Variance Reduction and Signal-to-Noise Ratio: Reducing Uncertainty in Spectral Ratios

Abstract

This paper uses an unusually large dataset to study scatter in site-effect estimation, focusing on how the events that increase uncertainty can be removed from the dataset. Four hundred seventy-three weak motion earthquake records from the surface and bedrock of a 178-m-deep borehole in Aegion, Gulf of Corinth, Greece, are used to evaluate spectral ratios. A simple statistical tool, variance reduction (VR), is first used to identify two groups of events that lie closest and farthest from the average, which is considered here as the initial best estimate of the site response. The scatter in the original dataset is found to be due to the group of events with smallest VR. These events can be removed from the dataset in order to compute a more reliable site response. However, VR is not normally used to choose records for siteeffect studies, and it cannot be applied to the usual small datasets available. The signal-to-noise ratio (SNR) is normally used to this end, for which reason we investigate whether SNR can be used to achieve similar results as VR. Signal-to-noise ratio is estimated using different definitions. Data selection based on SNR is then compared to that using VR in order to define an SNR-based criterion that discriminates against events that, according to VR, increase scatter. We find that defining the SNR of a surface record as the mean value over a frequency range around the resonant peak (here, 0.5-1.5 Hz) and using a cutoff value of 5 may be used in this case to exclude most events for which VR is small. This process is also applied to the downhole station, where we obtain similar results for a cutoff value of 3.

A Study on the Variability of Kappa ( ) in a Borehole: Implications of the Computation Process

“…In previous instrumental and numerical studies of site amplification at CORSSA (Ktenidou, 2010;Ktenidou et al, 2011), however, we found that though there is a reflected wave field at the downhole station, it is relatively weak. It causes a slight trough in the transfer function at the fundamental frequency and some fluctuations due to destructive interference, but the horizontal-to-vertical empirical spectral ratios (HVSR) remain relatively flat and very near unity, with values ranging from 0.7 to 1.5, up to 100 Hz (a relatively flat HVSR near unity has been considered to indicate the lack of site effects at a reference station, e.g., Raptakis et al, 2005).…”

Stochastic Modeling as a Method of Arriving at Higher Frequencies: An Application to κ Estimation

“…In Experiment III, the setup of Experiment II is repeated using real rather than synthetic signals. We choose a data set of accelerometric recordings that has been used exhaustively for engineering seismology purposes in order to study site amplification and site attenuation ( κ ), namely, that of Ktenidou et al (), Ktenidou et al (), and Ktenidou et al (). These data come from the Corinth Soft Soil Array (CORSSA), a vertical accelerometric array in the Gulf of Corinth, Greece (Pitilakis et al, ) and present the advantage of being recorded in a very low‐noise environment, at a high sampling rate (200 Hz), and by a sensor whose flat response out to very high frequencies enables signal analysis out to almost 80 Hz.…”

Accelerometric Data and Web Portal for the Vertical Corinth Gulf Soft Soil Array (Corssa)

“…It was developed to facilitate the study and modelling of the impact of the near-surface soil formations to the seismic wave field. It involves an infrastructure that has already served several scientific projects (http://crlab.eu) and motivated international publications (e.g., Pitilakis et al, 2004;Ktenidou et al, 2011Ktenidou et al, , 2013Ktenidou et al, , 2015. …”