Bob Hardage1, Michael DeAngelo1, and Diana Sava1
Search and Discovery Article # 40234 (2007)
Posted April 12, 2007
*Adapted from the Geophysical Corner column, prepared by the authors, in AAPG Explorer, March, 2007, and entitled “ Why Do P-Wave Wipeout zones Occur?”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, The University of Texas at Austin ( )
One hydrocarbon exploration application that has caused multicomponent 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 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.
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.
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.