Free and improved computer codes for HVSR processing and inversions

The investigation of seismic ambient noise (microtremor) in spectral ratio form, known as HVSR, is extremely popular nowadays both to investigate large areas in a reduced amount of time, and to leverage a wider choice of low cost equipment. In general, measurements at multiple locations are collected to generate multiple HVSR curves which can individually be inverted to retrieve local subsurface elastic properties. Recently, however, there has been an increasing interest on spatially correlating informative content from different locations. In this perspective, and in the broader context of a hydro-geological survey conducted at the Serravalle Sesia sedimentary basin (Italy), we collected and investigated 19 microtremor measurements with the objective of mapping the bedrock depth. In general, this requires manually extracting the main resonant frequency from individual curves and then “building” the desired map through a combined use of several software applications often resulting in a very time consuming workflow, especially for large surveys. Therefore, we developed and tested a new computer program, “OpenHVSR - Processing Toolkit”, specifically engineered to process and display the informative content of data in 2D and 3D, to optimize the investigation of spatially varying properties. The obtained HVSR curves were then inverted using OpenHVSR, the counterpart code developed previously for inversion.