What is "Equivalent Source Gridding"?

Equivalent Source Gridding (ESG) is the next generation of gridding routines for aeromagnetic and gravity data. Developed by PRJ, ESG is now available for the processing of your new and existing aeromagnetic or gravity data sets. ESG not only produces superior contour maps, but also better shaded images, and the resultant grid is more faithful to the geology. Operator and filter maps are therefore substantially more accurate.

BENEFITS OVER CONVENTIONAL GRIDDING

• Fewer "herring bones"
• Fewer line anomalies
• Fewer "dimples" or "pock marks"
• Fewer "ripples" in shaded relief maps
• More accurate operator and derivative maps that better represent the underlying geology and that contain fewer processing artifacts
• Noise due to variable elevation is gone
• The data can easily be contoured on any surface, either a fixed elevation or draped surface

Gridding Using Equivalent Sources

The search for better ways to reduce data to a regular grid for contouring, image display or two-dimensional processing has a long history in geophysics. Early attempts were limited by the capacity of the computers of the time; in the early 1970's sufficient computer capacity became available to allow a serious search for techniques that were in some sense optimal. This resulted in the near-universal adoption of minimum-curvature gridding of potential fields in exploration geophysics. Minimum curvature methods produce what is in some sense the smoothest surface that passes through the data, and can be implemented efficiently in finite-difference form.

Despite these advantages, minimum-curvature gridding has some drawbacks which have become increasingly apparent as imaging methods have gained popularity in the potential fields community. Two kinds of artifacts are often seen on such images: "pock marks" surrounding individual data points, and line anomalies. The former usually appear on gravity maps, and are believed to be generally associated with lack of near-station terrain corrections. The line anomalies are often due to small elevation deviations in the flight lines from nominal specifications. These artifacts, while subtle, can make an image presentation of the data difficult to work with. Thus, a new approach to gridding again is needed.

Over the past few years, there has been a renewed interest in equivalent source methods, largely rejected in the 1970's as being impractical for the computers of the day. Two events have occurred to change the balance: computer capacity has increased rapidly, and schemes which minimize computational effort have been developed. Recently, Mendonça and Silva have published a gravity equivalent source algorithm which is quite attractive. PRJ has now developed a system based on the magnetic equivalent of the technique of Mendonça and Silva.

Basically, the method computes a magnetic layer below the data which reproduces the data almost exactly. Once this is done, new values can be calculated using the equivalent layer at any desired location and elevation. In particular, a survey can be brought to either a perfect constant barometric elevation or to a perfect drape. This completely eliminates line anomalies that are associated with elevation deviations. In addition, the field can be re-computed at any desired inclination and declination; reduction to the pole is thus a straightforward matter once the equivalent layer is defined.

Because the equivalent layer is saved after the initial run, any number of maps at different elevations can be computed quickly and easily. The equivalent layer thus forms the basis for a whole suite of interpretive tools, avoiding some of the artifacts associated with Fourier transform interpretation processing.