Research Of Encoding Excitation Techniques For Seismic Sources

Vibroseis source is the most successful seismic source in land seismic exploration.Different from the single-pulse source like explosive which excites high-powerseismic pulse in very short time, Vibroseis source excites long-time and small-powercontinuous sine waves as the equivalence of spike pulse excited by pulse sources. Forthe original seismic record from Vibroseis acquisition, reflection information is mixedand unable to distinguish, while by cross-correlating the long seismic record with thepilot signal generated from Vibroseis vibrator, the long record are compressed into aseismic response of a single-pulse source, and then reflection information is recovered.As Vibroseis develops up to now, the main form of Vibroseis sweep signal is stillthe linear sweep frequency Chirp signal whose correlation wavelet has distinct side-lobe effect, thus, serious correlation noises will appear in decoded seismic profile andthe resolution of seismic profile is reduced. The objective of this thesis is to developcoding schemes for Vibroseis with ideal correlation characteristic which willfundamentally solve the side-lobe effect and correlation noises problems of Chirpsweep and increase the resolution and signal to noise ratio of seismic profiles.On account of the two type of seismic source, the continuous Vibroseis source andpulse-coded impact source, brand new signal coding schemes are given, whichexpands the applications of coding techniques for various kinds of seismic sources.Main contents and achievements are：1. Numerical simulation for single pulse source and Vibroseis Chirp sweep seismicresponses are conducted on the basis of the Vibroseis theory. The research on coding techniques for seismic source is part of seismic forward problem which is to solve theseismic response profile of a given source signal propagating through the establishedlayer model. First of all, finite difference format of seismic response numericalsimulation is given and then a three layer horizontal model is established with initialvalue condition, boundary condition and joining condition given for the2-d p-wavemathematical models. Since the seismic response of a single pulse source is the basisreference for verifying the quality of a new seismic source coding scheme, seismicnumerical simulation for a single pulse source using the Ricker wavelet is conducted,and the seismic response profile has clear and distinguishable seismic events of directwave reflected wave with the visible multiple underneath the profile and the wholeprofile has high signal to noise ratio. Then numerical simulation for conventionalChirp sweep is conducted and the result show that there is strong correlation noisesparallel to the direct wave with obvious side-lobe effect on both sides of the directwave, furthermore the multiple underneath the profile is submerged by strong noiseswhich result in a low signal to noise ratio profile and the quality of the profile cannotcompare with the single pulse situation.2. Ternary pseudorandom coding scheme for Vibroseis source is given. Ternarypseudorandom sequence is constructed for the use of modulation of Vibroseis sweepsignal, then n order q-ary m-sequence in finite domain GF (q)is generated from themodules q operation of an n order primitive polynomial in finite domain GF (q)by agiven nonzero initial vector. Ternary pseudorandom sequence consist of1,0,-1internary domain is produced through the mapping relation from multi-ary domain toternary domain and then the ternary pseudorandom sweep signal for Vibrosies sourceis generated from the sinusoidal carrier modulation of the ternary pseudorandomsequence. The numerical simulation result show that the correlation noises in thedecoded seismic profile has been remarkable attenuated in comparison with binarycase, and the multiple underneath the profile is also visible,which effectively increase the signal to noise ratio of seismic profile. In the meantime, ternary aperiodicpseudorandom coding scheme has also been researched.3. Brand new matched sweep method for Vibroseis source is researched. In order tofundamentally suppress the correlation noises due to the side-lobe effect of thecorrelation wavelet and increase the resolution and signal to noise ratio of seismicrecords, matched sweep method for Viborseis source is proposed. The realization ofmatched sweep method is through the excitation of a pair of correlative matchedsweep signals and the composition of the correlative matched seismic records.Matched pseudorandom sequence pair used for generating Vibroseis sweep signal isgiven, and matched sweep signals are generated by modulation of the matchedpseudorandom sequence pair. The numerical simulation result the comprehensivedecoded seismic profile of matched sweep can rival the seismic response profile ofsingle pulse source, which fundamentally suppress the correlation noises and theoutdoor excitation and acquisition experiments verify the superiority of matchedsweep method.4. Swept impact method of pulse-coded source is researched. The swept impactmethod is the combination of the linear sweep of Vibroseis and the random impact ofMini-Sosie, nevertheless, the choice of swept impact parameters can significantlyaffect the final decoded seismic profile in actual application. The numericalsimulation results reveal the affection by selecting different impact number andincrement of time interval; in the meantime, appropriate swept impact parametersthrough which high resolution and signal to noise ratio seismic profile can be obtainedare given.5. Matched impact method for pulse-coded seismic source is researched on thebasis of matched sweep method for Vibroseis source. Matched sweep signals areseparated into matched impact signal pairs which have a single impact state and can be excited by pulse-coded seismic source in sequence. The numerical simulationresults indicate that matched impact method can effectively avoid the side negativeeffect brought from inappropriate parameter setting up in the case of swept impact,and the comprehensive decoded seismic profile demonstrates high signal to noiseratio and resolution commensurate with the single pulse seismic profile. The outdoorexcitation and acquisition experiments verify that the matched impact method is aneffective coding scheme for pulse-coded seismic source which is superior to the sweptimpact method. The matched sweep method for continuous Vibroseis source and thematched impact method for pulse-coded seismic source compose a complete matchedexcitation technique for seismic sources in company which is an advanced andeffective coding scheme.The main innovative achievements of this thesis are:1. Ternary pseudorandom single excitation and acquisition coding scheme forcontinuous Vibroseis source is proposed which improve the correlation noise problemin the case of binary m-sequence coding scheme and enrich the single excitationcoding schemes for Vibroseis source.2. Brand new matched sweep method for Vibroseis source is proposed. After theprocesses of excitation, acquisition and decoding for the matched sweep sub-signalsmodulated from the matched pseudorandom sequences, the comprehensive decodedseismic profile is of extremely high resolution and signal to noise ratio.3. On the basis of the research on the existing parameter selecting problem forpulse-coded seismic source, matched sweep method for continuous Vibroseis sourceis extended to the signal design of pulse-coded seismic source, which has substantialtheoretical and realistic significance particularly for shallow high resolution seismicexploration which is appropriated for pulse-coded seismic source...