A mobile multi-depth borehole sensor set-up to study the surface-to-base seismic transfer functions

The best method to evaluate the seismic site response is by means of borehole vertical arrays that use earthquake records from different depths. In this paper we introduce the implementation of a single borehole sensor system (synchronized to a sensor on the surface) that is fixed at variable depths within a single well. This system is used for recording small amplitude earthquake signals at variable stiffness conditions in depth to compute empirical borehole transfer functions. The computed average empirical borehole transfer functions allow the estimation of an S-wave velocity model that is constrained using the frequency peak observed in the H/V ratio curve.Pairs of surface and borehole earthquake records were obtained with the borehole sensor placed at − 10, −20, −50, and − 100 m depth in a test site in Managua, Nicaragua. The average velocity of the final model down to − 100 m appeared to be in good agreement with the average velocity computed via cross-correlation using the surface and borehole signals. Likewise, an inverted MASW profile and H/V ratio at the same site agree with the S-wave velocity model obtained.

► We obtain empirical surface-to-base transfer functions relative to various depths using a single borehole sensor.
► Earthquakes recorded using the multi-depth set-up provided an S-wave velocity model.
► The obtained velocity model agrees with the one obtained from surface waves tests.
► The depth of the highest velocity contrast is constrained using an H/V ratio curve.
► The modes in the empirical and theoretical 1-D transfer functions compares well.