ION supports your toughest geologic challenges by offering multiple isotropic and anisotropic tomography solutions depending on geologic challenge: narrow azimuth (NAZ), wide azimuth (WAZ), multi-azimuth (MAZ), parametric and general move out tomography.
Narrow, Wide, and Multi Azimuth
Seismic tomography is a technique used to measure and display the 3D distribution of velocities in the Earth using multiple sources and receivers. The main difference between our NAZ tomography and MAZ, or WAZ tomography is that the equation used to derive the velocity field updates is better constrained by azimuth, thus allowing for more precise velocity determination in complex geology.
Parametric and General Move Out Autopicking
Autopicking (APK), either parametric or general, is used to pick the residual move out of events on the depth or time migrated gathers in an attempt to correct the velocities across source to receiver distances and flatten the gathers. Our imaging services team offers both techniques as each has its unique merits:
- Parametric APK – Produces a smooth background model, improves ray illumination, and produces robust picks in low signal-to-noise ratio areas
- General Move Out (GMO) APK – Produces more accurate picks in complex residual move outs caused by small-scale velocity anomalies or sharp lateral velocity variations
ION’s Q Tomography leverages prestack migrated common image gathers for estimating Q attenuation. As Q field estimates are often unstable. The Q tomography has a choice of two industry best practice, frequency-based calculations (log spectral ratio and centroid frequency shift) to provide a deterministic correction based on either specific horizons or generalized moveout picks to confidently estimate attenuative travel time. From there a deterministic model of attenuation can be built by back projecting the compensated horizons and picks along ray-paths derived from the 3D velocity model. This robust Q tomography approach is designed to deal with spatially variant effects and works in concert with our suite of time/space variant 3D Q migrations.
Using Constraints to Improve Modeling
In order to generate higher resolution, more geologically realistic models from tomography, our imaging team applies constraints at three levels.
- Event level - using a wavelet-by-wavelet autopicking algorithm, each wavelet is subject to three major constraints: 1) inter-trace depth differences, 2) trace shapes, and 3) depth variance
- Trace level - we correlate events across their offsets from trace to trace, resulting in a semblance measurement that is used as another constraint
- Geology level - we focus on constructing a model with minimal variance along predefined geological dips
As a result of these workflows, we rapidly obtain accurate velocity models with a high degree of confidence.
The Benefits of Tomography
Whether applying parametric or GMO APK to NAZ, WAZ or MAZ data, our workflow fulfills the requirement to build realistic models in complex geologic regimes by accounting for attenuation and other Earth properties as well as geological constraints. Highly automated and iterative, the workflow generates more accurate picks yielding more realistic updates with optimal resolution and minimized artifacts to utilize for seismic inversions. The final outcome:
- The most accurate and representative velocity model possible
- More accurate Earth models
- Helps resolve complex geologies
The significant improvement with advanced tomography using GMO with auto-picking is evident on these images with dramatic improvements to the “flattening” of events on the gathers after GMO compared to conventional tomography.
Mouse over for before/after comparison.
- Multi-Azimuth Anisotropic Tomography and PreSDM of a North Sea Streamer Survey - EAGE Technical Paper (PDF)
- Improving Gather Picking for Tomography in Complex Velocity Models, a Case Study on Shale Diapirs - EAGE Technical Paper (PDF)