Breakthrough acquisition and technologies for subsalt imaging

Abstract

The recently introduced method of wide-azimuth data acquisition offers better illumination, noise attenuation, and lower frequencies to more accurately determine a velocity field for imaging. For the field data experiment to demonstrate the technologies, we used a Gulf of Mexico (GOM) wide-azimuth data set that allows us to take advantage of possible low frequencies, relatively large crossline offsets, and increased illumination. The input data was processed through true 3D azimuthal surface-related multiple elimination (SRME) with zero-phasing and debubble. Eliminating the surface-related multiples aids the velocity determination and helps uncover the subsalt sediments at the final imaging stage. After the initial velocity derivation, which was constrained to wells and geology, full-waveform inversion (FWI) was used to further update the velocity field to achieve an enhanced image. The methodology used follows the top-down approach where suprasalt sediment model is developed followed by the top of salt, salt flanks, base of salt, and finished with a limited subsalt update. To approximate the observed data by using an acoustic inversion procedure, the propagator includes effects of attenuation, anisotropy, acquisition source, and receiver depth. The geological environment is salt related, which implies that the observed data is highly elastic, even though it is input to an acoustic full waveform inversion. To use the proper constraints for the inversion, layer-stripping method is used to develop the high-resolution velocity field. The inversion stages were carefully quality controlled through gather displays to ensure the kinematics were honored. We then demonstrated the benefit of the FWI velocity field by comparing the images derived with the traditional ray-based tomographic velocity field versus the velocity field derived by FWI performing reverse time migration to produce these images. Finally, the images were compared at key well locations to evaluate the robustness of the workflow.

Tools and Techniques: Marine Seismic Methods

“…In combination with the evolution of waveequation-based imaging algorithms (such as OWEM and RTM), data acquired in various wide and RAZ acquisition configurations, involving one or several vessels (e.g., RAZ, WAZ, and coiled shooting), have resulted in superior subsalt images (e.g., Vigh et al, 2011). In combination with the evolution of waveequation-based imaging algorithms (such as OWEM and RTM), data acquired in various wide and RAZ acquisition configurations, involving one or several vessels (e.g., RAZ, WAZ, and coiled shooting), have resulted in superior subsalt images (e.g., Vigh et al, 2011).…”

“…The resulting images are significantly improved and are capable of, for example, resolving many complex subsalt plays for the first time (e.g., Vigh et al, 2011). The resulting images are significantly improved and are capable of, for example, resolving many complex subsalt plays for the first time (e.g., Vigh et al, 2011).…”

“…FWI is an iterative method where seismic data are synthesized (modeled) using, for instance, a finitedifference method on an initial earth model. However, recently, the method has been successfully applied in wide range of hydrocarbon exploration projects (e.g., Sirgue et al, 2011;Vigh et al, 2011). After several iterations, the FWI method provides an Earth model that is increasingly capable of generating a better fit between synthetic and real data.…”

Combination of improved acquisition system, processing, velocity model and migration for seismic imaging in areas of intense salt tectonics

“…14. Результаты WEM узкоазимутальных данных после 2D SRME (а) и RTM широкоази-мутальных данных после 3D SRME (б) [Vigh et al, 2011]. появилась возможность выявить и за-картировать положительную структу-ру на большей глубине. Эта структура в дальнейшем оказалась продуктивной.…”

How New Data Acquisition Technique Can Complement Full Waveform Inversion and Lead To Improved Subsalt Imaging

“…Over the past few years, 3D FWI with the acoustic wave equation has been applied to real datasets in marine (Plessix, 2009, Sirgue at al., 2010, Vigh et al, 2010, 2011 and in land (Plessix, 2010) environments. These results have demonstrated that FWI can be used to update the velocity if the acquired data contain low enough frequencies and long enough offsets.…”