The Geophysical Corner is a regular column in the EXPLORER, edited by Bob A. Hardage, senior research scientist at the Bureau of Economic Geology, the University of Texas at Austin. This month’s column, part 1 of a two-part series continued in August, is titled “High Resolution P-P Imaging of Deepwater Near-Seafloor Geology.” Part 2
Technique Improves Deep Imaging
Multicomponent seismic data have unique value for studying near-seafloor geology in deepwater environments. When properly processed, P-P (compressional) and P-SV (converted-shear) images made from 4-C seismic data acquired in deep water with seafloor sensors show near-seafloor geology with amazing detail.
This article is the first of two that describe how improved imaging of near-seafloor, deepwater strata can be achieved with conventional multicomponent seismic data.
This article focuses on P-P imaging; next month’s article will focus on P-SV imaging.
In deepwater multicomponent seismic data acquisition, there is a large elevation difference between source stations (an air gun at the sea surface) and receiver stations on the seafloor.
Conventional processing of deepwater 4-C seismic data involves a wave-equation datuming step that transforms the data to a domain in which sources and receivers are on the same depth plane. This step effectively removes the water layer and allows the data to be processed as if the source was on the seafloor.
This adjustment of source-receiver geometry also allows deepwater multicomponent data to be processed with software already developed for shallow-water environments where marine multicomponent data acquisition technology was originally developed and applied.
An example of a good-quality, deepwater P-P image of near-seafloor geology made with this wave-equation datuming approach is shown as figure 1a. This image shows local geology associated with a fluid-gas expulsion chimney that extends to the seafloor.
If a person wishes to study near-seafloor strata, a new approach to P-P imaging of deepwater multicomponent seismic data is to not eliminate the large elevation difference between sources and receivers but to take advantage of that elevation difference.
The objective is to process deepwater multicomponent data similar to the way vertical seismic profile (VSP) data are processed, because VSP data acquisition also involves large elevation differences between sources and receivers (figure 2).
Users of VSP technology know VSP data provide high-resolution images of geology near downhole receiver stations. That same logic leads to the conclusion that deep-water multicomponent seismic data processed with VSP-style techniques should yield higher resolution images of geology near deep seafloor receivers.
The P-P processing illustrated here can be done with either 2-C or 4-C seafloor sensors. The fundamental requirement is to acquire data with a sensor having a hydrophone and a vertical geophone.
The seafloor hydrophone response (P) and the seafloor vertical-geophone response (Z) are combined to create downgoing (D) and upgoing (U) P-P wavefields as:
“F” defines the incident angle at which the downgoing compressional wave arrives at the seafloor. Once this wavefield separation is done, deepwater multicomponent seismic data are defined in terms of downgoing and upgoing wavefields, just as are VSP data.
Having access to downgoing (D) and upgoing (U) wavefields means sub-seafloor reflectivity can be determined by taking the ratio U/D. This reflectivity wavefield is then segregated into stacking corridors, and data inside these corridors are summed to create image traces just like VSP data have been processed for the past 20-plus years.
Figure 1b shows a P-P image made with this technique using the same deep-water data displayed in figure 1a. The improvement in resolution is obvious.
Applying this VSP-style imaging technique to deepwater multicomponent seismic data is proving to be invaluable for gas hydrate studies, geomechanical evaluations of deepwater seafloors and other applications where it is critical to image near-seafloor geology with optimal resolution.
Every seismic data-processing technique, however, has constraints and pitfalls. Two principal constraints of the technology described here are:
- There has to be a significant difference between the elevations of sources and receivers. The technique is not appropriate for multicomponent seismic data acquired in shallow water.
- The improvement in image resolution over that of production processing of marine multicomponent seismic data diminishes as the image space extends farther (deeper) from the receivers. At significant sub-seafloor depths, production-style, wave-equation-datuming-based, P-P imaging (figure 1a) is equivalent or superior to the VSP-style imaging described here.
Information about this technology is available at www.beg.utexas.edu/indassoc/egl/.
WesternGeco provided the seismic data used in this research. Research funding was provided by Minerals Management Service (Contract 0105CT39388) and DOE/NETL (Program DE-PS26-05NT42405).