Refraction and amplitude attenuation tomography for bedrock characterization: Trabzon case (Turkey)


BABACAN A. E. , GELİŞLİ K. , Tweeton D.

ENGINEERING GEOLOGY, cilt.245, ss.344-355, 2018 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 245
  • Basım Tarihi: 2018
  • Doi Numarası: 10.1016/j.enggeo.2018.09.008
  • Dergi Adı: ENGINEERING GEOLOGY
  • Sayfa Sayıları: ss.344-355

Özet

The purpose of this study was to characterize the geomaterials and rock quality of Eocene-Neogene aged volcanic sedimentary rocks in Northeastern Pontides, Trabzon, Turkey. Geomaterials characterization is a basic step for constructions, structural design, excavation and seismic microzonation studies. Volcanic sedimentary rocks have a wide distribution in the Trabzon and are foundation rock for many buildings and roads. They are also frequently used for surface work such as road pavement. Refraction tomography and amplitude attenuation tomography were applied to map P-wave velocities (Vp) and attenuation coefficients (a) to determine rock quality and construction plans. A 3-D Vp and a map was created from parallel lines of 2-D seismic data. Multichannel analysis of surface wave (MASW) was also used to determine shear wave velocities (Vs). Four boreholes close to our seismic lines were used to determine the underground structure of the layers, rock quality designation (RQD), and weathering degree. Laboratory tests were conducted on cylindrical specimens obtained from the boreholes to measure ultrasonic pulse velocities (UPV) for saturated and dry conditions of basalts. We also calculated the uniaxial compressive strength (UCS), water saturation, water absorption in weight, and effective porosity of samples. The results indicated the studied area consists of three layers. The surface layer was classified as topsoil with Vp from 0.5 to 0.8 km/s and thickness from 0.8 to 4 m. The second layer was weathered basalt having Vp from 1.6 to 2.3 km/s and a thickness 2-6 m. The bottom layer was identified as slightly weathered and massive basalt. Vp of this bedrock ranges from 2.4 to 3.8 km/s with beginning at depth of about 4 m. The average Vs from MASW was 0.95 km/s. 1/a values were 8 and 23 for weathered and slightly weathered or massive basalt, respectively. Regression analysis assessing the relation among UPV, UCS, and porosity indicated a high correlation among them. The seismic velocities, 1/a values, boreholes logs, and laboratory tests show that the bedrock is shallower and rock quality is better at the northern part of the study area. Therefore, the northern part seems more suitable for constructions. The combination of MASW and seismic refraction plus traveltime and amplitude attenuation tomography revealed fractured and weathered zones more reliably and with greater detail than any one method could have done.