Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-10-23
2004-07-20
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06766256
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for generating and processing simultaneous vibratory seismic data.
BACKGROUND OF THE INVENTION
In seismic prospecting using simultaneous vibratory techniques, a series of seismic energy sources or vibrators are employed to transmit seismic signals into the earth. Part of these signals are reflected from interfaces between subterranean strata, and/or refracted within strata, back to the surface of the earth, where they are detected by one or more receivers. The time taken for a signal to pass from a particular vibrator to a particular receiver gives an indication of the length of travel of the signal between that vibrator and that receiver, from which the structure of geological formations may be deduced.
Vibrators generally consist of a base plate in contact with the ground, with a large weight applied to this plate. The seismic energy is transmitted to the ground by applying a vibratory force to the plate. Thus if a constant weight of 30 tonnes is applied to the metal plate, a vibration with amplitude 20 tonnes can be applied to the vibrator, ensuring that there is always a positive force against the ground.
Many modern vibratory seismic surveys are performed by simultaneously imparting energy into the earth from multiple source locations, so that each receiver will detect refracted and reflected energy which has been emitted by the whole series of vibrators. The data recorded at each receiver must then be processed so that the signal due to each individual vibrator can be separated out.
This is generally achieved by each vibrator performing multiple “sweeps” or “shots”, where the phase of the signals emitted by the vibrators are varied between vibrators and between shots. In its simplest form, this can be illustrated by the case of two vibrators, twice operated simultaneously. If they are operated in phase with each other for the first sweep, but 180° out of phase for the second sweep, the receiver will record two signals. These signals may be added together to determine the signal arriving from the first vibrator, or subtracted to determine the signal arriving from the second vibrator.
In general, in order for the data arriving at each receiver to be decoded so that the contribution of each vibrator can be determined, there must be at least as many shots as there are vibrators. For each shot by each vibrator, a waveform is applied to that vibrator by the control mechanism. This waveform is normally a constant amplitude swept-frequency “chirp”, with tapered ends due to the fact that the amplitude has to be ramped up at the start and ramped down at the end, and is known as the “pilot sweep”. In modern seismic operations, the pilot sweep almost always begins at low frequency and finishes at high frequency, and the frequency normally increases linearly with respect to time.
In practice, the waveform actually applied to the ground by the vibrator is not quite the same as the waveform applied to the vibrator. Deviations from the pilot sweep are inevitable. These deviations have two characteristic forms:
1. As well as applying a force at the desired frequency (known as the fundamental), the vibrator also applies a force at integer multiples of that frequency (known as harmonics).
2. The force applied at the fundamental frequency deviates in amplitude and/or phase from the pilot sweep.
At each receiver, in order to separate out the signal from each vibrator, it is necessary to use some approximation of the signal provided by each vibrator. In the past, two methods have chiefly been used.
The first assumes that the vibrator force follows the pilot sweep exactly, and separates out the earth response due to the vibrator at each vibrator location accordingly. If such a method is used, the data at each receiver, or geophone, can be correlated with a single representative pilot sweep before separation. No measurements need to be made on the vibrator, and the procedure is robust. Non-linear effects such as a non-linear earth response or data clipping during acquisition will not leave significant artifacts.
The main disadvantage with this method is that deviations of the force applied at the fundamental from the pilot sweep can lead to significant cross-coupling of the vibrators, i.e. the signal from one vibrator being ascribed to another. This occurs if each vibrator has a different deviation from the pilot sweep. This method can also lead to timing errors for the individual vibrators.
The second known method uses a measurement of the force applied to the earth by the vibrator. Thus, instead of using the pilot sweep, the real force applied by the vibrator is used to derive an inversion operator. This removes the main disadvantage of using the pilot sweep, but itself has other disadvantages. The whole inversion procedure must be performed on uncorrelated data, the entire waveform of each shot must be acquired for each vibrator and each shot, and the method is sensitive to the non-linear effects described above.
In general, the data acquisition and much of the processing are performed separately. The data recorded by the receivers may be partially processed as it is received, but normally either raw or partially processed data is saved onto tapes which are then transferred to a central data processing unit.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention provides method of processing seismic data, said seismic data having been obtained by:
performing a plurality of sweeps, wherein each sweep comprises generating seismic signals in the earth using a plurality of vibrators by applying a pilot sweep waveform to each vibrator, each pilot sweep being a waveform of changing frequency;
measuring the force applied to the earth by each vibrator to determine a measured force waveform; and
measuring the seismic signals at one or more locations remote from the vibrators;
said method comprising:
filtering the measured force waveform to remove harmonics of the pilot sweep and thus determining a filtered force waveform;
generating an inversion operator from the filtered force waveform for each vibrator; and
applying said inversion operator to the measured seismic signals to determine the contribution of each vibrator to the seismic signals.
The force waveform applied to the earth may be measured by special sensors, although preferably it is measured by derivation from the signal used by the vibrator controller to control the vibrator output. This may be a weighted sum measurement, consisting of a linear combination of signals from accelerometers on the vibrator.
DETAILED DESCRIPTION OF THE INVENTION
The filtered force waveform may be determined by the application of a time varying notch filter to the measured force waveform, but more preferably the measured force waveform applied to the ground is filtered by cross-correlation with the pilot sweep followed by application of a time window to remove the harmonics of the pilot sweep. Each measured waveform may preferably be cross-correlated with the same pilot sweep. The time window is preferably applied around zero time.
The measured seismic signal is preferably also filtered by cross-correlation with the pilot sweep followed by application of a time window before the application of the inversion operator.
The pilot sweep preferably comprises a waveform having a substantially constant amplitude envelope with tapered ends, within which the frequency increases with time.
The number of sweeps may be the same as the number of vibrators. In another embodiment, the number of sweeps is greater than the number of vibrators, and the noise on each measured seismic signal is estimated and used in the determination of the inversion operator.
In a preferred embodiment, the invention provides a method of performing a seismic survey, comprising:
performing a plurality of sweeps, wherein each sweep comprises generating seismic signals in the earth using a plurality of vibrators by applying a pilot sweep waveform to each vibrator, each pilot sweep being a waveform of changing frequency;
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Batzer William B.
McElheny Jr. Donald
Ryberg John J.
Schlumberger Technology Corporation
Wang William L.
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