Statistical Decoding and Imaging of Multi-shot and Single-Shot Seismic Data
We here disclose new statistical based methods for decoding and imaging of multisweep multishot data.
Standard single shot data are treated as a particular case. The disclosed methods can be used to decode seismic convolutive mixtures, underdetermined mixtures, and single mixture data. The algorithms disclosed here are designed for decoding multishot data and imaging both single shot data or decoded data and multishot data.
The decoding process consists of reconstructing single shot gathers from multishot gathers, i. e. reconstructing data as if the acquisition were performed in the present fashion, in which waves are generated from a single shot location, and in which the response of the earth is recorded before moving on to the next shot location.
The imaging process consists of reconstructing the model of the subsurface from either single shot data, or decoded data or directly from multishot data. The removal of multiple reflections due to bounces at the sea surface, the decomposition of data into upgoing and downgoing waves, and the decomposition of data into P wave and S wave reflections are all considered part of the imaging process.
Imaging of Multishot Seismic Data
We here disclose methods of imaging multishot data without decoding. The end products of seismic data acquisition and processing are images of the subsurface.
When seismic data are acquired based on the concept of multishooting (i.e., several seismic sources are exploited simultaneously or near simultaneously and the resulting pressure changes or ground motions are recorded simultaneously there are two possible ways to obtain images of the subsurface. One way consists of decoding multishot data before imaging them that is the multishot data are first converted to a new dataset corresponding to the standard acquisition technique in which one single shot at a time is generated and acquired and then second imaging algorithms are applied to the new dataset.
Actually all seismic data processing packages available today require that multishot data be decoded before imaging them because they all assume that data have been collected sequentially.
Coding and Decoding: Seismic Data Modeling, Acquisition and Processing
A method for coding and decoding seismic data acquired, based on the concept of multishooting, is disclosed.
In this concept, waves generated simultaneously from several locations at the surface of the earth, near the sea surface, at the sea floor, or inside a borehole propagate in the subsurface before being recorded at sensor locations as mixtures of various signals. The coding and decoding method for seismic data described here works with both instantaneous mixtures and convolutive mixtures. Furthermore, the mixtures can be underdetemined [i.e., the number of mixtures (K) is smaller than the number of seismic sources (I) associated with a multishot] or determined [i.e., the number of mixtures is equal to or greater than the number of sources).
When mixtures are determined, we can reorganize our seismic data as zero-mean random variables and use the independent component analysis (ICA) or, alternatively, the principal component analysis (PCA) to decode. We can also alternatively take advantage of the sparsity of seismic data in our decoding process.
When mixtures are underdetermined and the number of mixtures is at least two, we utilize higher-order statistics to overcome the underdeterminacy. Alternatively, we can use the constraint that seismic data are sparse to overcome the underdeterminacy. When mixtures are underdetermined and limited to single mixtures, we use a prior knowledge about seismic acquisition to computationally generate additional mixtures from the actual recorded mixtures. Then we organize our data as zero-mean random variables and use ICA or PCA to decode the data. The a prior knowledge includes source encoding, seismic acquisition geometries, and reference data collected for the purpose of aiding the decoding processing.
Multi-Shooting Approach to Seismic Modeling and Acquisition
A multi-shooting approach to seismic modeling and acquisition where several shot gathers can be generated simultaneously. The method is called a multi-shooting approach to seismic modeling and acquisition.
A multi-shooting modeling method is disclosed which may be carried out on a computer system in either an explicit or an implicit manner. A multi-shooting acquisition method is also disclosed.
The finite difference technique has the ability to generate elastic waves from several locations and at different time intervals simultaneously. The multi-shooting approach is based on the property of the finite difference technique. The decoding solution exploits the fact that specific time delays can be imposed between the various shots and that two closely space shooting points produce nearly identical responses.
Scattering Diagrams in Seismic Imaging
The analysis of scattering diagrams of the correlation-type representation theorem in inhomogeneous media is improved by the use of virtual events. Virtual events here are events which are not directly recorded in standard seismic data acquisition, but the assumption of their existence permits the construction, of internal multiples with scattering points at the sea surface.