THE REDUCTION OF SEISMIC OBSERVATIONS 311

times or from general geologic information, are considered of
sufficient interest to be converted to depths.

First, the surface corrections are made by the use of a datum
level (method 3, page 308). These corrected reflection times
for each detector trace are plotted, instead of actual depths, on
the usual profile, the horizontal positions being the usual " center
points " midway between shot point and detector.

The corrected times T0 can be plotted conveniently and rapidly
by use of a " fiducial time " F. This time is an arbitrary reference
chosen near the average reflection time of the events being

SHOT

MOMENT .

FIDUCIAL MARK
REFLECTION EVENT

m

FIG. 141.—Tape markings and notation for calculating reflections by the "fiducial
time'* method.
mapped and for convenience is an integral number of seconds
or tenth-seconds. For each detector trace (Fig. 141) a fiducial
mark is placed at a time tm given by
ft. = F + C3; C3 = t. - F (286)
where C3 is the surface correction given by Eq. (285). Then,
since To = tr — Cs, it follows that
To = (tr - Q + F (287)
The time, tr — tm, can be scaled off conveniently on the record
with dividers and plotted on the time profile with reference to the
fiducial time F to give a plot of the corrected reflection time.
It is evident that for a given velocity distribution, the fiducial
time F corresponds to a constant reference depth. Depth differ-
ences below or above that reference depth are determined from
the time difference, tr — &*, multiplied by the velocity pertinent
to the part of the geologic section involved. If reflections are
read over a large range of reflecting times, it is convenient and
desirable to use several fiducial times for reflections from different
parts of the record. Total depths will be those corresponding