Why Do P-Wave Wipeout Zones Occur?
One hydrocarbon exploration application that has caused multi-component seismic data to be acquired across several offshore areas is the ability of the converted-S mode to image geology inside broad, thick intervals of gas-charged sediment where P-P seismic data show no usable reflections.
The term P-wave wipeout zone is often used to describe this imaging problem.
Numerous examples of P-wave and S-wave images across P-wave wipeout zones have been published, but the rock physics cause of the P-P imaging problem usually is not discussed.
One example of differences between P-P and P-SV images of stratigraphy and structure inside gas-charged sediment is shown in figure 1.
The P-wave wipeout zone shown here extends about two kilometers (from CDP 10,000 to CDP 10,150) and is small compared with some P-wave wipeout zones, which may span several tens of kilometers.
Visual inspection of these images shows that the P-P mode provides poor, limited information about geological structure, depositional sequences and sedimentary facies inside the image space dominated by gas-charged sediment between coordinates 10,000 and 10,150.
Conventional seismic stratigraphy (P-P mode only) would have little success in analyzing geological conditions within this poor-quality P-P image area. In contrast, the P-SV mode provides an image that is sufficient for structural mapping, as well as for analyzing seismic sequences and seismic facies.
These increased interpretation options are obvious advantages of multicomponent seismic data and elastic wavefield stratigraphy over single-component seismic data and conventional P-wave seismic stratigraphy in regions where gas-charged sediments are common.
A simple Earth model consisting of a shale layer atop a sand layer can be used to evaluate P-P and P-SV reflectivity behaviors for the types of siliciclastic rocks that occur across the Gulf of Mexico, where P-wave wipeout zones are common.
Two pore-fluid situations will be considered:
- Both layers have 100 percent brine saturation.
- Both layers have a mixed pore fluid of 80 percent brine and 20 percent gas.
Well-established rock physics theory can be used to determine seismic propagation velocities and bulk densities for these fluid-sediment conditions.
P-P and P-SV reflectivity curves calculated for typical pore-fluid conditions are shown in figure 2.
When the pore fluid is 100 percent brine, P-P and P-SV reflectivities have opposite algebraic signs but are approximately the same average magnitude (about 5 percent) for incidence angles ranging from 0 to 25 degrees (panel a). When the pore fluid changes to 20 percent gas (panel b), P-SV reflectivity is unchanged, but P-P reflectivity has a smaller magnitude and undergoes a phase reversal that essentially eliminates the P-P response across the first 30 degrees of the incidence-angle range.
P-SV imaging, thus, is not affected by the gas-charged sediment, but P-P imaging is seriously degraded. The effect on P-P and P-SV images would be similar to that exhibited by the data in figure 1.
Simple reflectivity analysis thus often explains much of the reason for degradation of P-P signal inside regions of gas-charged sediment and for the lack of negative impact of gas-charged sediment on P-SV signal.
One conclusion is that multicomponent seismic data and elastic wavefield stratigraphy are not just helpful for studying geological conditions across P-wave wipeout zones; they are essential.