Precise structural models utilizing 3D gravity and tensor analysis
Gravity surveys can now record changes in gravity in all three dimensions. This data is called gravity TENSOR data. Gravity tensor data provides much higher resolution than conventional gravity data. Tensor data can be used to constrain seismic data that has been diffused or absorbed by salt or igneous rock formations (see above). In many geologic environments the result is a much more reliable, higher-resolution structural model.
Gradient tensor technology captures much more detailed and directional information about the gravity field
In the last decade, new instruments have made it possible to measure the direction of the gravity gradient as well as its magnitude. These instruments record the individual components of the overall gravity field in three dimensions. This technology is called gravity gradiometry.
The resolution of gravity gradiometry data is much greater than conventional high-resolution gravity data—five times greater bandwidth (~600m vs. 3km). The measurements can be accurate to one part in 1011 (one part in 100 billion). In addition to improving overall resolution, the increased bandwidth also permits capture of high-frequency, short-wave components that help resolve shallow and intermediate structures.
The way gradient tensor data is collected also improves accuracy. Aerial surveys run overlapping orthogonal patterns that provide many common points for data comparison. This can be used to remove any bias drift in the data.
Mathematics used with gradiometry are called TENSOR ANALYSIS
The directional components of gravity can be expressed as a 3×3 matrix. This matrix is called the GRADIENT TENSOR, hence the name of the analysis. Of the nine elements of the tensor matrix, five are independent and are used for interpretation applications.
Different components reveal different features
The different components help to reveal different features. For example, one component which is similar to conventional gravity measurements helps position structural elements and relate them to subsurface geology. Two others help define edges. The remaining two help define corners. All five are used to define center of mass.
Reiterative process builds structural model
Gradient data is used with other types of data to interpret structure. For example, it is used to constrain seismic data. The resulting analysis, in turn is used to constrain gravity data. This process is reiterated until it creates a structural model that fits well with both gravity and seismic data.
Gradiometry aids interpretation around salt or igneous structures
Salt bodies interfere with seismic imaging because they absorb, redirect and reflect seismic pulses. The same is true for igneous interfaces and intrusions of basaltic materials. For many years, gravity gradient data has been used to constrain and enhance seismically-derived interpretation in these situations.
Three-dimensional data expressed as a “gradient tensor” help to define vertical and horizontal extent of salt or basaltic structures as well as sub-salt or sub-basalt structure.
Tensor analysis applications
IGC recommends that the interpretation of Tensor data be done on a proprietary basis. Understandably it requires expertise and is time intensive. Tensor analysis serves a number of purposes.
- It provides cost-effective, prospect-level-resolution regional mapping.
- With 2D seismic control, it permits 3D mapping. This can provide an alternative to lower-cost but less-informative 2D modeling and much more expensive 3D modeling.
- In pre- and post-stack-depth-migration reimaging of seismic data, it provides greater initial accuracy and helps bring more rapid convergence (fewer iterations). This increases the value of the seismic data and reduces processing costs.