If the seismic tool is intrinsically unaware of geology, why do we depend so heavily on the seismic tool to provide geological answers?
The seismic tool, sonic tool, and the check-shot survey are tools designed to record interval transit times. Geologists do geology. Geology is an observational process from which geological interpretations are drawn. Mechanical tools make measurements; such tools lack observational capability.
Intuitively, three parallel and contiguous processes occur throughout the course of depth map creation. One is a geological process that involves observation and assessment of geological data (well cuttings, regional and local geological settings etc.). The second is a geological assessment of interpreted data (seismic data, log formation etc.) collected to support such observations. A third process is a digital process that integrates the observational and interpretational processes.
All mechanical tools are intrinsically unaware of geology. If this is true, the depth equation, being digital, must also be intrinsically unaware of geology.
For lack of adequate volumes of velocity data, the industry has elected to follow a digital process in the preparation of depth maps. Nonetheless, this process has become the norm insofar as converting seismic time data to depth is concerned. Unfortunately, this path requires a number of serious compromises. These are:
- The depth equation (D=V*T): Substitution of the independent velocity vector (well velocity) for a dependent velocity vector (stacking velocity).
- Integration of time domain and depth domain data in the time domain.
- Geologic intervention is restricted.
Resolution of compromise 1 leads to resolution of the other compromises. Resolution of compromise 1 requires obtaining velocity data from either check-shot surveys or sonic logs.
- Establish domains:
- Assign the interpretive process (seismic data) to the time domain.
- Assign the observational process (well data) to the depth domain.
- Assign all digital processes to the digital domain. This domain oversees the integration of the time and depth domains.
- Integrate the time and depth domains in the depth domain.
- Equate the wells in the time domain to the wells in the depth domain. This is a simple ratio calculation. The only valid data in the time domain are those 2WT data points that correspond to 2WT data points in the depth domain.
- Generate a dimensionless contour map of the ratios.
- Multiply the time domain map by a contoured ratio map.
- Multiply the depth domain velocity map by the integrated depth domain time map to create a geophysical/digital depth map in the depth domain.
- Create a geological depth map by incorporating, in the digital depth map, a geological depth model that is suitable to local and regional geological requirements.
- Confirm the suitability of the geologic depth model by dividing the geologic depth map by the integrated time map to obtain a revised velocity map.
- Check the resultant velocity map for anomalous velocity contours.
- Adjust the geologic depth model to remove any velocity anomalies.
- Present the following maps in support of any drilling recommendation:
- Time domain 2WT
- Integrated depth domain 2WT
- Geological velocity
- Geological depth.
The question of adequate sources of well velocities arises.
From a theoretical standpoint the continuous sonic should be the best, and most complete form of velocity information possible to obtain at a given location. (Lindseth, 1968). Sonic logs represent a large, scarcely utilized source of valid velocity data.
Key to this discussion is recognition that all mechanical logging tools and the depth equation are intrinsically unaware of geology. Sonic log recordings are valid in as much as they fulfill the mathematical requirements of the depth equation. Processing of sonic log data for the effects of borehole conditions and tool travel speed is also possible. This process is restricted to sonic logs recorded in vertical wells because deviation requires redefinition of the velocity vector. The geologic depth model will accommodate deviated wells.
About the Author(s)
Robert Pickering is President of LookBack Exploration Ltd, specializing in reassessing plugged and abandoned wildcat wells.
Roy O. Lindseth (1968) 8. Velocity Analysis. Digital Processing of Geophysical Data – A Review: pp. 8.1-8.26.