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The construction of time sections using the GRM is a very convenient and powerful intermediate stage in the processing and interpretation of seismic refraction data.

The time sectioni s an orthogonalp lot of time-depthsa nd half-interceptt imes below the points on the surface to which each refers. The horizontal axis represents the seismic line on the earth's surface. For a geophone time-depth [equation (10)], the reference point is the midpoint of XY; for a time-depth near a shotpoint [equation (15)], it is 1/2 XY from that shotpoint toward the reverse shotpoint; for the half-intercept time [equation (39)], it is below the shotpoint. The vertical axis has the units of time positive downward.

Time sections have been used previously in seismic refraction processing for the adjustment of delay times (Wyrobek, 1956; Pakiser and Black, 1957; Layat, 1967). However, the most common use of time sections is in the seismic reflection method (Dobrin, 1976, p. 236).

The time section is an extremely convenient work area for seismic refraction processing because (1) it is not affected by uncertainties in determining velocities above refractors, (2) it can be constructed while still preserving consistency with the original traveltime data, and (3) it provides a criterion for field work requirements.

For the calculation of a time-depth using equation (10), the only velocity required is that of the refractor. This can be readily obtained using equation (6). The velocities of layers above the refractor being mapped are not required to form equation (10).

However, to convert time-depthsin to depthsu singe quations( 12)

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