Exploration offshore Angola has seen a recent shift in exploration emphasis, with interest moving from postsalt sedimentary sequences to the presalt section. To optimize seismic data quality and accuracy over target areas, PGS’ data library utilizes high-end imaging solutions to enhance resolution of the Mesozoic sedimentary architecture.
Revealing Shallow Geology in Shallow Waters of the Kwanza Shelf with Enhanced Processing Flows
On the Kwanza Shelf, shallow water environments present an imaging challenge that can be resolved by enhanced processing flows, such as Complete Wavefield Imaging (CWI) (see below), which utilizes the up-going and down-going seismic wavefields. The workflow is being carried out on PGS’ Kwanza Shelf MultiClient 3D GeoStreamer data where multi-sensor recording enables high-resolution velocity model building and depth imaging using reflections, refractions, and multiples.
Primary reflections, as recorded by multi-sensor streamers, are used to resolve the long-wavelength velocity trend and shallow gradients using reflection tomography. The resulting velocity model is then used by Full Waveform Inversion (FWI) for further refinement. PGS FWI uses the full wavefield to overcome the restrictions posed by conventional reflection tomography and as such is able to successfully resolve small-scale velocity features.
The rich low frequencies of multi-sensor recordings help mitigate cycle-skipping in FWI, and enable a high-resolution velocity model to be built using both refractions and reflections. Separated Wavefield Imaging (SWIM) exploits the separated wavefields and uses sea-surface reflection which results in high-resolution imaging of shallow stratigraphy.
Due to the lack of near offsets and angular illumination in areas of shallow water, conventional depth-migrated gathers and stacks are unsuitable for validating the shallow overburden model provided by FWI. Separated Wavefield Imaging creates high-resolution stack images and well sampled angle gathers by using sea-surface reflections to compliment primary reflection imaging. By using the separated up-going and down-going wavefields, uniquely recorded by multi-sensor streamers, SWIM provides enhanced subsurface illumination and improved angular coverage for shallow data.
Tackling Postsalt Sediments with Innovative Velocity Model Building
PGS’ Wavelet Shift tomography (hyperTomo) is used with FWI to optimize presalt velocity model building, resulting in improved imaging and a greater understanding of geological complexity in areas such as Block 16. Wavelet Shift tomography enables quick cycle velocity updates, using wavelet attributes and semi-automated inversion loops. PGS’ FWI algorithm inverts for the full wavefield, making use of both refractions and reflections for an updating the velocity model. PGS has devised a modified Inverse Scattering Imaging Condition (ISIC) for FWI. It is based upon the ISIC IN PGS’ Reverse Time Migration (RTM) and removed the migration isochron from the velocity kernel. This prevents reflectivity leakage in the velocity model when updating sing deep reflections. FWI testing completed in parallel to hyperTomo updates allows for a seamless transition in the model building workflow.
Salt interpretation for velocity model building can be time consuming and costly. In areas such as Angola, this involves multiple RTMs and periods for interpretation. Using a conventional top down approach, with PGS’ Cylindrical Wave Reverse Time Migration, runtime is considerably reduced. The addition of cross-departmental expertise and semi-automation of auxiliary information helps reduce the project turnaround.
The final RTM using PGS’ Inverse Scattering Imaging Condition excludes the backscatter energy, producing an image using only the migration isochron. This removes low wavenumber noise that often contaminates RTM images, minimizing post-processing, and reducing the project turnaround. As RTM is not affected by ray-based limitations, top and base salt imaging is clearer and uncertainty in the underlying structure is reduced.
Kirchhoff PSDM depth slice (1200m) with the velocity model co-rendered. Input to FWI is shown on the left, and output on the right. FWI has successfully updated the shallow channel, inserting a slow velocity which had not been resolved previously.