Types of resistivity surveys There are several variations on resistivity surveys: 1.A “lateral profile” aims to locate anomalies, along a line or on a map 2.A “depth profile” aims to construct a vertical profile of subsurface resistivities and depths 3.A combination of the above, which aims to construct a “pseudo-section” or a “real section”

Lateral profiling In lateral profiling the apparent resistivity will respond to lateral changes in the true resisitivity. Two cases are illustrated: the Wenner profile, with four mobil electrodes, and the Schlumberger (gradient) array with two mobile electrodes.

Depth sounding Roughly 30% of the current penetrates below a depth approximately equal to the electrode spacing. Increased current penetration means apparent resistivity will be more affected by deeper layers.

Depth sounding In a two layer case, the apparent resistivity will either increase with spacing, or decrease, depending on the layer properties. In a three layer case the profile has several possible appearances.

Master curves (“type curves”) For a two layer model, it is possible to derive a relationship of the form: where k is a measure of the resistivity contrast, If we choose a value of k and plot the relationship, changing ρ 1 only changes the vertical scale, and changing h only changes the horizontal scale.

Master curves (“type curves”) Instead of plotting this relationship, it is more useful to plot log ρ a vs log a, since This means if ρ 1 changes, the plot is identical except for a vertical shift, and if h changes the plot is identical except for a horizontal shift.

Master curves (“type curves”) Recipe for using master curves: 1.Plot the data on the same logarithmic scale as the master curves 2.Shift the data to find the best fitting master curve 3.Read the vertical and horizontal shifts to find ρ 1 and h 4.Read the value of k to find ρ 2

Pseudo-sections On occasion a more extensive picture is required, combining lateral profiles with depth sounding. Imagine two lateral profiles, with different array spacings: only the larger array spacing will “see” a deeper anomaly.

Pseudo-sections We may wish to use a variety of spacings to build a more complete sub-surface picture. A special technique for plotting data from these surveys is commong. This is the “pseudo- section” – captures all data simultaneously

Confused? The best way to understand pseudo-sections is to plot one …

Given these dipole-dipole data, fill-in the missing column, and create a pseudo-section. What might be going on in this example?

Pseudo-sections A pseudo-section is not a true resistivity section – it is only a way of plotting the data. The data from a scale model illustrate the limitations – note the typical “inverted-Vee” shape of the result

Pseudo-sections Top: a pseudo-section from a groundwater application. Bottom: a “real-section” result using the same data.

Next lecture: Design of resistivity surveys 1.Survey objectives 2.Depth of target 3.Adequate signal strength 4.Sensitivity to target parameters 5.Symmetry of the response 6.Minimize manual repositioning of electrodes 7.Compatibility with “Induced Polarization” (see later)