Gravity/Magnetic Data Use Surges

By BRIAN S. ANDERSON and MARK E. WEBER

(Editor’s note: The Geophysical Corner is a regular column in the EXPLORER and is produced by the AAPG Geophysical Committee. This month’s column is part one of a series on “The Renaissance of Gravity,” titled “Recent Advances.”)

The question is: What happened?

The ongoing surge in use of gravity to the present historic high levels can be attributed to several recent key developments, including:

• The industry is involved in more challenging exploration plays than ever before.
• Increased costs of exploration and drilling.
• Major advances in data resolution.
• 3-D modeling software applications are now integrating seismic, gravity and magnetics on the same workstation.
• 3-D seismic has not answered all our questions.
• Cost-effectiveness of the gravity and magnetic techniques.

A Brief Look Back

Historically, gravity and magnetic data were primarily used as a basin reconnaissance tool for determining gross features of an area. Key elements are depth to high-density basement (often coincident with economic basement), sediment thickness, fault delineation, mapping of volcanics and salt modeling.

A common approach was to have a gravity and magnetic “guru” on staff, or as a consultant, who would disappear with all the data and return some time later with an “answer.”

If the results of the gravity and/or magnetic work did not agree with the seismic interpretation, the “guru’s answer” would generally be disregarded.

Although much good work was achieved, results had been limited by the resolution of the recorded gravity and magnetic data, and the lack of cohesive integration.

Advances in Resolving Power

At a recent technical meeting in Houston, Ed Biegert, non-seismic methods specialist for Shell Development, asked the question: “Why do we re-acquire gravity?”

His own answer to the question was: “For the same reasons we re-acquire seismic data.”

Although the gravity fields mapped in prior years have not changed, our ability to accurately measure and process gravity on a ship has improved dramatically – just as we have improved our ability to shoot, record and process seismic data (figure 1).

Recent advances in gravity measurement at sea include:

• Upgrading from analog to digital control and acquisition systems.
• Higher data sampling and recording rates (200 Hz sampling, 1 Hz recording).
• Precise DGPS positioning for removal of ship accelerations.
• More accurate measurements of water depth.
• New data processing developments (signal to noise enhancement, micro-leveling, etc.).

With these advances, industry has seen stunning improvements over data recorded as recently as 10 years ago. In many cases, there is an increase of up to 10 times the data per unit area in new surveys over older data, with a correspondingly higher level of confidence in interpreted geological results.

Many operators are routinely incorporating new high resolution gravity into their interpretation projects, particularly in the deep water Gulf of Mexico. This integration is facilitated by new workstation software applications (figure 2 - 16KB).

Gravity Data Accuracy

One very experienced oil company gravity and magnetic interpreter quotes the following approximate interpretable accuracies of data in the Gulf of Mexico:

1. Sidney Schafer Water Bottom Gravity (shelf to 600-foot water depths), over 2,000- to 4,000-foot horizontal distances; primary limitation is sampling – station spacing: 0.1 to 0.3 milliGal (mGal).
2. Legacy Deep Water Marine Data (over 10 years old) over 10,000-foot horizontal distances: 0.5 to 1.0 mGal.
3. New High-Resolution Deep Water Marine Data (1991 or newer), over 1,500- to 3,000-foot horizontal distances: 0.1 to 0.5 mGal.

The above examples are general estimates based on several criteria, including positioning, instrumentation, sampling, processing techniques and associated bathymetry accuracy. Recent work has shown that errors of 0.3 to 1.0 mGal or greater can be introduced into data due to use of incorrect water depth or positioning information.

The importance of data resolution makes a thorough investigation of the gravity data prior to interpretation a sound practice. As with any geophysical technique, ambiguities still exist, and the limitations of the technique should be thoroughly understood.

Just What is a MilliGal?

Not many of us have a good grasp of what this measurement unit of gravity means, or in more general terms, the impact of gravity data accuracy on geologic interpretations.

“We already have a gravity map” is often heard at oil companies.

The following is an exercise in converting gravity (milliGals) into a meaningful geologic quantity (thickness of salt – in this case, thickness of a salt lens).

Modeled Salt Thickness
vs.
Gravity Data Accuracy – Sensitivity Models

Using a generalized density vs. depth curve for the deep water Gulf of Mexico, we have constructed a series of sensitivity models for a salt lense, two miles in diameter (figure 3 - 16KB).

The salt was inserted into the density model at several depths. At each depth the thickness was varied to establish data points for a salt burial depth and thickness vs. gravity response chart (figure 4 - 32KB). The results of these models quantitatively demonstrate the need for accurate gravity data in deep water salt modeling.

Admittedly, this is an over-simplified example, but it is effective in demonstrating the need for good quality gravity data to obtain meaningful geological results.

To read the diagram, go to the x-axis (depth) and find the 7,500-foot depth point. Moving upwards on the chart to the 0.1 mGal data curve, we see on the y-axis that 0.1 mGal data, when modeled for salt at this depth, will provide approximately plus or minus 300 feet accuracy in modeled salt thickness.

For 0.2 mGal data this range grows up to 400 feet; for 0.5 mGal data results are plus or minus 1000 feet; and for 1.0 mGal gravity data (most older gravity data sets in the deep water) the results are plus or minus half a mile of salt!

The Present Economics

Of Gravity and Magnetics

New high resolution gravity data costs approximately $1,200 per Gulf of Mexico OCS lease block, or $12 per line mile for new 2-D high resolution data (e.g. gravity from TGS-Calibre Phase 45 Program).

Costs for 2-D models are in the $2,500 to $5,000 range, and full 3-D gravity and magnetic modeling studies can cost from $25,000 to $50,000 or more depending on the complexity of the model.

In terms of new data acquisition, crew and equipment costs are in the range of $1,500 per day or less. In areas like the deep water Gulf, many companies are finding this a worthwhile investment. When rig rates are pushing well over $100,000 per day, it is easy to understand why.

Figure 5 - High resolution gravity used to refine 2-D and 3-D seismic velocity
Petrocaem Display Courtesy CGG-Petrosystems

(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.)

Printed with permission of AAPG Explorer.