Data Analysis Products

Figure 5.11 through Figure 5.13 illustrate some of the fundamental data analysis products produced from constant separation traversing, vertical electric sounding, and azimuthal rotation resistivity surveys. The apparent resistivity, pa, measurements obtained along a transect during constant

Distance

FIGURE 5.11 A horizontal profile of apparent resistivity along a transect, which can be produced from data collected by a constant separation traversing survey.

Distance

FIGURE 5.11 A horizontal profile of apparent resistivity along a transect, which can be produced from data collected by a constant separation traversing survey.

separation traversing can be graphed to produce a profile of the lateral changes in pa found along the transect (Figure 5.11). In an agricultural setting, assuming the resistivity survey had a shallow depth of investigation (~2 m), the fairly sharp step increase in pa shown in Figure 5.11 most likely represents some spatially abrupt soil property change (e.g., a decrease in soil clay content over a short distance).

The measurements of pa acquired during a vertical electric sounding are usually plotted versus some fraction, 1/K, of the electrode array length (Figure 5.12). For the Schlumberger array, the K value is typically 2; therefore, pa in the case of a Schlumberger array is normally graphed with respect to half the array length. The plotted vertical sounding curve depicted in Figure 5.12, if obtained for agricultural purposes, might indicate a soil profile trend from the surface downward, in which soil resistivity first decreases with depth and then increases with depth. The presence of a clay-pan within the soil profile is one scenario that would account for this type of vertical resistivity trend. During the past, changes in resistivity with depth were determined using manual graphical procedures, where type curves usually representing simple two- or three-layer resistivity depth models were overlaid and fit to the measured vertical electric sounding plot. These older graphical procedures are rarely used today.

Forward computer modeling techniques superseded these graphical procedures and provided substantially improved capabilities for quantifying the vertical distribution of resistivity. The one-dimensional forward modeling approach involves the following four steps:

K" x Electrode Array Length

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