Rayleigh wave spectral distortions induced by an anticline structure

Abstract

SUMMARYSpectral anomalies in the seismic ambient noise are commonly observed above hydrocarbon reservoirs at frequencies of several Hertz. If properly understood, these anomalies could be a potential indicator for hydrocarbon reservoir exploration or monitoring. Under the assumption that ambient noise mainly consists of surface waves, previous studies showed that the geological structure, rather than the hydrocarbon content, could be the major cause of the observed anomalies for purely elastic waves. This work further explores how Rayleigh waves interact with an anticline structure, a typical geological trap for a hydrocarbon reservoir. We propose an approximate semi-analytical surface wave approach for modelling the spectral anomalies. It is based on the contribution of 1-D local solutions from two different models. The results are compared with those obtained via 3-D numerical simulations using the spectral-element approach in the case of a wavefield dominated by Rayleigh waves. Analysis of the anomalies generated by an anticline structure is performed for different source configurations. It shows that the semi-analytical method provides reasonable spectrum predictions for smooth layered models and under some restrictions on the source distribution. Such a tool allows us to run a larger number of simulations to explore the parameter space and finally provide a general law linking the anticline geometrical parameters and the elastic properties of the medium to the anticline-induced spectral anomaly for the case of fundamental mode Rayleigh waves.