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Abstract

Small-scale surface irregularities are defined as variations in thickness and/or velocity of near-surface layers (Gardner, 1967, p. 344) that extend over only a few geophone intervals at most. These irregularities are significant enough to invalidate interpolation between shotpoints, but are not sufficiently common to justify a field program which fully maps the surface layers.

Most interpretation methods aim to define an irregular refractor which is usually assumed to lie below uniform surface layers. However, the accuracy in defining deep refractors often depends upon the recognition and definition of small-scale surface irregularities. If the time anomalies caused by the surface irregularities are assigned to deeper layers, then the computed depths can be quite inaccurate, and the inferred refractor irregularities may not even exist. Furthermore, if XY spacings greater than zero are used in defining deeper layers, then depth anomalies computed from the time anomalies may be migrated away from their sources.

Because the surface layers usually have low seismic velocities, any variations in thicknesses or velocities produce time anomalies which can be many times larger than the anomalies produced by the same variations in layers nearer the refractor.

For example, at G in the model shown in Figure 3, a change of 1.2 m in thickness of the surface layer results in a variation of 1 msec in the arrival time. However, a change of 3.2 m in the thickness of the third layer is required for the same variation in the arrival time.

This effect can be even more accentuated in areas where the

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