Over many years, GEUS and the Geophysics Department has developed extensive expertise in seismic interpretation and mapping. Interpretation and mapping of key seismic horizons results in geologic models of the earth's subsurface that, when combined with other data types, provides an understanding of the geologic and tectonic development of an area.
These methods require knowledge and experience with all aspects of seismic methods, including acquisition and processing. At GEUS, seismic interpretation is essential for carrying out a wide range of core responsibilities, including regional mapping of important structures and key stratigraphic intervals that include potential hydrocarbon resources (oil and gas reservoirs), groundwater supplies, and geothermal reservoirs.
The models developed from seismic interpretation are used for basin modeling studies in the North Sea, for understanding the geological development of the areas offshore Greenland, and in many other areas where GEUS is responsible for providing independent assessments for other government agencies as well as technical analyses to private companies.
Seismic interpretation and mapping consists of several steps:
- Interpreting seismic horizons
With seismic methods, sound energy generated near the surface propagates into the earth and is reflected back to the surface by various geologic layers. The time it takes for the sound to travel down to a layer and back again to the surface is recorded. These seismic reflections result in different patterns that can be related to how the various layers formed and what they are made of (e.g. sand or mud). With seismic interpretation, the key horizons are and their geological significance are identified and picked on all the seismic data.
Once the key horizons are identified and picked, maps of the different layers are generated using different gridding and interpolating techniques. These provide a three dimensional view of the main surfaces and features of the subsurface geology.
- Depth conversion
With seismic methods, the time it takes for energy to travel from one place to another is the primary observation. Seismic horizon maps are thus typically shown in the travel-time to the subsurface layer. Using different methods to measure the velocity of the sound energy, it is possible to convert the time to depth, which allows the maps to be displayed in meters below the surface.
- Attribute analysis
The seismic data contains additional information beyond just how long energy takes to travel from one point to another - amplitude and phase information from the energy can be used to infer additional physical properties of the subsurface using techniques based on so-called seismic attributes. Because the impedance contrast for a layer containing gas is much larger than for a layer containing water, for example, amplitude anomalies can be used to identify variations in fluids within a layer. AVO analysis (amplitude variation with offset) can be used to infer the presence of oil and gas within different layers.
- Forward modeling
To verify a stratigraphic and structural model of an area, it is possible to calculate the seismic response expected based on the inferred lithologies and physical properties. These so-called forward models can be compared to the recorded data to assess similarities and differences and assign confidence levels to various parts of the interpretation.
Seismic interpretation and mapping is a cross-disciplinary exercise that requires input and cooperation with geologists, geophysicist, and engineers from all three of the energy departments at GEUS.