Multi-Component Technology
Multi Component Processing
The evolution of 3D multi-component, 4-component (4C) and 9-component (9C) processing and acquisition methods has increased the available seismic information about reservoirs. Processing techniques that generate time or depth images can enhance reservoir characterization, especially when shallow gas clouds may inhibit compressional wave propagation. PGS multi-component processing allows clearer imaging of reservoirs and discrimination of fluids and rock properties.
Example: Pseudo Offset Pre-stack Time migration (POMZ)
Pseudo Offset Migration is a pre-stack isotropic / anisotropic V(z) time migration for P-P and P-S converted wave data. It can be used for both land or marine VTI and HTI media. This technique postpones velocity analysis until after the migration step, thereby simplifying the processing challenge. In addition, it accommodates elevation (datum) differences between source and receiver, and irregular acquisition geometries. The main advantage for this approach is that it is less sensitive to errors in the migration velocity analysis, and it will rapidly obtain a high quality image of the P-S converted wave data. In addition, the algorithm has proven to be superior in the focusing of velocities and conversion points. The separation of positive and negative offsets in connection with dipping structures can also be avoided.
Various output results from converted-wave pre-stack time migrations.
P-wave DMO + PSI (top left), P-wave POMZ (top right), PS-wave DMO + PSI
(bottom left), and PS-wave PSPOMZ (bottom right). Both the P-wave and
PS-wave POMZ results are significantly better.
Dual Sensor Summation
Dual sensor summation removes water-born multiples through the combination of the hydrophone and geophone response in seafloor seismic data. This is effective in varied water bottom conditions, requires little interpretation, and does not perturb the relative amplitude of the primary events. Correct dual-sensor summation will improve the effectiveness of all subsequent processes. Two techniques are available for summation in PGS:
Surface Consistent Dual Wave field inversion Technique (SCDuWiT). This technique is designed to remove the receiver ghost from dual-sensor ocean bottom seismic data in an entirely surface-consistent manner
Ocean Bottom Seismic De-MULTiple (OBSDMult). Works by using the property difference between the hydrophone and geophone effect wave field spectrum
An example of the power of the dual-sensor summation is given below. The water bottom multiples are attenuated and the primary events are much clearer due to better signal-to-noise (S/N) ratio after summation.
Stack result for the Hydrophone only (left), the Vertical Geophone only (middle),
and the Summation of the Hydrophone and Vertical Geophone (right).
Example: Shear-wave splitting analysis and application
With the emergence of larger OBC datasets the last few years, converted wave data has been more often subject to shear-wave splitting analysis in order to estimate fracture/stress direction and its density. In the presence of fractures, shear waves can be polarized into fast (S1), and slow (S2) directions, where S1 is oscillating in the fracture direction and S2 oscillates orthogonal to the fractures. In OBC acquisition, the data is recorded on 3-component geophones, where usually the horizontal phones are rotated into a radial component and a transverse component, which is orthogonal to the radial component. For the isotropic case, all energy will be on the radial component while the transverse component only contains some noise. For the anisotropic case, both the radial and transverse component will contain energy with a certain amplitude and polarity distribution, which is given as a function of fracture direction and density.
The main impact of shear-wave splitting on converted wave data are less resolution due to recording of a superposition of S1 and S2 wavelets (S1 is faster than S2, which gives a smeared total wavelet), and reduced amplitudes, due to the fact that wavelets with opposite polarity are added. Fracture direction and density are very important for reservoir characterisation, and in the planning of well positions. In addition, compensation for the effect of shear-wave splitting will improve the resolution and enhance the signal of the converted wave data for further processing.
A layer-stripping approach is the preferred technique for shear-wave splitting analysis when fracture direction and density are depth variant.
When analysing a full data volume using the layer-stripping approach, the fracture orientation and delay time (density) can be estimated as a function of depth, and the influence of the fractured layers can also be removed from the converted wave data.
The figure below shows the radial and transverse component azimuth gathers from the radial component and the transverse component after orientation and rotation of the horizontal geophones. Note the oscillatory behavior of the radial component, and the strong energy on the transverse component due to the influence of the shear-wave splitting. After the correction, the radial component azimuth gather is flat, and the energy on the transverse is reduced (middle image). S1 and S2 energy components are displayed on the right, and note the slight time delay of events.
In the first two figures the Radial component is on the left and the Transverse
component is on the right. Azimuth gathers without correction (left), with correction
(middle). The figure on the right is the azimuth gathers of S1 (fast: left) and
S2 (slow: right).
Map of time delays (colors) and directions (arrows), which gives indications of
the fracture density and the directions of the fractures. The solution shown is
made up by 4 cable positions.
