tomography, Oil & Gas Glossary

What Is Tomography in Oil and Gas Exploration?

Tomography in oil and gas is a computational imaging technique that reconstructs the spatial distribution of a physical property — most commonly seismic P-wave velocity, but also resistivity, density, or attenuation — within the subsurface by inverting the observed travel times, amplitudes, or phase shifts of waves that have propagated along multiple different ray paths through the medium, using the principle that each ray path provides an integral constraint on the property distribution along its path and that a large set of crossing ray paths provides sufficient information to resolve the three-dimensional property distribution.

Key Takeaways

Seismic velocity tomography builds subsurface velocity models used for depth migration, pore pressure prediction, and structural interpretation from the travel times of seismic events recorded at multiple offsets and azimuths.
Reflection tomography uses surface reflection seismic data and residual moveout (RMO) errors to update the velocity model; refraction tomography uses diving waves and first arrivals from shallow surveys.
Full-waveform inversion (FWI) is an advanced form of tomography that fits the entire recorded waveform (not just travel times) to produce high-resolution velocity models at scales approaching half a wavelength.
Crosswell tomography places sources in one well and receivers in an adjacent well to image the formation between wells at higher resolution than is possible from surface seismic data.
Anisotropic tomography simultaneously inverts for both the velocity field and the anisotropy parameters (epsilon, delta in VTI media) to correctly handle the directional velocity variations in layered sedimentary rocks.

How Seismic Velocity Tomography Works

The seismic velocity model — the three-dimensional distribution of P-wave (and S-wave) velocity in the subsurface — is the single most important parameter controlling the accuracy of seismic depth migration and the reliability of structural interpretation. Inaccurate velocities cause reflectors to be imaged at incorrect depths, creates spurious structural closures or fails to image real ones, and distorts amplitude patterns used for fluid identification. Building an accurate velocity model requires systematic inversion of observed seismic data for the velocity distribution that best explains all the measurements.

Reflection tomography begins with an initial velocity model (typically derived from semblance analysis of conventional velocity scans) and computes the predicted travel times for all source-receiver pairs in the seismic survey. The difference between the predicted and observed travel times — the residual moveout — is used to update the velocity model in regions where the discrepancy suggests the velocity is incorrect. This update is performed iteratively using tomographic inversion algorithms (steepest descent, conjugate gradient, or LSQR) that minimise the total misfit between predicted and observed travel times across all ray paths. The tomographic update is spatially localised to the regions of the model that are illuminated by ray paths showing residual moveout, concentrating the velocity correction where the data demands it.

Tomography Applications Across International Jurisdictions

In Canada, velocity tomography is used extensively in WCSB 3D seismic processing for Foothills exploration where the complex thrust-fault structure and salt diapir geology require accurate depth velocity models to correctly position steeply-dipping reflectors in depth. AER exploration licence applications for deep Foothills wells must include depth-migrated seismic interpretation to demonstrate structural closure; the quality of the depth migration depends directly on the accuracy of the tomographic velocity model. In the East Coast offshore (Scotian Shelf, Orphan Basin, Grand Banks), velocity tomography corrects for the velocity effects of thick Tertiary sediment wedges and salt bodies above Jurassic and Cretaceous reservoir targets. Tomographic velocity models built from 3D seismic refraction data are also used to correct for near-surface velocity variations in the WCSB glacial till and permafrost areas that cause static time shifts and blur structural images in shallow gas plays.

In the United States, velocity tomography is the enabling technology for sub-salt imaging in the Gulf of Mexico deepwater, where the large velocity contrast between salt (4,480 m/s) and the surrounding sediments (1,500-3,000 m/s) creates complex ray bending that requires accurate 3D velocity models for successful depth imaging of sub-salt reservoir targets. BSEE technical regulations for OCS exploration seismic data do not specifically mandate tomographic processing, but commercial pressures to accurately image sub-salt prospects effectively require state-of-the-art velocity model building including tomography and full-waveform inversion. In Norway, velocity tomography on the NCS has evolved from simple layer-cake 1D models in the 1980s to full 3D anisotropic tomographic models for North Sea chalk reservoir imaging where chalk velocity anisotropy is a critical factor. In the Middle East, seismic velocity tomography of the shallow (<1,000 metre) section in the Rub' al Khali desert of Saudi Arabia removes the strong near-surface velocity anomalies from sand dunes and sabkha (salt flat) deposits that otherwise cause severe imaging problems in the deeper carbonate targets.

Fast Facts

The first application of tomography in medicine (the CT scanner) was in 1971; the first application of seismic tomography to oilfield imaging was reported in the early 1980s. By the 1990s, reflection tomography had become a standard component of seismic processing workflows for structurally complex areas. Today, 3D anisotropic reflection tomography is applied routinely to all major deepwater and Foothills exploration surveys, with computational requirements that have grown from days on a workstation to hours on a computing cluster as the algorithms and datasets have grown in complexity. Full-waveform inversion (FWI), the most computationally intensive form of seismic tomography, was impractical before the 2000s; today it is applied as a production service by major seismic contractors on datasets of hundreds of terabytes using high-performance computing grids with thousands of processors.

Full-Waveform Inversion as Advanced Tomography

Full-waveform inversion (FWI) extends reflection tomography from travel-time inversion to waveform inversion — the velocity model update is driven by fitting the entire recorded seismic waveform (phase, amplitude, and frequency content) rather than only the kinematic travel time information. Because the waveform carries information about velocity contrasts at scales approaching half a wavelength (approximately 5-25 metres at seismic frequencies), FWI can produce velocity models with spatial resolution 5-10 times higher than conventional tomography. This high-resolution capability makes FWI particularly valuable for imaging near-surface complexity (gas clouds, shallow channels, unconsolidated sediments) that conventional tomography cannot resolve and that causes severe imaging problems for deeper targets. FWI requires starting from an accurate initial velocity model (to avoid cycle-skipping artefacts where the algorithm converges to the wrong minimum) and computationally intensive iterative forward modelling using finite-difference wave equation solvers that simulate the entire seismic wavefield in 3D.

Tip: When evaluating the quality of a velocity tomography model for a prospect evaluation, test the model by predicting the seismic response at a well location and comparing to the actual well log synthetic seismogram. A velocity model that correctly images the reflectors at known well depths (confirmed by the well-tie) is likely accurate throughout the survey area; a model that shows 50-100 metre depth misties at the wells indicates the tomographic velocity update has not converged to a geologically reasonable solution and the structural interpretation built on that model is unreliable. Request the tomography residual moveout panels at the wells — if the residual moveout is not flat after the tomographic update, the iteration has not converged and additional tomographic passes or a different parameterisation is needed before the depth migration is accepted for geological interpretation.

Tomography is also referenced as:

Velocity model building (VMB) — the operational term used in seismic processing workflows for the iterative process of constructing and refining the velocity model used for depth migration; VMB encompasses both tomographic and non-tomographic methods (semblance picking, sparse layer model building) for velocity determination
Seismic inversion — used when the inversion derives impedance (velocity × density) rather than velocity alone; seismic inversion is sometimes confused with tomography, but inversion typically starts from a migrated seismic cube and derives rock property values, while tomography derives the velocity model used in migration — the two processes work at different stages of the seismic processing-interpretation workflow
FWI (Full-Waveform Inversion) — the specific advanced form of tomography that minimises waveform misfit; FWI is a separate and more computationally demanding process than reflection tomography and is always qualified by its full name or abbreviation

Frequently Asked Questions

What is the difference between refraction tomography and reflection tomography?

Refraction tomography uses the first-arrival travel times of seismic waves that travel along the top of the high-velocity basement or a high-velocity refractor and are refracted back to the surface — these are the "diving waves" or "head waves" that arrive before the reflected waves in a seismic record. Refraction tomography is most effective for determining near-surface velocity structure (top 500-2,000 metres) where the diving wave ray paths are concentrated. It provides smooth, long-wavelength velocity models appropriate for static corrections (removing near-surface time delays from seismic traces) but does not resolve detailed structure in the deep section. Reflection tomography uses the travel times of reflected waves from subsurface interfaces to image the velocity structure at depths below the refractor. It is effective throughout the full depth range of the seismic survey and provides the velocity model used for depth migration. Modern velocity model building workflows typically use near-surface refraction tomography to build the shallow velocity model (0-1,000 metres) and reflection tomography to update the deeper velocity model (1,000 metres to total depth of the survey), combining the strengths of both methods across the full depth range.

How does anisotropy affect seismic velocity tomography?

Sedimentary rocks are typically anisotropic — seismic waves travel at different speeds in different directions due to the layered fabric of the rock, aligned minerals, or aligned fractures. For marine sediments and shales, the dominant anisotropy is vertical transverse isotropy (VTI): P-wave velocity is higher horizontally than vertically in these fine-grained, horizontally-layered rocks. Standard isotropic tomography assumes seismic velocity is the same in all directions, which is incorrect for VTI media. An isotropic tomography solution will over-estimate the depth to reflectors (because the horizontally-travelling rays used to image steep structures have higher velocity than the vertical velocity assumed in the isotropic model), creating systematic structural depth errors. Anisotropic tomography simultaneously solves for the vertical velocity and the anisotropy parameters (Thomsen parameters epsilon and delta) that describe the VTI deviation from isotropy. This is more computationally demanding and requires data at multiple azimuths and offsets to resolve the anisotropy parameters independently of the velocity, but produces more accurate depth images in regions where VTI anisotropy is significant — which includes most of the world's deepwater turbidite and shale-dominated exploration provinces.