Subsoil structure using SPAC measurements along a line

The SPAC (SPatial AutoCorrelation) method was proposed almost 50 years ago by Aki (1957). This method allows a phase-velocity dispersion curve to be obtained from microtremor measurements using., an array of stations arranged in a circle. The subsoil structure is subsequently derived from the inversion of that dispersion curve. In this article we show that it is possible to get similar results using microtremors recorded along a line. We use microtremor records obtained by using four broadband seismographs disposed along a line, with different interstation spacings (5, 10, 20, and 40 in). Our data are precessed by using the standard SPAC procedure, with the exception of the azimuthal average. The final subsoil structure as determined from the inversion of the phase-velocity dispersion curve shows excellent agreement with previous results at the site of our measurements. Our results suggest that the use of the SPAC method is not restricted to a particular geometry of the array, provided that the basic requirement of stationarity is fulfilled.