Benin

In spring 2019, Geodynamics WorldWide carried out a passive seismic survey in Benin. The main objective is to confirm, or adjust, the depth-to-basement information obtained by the client’s g & g information at hand.

The study covered a total of 660 km², a grid of 32 points, of which 20 consisted of stand-alone stations, whilst the remaining 12 formed three different 4-points square array. The field operations were executed in 10 days. At client’s request, in due course of the field operations, the study programme was extended to cover additional area of interest for the client.
The results, with its technical report, were delivered to the client within deadline, i.e. 2 months after field acquisitions were completed.

The data acquired at the three 4 points-arrays were analysed by means of ESAC (Extended Spatial Autocorrelation), which is a passive seismic method, representing an evolution of the ReMi method (Refraction Microtremor), from which it differs in that the geophones/seismometers can be arranged according to any two-dimensional geometry. It is a methodology which exploits spatial auto-correlation through vertical sensors synchronization.
The HVSR method (Horizontal to Vertical Spectral Ratio) is based on the analysis of the spectral relationship between the horizontal (H) and vertical (V) components of ambient noise. The technique is aimed at determining the “ellipticity” curve (HVSR curve) from passive measurements of environmental vibrations on the three components. The maximum of the ellipticity curve identifies the resonance frequency of shear waves in the subsoil.