Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Patent
1997-09-23
1999-08-03
McElheny, Jr., Donald E.
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
G06F 1900
Patent
active
059337907
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods and apparatus for compressing seismic signal data, in particular data representing uncorrelated vibroseis traces.
BACKGROUND OF THE INVENTION
In land seismic acquisition (both on the surface and in boreholes) a common seismic source is vibroseis: A vibrating mass on a baseplate provides a seismic signal whose frequency is varied slowly with time. For example, the frequency might be swept between 10 and 100 Hz over 8-20 seconds. The two-way travel time to the target (for surface seismics), or one-way time (for borehole seismics) will be much less than this--in exploration typically 1-4 seconds. The duration of the trace transmitted from each geophone must be at least the sweep time plus the travel time. If each trace is to be individually recorded far more data points will be required than for an impulsive source, where the trace duration is the travel time.
Because of this in most circumstances the raw traces are not recorded. The traces are cross-correlated with a trace representing the nominal sweep and (if a number of sweeps have been made from one site or closely spaced sites), these cross-correlations are summed. Only the summed cross-correlations between zero and the travel time are recorded.
It is therefore an object of the invention to provide methods and apparatus for compressing seismic data, in particular seismic data which represents vibroseis traces.
SUMMARY OF THE INVENTION
The objects of the invention are achieved by methods as set forth in the appended claims.
It is an important feature of the invention to split the original data set into several bands by steps of filtering and subsampling, and repeating those steps until the desired number of subset is generated. Each subsequent subset or higher band is generated by applying a filtering step to the remaining signal of the previous subsampling.
It is seen as a further important element of the invention that for each band, a requantization level is chosen. Thus the requantization levels effectively depend on two variables, i.e., band number and time. As a result, they can be represented by a two-dimensional template, hereinafter also referred to as requantization template.
In principle the splitting in subset is an arbitrary process depending only the definition of the filters. In case that those filters are designed as band-pass filters, the splitting leads to a frequency splitting of the original data set.
Further embodiments of the invention differ in how those requantization levels are derived.
In another preferred embodiment a further compression step is applied to the subsets.
The data sets generated by the compression are particularly useful for data transmission, e.g. from the seismic receiver (geophones, hydrophones) to a first data gathering and analysing station, which could be mobile and located close to the receivers, or from a mobile station to a data centre. The compressed data can transmitted via cables, radio transmission, or satellite. The invention can also be advantageously used for storing seismic data.
The invention further describes how to compress the original vibroseis trace in such a way that the windowed cross-correlation of the decompressed trace and the sweep is identical to the cross-correlation of the original trace and the sweep, but the remainder of the decompressed trace is of reduced accuracy. There is flexibility in the invention. For instance if noise in a particular frequency band can be removed before correlation, this noise can be recorded with the required accuracy over the whole acquisition time.
The method to be used is very similar to that employed in compression of audio data using band-splitting. In these techniques the audio signal is split into a number of frequency bands. The ear's sensitivity to different frequency bands varies, and so the original data quantization level is retained only in the band of greatest sensitivity. The quantization level of the data in the other bands is increased to the highest level at which there is no perceivable de
REFERENCES:
patent: 4312050 (1982-01-01), Lucas
patent: 4509150 (1985-04-01), Davis
Lee Peter Y.
McElheny Jr. Donald E.
Schlumberger Technology Corporation
Smith Kieth G.W.
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