A review of seismic geophysical testing in Iran for building near-surface velocity models

Abstract

It has long been understood that the near-surface structure of the earth affects several aspects of civil engineering practice, particularly in seismic applications. Therefore, the implementation of a competent subsurface exploration program not only addresses the need for comprehensive site characterization but also reduces the devastating consequences of seismic activities. Typical geotechnical practice for subsurface exploration has often relied on a combination of drilling, in situ testing such as standard penetration testing and cone penetration testing, and laboratory testing of field samples. However, geophysical techniques have been developed over the years to evaluate near-surface velocity models as a proxy for subsurface stiffness in seismic applications. In this regard, seismic geophysical methods such as refraction, reflection, downhole and crosshole testing, spectral analysis of surface waves (SASW), and multichannel analysis of surface waves (MASW) have continued to receive increased attention due to their advantages over traditional geotechnical subsurface exploration techniques. This has particularly been the case in the United States and in more developed countries across Europe and Asia. However, other countries have been slower to adopt such methods, and a comprehensive review of their domestic research literature can highlight potential improvements in their standards of practice for site characterization. Considering Iran, the domestic literature on seismic near-surface geophysical methods was investigated by examining the temporal variation of several parameters of interest, including the number of published studies, methods used (e.g., active versus passive), depth of investigation, geometry of subsurface profiles developed, and the desired application. Based on this review, it was noted that more than half of the seismic geophysical studies in Iran are exclusively relying on active approaches. Of these active-based methods, more than half rely on conventional seismic refraction. Moreover, approximately 75% of studies target depths equal to 30 m or shallower, and the overwhelming majority of publications (more than 80%) develop 1D profiles. Finally, it was observed that a lag time of multiple years was often required for state-of-the-art methods to be used in Iran, particularly in the case of refraction microtremor, SASW, and MASW. This lag time also implies that these methods have not been used domestically in certain applications where other countries have grown more accustomed to using these methods. Based on results of the increased rate of usage in recent years, it is expected that seismic geophysical testing, especially surface-wave methods, will become more common in Iran and that researchers as well as practitioners will adopt these methods for near-surface site characterization and other applications.