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Figure Captions

Example 

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 multi-component 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. 

Model 

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: 

1)      Both layers have 100 percent brine saturation. 

2)      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 (Figure 2a). When the pore fluid changes to 20 percent gas (Figure 2b), 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.

Conclusion 

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 multi-component seismic data and elastic wavefield stratigraphy are not just helpful for studying geological conditions across P-wave wipeout zones; they are essential.

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