Advanced seismic technology to detect gas deposits

RENEWED interest in multi-component seismic data occurred during the late 1990'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-z 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 are primarily recorded on the hydrophone (pressure) and on the vertical geophone (z) component..OBC P-wave data has three advantages over regular streamer data: a) because it is far away from the swell action the signal-to-noise ratio is in general excellent; b) the bandwidth of the recorded data is broader thus improving the resolution of the subsurface geology; c) 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 loss (attenuation). Shear waves are unaffected by pore fluids since shear-waves propagate through the rock matrix and not within the fluid-filled pore space.
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