Data processing: measuring – calibrating – or testing – Testing system – Signal generation or waveform shaping
Reexamination Certificate
2003-02-26
2004-10-19
McElheny, Jr., Donald (Department: 2857)
Data processing: measuring, calibrating, or testing
Testing system
Signal generation or waveform shaping
Reexamination Certificate
active
06807508
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to a device intended for land seismic prospecting by simultaneous emission in the ground of seismic signals emitted by several vibrators or groups of vibrators, the signals being obtained by coding a signal by pseudo-random sequences, and notably a periodic signal phase modulated by such sequences.
2. Description of the Prior Art
There are well-known land seismic prospecting methods comprising transmission in the ground, for several seconds, of a periodic signal whose frequency varies continuously within a frequency band, reception by pickups of the signals reflected by underground reflectors and recording of the received signals. As a result of the emission time, the signals picked up are combinations of signals reflected by reflectors arranged at very different depths. The image of the various reflectors in the subsoil can only be recovered by processing the signals picked up, including correlation thereof with the signals emitted. The processing result is identical to that obtained by convolving the autocorrelation function of the signal emitted by the reflection coefficients of the various reflectors. The seismic trace obtained is the image of the interfaces between the various geologic layers midway between the emission and reception points. Such a method is for example described in U.S. Pat. No. 2,688,124.
This method however has some drawbacks. The autocorrelation function that is obtained in this case exhibits secondary peaks on either side of the main peak, whose amplitude is quite significant. Furthermore, a time interval at least equal to the loop travel time of the waves emitted down to the deepest reflector in the zone explored, referred to as “listening interval”, has to be provided between two successive emission sequences so that the strong signals picked up at the beginning of the corresponding recording sequence cannot conceal the weaker signals coming from more distant reflectors, picked up at the end of the previous recording sequence. The necessary emission interruptions during a relatively long listening time interval have the effect of limiting the energy transmitted.
There is also another well-known method wherein various vibrators emit simultaneously signals with an identical frequency sweep, the lag between their emissions being at least equal to the listening time.
Pseudo-random Binary Sequences
Other known land seismic prospecting methods use a technique that is well-known in the field of communications and radars. They comprise using periodic sources that transmit signals obtained by modulating the phase of a periodic carrier signal by a binary signal or pseudo-random code consisting of a sequence of elements that can take on two logical values 0 or 1. As shown in
FIG. 1
, the order of succession of these values is selected so as to have a random character.
When, for such a code, any sequence of n successive bits (n integer) can be repeated identically only after a sequence of (2
n
−1) bits, the sequence is referred to as “Binary Sequence of Maximum Length” (BSML). These pseudo-random binary sequences can be used for phase modulation of a periodic signal, each element of the sequence being associated with a period of the signal, by keeping or by inverting the sign according to whether it is a 1 or a 0. The term “elementary sequence” designates a portion of the periodic signal modulated by a binary sequence of (2
n
−1) terms, of length (2
n
−1)Tc, where Tc is the period of the carrier signal.
Correlation, by an elementary sequence, of a signal consisting of the repetition of this elementary sequence modulated by the elementary sequence (
FIG. 3B
) gives (
FIG. 3C
) peaks spaced out (in time) by the length of sequence Ts and a minimum (or even zero) level between the peaks or, more exactly, over the length of the sequence minus the period of the carrier Tc. The ratio of the correlation peak to the correlation noise is equal to the number of terms of the sequence.
Such methods are for example described in U.S. Pat. Nos. 3,234,504, 3,264,606, 4,034,333 and 4,069,470.
U.S. Pat. No. 3,234,504 describes a seismic prospecting method wherein modulated signals without cusps, whose amplitude spectrum is centered on the zero frequency, are applied to a vibrator.
U.S. Pat. No. 4,780,856 filed by the assignee describes a marine seismic prospecting method wherein the emission means comprise at least one vibrator towed by a boat progressing continuously along a seismic profile to be studied, emitting an uninterrupted series of sequences consisting each of a periodic carrier signal phase modulated by a pseudo-random binary coding signal of maximum length. The signals reflected by the discontinuities of the medium are correlated with the coded signals emitted so as to obtain correlation peaks at time intervals shorter than or at most equal to the period of repetition of the successive emission sequences. The periodic source can be the sole source, correlation being carried out between the signals received and alternately two sequences of signals emitted deducible from one another by a time lag shorter than the period of repetition of the sequences. A lag equal to the half-period of repetition of the sequences of signals emitted is for example selected.
It is also possible to use at least two periodic sources emitting simultaneously sequences of identical signals but with a time lag between them, and a correlation is established between the received signals, which correspond to the signals emitted simultaneously by the sources, and at least one sequence of coded signals, so as to alternately obtain correlation peaks corresponding to each periodic source.
The use, in land seismic prospecting, of vibrators emitting simultaneously or insufficiently separated in time has drawbacks linked with various factors: the autocorrelation noise, the harmonics and the slow waves.
Autocorrelation Noise
The vibroseismic signal is compressed by correlating the signals recorded by the signal controlling the vibrator (or by a combination of the signals of the plate and mass accelerometers of the vibrator). The equivalent of the correlation of the series of reflection coefficients is thus obtained by autocorrelation of the emitted signal. The emitted signal is generally a frequency linear sweep whose amplitude spectrum has either a crenellated shape, or it is preferably a bell curve to reduce the amplitude of the bounces.
The question of autocorrelation noises arises for all the vibroseismic records. The bounces decrease with time as a function of 1/t substantially. For an isolated record, the autocorrelation bounces of the large values at the beginning of a trace are sufficiently attenuated when the weakest reflections at the end of a trace reappear. In the case of a frequency slip sweep, recording is semi-continuous and the bounces of the large values are found in front and behind, and they can interfere with the low values of the deep reflections of the previous shot if the slip time is insufficient.
Harmonics
For an isolated vibroseismic record, the harmonic distortion adds oscillations to the correlated signal. If the sweep is carried out from the low frequencies to the high frequencies, the oscillations due to the correlation of the harmonics by the control signal are precursors. Thus, except for the closest traces comprising the surface noise, the noises due to the correlation of the harmonics mix with an earlier, therefore in principle stronger signal. For continuous slip sweep type records, the noise due to the harmonics of the early arrivals of a shot can be superposed on the late and therefore weaker arrivals of the previous shot.
Slow Waves
If the time interval between the start of two successive shots decreases, there is a risk the slowest waves of a shot, air waves and surface waves, may be found on the next shot. The sweeps being identical from one shot to the next, the air wave and the surface noises will be compressed similarly on the two shots.
Minimum
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
McElheny Jr. Donald
LandOfFree
Seismic prospecting method and device using simultaneous... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Seismic prospecting method and device using simultaneous..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Seismic prospecting method and device using simultaneous... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3284738