Renewed interest in multi-component seismic data occurred during the late l990's primarily due to the development of Ocean-Bottom-Cable (OBC) seismic recording. OBC acquisition is particularly useful when producing fields contain heavy infrastructure (production facilities and pipelines) such that towed streamer data cannot be acquired. Four-component (4-C) OBC data(pressure, and x-y-2 components of particle velocity or acceleration are measured) record compressional waves (P-waves) and converted waves (P-S waves). The latter cannot be recorded with streamer as they do not propagate in water.
Conventional towed streamer seismic data are processed and imaged for the compressional-wave (P-wave) reflections. A P-wave reflection propagates from the seismic source downward to, and upward from, subsurface reflectors as compressional waves. In the case of OBC data. P-wave particle motion is parallel to the direction of the wave propagation, and P-waves me primarily recorded on the hydrophone (pressure) and on the vertical geophone (z) component.
OBC P-wave data has three advantages over regular streamer data:
- Because it is far away from the swell action the signal-to-noise ratio is in general excellent
- The bandwidth of the recorded data is broader thus improving the resolution of the subsurface geology
- The two measurements of pressure and particle motion allow us to separate the up going wave-field from the down going wave field thus eliminating unwanted effects in the image.
Converted wave (C-wave) data are recorded on the horizontal (x-y) receiver components available from 4-C OBC acquisition. C-waves are assumed to propagate downward from the seismic source as P-waves, reflect, and propagate upward to the seismic receivers as shear waves. Shear-wave particle motion is perpendicular to the direction of wave propagation (primarily in the horizontal plane), thus, the need to record the (x-y) components of particle motion for C-wave processing and imaging. Shallow gas accumulations generally distort P-wave reflection data, while C- wave reflection data is often unaffected. P-wave propagation velocities decrease when the pore fluids change from brine to gas producing reflection travel time delays and an associated, in general, amplitude and high-frequency low (attenuation). Shear waves are unaffected by pore fluids since shear-waves propagate through the rock matrix and not within the fluid-filled pore space.
An example of a fully preprocessed and pre-stack time migrated P-wave seismic section is shown in Figure 1 (left). A shallow gas accumulation produces a "dim zone" having an inverted V-like shape in the center of the profile. Reflections within the dim zone are much weaker in amplitude, considerably lower in temporal frequency, and delayed in travel time relative to the reflections which have not propagated through the shallow gas. The equivalent C-wave section composed of P-S reflections recorded by the horizontal receiver components. The reflection signal strength, higher temporal frequency, and lack of travel time delays within the gas shadow zone when compared with the P-wave data. The down going P-wave of the P-S reflections can be affected by the shallow gas, but the P-S image is much more interpretable than the P-wave image.
A number of papers present examples of C-wave imaging through gas clouds during the late 1990's and early 2000's in the North Sea (Brzostowski et al., 1999; Granli et al,. 1999; MacLeod et al., 1999; McHugo et al., 1999; Rognoe et al., 1999; and Thomsen et al., 1997) and Gulf of Mexico (Knapp et al., 2001). Offshore Nigeria is also a "shallow gas cloud province". In addition to the benefits of OBC P-wave data, increased use of OBC acquired C-wave data is expected to produce improved images of the true subsurface geologic structure where these shallow gas accumulations exist.