Tomography Definition / Meaning
Tomography is a geophysical imaging technique that reconstructs a detailed, three-dimensional representation of subsurface rock properties by analyzing how transmitted energy (such as seismic waves, electromagnetic fields, or electrical currents) propagates through the Earth. In petroleum exploration and geology, tomography provides a critical tool for mapping complex geological structures, identifying reservoir boundaries, and improving the accuracy of subsurface models used for drilling and production decisions.
Principles of Tomography
The fundamental principle involves measuring energy that travels between multiple sources and receivers placed on the surface, in boreholes, or across a survey area. Each recorded signal carries information about the medium through which it passed. By analyzing differences in travel times, amplitudes, or waveforms across many ray paths, a computer algorithm solves an inverse problem to reconstruct the distribution of a physical property (e.g., seismic velocity, attenuation, or electrical resistivity). The result is a grid of values that can be displayed as cross-sections or 3D volumes, analogous to a medical CT scan but for the Earth.
Types of Tomography in Petroleum Exploration
| Type | Energy Source | Primary Application |
|---|---|---|
| Seismic Traveltime Tomography | Refracted or reflected seismic waves | Velocity model building for depth imaging |
| Seismic Waveform Tomography | Full waveform data (including amplitudes and phase) | High-resolution velocity and density models |
| Crosswell Tomography | Seismic or electromagnetic sources between boreholes | Interwell reservoir characterization |
| Borehole-to-Surface Tomography | Sources in borehole, receivers on surface (or vice versa) | Near-wellbore structural detail |
| Electrical Resistivity Tomography (ERT) | Electrical current injected into ground | Fluid saturation and fracture detection |
Applications in Exploration and Geology
- Velocity Model Building: Traveltime tomography is routinely used to refine the velocity field for pre-stack depth migration, reducing depth uncertainty in target horizons.
- Reservoir Characterization: Crosswell tomography can map the distribution of porosity, permeability, and fluid types between wells, aiding in reservoir simulation.
- Structural Imaging: Tomography helps delineate faults, salt bodies, and other complex structures that distort seismic waves.
- Time-Lapse Monitoring: Repeat tomography surveys can track changes in reservoir properties caused by production or injection.
Workflow and Integration
A typical tomography workflow in exploration follows these steps:
- Data Acquisition: Seismic or other energy is generated and recorded over a grid of source-receiver pairs.
- Traveltime Picking: First arrivals or reflection events are identified and digitized.
- Inversion: A mathematical algorithm iteratively adjusts the velocity or property model to minimize the difference between observed and calculated traveltimes.
- Quality Control: Model resolution and uncertainty are assessed using checkerboard tests or ray-coverage analysis.
- Interpretation: The final tomographic image is combined with other data (well logs, core, seismic amplitudes) to guide drilling locations.
Advantages and Limitations
Advantages include the ability to resolve structures that are invisible to conventional seismic processing, especially in areas with strong velocity variations (e.g., sub-salt or carbonate reefs). Tomography also provides quantitative property values rather than just qualitative images. Limitations include high computational cost, sensitivity to noise in picked traveltimes, and the non-uniqueness inherent in all inverse solutions. Modern software and cluster computing have made 3D tomography practical for most large-scale exploration projects.
Usage Example: In a deepwater Gulf of Mexico field, seismic traveltime tomography was applied to iteratively refine the velocity model for a subsalt target, reducing depth uncertainty from 200 feet to under 50 feet and enabling a successful discovery well.
Overall, tomography remains an essential component of the geophysicist’s toolkit, bridging the gap between raw seismic data and reliable earth models that drive exploration success.