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. Plain Language Summary The autocorrelation, correlating a signal with itself, of ambient seismic noise permits to achieve an approximation of the zero-offset reflection response of the structure beneath a seismic station. With this approach no active source or earthquake is needed and few days of continuous noise recordings are usually sufficient to identify reflections of discontinuities at crustal scale. Autocorrelations of ambient noise have already been successfully applied to retrieve P wave reflections for crustal scale imaging. In this article, we also use ambient noise autocorrelations but use higher-frequency bands with the main objective to resolve and map the Paleozoic basement of the Ebro Basin in North Spain. For this purpose we tested two different autocorrelation methods. Both methods, the classical approach and the phase cross-correlation, are based on independent strategies. We finally use the phase cross-correlation approach since it does not depend on signal amplitude information and since it proved to be the more efficient approach to retrieve the small-amplitude reflections from the basement. We further analyzed the time and frequency stability of these signals and used information from neighboring well logs to reduce ambiguities in the data interpretation. The well data were essential to correctly identify the signals and to finally map the Paleozoic basement in between and beyond the well positions. At the end, we discuss the new Ebro Basin map and the performance of our approaches.