First breaks

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
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.

Figure 3.4-10  A near-surface model for statics corrections when shots are situated below the weathering layer. Here, S = shot, ES = elevation at the shot station on the ground, R = receiver, ER = elevation at the receiver station on the ground, T = surface topography, B = base of weathering, D = datum, ED = datum elevation, vw = weathering velocity, and vb = bedrock velocity.

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.

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First breaks
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