The results of electrical soundings are represented in graphe in which the half length of the configuration AB/2 = OA is plotted on the abcissa and the corresponding apparent resistivity is plotted, on the ordinate. The scale of both axes are logarithmic.

Diagram bilogarithmic for the representation of the electrical sounding

Download bilogarithmic diagram

Such a scale is a natural choice for some reasons:

• This scale produced the same graphical variation for a relative variation of the variable as the precision expected in resistivity as in depth, being more logically in relative values.
• The effect of a given structure diminishes with depth, and at the same time there is a loss of precision with which we can determine its dimensions. For near surface problem a precision inferior to 1 m could be requiered, while when we want the determination of the depth of the bedrock at hundred meters, the precision could be 10 meters.
• The same reasoning is true in the case of the resistivity. We can hope to determine the resistivity of a conductive clay to within one ohm-meter, whereas such precision would be completely out of question for a resistant limestone.

A second advantage of the logarithmic scale is:

• If we multiply all the electrode separations and bed thicknesses involved by the same factor the curve obtained is the same and simply translated parallel to the abcissa.
• If all of the resistivities in a given geological section are multiplied by the same factor, the result is a simple translation of the original curve parallel to the ordinate.
• To facilitate comparison of the field curves with theoretical curves.