Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for redatuming and binning seismic data collected with different datums for improving a final image of a surveyed subsurface in a time-lapse or 4D project.
Discussion of the Background
Marine seismic data acquisition and processing generate an image of a geophysical structure (subsurface) under the seafloor. While this image/profile does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 11 located on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14, or may have spatial arrangements other than horizontal, e.g., variable-depth arrangement. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until, eventually, a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
Alternatively, ocean bottom cables (OBC) or ocean bottom nodes (OBN) and seismometers (OBS) may be used to record the seismic data. FIG. 2 shows an OBC 30 that includes plural receivers 32 distributed on the ocean bottom 20. The plural receivers 32 are connected to each other with a cable 33 that may also be connected to a data collection unit 34. Various means (e.g., underwater vehicle) may be used to retrieve the seismic data from the data collection unit 34 and bring it on the vessel 10 for processing.
One or more of the above-noted techniques may be used to monitor a producing reservoir. For these instances, the goal of 4D processing is to determine how and where earth properties change by evaluating differences in processed seismic data acquired at different times, usually before (i.e., the baseline survey) and after (i.e., the monitor survey) a period of fluid production from a petroleum reservoir. Success of 4D processing depends on the accuracy with which differences in acquisition or subsurface changes not related to fluid production are compensated for during data processing and imaging, in order that 4D noise (the difference of migrated images not related to fluid production) is kept reasonably quiet. Relevant sources of 4D noise include differences in wavefield sampling caused by different survey acquisition parameters between baseline and monitor.
An important step of 4D processing is to select subsets of the base and monitor data that maximize the similarity of information content and wavefield sampling between them. It is common to achieve this by 4D-binning (e.g., U.S. Patent Application Publication No. 2008/0170468A1, herein “'468”), wherein traces from the base and monitor surveys are selected for further processing if they satisfy a set of criteria designed to assess their degree of similarity. These 4D-binning criteria usually assess similarity using surface attributes of the baseline and monitor surveys, for example the geographic position of traces defined by shot and receiver locations, or by mid-point location and/or offset and/or azimuth. Note that geographic position in this context means the X and Y coordinates of the sources and receivers in a plane substantially parallel with the earth's surface or ocean bottom and does not include a depth (Z coordinate) of the sources and/or receivers.
The traditional approach of '468 works well when the source and receiver datums of the baseline survey are the same as those in the monitor survey, in which case similarity of surface attributes is an accurate proxy for similarity of wavefield sampling and information content of the two datasets. One example of this is a towed-streamer baseline survey and a towed-streamer monitor survey, in which shots and receivers lie on an approximately constant datum near the sea-surface.
However, when the shot or receiver datums of the baseline survey are different than those of the monitor survey, similarity of surface attributes (e.g., shot and receiver positions) no longer represents similarity of information content or of wavefield sampling because the reflection points RP and incidence angles IA of the two datasets are different for the same shot and receiver locations as illustrated in FIGS. 3A-C. In this sense, FIGS. 3A-C show a source S and a receiver R having a same offset distance (distance between the source S and the receiver R in the XY plane) but the reflection points RP and incidence angles IA do not spatially coincide. FIG. 3A illustrates a towed-streamer geometry, FIG. 3B illustrates OBN geometry with the downgoing wavefield and FIG. 3C illustrates the OBN geometry with the upgoing wavefield.
This problem was recognized for 4D processing of towed-streamer and ocean-bottom data by Bovet et al. (2010, “Ocean bottom node processing reconciliation of streamer and OBN datasets for time lapse seismic monitoring: the Angolan deep offshore experience,” SEG Expanded Abstracts, 3751-3755), Lecerf et al. (2010, “WAZ mirror imaging with nodes for reservoir monitoring, Dalia pilot test,” Expanded Abstracts of the 72nd EAGE Conference and Exhibition) and Boelle et al. (2012, “A large-scale validation of OBN technology for time-lapse studies through a pilot test, deep offshore Angola,” The Leading Edge, 31, 397-403). The solution proposed by these documents is simply to image the downgoing part of the ocean-bottom wavefield (after wavefield separation) since its subsurface properties are closer to those of the towed-streamer wavefield for the same source and receiver locations. However, these approaches still suffer from mismatching between the base and monitor datasets.
Thus, there is a need for a new method that accounts for the unrepeatable wavefield sampling prior to applying known 4D data processing techniques.