Summary
In this paper we investigate the use of multiples in a novel, closed-loop imaging concept, being full wavefield migration. The use of multiples will enlarge the illumination of the subsurface and can be of crucial importance when primaries are not properly measured due to acquisition constraints and background noise. In this paper it is shown that the full wavefield migration process can image subsurface structures via surface and via internal multiples. Both situations can relieve the requirements for dense, symmetric acquisition geometries.
Introduction
In most of today’s seismic imaging algorithms only the primary reflections are considered and multiple reflections are discarded as noise. In the recently proposed method of full wavefield migration (FWM) (Berkhout, 2012, 2014b) all higher-order scattering is taken into account in the imaging process. In this way illumination is extended and it no longer produces spurious imaging artifacts (Davydenko and Verschuur, 2014). This method involves an inversion process, where a recursive modeling method is used to predict the measured reflection response – including all its higher-order scattering effects – based on estimated reflectivity values and a background velocity model.
Figure 1: Wavefields and operators in the full wavefield modeling (FWMod) process. The reflectivity operators (Rn and R? define the angle-dependent reflection from wavefields incoming from above and below the depth level, respectively. Operators W+ and W- describe the propagation of the wavefields between two depth levels in a downward and upward mode, respectively.
TheFWMprocess has two advantages compared to data-driven imaging or inversion methods, like via the inverse scattering series (Weglein et al., 2010) or Marchenko redatuming (Broggini et al., 2012;Wapenaar et al., 2012): because of it’s closedloop implementation it does not require densely sampled sources and receivers (Berkhout and Verschuur, 2014) and it will actively use multiples in the imaging process (Davydenko and Verschuur, 2013). In this paper we will focus on the last two features of the FWM scheme, as using multiples in imaging will become crucial for cases where primaries are very weak or not measured at all due to constraints in the acquisition geometry or in the signal-to-noise ratio. In this paper it will be shown that the use of multiples will play an important role in imaging and, thereby, in future acquisition design.