First breaks

ADVERTISEMENT
From SEG Wiki
Revision as of 10:27, 19 September 2014 by Ageary (talk | contribs) (removed links)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search
Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


The refracted energy associated with the base of the weathering layer often constitutes the first arrivals on a shot gather. The onset of these first arrivals is referred to as the first break.

First breaks occur in varying degrees of quality — depending on the source type and the near-surface conditions. The common-shot gather shown in Figure 3.4-3 has first breaks with clear onset. Deviations from the linear trend of the first-break times may largely be attributed to elevation differences along the shot profile. Figure 3.4-4 shows a record with first breaks associated with a prominent refractor. In Figure 3.4-5, note a shallow and a deep refractor. Figure 3.4-6 shows a shot record in which automated procedures would largely fail to pick the first breaks. Figure 3.4-7 shows a shot record with first breaks that can be detected easily by automated procedures. From the first breaks on the left, one can infer near-surface irregularity — either in the form of a variable refractor shape or velocity variations in the near-surface layer. The right-hand side shows the presence of a distinct refractor. Figure 3.4-8 shows a shot gather recorded with a vibroseis source, which often produces poor first breaks compared to a dynamite source. A similar situation exists in the record shown in Figure 3.4-9 — it is not simple to detect the first breaks. The remainder of the sidelobes from sweep correlation masks the onset of the first arrivals.

First-break picking can be done automatically, interactively, manually, or as a combination thereof. To make reliable picks, first apply linear moveout (LMO) to the data. Once picking is done, the LMO correction is reversed. Note that effectiveness of both reflection- and refraction-based methods of statics corrections depends on the reliability of the picking process. Apart from the signal-to-noise ratio, indistinct first breaks (such as in vibroseis) sometimes can make picking consistent first breaks difficult.

The first-break picks associated with the refracted arrival times are then used in an inversion scheme to estimate the near-surface model parameters. In this section, we discuss ray-theoretical methods such as plus-minus and its generalized form, the reciprocal method, and the least-squares inversion methods. The basic assumption made is that the refractor is flat or nearly flat, with a smoothly varying shape along the seismic profile. As demonstrated by the field data examples, these methods appear to remove medium- to long-wavelength statics anomalies associated with various types of near-surface models. Combined with the reflection-based residual statics corrections to resolve any remaining short-wavelength statics variations that affect the stack quality, we get a final stacked section ready for poststack processing.

See also

External links

find literature about
First breaks
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png