Contributed by PGS

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PGS Ultima is a game-changing realization of several decades of industry ambition. It is a move away from traditional sequential processing to a simultaneous inversion combining velocity model building and least-squares imaging.

The main advantage of PGS Ultima is that an accurate velocity model and reflectivity image can be derived faster, as the simultaneous inversion resolves both using the same raw seismic data as input. Most time-consuming processing steps in a seismic sequence can be bypassed allowing a significant turnaround time reduction in imaging projects by more than 50%.
A key technical aspect of PGS Ultima (Figure 1) is the absence of leakage between parameters, so both products of the simultaneous inversion are not contaminated by each other, unlike many current industry approaches. Another differentiator is the implementation of a single modeling engine allowing the full wavefield to be modeled including reflections, multiples, refractions and diving waves.

PGS Ultima provides companies with more time to analyze their data and therefore reduce their risk in field exploration, development or production.

Figure 1. PGS Ultima – Simultaneous inversion workflow
Figure 1. PGS Ultima – Simultaneous inversion workflow

Simultaneous Inversion Speeds up Processing and Delivers Reliable Products

Traditional cascaded imaging workflows take time. Full waveform inversion (FWI) is normally run at the beginning of a processing sequence using raw data and has become the main tool for velocity model building. Least-squares migration (LSM) requires an accurate velocity model and fully preprocessed data, so it happens at the end of the preprocessing sequence and after the velocity model building stage. When applications rely on the products of earlier steps we end up with time-consuming cascaded workflows.

The simultaneous nature of PGS Ultima means FWI and LSM are now performed in a single framework. It uses raw seismic data, models the full acoustic wavefield, and outputs accurate velocity models and reliable reflectivity.

Suitable for Use on Any Data

PGS Ultima can be used on any data, streamer or OBS. There are no limitations on acquisition geometries to get accurate and deep models, as the code uses the entire wavefield, reflections, multiples, refractions, diving waves and turning waves.

A Different Approach to a Long-standing Problem

PGS Ultima uses a unique implementation of scale separation, so there is no parameter leakage, and the velocity model and reflectivity are not contaminated by each other. This is unique in the industry and is achieved with a robust imaging condition that performs the scale separation and hence removes completely the leakage between these two parameters in the multi-parameter inversion. Another key implementation is the vector reflectivity modeling engine allowing the entire wavefield to be modeled and reducing the need for long offset acquisition.

Save Time with PGS Ultima

Only limited processing is needed for PGS Ultima, the input required is a starting velocity model and raw gathers. This means time is saved on preprocessing steps like designature, deghosting and demultiple. By doing the inversion for velocity and reflectivity simultaneously not only does this save time but it can deliver an even better image.

Synthetic Tests Validate PGS Ultima

A modified version of the SEG/EAGE overthrust model is used to demonstrate the benefits of the simultaneous inversion. The true velocity and reflectivity models are shown below in the left column (Figure 2). A heavily smoothed velocity field is used as a starting model along with raw gathers (middle column). The results of the simultaneous inversion are shown in the right column. The inverted velocity correctly retrieves the detailed features in the velocity model. Similarly, the inverted reflectivity is equivalent to the true one. 

Figure 2. Overthrust model synthetic test example.
Figure 2. Overthrust model synthetic test example. True velocity and reflectivity models (left column). Initial velocity model and input raw shot gathers (middle column). PGS Ultima final velocity model and reflectivity (right column). 

Field data example from the Gulf of Mexico

The field data example in Figure 3 is from the deepwater Gulf of Mexico, De Soto Canyon area. The data is multisensor GeoStreamer with a maximum offset of 12 km. The input shot gathers were not muted and the full wavefield was used for the inversion. Figure 3a shows the initial velocity model while Figure 3c displays the reflectivity from the first iteration of the inversion, which is equivalent to RTM image using the starting velocity model. Since the data contained multiples, crosstalk artifacts are observed in the RTM image and are indicated by the yellow oval. The results after several iterations of simultaneous inversion using PGS Ultima are shown in Figures 3b and 3d. The inverted models clearly display higher resolution, better balanced amplitude and a reduction of the crosstalk in the final reflectivity model.

Figure 3. De Soto Canyon field data example.
Figure 3. De Soto Canyon field data example. (a) Initial and (b) inverted velocity models. (c) Vertical reflectivity from first iteration. (d) Final inverted vertical reflectivity. Notice the crosstalk reduction as the yellow oval indicates.

Future Vision

PGS has the ambition to leverage PGS Ultima to reduce project turnaround by at least 50% over today’s standards. Whilst PGS Ultima is available on projects today, the technology will continue to be developed with the intention to:

  • Extension to include pre-stack gather output;
  • Include update of the anisotropic terms within the inversion; and
  • Validate amplitude fidelity giving confidence when inverting for rock properties.