Nongeometrically converted shear waves in marine streamer data

Abstract

Under certain circumstances, marine streamer data contain nongeometrical shear body wave arrivals that can be used for imaging. These shear waves are generated via an evanescent compressional wave in the water and convert to propagating shear waves at the water bottom. They are called "nongeometrical" because the evanescent part in the water does not satisfy Snell's law for real angles, but only for complex angles. The propagating shear waves then undergo reflection and refraction in the subsurface, and arrive at the receivers via an evanescent compressional wave. The required circumstances are that sources and receivers are near the water bottom, irrespective of the total water depth, and that the shear-wave velocity of the water bottom is smaller than the P-wave velocity in the water, most often the normal situation. This claim has been tested during a seismic experiment in the river Danube, south of Budapest, Hungary. To show that the shear-related arrivals are body rather than surface waves, a borehole was drilled and used for multicomponent recordings. The streamer data indeed show evidence of shear waves propagating as body waves, and the borehole data confirm that these arrivals are refracted shear waves. To illustrate the effect, finite-difference modeling has been performed and it confirmed the presence of such shear waves. The streamer data were subsequently processed to obtain a shear-wave refraction section; this was obtained by removing the Scholte wave arrival, separating the wavefield into different refracted arrivals, stacking and depth-converting each refracted arrival before adding the different depth sections together. The obtained section can be compared directly with the standard P-wave reflection section. The comparison shows that this approach can deliver refracted-shear-wave sections from streamer data in an efficient manner, because neither the source nor receivers need to be situated on the water bottom.

A petrophysical approach to the investigation of shallow marine geology

“…Shear‐wave velocity data normally cannot be recovered from standard marine reflection seismic data, although methods of recovering Vs calculations from multichannel seismic data have been reported (Gettrust and Rowe ; Drijkoningen et al . ), and Vs is a product of some marine broadband seismic techniques and ocean‐bottom cable (OBC) seismic acquisition (Hardage et al . ).…”

“…Shear-wave velocity data normally cannot be recovered from standard marine reflection seismic data, although methods of recovering Vs calculations from multichannel seismic data have been reported (Gettrust and Rowe 1990;Drijkoningen et al 2012), and Vs is a product of some marine broadband seismic techniques and ocean-bottom cable (OBC) seismic acquisition (Hardage et al 2006). Top-hole wireline logging rarely, if ever, records Vs, but Vs may be a component of geotechnical surveys (e.g., Digby 2002; Peuchen, Assen and de Ruijter 2002; Digby 2012), its primary use being the assessment of the small shear-strain modulus (Look 2014; Zuccarino et al 2015) and the constrained modulus (Lambe and Whitman 1969).…”

State‐of‐the‐art remote characterization of shallow marine sediments: the road to a fully integrated solution

“…; Drijkoningen et al . ). As S‐waves are transmitted through the grain skeleton, they provide a more direct link between the geotechnical and geomechanical parameters than P‐waves.…”

“…Over the following sections, each of these parameters is considered in turn using a range of different example datasets However, a major limitation with the quantitative methods applied to the marine shallow subsurface is that S-waves or surface waves are only rarely considered. Notable exceptions are the multi-model surface wave inversion work of Socco et al (2011) and Vanneste et al (2011) and some limited mode-converted wave analysis (Bohlen et al 2004;Allouche, Drijkoningen and van der Neut 2010;Allouche et al 2011;Drijkoningen et al 2012). As S-waves are transmitted through the grain skeleton, they provide a more direct link between the geotechnical and geomechanical parameters than P-waves.…”

Air Gun near the Sea Floor as Shear-wave Source?

“…Still, S waves also have some difficulties, one of them being that it is cumbersome (and therefore expensive) to generate these waves at the sea floor. Recently (Allouche et al 2011, Drijkoningen et al 2012 it has become clear that it is possible to create shear waves at the sea floor via an airgun that is deployed near the sea floor. The propagating part of the P wave in the water converting to a propagating shear wave at the sea floor is small in amplitude.…”