A RAPID GRAPHICAL METHOD FOR THE INTERPRETATION OF A‐ AND H‐TYPE ELECTRICAL SOUNDINGS

Abstract

Three‐layer vertical electrical sounding (VES) curves that register the effect of a highly resistive electric basement are of either the minimum (H) or the double‐ascending (A) type. The correct interpretation of these two important types of VES curves depends largely on the successful determination of the resistivity of the second layer. This resistivity can sometimes be evaluated from electric logs, from a comparative study of field VES curves, from electrical measurements on outcrops, or from parametric VES curves. A new single auxiliary curve has been devised to help interpret both these types of VES curves, provided that the resistivity of the second layer is known. The thickness of the middle layer is determined accurately and rapidly by the value of the abscissa at the intersection of the total longitudinal conductance line S with the new auxiliary curve. The auxiliary curve is used to interpret an H‐type curve, an A‐type curve, and a five‐layer (KQH‐type) parametric VES curve. Analysis of the KQH parametric curve shows that the resistivity of the third layer may vary by more than a thousandfold without affecting the shape of the curve, whereas the resistivities of the second and fourth layers may correspondingly vary by less than one order of magnitude.