Interpreting multicomponent seismic data in the Gulf of Mexico for shallow sedimentary properties: methodology and case history

We developed a methodology for manually establishing tie points of depth-equivalent surfaces in P-P and P-S seismic data volumes derived from a 4-C ocean bottom seismic survey using seismic attribute volumes viewed in time slices. These tie points were used as a basis for establishing interpretations of depth-equivalent surfaces throughout the volumes that were then used as a basis for depth registration of several 2-D sections throughout the volume. We examine the Vp/Vs ratios derived from this interpretation. While these ratios are physically reasonable, they are averaged over several stratigraphic sequences and do not provided enough detail to represent the true interval Vp/Vs values of a given sequence. However, we use these Vp/Vs ratios to correct the interpretation and perform an initial registration of the P-S volume to P-P two-way time. An analysis of the registered, or “warped”, P-S volume shows the limitations of this simple technique, and from this we infer what processes must be addressed for a robust method of registration. Introduction The work summarized here is part of a larger study undertaken to develop methods of multicomponent data analysis for the detection of gas hydrate prospects in the northern Gulf of Mexico. Methane gas hydrates deposits occur within a narrow window of near ocean-bottom strata extending no more than a few hundred meters below the seafloor, with size and extent dependent on local temperature and pressure conditions in the region. In many regions of North America, including the southern Gulf of Mexico (GOM), Alaska, and the Atlantic and Pacific coasts of the United States, known gas hydrate deposits can be identified on P-wave seismic surveys by a diagnostic bottom-simulating reflector (BSR) at the base of the frozen hydrates. In the northern GOM, gas hydrates do not exhibit the classic BSR. In this region, the known hydrate deposits have no known distinctive seismic signature, but they do tend to be associated with shallow “gas clouds” which show up as no-data or poor-data quality zones on traditional Pwave seismic surveys. Converted-mode shear waves have already been proven a useful tool for imaging through gas cloud regions (Thomsen et. al, 1997) and specifically in the northern GOM for the purpose of identifying deep targets of interest for oil and gas production (Cafarelli et al, 2000, Knapp et al, 2001). We consider the identification of shallow gas clouds for hydrate detection another possible application for this type of multicomponent seismic data. As part of this study, we introduced the use of a set of multicomponent seismic data designed for deep target exploration as a tool to aid in the identification of possible hydrate prospects and other possible engineering hazards in the upper 1000m of strata across a study region. It should be noted that this study area of the northern GOM shelf is not itself a candidate for hydrate deposits. Large regional 3-D multicomponent data sets of such environments in the deepwater GOM are not available to us at this time. However, the data provided to us for this study provide an opportunity to demonstrate the utility of multicomponent data for the purpose of analyzing nearseafloor sediments and elastic properties which can be extended to other deepwater environments. 4-C Seismic Data The data provided us were from a multi-client 4-C ocean bottom cable survey acquired over 46 OCS blocks of East Cameron South, offshore Louisiana in 1999 and 2000, covering approximately 1000km. The survey is located on the continental shelf in water depths of approximately 100m. The data were processed as a pure-mode P-wave (P-P) volume created from vertical geophone velocity and hydrophone pressure data, and as a radially rotated converted shear mode (P-S or C-wave) volume created from the horizontal velocity phones. For more details of the survey design and processing, refer to Knapp et. al (2001). We were given a P-P wave volume with an extent of 1s in two-way traveltime, and a P-S wave volume extending to 2s so as to allow us enough data to complete our shallow regional studies and protect the confidentiality of the deeper data currently in use for the location of commercial hydrocarbon prospects.