PGS Access Westralia: Evolution of Seismic Imaging
Exploration with 2D seismic continues to be an integral part of early evaluation for frontier hydrocarbon plays around the world. Large scale regional 2D structural framework surveys continue to provide additional insight into known trends and are regularly referred to for clarity and ground-truthing when a new petroleum play is proposed.
The 11 776 km of data selected for the PGS Access Westralia study has been reworked from field tape with an emphasis on deep structural imaging to unlock previously unrecognised structural settings. The reprocessed seismic imagery provides a greater understanding of the underlying architecture and tectonic evolution of the constituent basins comprising the Westralian Superbasin geological province.
The seismic data was first processed in 2008/2009 using industry best practice workflows common at this time. This included a general denoise, linear noise attenuation, SRME, Tau-P deconvolution and a high resolution Radon demultiple. A subset of the data within the shallow water region of the Beagle and Dampier sub-basins was then re-processed in 2013. This reprocessing utilised an improved de-signature approach and a deterministic shallow water demultiple technique, and focused on low frequency preservation. Both processing efforts achieved good results, but new as new processing methodologies improve, especially with the advent of widespread use of Pre-Stack Depth Migration, the data reprocessing in 2015/2016 has provided opportunities for further improvements.
The main challenges for the data processing lie in the variability and complexity of the geology, and the limited (2D) recorded wavefield. These include:
- Very shallow water bottom generating high frequency, high energy reverberations
- Deep and often rugose sea floor generating complex 3D multiple fields
- Shallow sub-surface high reflectivity layers generating complex diffraction multiples
- Significant out-of-plane energy (primary and multiple)
As many of the lines traverse the varying regimes, often all of the above challenges are present on individual lines.
PGS Access Westralia utilises the latest dual-sensor wavefield separation techniques for deghosting GeoStreamer data, and the latest evolution of wave equation based receiver deghosting for conventional single-component data. Source deghosting allowing for emergence angle, and source designature performing zero phasing and de-bubbling have also been optimised to maximise the data bandwidth, particularly on the low frequencies.
The demultiple sequence has been improved significantly with the use of a multi-model adaptive subtraction algorithm, working with complimentary wave-equation and convolutive SRME multiple models. The wave-equation multiple modelling is well suited to shallow water multiple prediction, and works well with the higher frequency multiple trends, while the SRME multiple modelling competently handles the longer period multiple trends.
Figure 1: Input shot gather, WE multiple model, SRME model, Amplitude spectra of WE and SRME models
Additional demultiple techniques applied to the current re-processing include a muted SRME application and high resolution Radon demultiple. The diffraction multiples have also been suppressed to provide additional noise removal. The result of the revised processing sequence is a cleaner, more frequency-rich input dataset ready for imaging.
The imaging objectives of the PGS Access Westralia Study were to recover new insights relating to the deep crustal scale reflectors, and to provide a regionally consistent 2D framework across the laterally variable velocity regime of the North West Shelf. In order to achieve this, a pre-stack depth migration approach was required. Velocity models have been built using regionally consistent tomographic velocity model updating techniques, and utilising key wells and horizons. Final imaging algorithms include Kirchhoff, RTM and PGS Beam migrations, each of which provides unique imaging merits.
Figure 2: Legacy PSTM stack, PGS Access Westralia KPSDM
The re-processing has provided the expected uplift over previous processing efforts. The deep structures that underpin the region have been more clearly resolved, and can now be more confidently defined in conjunction with the regional gravity and magnetics interpretations. For a deeper understanding of the formative processes of the North West Shelf, PGS Access Westralia is delivering results.