Node Processing
Autonomous node hardware for the recording of seismic data on land, in transition zones and in water depths up to 3000 meters is manufactured by FairfieldNodal. As these systems have been developed, FairfieldNodal also developed unique data processing techniques to take advantage of these new recording system capabilities, thus improving the quality of 2D and 3D data volumes.
The FairfieldNodal family of nodes, Z Land®, Z700® and
Z3000® nodes, records data continuously, from the moment they are
initialized on the ground or the seafloor, until they are successfully
retrieved and downloaded. These and other design features enable
FairfieldNodal systems to cost-effectively record improved seismic data
quality in all environments from land to 3000 meters of water.
FairfieldNodal’s Data Processing teams have significant experience in
handling data from these systems, plus we have developed new
technologies such as, but not limited to, Wavefield Separation and
Wavefield Extrapolation for multiple attenuation and mirror
migration. This software specifically applies to the geophysical
issues associated with the sources being on the surface and the
continuous recording device being located on the sea bottom.
As of this writing, FairfieldNodal have been the only contractor to utilize new state-of-the-art data processing techniques to enhance 3D data quality by the innovative development of new software to take advantage of this revolution in data acquisition technology. This technology has been applied to node data acquired for major oil international companies and has been very well received.
Wavefield Separation
The separation of seismic recordings into upgoing and downgoing wavefields is a primary goal in marine multicomponent data processing. The quality of the separation is degraded, however, when the vertical geophone contains shear related noise and other non P-wave energy.
At FairfieldNodal, we remove such energy using a patented frequency-sensitive 3D tau-p technique, which is designed to identify reliable p-wave energy and to tune the data accordingly. The result is a vertical geophone which has the same signal to noise content as the hydrophone, but which maintains phase discrimination of upgoing and downgoing waves. The hydrophone, itself, is unchanged by the process. Subsequent summation and subtraction of components yield wavefields which are well separated and optimal for downstream processing.
Wavefield Extrapolation
Wave equation surface multiple attenuation (WEMA ) is a model driven multiple attenuation technique and therefore depends on an earth model. The earth model must contain the reflector(s), whose multiples are to be attenuated.
The measured wavefield is propagated from the surface to the target reflector(s) and back, typically using finite difference wave equation modeling code. This way the primaries are converted into first order multiples, 1st order multiples into 2nd order multiples etc. The modeled multiples are then adaptively subtracted. Requirements for data preparation (regularization, interpolation) are similar to SRMA. However requirements for crossline acqusition footprint in case of crossdip might be less than for SRMA.
Mouse over figure 2 to see figure 3.
Mirror Migration
In deep water Ocean Bottom Node (OBN) seismic acquisition, each node records reflected energy from the subsurface more than once. Seismic energy can arrive directly at the seafloor (up-going wave) or after a near total internal reflection at the air-water interface (down-going wave). Seismic recording with both hydrophone and geophone in each node allows for the separation of the seismic wavefield into up-going and down-going components, which can be processed separately. The two wavefields follow different paths providing different illumination of the subsurface. Mirror migration is a technique to image the down-going wavefield after the internal reflection at the air-water interface and generally produces the preferred image because of the better illumination of the shallow horizons from the perspective of the virtual node position.
To summarize:
Node acquisition and processing has the following advantages over conventional seismic:
- Longer Offsets
- Full Azimuth
- Four Component
- Proven Repeatability for 4D
- Easy and Cost Effective for Congested Areas
- Cost Effective Migrations using Shot/Mirror RTM Options
- Outstanding Imaging Results