22 novembre 2012 ( dernière mise à jour : 30 août 2016 )
Subject : Analysis and Inversion of Surface Waves in an Oil & Gas Context
Supervisors : Romain BROSSIER, Jean VIRIEUX, and Jean-Luc BOELLE (TOTAL E&P, Pau)
In oil and gas exploration, surface waves (also known as Rayleigh waves or ground roll) are usually considered as noise or more precisely, as an undesired signal. A significant effort is applied to attenuate them at both acquisition and processing steps, as their high amplitude, relatively low frequency and slow apparent velocity mask valuable signals coming from deeper in the subsurface.
However these waves may provide useful information on properties of the near subsurface. Applications considering 1D models have taken advantage of the dispersive property of surface waves, to obtained velocity profiles of the near surface from the inversion of dispersion curves (Socco et al., 2010). Surface waves have been studied and used at the near-surface scale down to few tens of meters, such as in civil engineering, as well as on a much larger scale in global seismology to provide information on the Earth’s eigenmodes and internal structure.
The continued high demand for oil & gas has driven the petroleum industry to move towards increasingly difficult exploration in areas of higher complexity. There is a growing interest to develop high quality imaging methods to deal with this. In particular, retrieving near surface velocity properties would be very significant in complex geological environments to complement and support the imaging of deep reflection signals coming from reservoirs.
This is especially important for onshore cases where a near surface heterogeneous weathered layer or complex mountainous topography may be present, as well as in shallow marine environments in the presence of unconsolidated interface layers. In such scenarios surface waves may dominate the seismograms, and make the extraction of information difficult for classical processing. It is therefore important to analyze surface waves for two main objectives : first, to determine if they can provide useful and usable information about physical properties of the near surface ; and second, to understand how one may attenuate them efficiently at the processing step.
This study aims to explore the use of surface waves in oil & gas exploration, and to investigate if they can be useful in characterizing the near surface and obtaining useful information about shallow structures on an intermediate scale, up to a few hundred meters depth.
In this study a full-waveform inversion (FWI) approach will be used, allowing the limitations of conventional methods of surface waves analysis in near-surface applications to be overcome and extend the study beyond the 1D case.
FWI is a data-fitting method that takes advantage of the full complexity of seismograms, and models the whole seismic wavefield (Virieux & Operto, 2009). The misfit between modeled and observed data is then minimized to solve the inversion. Although FWI can produce highresolution results, there are many difficulties such as the dependency on a starting model, the need of low frequencies and different methods of misfit calculation that also need to be confronted. Futhermore, most of the FWI applications have been tackled with an acoustic approximation up to now, neglecting elastic effects, and therefore surface waves.
The first step of this study will be to quantify the misfit between complex signals such as surface waves, so that the FWI procedure can take advantage of specific properties of these waves and exploit the information they contain. In addition, the sensibility of surface waves to 1D and 2D structures will also be investigated. Finally, a processing and inversion procedure that fully benefits from information provided by surface waves will be investigated.
The optimal procedure will be tested on simple to more complex synthetic data, and ultimately applied to real datasets in order to characterize the near surface at the oil & gas exploration scale.
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