THE REDUCTION OF SEISMIC OBSERVATIONS

305

weathering shot-time plot (Fig. 136 or 137); and the JTs, which
are given by the velocities involved [Eq, (275)].

WEATHERING CORRECTIONS FOR REFLECTION SHOOTING
In reflection shooting, the detector spread is usually close to
the shot point so that the first arrivals on the reflection records
can be used to calculate the weathering. The shot is nearly
always placed below the surface zone. Therefore, the first
arrivals reach the detectors by the dotted wave paths (Fig. 139).

TTOTyMPLANE.

SEA LEVELjrf^t__/ Jl \ f _

FIG. 140.—Reflection weathering correction notation.

The first arrival times to the individual detectors are indicated
by the crosses of the time-distance diagram of the upper part of
the figure. The average straight line through these points has the
slope 1/Fi. If the dashed line parallel to this is drawn through
the origin, the individual intercept times, such as fc, will be the
times above this line. These intercept times will be equal to
the individual delay times D> associated with the thickness of
the surface zone under the corresponding detector. Thus, if this

thickness is Wet,

cos i

= W.d

V0

and

cos i

(276)