A B S T R A C TElastic full waveform inversion of seismic reflection data represents a data-driven form of analysis leading to quantification of sub-surface parameters in depth. In previous studies attention has been given to P-wave data recorded in the marine environment, using either acoustic or elastic inversion schemes. In this paper we exploit both P-waves and mode-converted S-waves in the marine environment in the inversion for both P-and S-wave velocities by using wide-angle, multi-component, ocean-bottom cable seismic data. An elastic waveform inversion scheme operating in the time domain was used, allowing accurate modelling of the full wavefield, including the elastic amplitude variation with offset response of reflected arrivals and mode-converted events. A series of one-and two-dimensional synthetic examples are presented, demonstrating the ability to invert for and thereby to quantify both P-and S-wave velocities for different velocity models. In particular, for more realistic low velocity models, including a typically soft seabed, an effective strategy for inversion is proposed to exploit both P-and mode-converted PS-waves. Whilst P-wave events are exploited for inversion for P-wave velocity, examples show the contribution of both P-and PS-waves to the successful recovery of S-wave velocity.
I N T R O D U C T I O NFor the characterization of both a hydrocarbon reservoir and its overlying geology, a depth model quantifying the elastic parameters is a more desirable product than a qualitative stacked image of reflectivity in time. Variation in depth rather than time is more meaningful to geologists and drilling engineers alike, whilst elastic parameter quantification can be used to deduce the lithology, fluid content and pore pressure of rocks (e.g., Tatham and Stoffa 1976).Full waveform inversion, in which the aim is to find the true Earth model from an observed seismic dataset, provides an alternative approach to stack based processing for the analysis of seismic reflection data. Using iterative optimization methods, inversion aims to recover the true model by relating the differences in synthetic and observed data to appropriate model updates. Whilst tomographic inversions (e.g., Zelt and Smith 1992;Hobro, Singh and Minshull (2003)) use picked * Now at: BG Group, 100 Thames Valley Park, Reading, RG6 1PT, UK travel-time data alone, full waveform inversion in the manner of Tarantola (1986) and Mora (1987) offers a more complete approach, exploiting both traveltime and waveform attributes such as amplitude, phase and frequency content. The full two-way elastic wave equation must be solved through methods such as finite-differences, allowing calculation of required traveltime and waveform information for all events within the wavefield. Though computationally intense, full waveform inversion is sensitive to elastic parameters of the Earth at shorter wavelengths than traveltime inversion which is restricted by the high frequency approximation. Moreover, waveform inversion is largely data driven, without...