Exploration for a cave by magnetic and electrical resistivity surveys: Ayvacik Sinkhole example, Bozdag, Izmir (western Turkey)


BALKAYA Ç., Gokturkler G., Erhan Z., Ekinci Y. L.

GEOPHYSICS, cilt.77, 2012 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

Geophysical survey techniques have been successfully applied to near-surface cave detection in karstic terrains. We used magnetic and electrical resistivity surveys to delineate the karstic structure of the Ayvacik Sinkhole, which may be considered to be a vertical cave. The magnetic-total-field-anomaly map helped reveal the metamorphic and sedimentary units in the study area. The total-horizontal-gradient map, which was based on a calculated pseudogravity anomaly, successfully identified the contact between the limestone unit and the cave system. Using these results, we positioned and carried out a vertical electrical sounding (VES) survey with a Schlumberger array along a line that consisted of 11 stations. The VES data were then processed using a ID global optimization technique, which used a genetic algorithm and a 2D linearized least-squares algorithm. The results were generally in good agreement with each other, and together they pointed out three geologic layers: (1) an overburden layer (>316 Omega m), (2) an approximately 25-m-thick alluvial fill (100-316 Omega m), and (3) a limestone unit (316-3162 Omega m); and also suggested the existence of a high-resistive anomaly (>15000 Omega m), possibly a karstic cave, located at the depth of approximately 40 in. Also, the results suggested that the buried limestone unit had an undulating karstic topography including a probable pinnacle structure. A synthetic modeling study was carried out, and it validated the reliability of the results. Finally, our findings indicated that the geophysical survey techniques used here were successful in detecting a cave located deep enough to make human exploration difficult.