Bedrock Depth Calculation of Çanakkale (Turkey) Basin Using Rayleigh Ellipticity and Microgravity Survey

Abstract

It is a well-known fact that the local ground effect plays an important role in the damage to structures during earthquakes. In order to define the soil-structure interaction correctly, the geometry of the tight and loose soil layers in the soil should be well known. In addition, the impedance contrast due to local ground effects can be deeper than the first 30 m, which is accepted as the engineering depth, and in this case, the Vs,30 parameter is disabled. For this reason, the location of engineering bedrock and seismic bedrock gains importance. The variation of shear wave velocity with depth is a key parameter for seismic field effect estimation. In addition, the separation of low-density layers close to the surface and higher-density bedrock can be easily done by the gravity method. Bedrock/seismic foundation levels were determined by using these two methods together for Çanakkale soils, which are known to have thick alluvial thickness and liquefaction problems. Microgravity measurements were made at 100 points and the anomalies obtained were modelled in three dimensions. In light of the data obtained from this study, it has been determined that the thickness of the loose basin unit in the study area is in the range of about 50-300 m. Ambient vibration was recorded with the help of a velocity seismometer at 146 points, and S velocity (Vs) profiles were obtained by using the inversion of the observed ellipticity curve. In addition, the velocity-depth change rate structure of the basin is defined depending on the Vs velocities on the 2D gravity model made along the two profiles that cut the basin in N-S and E-W directions. Along these profiles, the frequency range has been determined as 0.8-1.3 Hz in the deep parts of the Basin and 1.5-11.0 Hz in the shallow parts.

Keywords

• bedrock
• microgravity
• rayleigh ellipticity
• s-velocity
• seismic foundation