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Interpretation (Correlation): Decisions and Parameters 

The first decision is what seismic event to pick. Figure 1 shows a geological marker in two wells; a continuous reflection approximately ties the two markers, but in one well the marker is on a positive-to-negative zero crossing, while it is close to a peak in the second well. A synthetic seismogram might help, but that is beyond the scope of this article. Often the interpreter can simply decide on a continuous event to pick close to the marker to be mapped. 

Most reflections are composite; the perfect phase point to pick is uncertain. All interpretation systems can "snap" to, or follow, maximum negative, maximum positive or zero crossings (going negative with increasing time, and going positive). In Figure 1 the reflection is picked on the peak of an event, and on the positive-to-negative zero crossing. Each horizon is an automatic track along a line from a single seed point on one trace. 

Any interpretation system has parameters that can be set to specify the way in which the correlation from trace to trace is done. These may include:

• Maximum time (or depth) change from one trace to the next.

• Whether to pick the largest event within this limit, or the closest.

• Maximum amplitude change from one trace to the next.

• Whether to follow an event, or to cross-correlate.

If the parameters are too restrictive, the tracking leaves gaps. If the parameters are too loose, it makes mistakes.

Automatic Tracking for 3-D Data 

For 3-D data, automatic tracking from one trace to the next can be extended throughout the data volume. Figure 2 shows the result of automatic picking of the peak. Where the event mapped becomes less obvious, the automatic picking breaks down. 

The conventional seismic data may not give the most accurate picture of geological structure when you pick on a constant phase point (peak, trough or zero crossing), especially if the reflection is weak. Seismic attributes allow removing the amplitude information to make all reflections the same. The instantaneous phase attribute does this, but is discontinuous where the phase passes 180 degrees (it jumps to -180 degrees). 

A more elegant attribute is cosine of instantaneous phase, which is -1.0 for both 180 degrees and -180 degrees (Figure 3). Notice there are differences of up to 1.1 meters (1.5 meters, or five feet -- a significant depth error in many prospects) between the two tracked horizons -- green (picked on conventional data) and red (picked on cosine-of-phase). 

Workstations interpolate between samples using a spline function, so the peak or zero-crossing is picked with a much greater precision than the sample interval (two meters in this data). This difference between the two horizons is real. Automatic tracking is at least an order of magnitude more accurate than manual picking; such a small time difference would never be detected with manual timing. 

Along with the structure map, we can get a reflection amplitude map, either trace amplitude or the reflection magnitude. Reflection amplitude measurements are impractical with manual picking. 

With the cosine-of-phase data volume, details differ (compare Figure 4 and Figure 2). Either of these maps show an excellent picture of the structure in most places, and we have achieved it by manually identifying the event on one trace only.

The results vary with which point on the reflection is used for picking. In this example, automatic picking fails over a larger area if the positive-to-negative zero crossing is used on the conventional data (Figure 5). There are fewer gaps with cosine-of-phase (Figure 6), but there are obvious errors in several areas.

Alternative to Automatic Tracking 

For 3-D, an alternative to automatic tracking is to pick manually a subset of the data, then interpolate. This is particularly attractive if automatic tracking is unreliable because the event is weak or the horizon is extensively faulted. In this case the steps to producing both a structure map and an amplitude map are:

1. Manually pick a subset of the data, such as every tenth line and crossline, by displaying the lines on the screen and picking exactly where you want the horizon, using automatic tracking along the line, or point-to-point interpretation.

2. Interpolate between the picked lines.

3. Snap to a peak, trough or zero crossing.

4. Smooth if needed.

5. Map and extract amplitudes.

This alternative technique can be used over the whole of a 3-D survey or only over parts where automatic picking is unreliable and the results merged with automatic picking in areas where that technique is reliable.

Concluding Comments 

Automatic tracking of seismic horizons in good quality data from a few seed points is a powerful tool for rapidly completing interpretation of 3-D seismic data volumes. It reveals geology with much greater precision and detail than manual interpretation can.

However, there are pitfalls in its use; it is less than reliable unless the interpreter understands the geology and restricts the automatic picking by using parameters chosen to minimize mis-picking. Interpolation is often a good alternative for 3-D if automatic tracking will not work reliably

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