Estimating S-Wave Attenuation in Sediments by Deconvolution Analysis

Seismic-wave attenuation in near-surface deposits is a key factor in site-effect modelling and local seismic hazard assessment. We investigate S-wave attenuation at station CTL8 of the Italian National Seismic Network, located in the Po Plain (northern Italy), by applying the borehole-to-surface deconvolution technique and comparing the results with estimates obtained using the kappa-based approach. The station is equipped with a surface accelerometer and a borehole velocimeter installed at 162 m depth, providing a suitable configuration for near-surface attenuation studies.

The analysed dataset consists of 109 pairs of surface and borehole recordings selected for their high signal-to-noise ratio, associated with local earthquakes with magnitudes between 3.0 and 5.8 and epicentral distances ranging from 36 to 256 km. Assuming predominantly vertical S-wave propagation between the borehole and the surface, identical time windows around the S-wave arrival were selected on the transverse component. The orientation of the borehole sensor was determined using tele-seismic events and corrected prior to the analysis.

Following the deconvolution procedure, up-going and down-going S-wave pulses were successfully isolated in the time domain. The spectral ratio between these pulses was used to estimate attenuation, yielding a surface-borehole kappa difference of Δκ₁₆₂= (11.3 ± 1.1) ms. The time separation between the pulses also allowed the estimation of the time-averaged S-wave velocity between the borehole and the surface, resulting in Vs₁₆₂ = (364 ± 7) m/s.

The results are consistent with previous estimates obtained at the same site using standard kappa-based methods and with synthetic deconvolution signals derived from a previously developed velocity profile. These findings indicate that borehole-to-surface deconvolution is a reliable and complementary tool for estimating near-surface attenuation and average S-wave velocity, provided that sufficient borehole depth and data quality allow a clear separation of the up-going and down-going wavefields.