Marine Multicomponent Seismic-Acquisition Technologies
Authors: Caldwell Jack 
Published: October 2008

Marine multicomponent seismology has seized the geophysical industry's fancy since it made its semicommercial emergence in the North Sea in the autumn of 1996. This technology involves the recording of S-waves, in addition to the conventional P-waves, by deploying on the seafloor sensor packages containing hydrophones plus 3-component geophones. New processing algorithms have had to be developed to properly process these data. The major uses of this type of data have been (1) to image beneath gas clouds and chimneys, (2) to better infer lithologic and pore fluid information some distance away from wells, and (3) to image beneath high velocity bodies such as salt and basalt masses. The future looks bright for this technology, for reservoir characterization, for seismic time-lapse monitoring (4D), for reservoir management, and perhaps even for exploration.

On the order of 110 4-component marine surveys have been acquired since September, 1996, the vast majority of them being 2D. However, the number of 4C 3D surveys is growing at a faster rate than the number of 4C 2D surveys, the processing of the first handful of 3D 4C surveys has been completed, and the information content of these data sets has been very high, and very useful. 4C surveys have been conducted in the North Sea, the Far East, offshore West Africa, Cook Inlet, the Middle East, and in the Gulf of Mexico. The overall experience is that the data quality in all locations is relatively high, the acquisition systems couple reasonably well to the seafloor, and the bulk of the modeconverted S-wave energy derives from P-waves modeconverting to S at depth, and not at the seafloor.

Four fundamentally different acquisition systems are used in the industry today to acquire the P- and S-wave data: three different cable-based systems, as well as one node-type system. There are at least three different modes of deployment of the cable systems: dragging the cable into place, draping it under tension, and draping it not under tension. A remotely operated vehicle (ROV) emplaces the node-type system. While data quality certainly is not uniform across the industry at the present time, it is not clear yet how system construction and deployment directly affect the data quality. More analysis and research is needed in this area, and is forthcoming no doubt.

The cost to acquire 3D marine multicomponent data is 3-6 times more expensive than conventional towed streamer 3D, but that is expected at this stage of the development of the technology. The cost will decrease fairly dramatically in the future as capacity and production rates increase.

What Is Marine 4-Component Seismology?

In the context of this article, marine 4-component seismology (M4C) is the technology that places 4-component sensing systems on the seafloor so as to record the full vector wavefield of passing stress waves. In a simplistic sense, this means the recording of both compressional waves (P-waves) and shear waves (S-waves). Further, M4C supplies sufficient data processing to make interpretation of the data possible. Finally, it provides for more complete interpretation of reservoir properties, including reservoir boundaries, than is provided by the more conventional 1- or 2-component (Pwave) technology that is far more prevalent in the offshore industry today.

The four components of M4C are a hydrophone, a vertical geophone, and two horizontal geophones oriented perpendicular to each other, and all four are included in each receiver group. Conventional marine towed-streamer systems are 1-component systems and use only hydrophones as the sensor of seismic signals, and record directly only P-waves. onventional OBC (ocean bottom cable) systems e