Procesamiento De Datos Sísmicos

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Where complete refraction profiles from zero off­ set to large offsets are available, playback of the data with judicious selections of filters and AGC may allow one to correlate reflection events with rcfrac­ tion events, thus adding useful information to each type of interpretation. Often the most prominent reflections will not correspond to the most promi­ nent refractions.
Anotheruseful technique is to display the data as
a reduced refraction section where arrival times have been shiftcd by thc amount xjVR• where VR is near tbc refractor velocity. If VR wcrc exactly equal to the refractor velocity, the residual times would be the delay times (wlúch will be discussed in §4.9.3), and relief on the reduced refraction section would corre­ late with refractor relief(although displaced from

Seismic methods

Figure 4.74 illustrates a method of obtaining the correction for 10 , the shotpoint arrival time. Ed is the elevation of the datum, E, the elevation of the
surface at the shotpoint, D, the depth of tbe shot below thc surfacc, and tuh is the uphole time (§4.5.2e). The deviation of reflection paths from the vertical is usually smallenough that we can regard the paths as vertical. Therefore, the time required for tbe wave to travel from the source down to the datum is tu,, where

( 4.86) Similarly, the time for the wave to travel up from the
datum to a geophone on the surface at B is tJ.t

the subsurface location of the relief). However, cven
if VR is only approximately correct, theuse of re­ duced sections considerably improves the pickability of refraction events, especially secondary refractions.

whcre

8

At 11 = At, + tub ( 4.87)

4.7. DATA PROCESSING

The correction At0 for the traveltime for a geophone at the sourcepoint is then

At 0 = At, + At 8 = 2 At, + tub

4.7.1. Data Reduction

Variations in tbe elevation of the surface atfect trav­eltimes and it is necessary to corree! for such varia­

2( E, - D, - Ed)
=
VH

+tub

(4.88)

tions as well as for changes in tbe near-surface low-velocity-layer (LVL). Usually a reference datum is selected and corrections are calculated so tbat, in etfect, tbe shotpoints and geophones are located on the datum surface, and it is assumed that conditions are uniform and that there isno LVL material below the datum level. The reference datum is usually horizontal, but where elevation variations are large, a tilted datum or a ftoating datum (one having the generalized relief of the surface but with smaller magnitude) may be used.
Many methods exist for correcting for near­ surface etrects. These scbemes are usually based on (1) uphole times, (2) refractions from the base ofthe LVL, or (3) the smoothing of retlections. We shall describe several of these metbods tbat are simple to apply and adequate to cover most situations. Auto­ matic statics-correction schemes, which usually in­ volve statistical methods of smoothing reftections, will be discussed in Section 4.7.5d. We shall assume that Vw and VH, the velocities in the LVL and in the layer just below it, are known;they can be found from an uphole survey or the refraction first breaks, as will be discussed later in the section. In what follows we assume that the sbot is placed below the base of the LVL; if this is not true, modifications bave to be made in the equations in this section (see problem 10).

Subtraction of At0. from the arrival time 10 is equiva­
len!to placing the sbot and the shotpointgeophone
group on the datum plane, thereby eliminating the effect of the low-velocity layer iC the shot is beneath the LVL. At times the shot may be so far below the datum plane that At, will be negative.
When Equation (4.56) is used to calculate dip, the
dip moveout must be corrected for elevation and weathering. The correction to the dip moveout, tlt,, often called the differential...