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    Exploration for a cave by magnetic and electrical resistivity surveys: Ayvacik Sinkhole example, Bozdag, Izmir (western Turkey)
    (Soc Exploration Geophysicists, 2012) Balkaya, Caglayan; Gokturkler, Gokhan; Erhan, Zulfikar; Ekinci, Yunus Levent
    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.
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    Upper crustal P-wave velocity structure of Kii Peninsula, SW Japan by first-arrival traveltime inversion of the Kawachinagano-Kiwa refraction profile
    (Wiley-Blackwell, 2011) Bekler, Tolga; Gokturkler, Gokhan
    We investigated the upper crustal P-wave velocity structure down to 5 km in the Kii Peninsula, SW Japan by a tomographic inversion of the first-arrival traveltimes from the seismic refraction experiment performed by the Research Group for Explosion Seismology (RGES) in 1988. The observations were carried out on a profile running in the N-S direction from Kawachinagano, Osaka Prefecture to Kiwa, Mie Prefecture. The profile extends for about 65 km across the major geological zones, which characterize the geological features of SW Japan. Six shots were fired and the generated seismic waves were recorded at 86 temporary observation sites. The inversion scheme was applied to 359 first-arrival times to delineate the velocity structure along the profile. In the tomographic scheme, velocity estimation was achieved by an iterative, linearized least-squares inversion. The Jacobian matrix was constructed via a finite-difference approximation by perturbing the slownesses of the cells instead of performing ray tracing. Traveltime calculations were carried out by using a fast finite-difference eikonal solver. Velocity updates were obtained by a matrix inversion algorithm using a conjugate gradient least-squares scheme. In addition, model covariance and model resolution matrices were obtained to assess the velocity image. Two low-velocity zones observed in the northern and southern parts of the profile are the most prominent features of the tomogram. The P-wave velocity structure is generally consistent with the surface geology, and the fault zones associated with the Median (MTL) and Gobo-Hagi (GHTL) tectonic lines across the peninsula.