Mapping the Basement of the Ebro Basin in Spain With Seismic Ambient Noise Autocorrelations

Single‐station autocorrelations of seismic ambient noise allow to obtain the shallow subsurface reflection response without the need of any active source or earthquake. Recently, crustal‐scale studies successfully applied this method to map the crust‐mantle boundary. These studies employed different processing methodologies to stabilize and identify the weak‐amplitude reflections. Here we also analyze noise autocorrelations but use higher‐frequency bands to map the basement of the Ebro Basin in Spain. This basin has a nonuniform basement structure due to its complex evolution. We tested two autocorrelation methods (the classical and phase cross correlation) to retrieve the P wave reflection response for seismic broadband stations located in the basin. The phase correlation approach is amplitude unbiased since based on the instantaneous phase coherence of analytic signals. It is insensitive to the background seismicity from the Pyrenees and other noise and proved to be the more efficient approach to retrieve P wave reflections from the Paleozoic basement. The detected signals per station are quite stable and show only a small time variability. We also show that sidelobes caused by the convolution of a delta pulse at zero time lag with the effective noise source time function do not affect our measurements and compare the measured noise autocorrelations with synthetic data for lithological profiles from neighboring wells. The a priori information from well data was essential to identify the signals and to finally map the discontinuity in between and beyond the well positions. Our final result is a comprehensive map of the Paleozoic basement in the Ebro Basin.