Subsurface structure of seabeach revealed by seismic tomographic velocity model: An example of the Qingdao Shilaoren beach

doi:10.13745/j.esf.sf.2021.3.15

Abstract

Abstract:

Understanding the subsurface structure of seabeach is an important part of coastal zone research and fundamental to seabeach protection and coastal development. Current seabeach researches are mainly focused on beach surface dynamics, but the internal structure and basement morphology of seabeach is little studied. Although drilling and other conventional exploration methods can be used for structural analysis of shallow sand, they are inadequate for building a complete seabeach model from surface to the bottom of basement. Without such a model it is difficult to calculate the sandbody volume or study seabeach evolution; therefore, a wide-range, effective detection method is needed for the structural analysis of sand body. Here, as an example, we apply multiscale seismic tomography to the Qingdao Shilaoren beach, a typical seabeach on the granite rocky coast of southeastern Shandong Peninsula, to investigate its detailed subsurface structure from surface to the crystalline basement. By using the newly developed heavy hammer source, we obtained the seismic data with high signal-to-noise ratio at offset as far as 150 m under noisy beach environment. The multiscale seismic tomography method performed well in modeling the subsurface structure of the seabeach, which allowed us to establish a velocity model to a depth of ~50 m below the Qingdao Shilaoren beach for interpreting the internal structure of sand body. The resulting velocity structure model fits well with the previously reported sand depth and particle size variation results obtained from drilling experiments. The inverted velocity structure shows that the sand body beneath the north end of the survey line (coastal side) is thicker than that beneath the south end near the shoreline, with ~15 m thickness beneath the backshore and only 5 m beneath the foreshore. We infer that there may be underground vertical rocks and partially weathered rocks in the sand body. The uneven weathering resulted in a concave-shaped basement that might have stabilized the sand body. Our method can be used to quickly and effectively build velocity models of subsurface sand body and other subsurface structures of seabeach, which is significant to the study of seabeach structure and evolution and beach protection.

Key words: seismic tomography, beach, sand body, seismic hammer source, Qingdao Shilaoren beach

CLC Number:

GAO Fujian, ZOU Zhihui, WANG Yonghong. Subsurface structure of seabeach revealed by seismic tomographic velocity model: An example of the Qingdao Shilaoren beach[J]. Earth Science Frontiers, 2022, 29(5): 275-284.

Figures/Tables 11

Fig.1 Diagram of working area in Shilaoren beach

Fig.2 Filtering process of typical seismic data

Fig.3 Results of seismic first-arrival time picking

Fig.4 Diagram of multiscale parameterization of velocity model. Adapted from [18].

Fig.5 Checker board resolution test

Fig.6 Variation curve of sand particle size with depth measured by sampling in drill hole

Fig.7 Tomographic inversion results

Fig.8 Variation of travel-time residuals with iterations during tomographic inversion

Fig.9 Restoration test of velocity model

Fig.10 Inversion velocity contour profile and depth cross section show layered structure below beach

Fig.11 Sand-body structure beneath survey line in Qingdao Shilaoren beach inferred based on velocity model

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