ELECTRICAL RESISTIVITY TOMOGRAPHY FOR SUBSURFACE CAVITY DETECTION IN KARST TERRAINS: A CASE STUDY FROM THE UCTU IJI CHANGA CAVE (TENA, EASTERN ECUADOR)

Electrical Resistivity Tomography (ERT) has shown to be a helpful tool in near-surface cave detection, however, the high-cost of these equipment has limited their use in developing countries such as Ecuador. In order to solve this limitation, we have conducted a thorough study to assess the reliability of an Inexpensive Resistivity Device (IRD) in detecting the shallow subsurface Uctu Iji Changa Cave located in the Amazon karst system (Oriente Basin, Tena). An ERT study using both a typical and modified dipole-dipole arrays has been carried out along 13 profiles, which were interpreted corroborating topographic, stratigraphic and speleological data. The results were generally in good agreement with each other, and together they pointed out three geologic layers: (1) a top layer of ~2 to 9-m-thick (< 50 Ωm - limestone); (2) a middle layer of ~2 to 4-m-thick (51–80 Ωm - marlstone), which hosts the cave ; (3) another limestone intercalation (< 50 Ωm). The high-resistivity anomalies (80 – >100 Ωm) within the marlstone layer were interpreted to be the signal from the caves. Our material characterization shows well-defined ranges of possible resistivities, which is due to little seasonal variation during the year in Ecuador. The modified dipole-dipole array was superior to the typical one since it reached higher penetration depths without increasing noise error. Repeated and reciprocal ERT profiles were carried out validating the reliability of the results and the method. The ERT using IRD has therefore proved to be an appropriate technique for cavity detection in the Ecuadorian karst system. It is hoped that this technique can be widely used, solving the major challenges of karst surveying in Ecuador, such as the need for a national database of caves.