Seismic refraction and downhole survey for characterization of shallow depth materials of Bam city, southeast of Iran



Seismic refraction and downhole survey were employed to study dynamic characteristics of subsurface materials in Bam city, southeast of Iran. The data acquisition was performed at 160 P and S-wave refraction stations and 15 boreholes in the city. To derive velocity depth sections along these profiles as well as to perform downhole diagrams, SeisImager software was used. Based on the obtained values different maps consists of iso–depth, iso-velocity and iso-Poisson’s ratio maps of city were prepared. The obtained results from the velocity variations of the P and S waves, three layers were identified. The first layer has low velocity, the second layer with medium velocity and the third layer has relatively high velocity values. The thickness of the first layer increases from the south west towards the north east of the study area while the thickness of second and third layers decreases from south west towards the north east of the study area, where in the south west of the region because of the thickening of the third layer, even using the far shot data of seismic measurement, the thickness of this layer was not detected. In the other word in the most part of the north east of the region, only two layers with low and medium velocity were determined. Attentive to the velocity distribution of the P and S wave velocity, the Poisson’s ratio distribution as well as attenuation coefficient obtained for the identified layers of the area under investigation in the Bam city, it is concluded that the low velocity layer with high thickness and attenuation coefficient of the first layer can be attributed to be the cause of the strong motion vibration of the soil due to Bam earthquake on 26th December 2003 in the city.