ENHANCED LINEAR FEATURE ANALYSIS

The horizontal gradient has been used for some time to delineate structural contacts from aeromagnetic data. It can be shownthat for a high-angle contact the horizontal gradient of the reduced-to-pole magnetic field has its maximum at the contact location. Finding the contact locations then becomes a matter of locating these maxima. One approach to this has been given by Blakely and Simpson (1986), but this algorithm gives just a fence line which can be visually difficult to follow.

EDCON-PRJ has developed an alternative method, Enhanced Linear Feature Analysis, which provides not only a fence line plot but a visually appealing map which quantitatively depicts the sharpness of the contact. Based on curvature calculations similar to those recently described for seismic data by Roberts (2001), the method lends itself well to color plotting for rapid assessment.

To illustrate, we show the results obtained from an aeromagnetic survey of part of the Green River Basin in Wyoming. The data were acquired on northeast-southwest traverse lines at approximately a -mile line spacing and northwest-southeast tie lines at a spacing of approximately 2¼ miles at a flight height of 500 feet.

Figure 1 shows the reduced to pole anomalous total magnetic intensity for the survey; Figure 2 shows the horizontal gradient of the reduced to pole anomalous total magnetic intensity.

Figure 1. Reduced to pole anomalous total magnetic intensity from the aeromagnetic survey. Contour interval 20 nT.

Figure 2. Horizontal gradient of the reduced to pole anomalous magnetic intensity for the aeromagnetic survey. Contour interval 2 nT/km.

Figure 3 shows the boundary points calculated from the horizontal gradient (Figure 2) of the aeromagnetic data.

Figure 3. Boundary points calculated from the horizontal gradient of the aeromagnetic data.

Figure 4 shows the Enhanced Linear Feature Analysis of the horizontal gradient from Figure 2. In this plot, negative values, which correspond to valleys inthe horizontal gradient, have been suppressed. Note that the lineations, visible in the horizontal gradient, are nowextremely obvious.

Figure 4. Enhanced Linear Feature Analysis of the horizontal gradient from the aeromagnetic survey.

While the boundary point plot (Figure 3) and the Enhanced Linear Feature Analysis (Figure 4) contain exactly the same information as far as locating thecontacts is concerned, the Enhanced Linear Feature Analysis plot is far easier to follow visually and contains additional information about the sharpness of the contact that is missing from the boundary point plot.

References:

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, 51, 1494-1498.

Roberts, A., 2001, Curvature attributes and their application to 3D interpreted horizons: First Break, 19, 85-100.