In 2007, ConocoPhillips conducted field experiments designed to evaluate the data quality of multi-offset VSPs acquired by a single vibrator and simultaneous ...
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Validating land data quality of simultaneous multiple vibrator acquisition Stephen K. Chiu*, Simon A. Shaw, Peter M. Eick, P.G. and Joel D. Brewer, ConocoPhillips
Summary
Field experiments
In 2007, ConocoPhillips conducted field experiments designed to evaluate the data quality of multi-offset VSPs acquired by a single vibrator and simultaneous multiple vibrators. To check the repeatability of vibrator sources, we recorded 8 repeated sweeps at the same source location for both acquisitions. The data quality is consistent from sweep to sweep at the same source location showing good repeatability of vibrator sources. Inverting 8 repeated sweeps simultaneously by a least-squares approach produces a solution that is very comparable to an average solution derived from inverting each sweep separately. In some cases, the least-squares solution tends to handle the ambient noise better and gives a slightly better solution than the average solution. The analyses of downgoing and upgoing VSPs demonstrates that simultaneous multiple vibrator acquisition yields equivalent data quality when compared with a single vibrator and cross-talk artifacts generated by simultaneous multiple vibrators are minimal in this case.
In 2007, we conducted field experiments to evaluate the data quality of multi-offset VSPs acquired by a single vibrator and four simultaneous vibrators. There are 23 source locations (Figure 1) with various offsets and azimuths from the well. The VSP consists of a downhole vertical cable of 80 stations with 50 foot spacing between receivers. The receiver depths range from 2974 to 6923 ft. We acquired 23 VSP experiments using a single vibrator, and repeated the same number of experiments using 4 simultaneous vibrators with phase-encoded sweeps at the same source locations. We used an upsweep from 6 to 110 Hz with a sweep length of 24 seconds and a 4 second listening time. At each source location, we recorded 8 sweeps for both a single vibrator and 4 simultaneous vibrators.
Introduction Recent developments in land seismic field operations utilize simultaneous multiple sources to reduce acquisition cost and increase productivity. Early work of Pritchett (1991) used the theorem of superposition to successfully separate data from two simultaneous vibrator sources. He employed the first set of vibrators to sweep with a positive polarity, and the second set of vibrators to sweep with a negative polarity. Sallas et al. (1998) developed an inversion approach (High Fidelity Vibratory Seismic, or HFVS) to separate the data from multiple vibrators operating simultaneously with phase-encoded sweeps emitted from each vibrator. Moldoveanu et al. (2000) conducted two VSP experiments to compare the data quality acquired by a single vibrator and simultaneous multiple vibrators using HFVS. They concluded that the HFVS method produced an enhanced resolution of final VSP images when comparing with conventional offset VSP acquired by a single vibrator. However, the enhanced images from the HFVS method were probably due to the difference in processing sequences as Krohn and Johnson (2006) demonstrated that there was little difference in data quality from two VSP experiments that were acquired individually and simultaneously using 2 and 4 vibrators.
Using all 23 VSP data sets, this paper addresses two of our objectives in the larger field program, namely to investigate at the same source location: (1) whether it is better to invert 8 sets of single vibrator data individually and produce an average solution by stacking, or invert all 8 sets simultaneously to produce a least-squares solution; (2) evaluate the data quality between single and four simultaneous vibrators by analyzing amplitude and phase spectra of the downgoing and upgoing VSP data, and assess cross-talk artifacts generated by simultaneous multiple vibrators. Stacking versus the least-squares solution Cross correlation is a standard method to extract seismic signals from vibratory data. However, the correlation method fails to extract signals from simultaneous multiple vibrators. We use an inversion method (Chiu, et al., 2005) to invert both the single and simultaneous multiple vibrator data to avoid the discrepancies caused by different extraction algorithms. For the single vibrator data there are 8 repeated VSPs acquired at the same source location. Each VSP is inverted separately to generate 8 VSP data sets (Figure 2). The data quality is very consistent from sweep to sweep showing good repeatability of vibrator sources, but the background ambient noise tends to change from sweep to sweep. These 8 VSPs are stacked to produce an average solution. An alternative way is to invert all 8 data sets simultaneously to produce a least-squares solution. The least-squares solution is almost the same as the average solution (Figures 3a and b) and their differences are insignificant (Figure 3c) in this case.
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Validating simultaneous multiple vibrator
We repeat the same analysis on the simultaneous multiple vibrator data and can draw similar conclusions as in the single vibrator case (Figures 4a and b). In this case, their differences mainly come from the ambient noise (Figure 4c) because the ambient noise is unpredictable. The leastsquares solution is more efficient in data processing because it is one-step process rather than two or more steps in the average solution. In addition, it appears to handle the ambient noise slightly better than the average solution in this case. The tube waves are generally more visible on simultaneous multiple vibrator data at the near- and midoffsets (Figures 3b and 4b). We postulate that the ground force estimate put out by the vibrator controller may not always be a good approximation of the actual vibrator source signature that transmits to the ground. Inverting the simultaneous multiple vibrator data using ground force estimates instead of actual vibrator source signatures produces imperfect source separations that could contribute the presence of more tube waves. To investigate the uncertainty of the ground force estimate, Shan et al. (2009) employed a load cell system to measure the vibrator source signatures and compared with conventional weighted-sum ground force estimates. They concluded that the conventional ground force estimate was questionable. We plan to further investigate the effect of the uncertainty of the ground force estimates on source separations. Comparisons of single and simultaneous multiple vibrators There are many ways to compare single and simultaneous multiple vibrator data. The approach we adopt here is to examine the amplitude and phase spectra of the inverted raw data that have minimal processing steps to ensure that the fidelity of amplitude and phase are preserved as much as possible in both cases. First we analyzed the downgoing VSP by extracting the direct arrivals of both data sets. Figures 5a and 5b represent typical data quality among all VSP data. No amplitude scaling is applied to the data sets. There is little difference between these two data sets (Figure 5c). The close matching of amplitude and phase
spectra at three depth levels of 2974, 4973 and 6923 ft (Figure 5d) indicates that simultaneous multiple vibrator data with a proper phase-encoding scheme reproduce equivalent data quality as a single vibrator. The cross-talk artifacts generated by simultaneous multiple vibrators are minimal in this case. The second step is to extract upgoing VSP data between single and simultaneous multiple vibrator data. The primary reflections between these two data sets match reasonably well (Figures 6a and b). Their differences (Figure 6c) are probably due to ambient noise and the approximation of true vibrator source signatures by ground force estimates in the source separation process. The amplitude and phase spectra at depth levels of 3524 and 5573 ft also deviate slightly from each other (Figure 6d). The general conclusion after analyzing all VSPs is that the data quality is equivalent between single and simultaneous multiple vibrator acquisitions and the crosstalk artifacts generated by simultaneous multiple vibrators are minimal. Conclusions The least-squares solution is almost the same as the average solution by stacking. In some cases, the least-squares solution appears to handle the ambient noise slightly better than the average solution. The analyses of downgoing and upgoing VSP indicate that simultaneous multiple vibrator acquisition achieves equivalent data quality as a single vibrator without sacrificing data fidelity and the cross-talk artifacts generated by simultaneous multiple vibrators are minimal. Acknowledgments The authors thank ConocoPhillips for the permission to publish this work. We would like to thank Phil Anno, Peter Cramer, Erik Keskula, and Hugh Rowlett for their suggestions. Special thanks are due to Michael Davidson and Charles Emmons for useful discussions, and ConocoPhillips L48 Business unit for their support.
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Validating simultaneous multiple vibrator
VSP well
Figure 1. VSP location map: square is well location, and star is source location.
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Figure 2. Eight VSPs at the same source location. Each is Inverted separately.
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Figure 3. Inverted VSP from a single vibrator at a offset of 264 ft from the well: (a) an average solution of 8 sweeps, (b) a least-squares solution of simultaneously inverting 8 sweeps, (c) differences between (a) and (b).
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b
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Figure 4. Inverted VSP from 4 simultaneous vibrators at a offset of 264 ft from the well: (a) an average solution of 8 sweeps, (b) a least-squares solution of simultaneously inverting 8 sweeps, (c) differences between (a) and (b).
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Validating simultaneous multiple vibrator
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d a
b
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Figure 5. Direct arrivals of downgoing VSP at a offset of 264 ft from the well: (a) from a single vibrator, (b) from 4 simultaneously vibrator, (c) differences between (a) and (b), and (d) amplitude and phase spectra at three depth levels of 2974, 4973 and 6923 ft. The red line is from single vibrator and the blue line is from 4 simultaneously vibrators.
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Figure 6. Upgoing VSP at a offset of 264 ft from the well: (a) from a single vibrator, (b) from 4 simultaneously vibrator, (c) differences between (a) and (b), and (d) amplitude and phase spectra at two depth levels of 3524 and 5573 ft. The red line is from single vibrator and the blue line is from 4 simultaneously vibrators.
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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2009 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Chiu, S. K., C. W. Emmons, and P. P. Eick, 2005, High Fidelity Vibratory Seismic (HFVS): robust inversion using generalized inverse: 75th Annual International Meeting, SEG, Expanded Abstracts, 1650–1653. Krohn, C. E., and M. L. Johnson, 2006, HFVS: Enhanced data quality through technology integration: Geophysics, 71, no. 2, E13–E23. Moldoveanu, N., M. Puckett, A. Campbell, and J. Meyer, 2000, High fidelity vibratory seismic applications for VSP: 70th Annual International Meeting, SEG, Expanded Abstracts, 1759–1762. Pritchett, W. C., 1991, An example of simultaneous recording where necessary signal separation is easily achieved: Geophysics, 56, 9–17. Sallas, J., D. Corrigan, and K. P. Allen, 1998, High fidelity vibratory source seismic method with source separation: U. S. Patent 5,721,710. Shan, S., P. M. Eick, J. D. Brewer, X. Zhu, and S. Shaw, 2009, Load cell system test experience: Measuring the vibrator ground force on land seismic acquisition: submitted to 79th Annual International Meeting, SEG.
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