Brian S. Anderson
Perhaps now more than ever, the old business adage that the best solutions are "fast, cheap, and easy" certainly applies to the oil and gas exploration industry. When properly integrated with seismic data, the gravity and magnetic methods provide one of the fastest, most cost-effective, and easiest ways to reduce exploration risk. In terms of risk reduction, it is difficult to get more value from less investment than with gravity and magnetic data. At 3% to 5% of seismic costs before seismic processing and interpretation, it is a claim that is difficult to challenge. This is particularly true in Southeast Asia, where seismic alone cannot always determine if structures are volcanic.
Recent developments in instrumentation, positioning, and data processing have dramatically increased the accuracy and resolution of current surveys, making it not only economically feasible, but technically desirable to re-acquire surveys recorded prior to about 1990. This higher quality data is fueling a renaissance in the applications for the data, as it is helping us to refine our interpretations of geology as well.
Seismic is a measurement of rock velocities, which vary with rock densities. Gravity measures rock densities, and sensitivity varies with depth. As a result, the laws of physics link seismic velocities and gravity densities. Density is the common term.
Velocity models: The initial velocity model
A recently commercialized research consortium developed a comprehensive software toolkit for the conversion of a properties volume (velocity) into a different properties volume (density) and back using a rich suite of tools and algorithms. Such developments now enable explorationists to incorporate results of non-seismic 2D and 3D modeling much more effectively into their seismic interpretations and depth imaging efforts.
Magnetic data measures rock susceptibilities, which are related to their iron content – basement, volcanics, and faults. Modern high-resolution marine and airborne magnetic data have effectively illustrated that the data can be useful for not only delineation of regional basement and structure, but also very subtle geologic features in the shallow (<10,000 ft or 3,000 m) sedimentary column as well.
Velocity models: shows larger salt features when enhanced with gravity data
Gravity and magnetics often provide the most benefit when seismic imaging is difficult. Using three independently measured geophysical properties enables construction of a highly constrained interpretation, increasing confidence, and decreasing risk. The benefits of integration have been illustrated in the following ways:
- Confirmation or enhancement of velocity models
- Enhanced seismic images
- High grading equally viable seismic interpretations
- Quality control of time to depth conversion
- Delineation of geologic bodies: extent, thickness, and origin
- Identification of volcanics, salt, carbonates, and other rock types
- Verification of dip angles for near vertical features
- Identification of subsalt or sub-volcanic sediment
- Lithological changes inferred by density change
- Mapping of faults or structures between seismic lines on 2D surveys
- Knowledge of structural relationships bet-ween sediment and basement geologic features.
Gravity and magnetic data, acquired simultaneously on a proprietary seismic survey or licensed from a multi-client library, are readily available for constraining seismic interpretations and enhancing velocity models used for advanced seismic processing methods such as pre-stack depth migration (PSDM). Final processed data is ready within 30 days after the last shot of the seismic survey, and multi-client gravity and magnetic data is available straight off the shelf.
Using modern software tools, a typical integrated seismic/gravity/magnetics interpretation project can be conducted in a matter of days or weeks, not months.
For every dollar spent to acquire (not process or interpret) seismic, it takes an additional 3-5 cents to acquire, process, and interpret gravity and magnetics data. An integrated earth model constrained by four independent geophysical data sets – seismic, well log, gravity, and magnetics – can greatly reduce exploration risk and yet can be achieved for less than a penny on the dollar as compared to drilling costs.
In the initial pre-stack depth migration (A), some mid-section and very deep reflectors show significant offsets.
Eliminating one well that is drilled into improperly interpreted volcanics would pay for gravity and magnetics on every exploration program that an oil/gas company conducts in a year, or more.
New technologies provide hassle-free gravity and magnetic data acquisition on 2D and 3D seismic surveys. These include a new magnetometer system, which is deployed from the air gun sub arrays, eliminating winches and tangled cables. Modern digital-control marine gravity systems can be effectively mobilized via crew boat or helicopter, eliminating the need for costly and time-consuming port calls for this purpose.
After gravity enhancement (B), misplaced reflectors were correctly positioned, velocity pull-ups corrected, and deep reflectors became continuous.
State-of-the-art dynamically linked software applications enable the seamless conversion of a seismic/well interpretation to an integrated earth model that is enhanced with the independent geologic constraints provided by the gravity and magnetics methods. Tight software integration with Landmark and GeoQuest seismic workstations and databases enables push-button transfer of interpreted seismic horizons and velocity data to integrated gravity/magnetic/seismic earth models, and vice versa.
The conventional method uses PSDM, an iterative approach, to converge on a final seismic velocity model. The conventional method is typically constrained only by seismic and well data. Additional constraints are valuable in reducing the number of iterations required to converge on a final velocity model or reducing the geologic uncertainty of the final PSDM image.
Magnetic data over a basalt terrain combined with seismic was interpreted to define the basement structure seen in this perspective view.
The gravity enhanced PSDM method begins like the conventional PSDM. The seismic velocities and well data are integrated to create an initial velocity model. The initial velocity model is used to create an initial PSDM image. The method then uses the additional geophysical constraints of gravity and magnetics.
At the center of the method is an integrated earth model. This earth model must honor the constraints imposed by all of the geophysical data. Typically the initial earth model is constructed from the seismic and well data. The model is then modified to honor the additional constraints provided by gravity and magnetics, which are particularly valuable in constraining the model where seismic and/or well data is not optimal.
In this case study from offshore Brazil, a long-offset (8,200 m) deep record (12 s) 2D seismic survey line was used. The initial velocity model shown was generated solely from the seismic velocities. The initial salt geometries are shown in orange with tops of salt at depths ranging from 4,000 ft to 6,000 ft. Also of note are the lateral velocity changes. The yellow-orange anomalies in this part of the model at depths of 11,000 ft were interpreted to be carbonates. This initial velocity model was used to generate the initial PSDM.
This initial PSDM ranges in depth from sea level to about 16,000 ft (4,875 m). Although this PSDM image is easily interpreted, some mid-section and very deep reflectors exhibit significant offsets. From this PSDM image alone it is not possible to unambiguously determine whether these offsets are geologic structures of velocity artifacts. Deeper in the section another strong reflector can be interpreted. Here the interpreter desires to know if this offset in the deep reflector is a fault or an artifact of an inaccurate velocity model.
Over three days, a series of gravity and magnetic models were created, and in an iterative manner, a convergence was reached on a revised earth model that honored the constraints of the gravity and magnetic data.
Integrated modeling results in a refined PSDM velocity model. Several major changes in the model can easily be observed. The reflector located below the base of shallow salt on the gravity-enhanced PSDM section moved more than 1 km in depth as a result of the modified salt geometries in the integrated earth model. Consequently, the structural high, or velocity pull-up was resolved. In addition, the deepest reflectors became more continuous over areas of the model where the shallow salt geometries were modified.
Imaging basalt with seismic data can be a formidable challenge. The initial model building included an area 120 km by 200 km offshore Brazil. Fourteen interpreted Fugro-Geoteam AS seismic lines were incorporated into the model, and Fugro-LCT gravity and magnetics data collected with the seismic lines were used to constrain the 3D combined seismic/gravity/magnetic model.
The magnetic data was interpreted to define the basement structure, which varied significantly from the initial basement interpretation based on the seismic data. In the initial 3D model, since the top of the volcanics was well imaged and the base was generally in question, the basalt layer was initially included as a 30-m thick "zero thickness layer." The modeling then went into a constrained inversion phase, using the residual gravity data to drive and constrain the structural inversion of basalt layer thickness over the model area.
To test whether the basalts were deposited from a local versus a distal source, two models were attempted. The first shows the result of a distal source, which seems to be a reasonable match to the observed gravity data.
When a local intrusive source is built into the model, it becomes more difficult to accommodate the additional subsurface densities into the gravity field, leading us to the conclusion that a distal source was more probable than a local source.
These models can be made increasingly complex, with additional well and perhaps 3D seismic data, as well as different assumptions about the density contrasts between basalt and sediments. These results can be of significant benefit in discerning the thermal maturation and history of a basin.
Anderson thanks Greg Lyman and Mark Weber of Fugro-LCT and Qingbo Liao of Paradigm Geophysical for their work on the gravity-aided PSDM case study. Thanks to Maryanne Parsons for her work on the 3D volcanics inversion case study.
Brian Anderson holds a B.Sc. in geological oceanography from the University of Washington and is vice president of marketing for Fugro-LCT. He has authored several papers on the integration of gravity and magnetic data with seismic, including case studies from many areas of the world.
Q: How can I know the sedimentary thickness in a basin? How can I better understand the geologic history of a basin?
A: Use magnetics to map the basement to determine the sedimentary thickness. Then, correlate basement faults with sedimentary faults to understand the history of the basin.
Q: How can I better understand the heat flow history of a basin?
A: Understand the impact of the oceanic and continental crust in the basin. These crustal types generally will exhibit differing density and magnetic susceptibility properties, and can be mapped using high-quality gravity and magnetic data.
Q: How can I better understand the hydro-carbon migration pathways of a basin?
A: Use integrated seismic/gravity/magnetic earth models to better understand the comp-lete earth model – Moho, crustal thickness, basement structure, sediments, and faults.
Q: How can I understand volcanics in an area and their affect on my prospect?
A: Use the magnetic and gravity data to map the volcanics and analyze alternative geologic sources for the volcanics.
Q: How can I best choose between two equally viable seismic interpretations? How can I have confidence in my seismic time-to-depth conversion?
A: Test the seismic interpretations and time-to-depth conversion using non-seismic geophys- ical constraints, such as gravity and magnetics.
Q: How can I know the accurate dip angle of near vertical faults/geologic body edges when my seismic image is not optimal?
A: Compute the dip angles from magnetics data or test the dip angles using non-seismic geophysical constraints such as gravity and magnetics.
Q: How can I optimize my seismic survey design to get optimal quality seismic data at the lowest possible cost?
A: Use low cost gravity/magnetic data to understand the basic geologic setting before shooting seismic data.