Guidelines and pitfalls of refraction microtremor surveys

Abstract

The geotechnical industry has widely adopted the refraction microtremor shear-wave velocity measurement technique, which is accepted by building authorities for evaluation of seismic site class around the world. Clark County and the City of Henderson, Nevada, populated their Earthquake Parcel Map with over 10,000 site measurements for building code enforcement, made over a 3-year period. 2D refraction microtremor analysis now allows engineers to image lateral shear-wave velocity variations and do passive subsurface imaging. Along with experience at a basic level, the ability to identify the “no energy area” and the “minimum-velocity envelope” on the slowness-frequency (p-f) image helps practitioners to assess the quality of their ReMi data and analysis. Guides for grading (p-f) image quality, and for estimating depth sensitivity, velocity-depth tradeoffs, and depth and velocity resolution also assist practitioners in deciding whether their refraction microtremor data will meet their investigation objectives. Commercial refraction microtremor surveys use linear arrays, and a new criterion of 2.2% minimum microtremor energy in the array direction allows users to assess the likelihood of correct results. Unfortunately, any useful and popular measurement technique can be abused. Practitioners must follow correct data collection, analysis, interpretation, and measurement procedures, or the results cannot be labeled “refraction microtremor” or “ReMi” results. We present some of the common mistakes and provide solutions with the objective of establishing a “best practices” template for getting consistent, reliable models from refraction microtremor measurements.

Introduction to the special issue of the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) international guidelines for applying noninvasive geophysical techniques to characterize seismic site conditions

“…Surface-based site characterization methods differ mostly in their data acquisition approach but share similar procedures for data processing and analyses (Socco et al 2010;Foti et al 2014;Yong et al 2019). Thus, the section addressing the application of techniques includes six select papers that can be generally categorized into two types: standalone single-method approaches (Louie et al 2022;Pancha and Apperley, 2022;Hayashi et al 2022;Hunter et al 2022;Molnar et al 2022) and the so-called multi-method approaches (Stephenson et al 2022). Within the standalone approach, each method can also be differentiated by whether it relies on either active-or passivesource energy.…”

“…Louie et al (2022, in this issue) and Pancha and Apperley (2022, in this issue) present a pair of papers focusing on the refraction microtremor (ReMi) technique. The Louie et al (2022) paper addresses common misapplications ("abuses") of this popular North American site characterization method that had been in practice since it was introduced by Louie ( 2001). In two parts, this paper first provides best practices in the form of guidelines and cautions against mistakes by avoidance of pitfalls inherent in all three site characterization phases (acquisition, processing, and analysis); the second half is devoted to discussions on extending the resultant 1D ReMi profiles into 2D cross sections, as well as on deep ReMi surveys to estimate the so-called depth parameters known as Z n (e.g., when n = 1.0, then Z 1.0 is the designation for depth to the 1 km/s iso-surface).…”

“…To this end, eleven papers were curated by the COSMOS guest editors. These articles are assigned to three main themes: the first topic articulates the best practice for applications of various site characterization methods (Louie et al 2022;Pancha and Apperley 2022;Hayashi et al 2022;Hunter et al 2022;Molnar et al 2022;Stephenson et al 2022); the second is agnostic to the aforementioned techniques and focuses on processing and analyzing data (Toro 2022;Vantassel and Cox 2022), including one paper on the role of analysts (Asten et al 2022); and the third involves reviews of select topics that are fundamental for consideration in all techniques (Gosselin et al 2022;Parolai et al 2022). All papers are aligned on issues relating to uncertainty, which are paramount to the practice of site characterization as performed at the time of publication.…”

Flexible multimethod approach for seismic site characterization

“…Passive surface-wave methods may, however, be effective at characterizing deep velocity structure in rural environments. Suburban sites may represent intermediate noise conditions compared to that of urban and rural settings; it is in these transitional environments that the assumption of ambient (isotropic) noise conditions may not be applicable given higher energy from several anisotropic sources that can dominate the environment in space or time, prompting a recent study by Asten et al (2022, this issue) and others to issue caution when applying passive methods (Hayashi et al 2022;Louie et al 2021;Pancha and Apperley 2021; all in this issue).…”

“…The accompanying papers in this special issue provide specific directions for each of the individual methods. For example, if using mHVSR refer to Molnar et al (2022); if using SH-body-wave data, see Hunter et al (2022); if using ReMi, see Louie et al (2021); guidelines on MAM can be found in Hayashi et al (2022). While not explicitly included in this special issue, guidelines for MASW and SASW can be found in Y13, Dal Moro (2014), , and references therein.…”