In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subterranean formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. One technique associated with geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downward into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downward through a body of water overlying the surface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, such as sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected and recorded by seismic sensors (also called seismic receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes.
Example seismic sources for generating the seismic signal in land seismic surveys include, without limitation, explosives, vibrators, or other devices capable of generating seismic waves. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types including, without limitation, a small explosive charge, an electric spark or arc, a marine vibrator or a gun. The seismic source gun may be a water gun, a vapor gun or an air gun. In many cases, the seismic source consists not of a single source element, but of a spatially-distributed array of source elements.
The appropriate types of seismic sensors are also diverse and may depend on the application. Example seismic sensors include, without limitation, particle velocity sensors, particularly in land surveys, and water pressure sensors, particularly in marine surveys. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, different types of sensors, such as pressure sensors and particle acceleration sensors, may be deployed together in a seismic survey, collocated in pairs or pairs of arrays.
The resulting seismic data obtained in performing the survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. For example, the processed seismic data may be processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image or otherwise characterize the geologic subsurface. Accurate characterizations of the geologic subsurface may greatly facilitate geophysical prospecting for petroleum accumulations or other mineral deposits.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
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