently developed time-domain multiscale waveform tomography (MWT) method is applied to synthetic and field marine data. Although the MWT method was already applied to synthetic data, the synthetic data application leads to a development of a hybrid method between waveform tomography and the salt flooding technique commonly use in subsalt imaging. This hybrid method can overcome a convergence problem encountered by inversion with a traveltime velocity tomogram and successfully provides an accurate and highly resolved velocity tomogram for the 2D SEG/EAGE salt model. In the application of MWT to the field data, the inversion process is carried out using a multiscale method with a dynamic early-arrival muting window to mitigate the local minima problem of waveform tomography and elastic effects. With the modified MWT method, reasonably accurate results as verified by comparison of migration images and common image gathers were obtained. The hybrid method with the salt flooding technique is not used in this field data example because there is no salt in the subsurface according to our interpretation. However, we believe it is applicable to field data applications.

3D Multisource Full‐Waveform Inversion using Dynamic Random Phase Encoding

We have developed a multisource full-waveform inversion algorithm using a dynamic phase encoding strategy with dualrandomization—both the position and polarity of simultaneous sources are randomized and changed every iteration. The dynamic dual-randomization is used to promote the destructive interference of crosstalk noise resulting from blending a large number of common shot gathers into a supergather. We compare our multisource algorithm with various algorithms in a numerical experiment using the 3D SEG/EAGE overthrust model and show that our algorithm provides a higher-quality velocity tomogram than the other methods that use only monorandomization. This suggests that increasing the degree of randomness in phase encoding should improve the quality of the inversion result.

3D Multi‐source Least‐squares Reverse Time Migration

We present the theory and numerical results for least-squares reverse time migration (LSRTM) of phase-encoded supergathers, where each supergather is the superposition of phasedencoded shots. Three type of encoding functions are used in this study: random time shift, random source polarity and random source location selected from a pre-designed table. Numerical tests for the 3D SEG/EAGE Overthrust model show that multi-source LSRTM can suppress migration artifacts in the migration image and remove most of the crosstalk noise from multi-source data. Empirical results suggest that multisource LSRTM can provide a noticeable increase in computational efficiency compared to standard RTM, when the CSGs in a supergather are modeled and migrated together with a finite-difference simulator. If the phase-encoding functions are dynamically changed after each iteration of LSRTM, the best images are obtained. The potential drawback is that the final results are very sensitive to the accuracy of the starting model.

Application of multiscale waveform tomography for high-resolution velocity estimation in complex geologic environments: Canadian Foothills synthetic data example

Seismic imaging in compressional belts such as the Canadian Foothills is very challenging due to complex geological structures, rugged surface topography, and highly variable near-surface conditions. Seismic sections across the Canadian Foothills are usually progressively more distorted when approaching the Canadian Foothills region. Figure 1 shows the degree of structural complexity and topographic variations which are in part responsible for the deteriorated imaging in the thrust belt. Accurate velocity models of subsurface structures are critical for improving seismic images of thrust belts in both the time domain (e.g., tomostatics) and the depth domain (e.g., prestack depth migration).

Demonstration of super-resolution and super-stacking properties of time reversal mirrors in locating seismic sources

We demonstrate with synthetic seismic data the superresolution and super-stacking properties of time reversal mirrors (TRM). Tests on synthetic data show that TRM has the potential of exceeding the Rayleigh resolution limit by a factor of more than 9. This property is accompanied by the fact that TRM has a significant resilience to strong noise. Computer tests validate these properties by accurately imaging the source location from passive seismic data with signal-to-noise ratio of about 0.001. Results also validate that TRM enhances signal by a factor pro-

Multisource Reverse-time Migration and Full-waveform Inversion on a GPGPU

Reverse-time migration (RTM) and full-waveform inversion (FWI) are a powerful but computationally expensive method. We propose to improve their efficiency by using a multisource method and a general-purpose graphics processing unit (GPGPU). In the multisource method, several sources are used simultaneously to compute the migration image and the gradient of the misfit function resulting to a reduction in computational costs of RTM and FWI. Combining the high-performance GPGPU, multisource RTM and FWI are two orders of magnitude faster than conventional methods.

Acoustic Multi-source Waveform Inversion with Deblurring

The theory of preconditioned multi-source waveform inversion is presented where many shot gathers are simultaneously back-propagated to form the multi-source gradient of the misfit function. Synthetic tests on 2D Marmousi model data show that multi-source waveform inversion using an encoded multi-source deblurring filter as a preconditioner can provide an accurate velocity model at 1/100 the computational cost of conventional waveform inversion.

Estimation of Hydro-fracture Source Location with Time Reversal Mirrors

We present a time reverse mirror approach for estimating hydro-fracture source locations. With this scheme, the passive seismic data generated by the hydro-fracture sources are crosscorrelated with the wavefield extrapolated from the VSP or seismic while drilling data, and the image shows the location of hydro-fracture sources. Only an estimate of the local velocity v(x, z) model around the VSP well is needed and this method is robust with respect to noise and limited source and recording apertures. Synthetic tests with the SEG/EAGE salt model demonstrate the potential for its use with enhanced oil recovery operations.

Computational Science and Engineering On-line: an integrated web-based environment for multi-scale modelling of complex reaction systems

We present a development of an integrated extendable web-based environment called Computational Science and Engineering On-line (CSEO) to include different fields of computational science. Our initial efforts are focusing on an integrated environment for multi-scale modelling of complex reacting systems from fundamental quantum chemistry with different entry points. CSEO provides an information management system that allows data flow from one application to another in a transparent manner. In addition, it provides a set of web-based graphic-user interfaces (GUIs) to different scientific applications. Current available GUIs are for quantum chemistry, thermodynamics and kinetics. Work is in progress to allow CSEO accessing resources from the computing grids using the Globus technology. CSEO can be accessed at http://cseo.net. It can also be hosted at different mirror sites.