Where do P-S conversions occur? Analysis of OBS-data from the NE Atlantic Margin

The vast majority of seismic surveys on the Norwegian continental shelf have been performed using a towed streamer. The reason for this method’s dominance is its efficiency in 2D and 3D mapping of sedimentary structures of importance in the exploration for hydrocarbons. It is, however, curious that the first seismic surveys performed on the Norwegian continental shelf were conducted in 1962 using multicomponent geophones placed on the sea-floor (Hirschleber et al. 1966). Today, this is state-of-the-art in seismic exploration. During the last decade there has been a steady increase in interest in multicomponent ocean-bottom seismograph (OBS) surveys, and during the last 5 years there has been an even greater increase in interest in marine multicomponent reflection surveys (marine 4C). For both types of survey we use conventional marine seismic sources, but the multicomponent sensors, the way the sensors are handled and the acquisition geometry, are very different in the two methods. OBS surveys are characterized by a coarse receiver grid and coarse shooting over a large offset range. A typical receiver distance is 5–30 km and a typical offset range is from zero to several hundred kilometres. The sensors are dropped from the sea-surface to the sea-floor. The interest in OBS surveys has two main causes: • Wide-angle reflections and refractions (P-waves) can be used to map structures beneath volcanic rocks (e.g. Mjelde et al. 1992, 1997). • Various kinds of converted waves recorded as P- or S-waves at the sea-floor can be used to constrain lithology and pore fluid (e.g. Neidell 1985; Berg et al. 1997; Digranes et al. 1998). Marine 4C surveys are characterized by a relatively dense receiver grid and dense shooting over a moderate offset range (Caldwell 1999). A typical receiver distance is from 25 to a couple of hundred metres and a typical offset range is from zero to five kilometres. Most types of 4C sensor are incorporated in cables which are placed on the sea-floor by specialized surface vessels. Other sensor types are planted on the sea-floor by a remotely operated vehicle (ROV). The data are mainly used for imaging using reflected P-waves (PP) and converted waves (PS), and a critical assumption is that the conversions take place at the reflecting boundary. There are many applications for marine 4C data (MacLeod et al. 1999; Rogno et al. 1999); the most important are: • Improved imaging in PP due to lower background noise level, better azimuth distribution and more possibilities for removal of receiver ghost and multiples. • Imaging in PS below shallow gas and for geological boundaries with low contrast in acoustic impedance for P-waves. For shallow structures one can also achieve better vertical resolution in PS than for PP. • Lithology and fluid characterization from combined analysis of PP and PS data. In the imaging of marine 4C data it is commonly seen that both vertical and horizontal resolution degrade more rapidly with depth for PS than for PP. S-wave absorption is assumed to have some influence on this, but it is reasonable to assume that absorption is just one of many contributing causes. The PS wavefield has a tendency to become more complicated with depth. There are many probable reasons for this, but the most obvious is that the wavefield passes one or several interfaces which produce strong, transmitted converted waves. These transmitted converted waves produce seismic events with nearly the same apparent velocity as the PS events reflected at the corresponding depth. In some cases, it is difficult to identify which events are converted at the reflecting interface, and which are converted through transmission at nearby interfaces. Incorrect interpretation might lead to inconsistency in the processing velocities and smearing of the resulting image. Event identification can be guided by forward modelling, but it would be helpful if we knew beforehand what interfaces are likely to be the strongest P-to-S conversion interfaces. In wide angle OBS data, similarities between P- and S-wave refractive events can be used for direct detection of layers with S-wave propagation. The present paper describes the identification of conversion boundaries in modelling of both unpublished (presented in University reports) and published OBS data. The database covers volcanic sedimentary basins, nonvolcanic sedimentary basins, uplifted continental shelf, normal oceanic crust and thickened oceanic crust. A total of 2485 identified S-wave arrivals from a total of 347 OBSs along 8023 km of 2D profiles have been interpreted and modelled. The data sets used were acquired on the Faeroe, More, Voring, Lofoten and western Barents Sea Margins, as well as in the north-eastern Barents Sea (Fig. 1). Our opinion is that the identification of conversion boundaries is more easily achieved with wide-angle OBS data than with the more conventional offset ranges used for processing of marine 4C data. However, the acquired marine 4C data usually contain long offsets, at least to one side of each receiver. We believe that the methodology presented in this paper can, in many cases, be used on marine 4C data, but this remains to be proven.