Geophysical investigation of a landslide: The Altindag landslide site, Izmir (western Turkey)

A geophysical survey including electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) was carried out to study a landslide site in the Altindag district of Izmir city, western Turkey. In the city, landslide is one of the causes of natural hazard, and one of the most important landslide sites is located in the Altindag district. The ERT studies were performed along four profiles over the landslide body in the directions of N-S and E-W. A Wenner-Schlumberger configuration was used during the resistivity measurements. The SRT survey was carried out using vertical geophones along a profile coincident with one of N-S trending resistivity profiles. Both the resistivity and the seismic data sets were processed by least-squares inversion techniques. In the first-arrival traveltime inversion of the surface refraction data, a method which is not based on ray tracing was used. Instead, the method used the functional description of traveltimes to generate the Jacobian and this matrix was calculated by using a finite-difference approximation based on the perturbation of the cell slownesses. Traveltime calculations were performed by an eikonal solver. Each technique produced useful results on the internal structure of the landslide, physical properties of the landslide material and sliding surface geometry. The landslide material was characterized by low resistivity and low seismic velocity. The resistivity results also indicated the zones associated with the high water and clay contents within the landslide mass. A layer of consolidated clastic rocks was considered to be the landslide bedrock, and it was represented by relatively high (moderate) resistivity and high velocity. Integrated interpretation of the resistivity and the seismic refraction data along one of N-S trending profiles helped us identify the geometry of the failure surface and the changes in the thickness of the landslide material. An undulating sliding surface is present along that profile and the landslide material gets thicker in the middle and the northern parts (the toe area) of the profile. Moreover, the water content is thought to have a key role in the mass movement in the landslide site. (C) 2008 Elsevier B.V. All rights reserved

Investigation of a shallow alluvial aquifer using geoelectrical methods: a case from Turkey

Near-surface geophysical methods are commonly used to solve a wide class of geological, engineering and environmental problems. In this study, a geoelectrical survey was performed to investigate an alluvial aquifer. The study area is located in the southwest of the Cubukludag graben, situated in the south of Izmir, Turkey. The geophysical studies included the electrical resistivity imaging and self-potential (SP) methods. The resistivity data were acquired along eight profiles in the northern part of the study area by a Wenner-Schlumberger electrode configuration and the data processing was achieved by a tomographic inversion technique. The SP data were collected by gradient technique along 16 profiles. Total field values were calculated for each profile by addition of the successive gradient values, then a total field SP map was obtained. The water-saturated zone in the northern part of the study area was clearly revealed by the electrical resistivity imaging and the SP survey yielded useful information on the subsurface fluid movement

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

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