RESISTIVITY PROSPECTING IN A UNITED NATIONS GROUNDWATER PROJECT OF WESTERN ARGENTINA*

Resistivity surveys including surface measurements and electric well logging are at present carried out as routine techniques by a Groundwater Project of the United Nations Special Fund in arid valleys of the Argentine eastern Andes.

The aims of the Project are the evaluation of ground water supplies in 5 pilot areas and the training of Argentine professionals in modern techniques of groundwater investigation.

In Zonda and Tulüm Valleys, one of the five pilot areas, the groundwater body is essentially represented by alluvium of San Juan River and upper Tertiary gravels lying upon impervious clayey Tertiary sediments.

Profiles of electrical soundings—spread in the valleys normal to the structural trends—were completed as a broad reconnaissance type investigation. In the outwash plain, a thick clayey overburden sometimes covered by saline material produces a masking effect on the deep sands and gravels and makes the evaluation of true resistivities difficult. Under certain conditions, appropriate use of electric well logging results helps solving the ambiguities found in the interpretation of the electrical soundings. Two examples are discussed.

Also a typical development of surface resistivity interpretation as a whole is presented. Emphasis is placed upon the significance of the transverse resistance values to better interpret the hydrogeological picture. The last stage in data processing, presented on a map of the resistive fill and the conductive basement, provides evidence of the strong structural control of the valleys and shows the main variations in the ground water body constitution.