Integration: A Practical View

By BRIAN S. ANDERSON, MARK E. WEBER and JOHN E. BAIN

(Editor’s note: The Geophysical Corner is a regular column in the EXPLORER and is produced by the AAPG Geophysical Integration Committee. This month’s column is part two of a series on “The Renaissance of Gravity,” titled “A Practical View of Integration Methods.”)

Even with the best quality 3-D seismic data, an interpreter can have a troublesome task in defining the salt/sediment boundary at the flanks of a salt dome, salt sheet or other complex structure.

For decades, gravity has been used in the Gulf of Mexico to address this problem. The major differences in how it was done then and how it is now done are twofold. It’s better today because of:

• Better acquisition technology and processed data.
• Truly integrated workstation software tools.

By incorporating a co-recorded data set with each data set (e.g. seismic & gravity) independently measuring a related property of the subsurface, the interpreter can place a much higher degree of confidence in the final geologic interpretation.

To quantify this observation, case studies show that incorporating 3-D seismic with high resolution gravity and magnetics can alter the base of salt interpretation by several thousand feet from the 3-D seismic interpretation alone.

Team-Oriented Exploration Tools

With the trend toward highly focused exploration teams, the smooth interaction and coupling of multiple geophysical disciplines is essential. Explorationists are expected to employ and be familiar with more disciplines on a continuing basis.

The development of workstation applications, which enable the interpreter to simultaneously refine the subsurface model using seismic, gravity and magnetic data, has been a giant step forward.

A flow diagram for such an application is shown in Figure 1.

In using a new software tool kit, high resolution gravity now can be applied to an increasing number of seismic velocity modeling projects. This technique is employed using the procedure outlined below.

1. High resolution gravity is recorded and processed along with the 2-D or 3-D seismic survey. Present techniques allow for delivery of processed gravity data in advance of, or in parallel with, processed seismic data delivery.

2. The seismic velocity data are used to create a corresponding density section (or volume, in the 3-D case) by means of a flexible velocity-density conversion tool kit, incorporating:

Figure 1 - Flow diagram for integrated seismic-gravity-velocity interpretation

• Gardner’s Equation.
• Nafe/Drake, Hilterman and other density-velocity relationships.
• Use of available empirical data (e.g. velocity logs, check shot surveys, gamma-gamma density logs, etc.).
• User defined conversion algorithms or formulae.
• Other approaches.

3. The density model can be as simple or as elaborate as the corresponding velocity model – up to and including a discrete value of density for each x-y-z node within the profile or volume of data.

4. Input of digital horizon data (again, 2-D or 3-D) as interpreted on the seismic workstation. The system incorporates a “universal translator” for the conversion of one type of horizon to another to accommodate company partner teams, etc.

5. Computation of the gravity field of the model, input of gravity data as recorded on the survey, and a direct comparison between the two fields.

6. Manipulation of the model using both forward modeling and inversion processes based on minimizing the misfits between model and measured gravity fields.

7. On completion of the modeling and/or inversion process, the revised earth model is converted into the velocity domain, providing an improved starting point velocity model for depth migration.

8. This iterative process and feedback loop continues throughout the seismic migration and interpretation process.

How it Works:
Gulf of Mexico Example

Figure 2  is a cross section through a full three dimensional model of a salt feature in the Gulf of Mexico. The density cube is derived from available well control. The top of salt is typically obtained from a simple initial stretch to depth from the time interpretation. Later – in the interpretive processing sequence – this is updated with the post-stack or pre-stack depth migration results.

The base of salt is input from an initial time interpretation. In many cases the initial base of salt interpretation is provided with confidence factors, e.g. a 10 might be assigned to high seismic confidence areas, a 0 being assigned to seismic blind zones, and grades in between. The gravity modeling can then be constrained by the high seismic confidence areas, and the low (seismic) confidence areas are then of most interest in the search for a better interpretation using gravity modeling results.

The density and velocity data are analyzed, typically using cross plots, and a function is derived to convert between the density and the velocity volumes. The gravity effect of the density volume is computed and compared with the observed gravity data, and the differences are resolved through a series of automated structural and density inversion techniques.

The final model should contain as much seismic-gravity constraint as possible for optimal results, often involving close interaction between the gravity interpreter and seismic interpreter at the same workstation.

Once the final density model is constructed, the density-velocity function is used to translate the alterations into an apparent velocity cube. Figure 3   is the final result of this process. Note the original outline of the salt body (prior to integration of the gravity and seismic results), shown as a white outline. In this case, several thousand feet of change in the base of salt are indicated through the multi-disciplinary approach, as compared with the seismic-only approach.

A full 3-D view of an integrated seismic-gravity model with well control is shown in Figure 4,  (page 12).

This process, in addition to providing important and independent corroboration and improvement to the seismic interpretation of the base of salt, also provides an important source of long wavelength velocity information beneath the salt masses.

This information, when injected back into the velocity model used for producing the final base salt and sub-salt images, can have a dramatic impact on the enhanced quality of the seismic processing results.

Economic Impact

In today’s team-oriented exploration environment, the availability and use of real-time interpretation software tools allow for the integration of gravity and magnetic data at the same workstation. This approach is now embraced by a growing number of oil companies for:

• Increasing confidence in their geologic interpretations.
• Decreasing risk.

To be most effective, the integration of gravity and magnetics must take place at the earliest stage of prospect development, and can continue throughout the exploration process.

(Editor’s note: Brian Anderson and Mark E. Weber are with FUGRO-LCT Inc., Houston. Brian Anderson can be reached via e-mail at banderson@lct.com. John E. Bain is with Galileo Geophysics.)

Printed with permission of AAPG Explorer.