Reflection seismology is the predominant geophysical method for hydrocarbon exploration. Seismic data are collected both at land and at sea. The most common seismic wavefield for hydrocarbon exploration recorded at the surface consists of compressional waves, also referred to P-waves.
However, the seismic wavefield that propagates through the Earth's subsurface consists of, depending on the source wavefield, compressional waves (P-waves), two types of shear waves (S-waves): (1) the SH-wave, where the particle motion is perpendicular to the vertical plane defined by the source, receiver and reflection point, and (2) the SV-wave where the particle motion is defined by the source-receiver direction. Figure presents a graphical summary of the components for the seismic wavefield Tatham and McCormack (1998).
Although, S-waves have been a very useful source of information in global seismology, the processing and imaging of S-waves is more challenging for hydrocarbon exploration. In the early 1950s, one of the first research studies demonstrated the potential use of S-waves for seismic exploration. Ricker and Lynn (1950) present their observation of mode-converted S-waves that were reflected from sharp interfaces at relatively shallow depths and observed at relatively large offsets. Most of the studies in the 1950's were directed to a complete understanding of the propagation of seismic waves, including P-waves and S-waves, as well as near-surface ground roll.
In the 1960s the vibroseis to generate shear-wave energy was introduced, these early S-wave vibroseis were employed in several studies. One of the objectives for these studies was to test for possible higher resolution in S-wave data. However, the attenuation of higher frequencies limited the wavelengths of S-waves roughly to those of P-waves, and the frequency bandwidth to almost one-half; therefore, there was no advantage on S-wave data over conventional P-wave data.
In the late 1960s and early 1970s the interest in S-wave exploration was revived because of the new interest for hydrocarbon exploration using seismic methods in stratigraphic traps, the interest in S-wave exploration was revived. During these times the S-waves were proved to be useful to distinguish P-wave bright spot between those caused by gas saturation from those caused by lithological variations. Also, the P-to-S velocity ratio was identified as a possible indicator of lithology. However, new problems arrived with new technology, as for example shear-wave static problems in the near surface and a smaller frequency content produced lower quality images compared with the conventional P-wave seismic exploration. At this time all the work done with S-waves for seismic exploration was done on land. By the 1980s the first reported S-wave seismic experiment was carried out on a marine environment, and also the first three-component source and three-component receiver was generated to create the first multicomponent seismic experiment.
Nonetheless, the satisfactory use of S-waves seismic was still doubtful. In the 1990s, the introduction of Ocean Bottom Cable (OBC), where the seismic cables are laid down on the sea bottom, the interest in shear-wave exploration rose again. A particular kind of OBC includes two seismic sensors, one hydrophone and one multicomponent receiver. The hydrophone provides pressure measurements while the multicomponent receiver, that consists of three geophones oriented in directions perpendicular to each other, measures the components of the elastic wavefield. This particular seismic acquisition method, known as Ocean Bottom Seismograph (OBS), yields satisfactory data quality for S-wave energy in a marine environment.
OBS seismic data provide the information to obtain compressional-wave images of the subsurface. These images usually have fewer water-bottom multiples than streamer seismic data. Because of the nature of the OBS data acquisition, combining the vertical component (Z-component) of the vector wavefield and the pressure component (hydrophone), a process known as PZ summation, produces PZ seismic sections, which are ideally free of water-bottom multiples. There are several issues with this topic, but since they are not the main point dissertation, they will not be discussed here.
Another advantage with OBS data is the analysis of shear-waves. Since the seismic source is acoustic, the only way to study shear-waves is through the conversion of P-waves into SV-waves at the reflection interface. Throughout this dissertation I will refer to these seismic sections as converted-wave data (PS data), where the downgoing path is a P-wave and the upgoing path is an SV-wave. The PS sections of the OBS seismic dataset have proved to be useful in several areas, as for example, the identification of gas seepages, P-wave reflections are disturbed by the presence of gas in the subsurface, however, S-waves help to clarify the subsurface image since they are not affected by the presence of gas. Another application is the identification of hydrocarbons, S-waves provide information on the nature of subsurface lithology and pore-fluid saturation; therefore, S-waves highlight those reservoirs that were previously undetectable by P-waves.
There are many aspects related with OBS data, that will be impossible to address in a single dissertation. Some of these issues are coupling of geophones with the ocean bottom, vector fidelity of the seismic wavefield, vector decomposition, PZ summation, among others.