Random and systematic navigation errors: how do they affect seismic data quality?
| Autor: | Dennis Fryar, David J. Monk, Josef Paffenholz |
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| Rok vydání: | 1994 |
| Předmět: | |
| Zdroj: | First Break. 12 |
| ISSN: | 1365-2397 0263-5046 |
| DOI: | 10.3997/1365-2397.1994030 |
| Popis: | A quantitative analysis has been performed to assess the effect of navigation errors on marine Seismic data. Two different types of errors were considered The first is a systematic error, that of a rotation of the streamer coordinates which could be caused for example by incorrect magnetic declination The second type models the random errors in the receiver positions due to the limited accuracy of the navigation network. The analysis is performed as a function of streamer feather angle, structural dip, and acquisition parameters. The effects on the seismic data are reported in terms of stack degradation and difference in the apparent NMO velocity. To assess the effect of a rotational error on the final image, we process a synthetic seismogram consisting of a dipping event and a diffractor through DMO and migration. Significant stack degradation as a consequence of a systematic rotational error is found only for lines shot in alternating directions in the presence of notable crossline dip. In all other cases the error is either absorbed by the NMO velocity or inconsequential because of small crossline dip. Stack degradation caused by random position errors are weakly dependent on the crossline dip and can be minimized by collecting the lines with the boat driving in the down dip direction. The changes in the resolution and position of the final image depend on the velocity which is used. For an inline of a survey shot along strike the use of true material velocity minimizes the impact of the rotational error while use of the apparent NMO velocity leads to a significant stack degradation. The misposition is about half a trace laterally and 5 and 10 ms in time for the true velocity and NMO velocity case, respectively. Other cases will be discussed. INTRODUCTION While the effects of streamer feather on binning and stacking of seismic data have been studied in the past (eg. Levin 1983, 1984), a quantitative analysis of the effects of an error in the measurement of the cable feather has not been made. Such a systematic error could be caused, for example, by incorrect magnetic declination and would result in a apparent rotation of the cable. More widespread are random errors in the receiver positions caused by the limited accuracy of the navigation measurements. Houston (1991) estimated the maximum uncertainty in cable receiver positions for a state of the art navigation system to be about 6 m. The high cost and reliability problems introduced by redundant navigat ion networks cal l for an investigation of the tradeoffs between operational cost and quality of the final seismic section. Cost pressures have led to a heightened interest to establish a link between the efforts to reduce the uncertainty in the receiver positions and increased image quality. Ursin-Helm et. al. (1992) studied the effect of infill on the quality of the final image and concluded that in their particular case an infill of 30 % Causes only minor improvements of the section quality. They also studied the effects of less accurate navigation by visual inspection of the final seismic data. In this paper we offer a quantitative analysis of how seismic data is affected by statistical or systematic rotational errors in the position of a marine seismic streamer. PROCEDURE Ray tracing is used to assess the effects of random errors and a particular systematic error, that of an apparent rotation of the cable. Synthetic CMP gathers are collected over a model consisting of one dipping event for different structural dips, bin sizes, and feather angles of a straight single cable. The least squares NMO velocity and the stack response are calculated for data collected with a particular rotation error and compared to the error free case. Two different shooting patterns are investigated. In the first case, all lines are shot in the same direction, while the second case consists of l ines shot in alternating directions. If the stack trace results from the summation of traces which were collected from different boat passes (with feather), then energy from an event in the subsurface will deviate from a perfect hyperbola (Levin 1984). The need for correction to an hyperbola has been examined in relation to velocity modelling and DMO by Meinardus and McMahon (1981). In this paper since we assume that the navigation errors are not known (and therefore not corrected) we do not apply the location correction required to correct for this effect, but rather analyze the results in terms of change to the velocity that is determined. A rotational error of up to +/2 deg is considered. Random errors in the receiver positions are implemented as normal distributions around the true positions which are assumed to fall on a straight line. The standard deviations increase linearly with the offset up to 25 m for the far offset (3000 m). A second set of experiments is used to assess the impact of a rotational navigation error on the resolution and position accuracy of the final migrated image. The synthetic constant velocity 3D model consists of a dipping reflector and a diffractor suspended 200 m above the plane. lnlines and crosslines are processed through 2D DMO and 2D migration, Final sections of error free data and data with rotation error are compared. |
| Databáze: | OpenAIRE |
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