An N-profile is simply a graphic presentation of N in the vertical. The profile is sometimes referred to as the N-curve or trace. It is constructed on graph paper, using the height of the N -values and the N -values themselves. See figure 6-3-8. Note that the heights are displayed vertically along the left side of the graph and the N -values are displayed horizontally along the bottom. After plotting all of the N-values, connect the values. Compute N-gradients for every layer, starting at the surface. Color code the zones or layers of subrefraction, superrefraction, and trapping conditions; shade subrefraction zones blue, superrefraction zones green, and trapping zones red.

After plotting the N values on the N-profile, use the N-gradient overlay to quickly determine refractive conditions without computing gradients. The overlay is shown in figure 6-3-9. Use it as follows:

1. Place the N-gradient overlay on the graph so the intersection of the N-reference line and the h -reference line is on the plotted N -value at the base of the layer with which you are working. Align the N -reference line with the height line on the graph, as shown in figure 6-3-10.

2. Determine the refractivity for the layer in question. For example, in figure 6-3-10, the

Figure 6-3-8.-Analyzed N-profile.

conditions in the layer from 1,500 to 2,000 feet are subrefractive.