FIGURE 5.14 Apparent soil electrical conductivity (ECa) maps of the same agricultural test plot from data collected with (a) the Veris 3100 Soil EC Mapping System and (b) the OhmMapper TR1. The ECa scales for each map are different, and the ECa values are in mS/m.

were collected by the Veris 3100 Soil EC Mapping System (continuous galvanic contact method) with its 2.1 m electrode array. The data used to produce Figure 5.14b were collected by an Ohm-Mapper TR1 (continuous capacitively coupled method) with a 5 m current dipole, a 5 m potential dipole, and a 1.25 m separation between the dipoles. The reported investigation depth for the Veris 3100 with its 2.1 m electrode array is 0.9 m, and the OhmMapper TR1 in the configuration described had a median investigation depth of 0.8 m. There was an interval of two days between the Veris 3100 and OhmMapper TR1 surveys, but soil temperature and moisture conditions did not change drastically during this time interval.

When compared to one another, the spatial ECa patterns exhibited by both Figure 5.14 maps appear consistent, and this observation is confirmed by a correlation coefficient (r) between the two maps that equals 0.79. The average test plot ECa from the Veris 3100 survey was 46.5 mS/m, and the average test plot ECa from the OhmMapper TR1 survey was 33 mS/m. The difference in the average test plot ECa may reflect dissimilarities between the Veris 3100 and OhmMapper TR1 systems in regard to the soil volume influencing the instrument response, the magnitude of effects due to small-scale features, the sensitivity to small changes in field conditions, the relative impact of unwanted electric signal (noise), procedures for calculating ECa, among others. Consequently, given a particular survey area, the horizontal pa or oa, ECa maps produced with different measurement systems having similar depths of investigation will usually display similar spatial patterns, but average pa or ca, ECa values may be significantly different. Importantly though, the pa or ca, ECa horizontal spatial patterns prove useful in assessing lateral changes in soil properties.

Two-dimensional resistivity (or electrical conductivity) depth sections characterize the distribution of resistivity (or electrical conductivity) with depth beneath a measurement transect along the surface. The resistivity (or electrical conductivity) values shown in a depth section are considered to represent true values, not apparent values. The data needed to create a depth section can be obtained several ways, such as through a set of vertical electric soundings conducted at a number of regularly spaced locations along a transect, several resistivity survey passes over a transect with a one-electrode array whose length is changed each pass, or one resistivity survey pass over the transect using several different length electrode arrays at once. The data acquired are then used as input for the forward or inverse computer modeling programs that generate the resistivity (or electrical conductivity) depth sections.

Two examples of EC depth sections are displayed in Figure 5.15. Each EC depth section is from a separate agricultural test plot. The two agricultural test plots are situated adjacent to one another

Line Distance (m) 13.0 23.0

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