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MJUTHUDS

The frequency with which drift observations must be made
depends on the stability of the instrument. If the drift curve is
quite regular and nearly linear, three or four drift station observa-
tions per day may be sufficient. For very accurate work it may
be necessary to take drift readings as frequently as once per hour
or to consider each station a drift station for the next station
made (the "loop" method of observation described below).

FIELD OPERATION OF GRAVIMETERS

The details of the field methods used for gravimeter surveys

^___^^—»^ ^ depend on the characteristics of

«^-£*^ a ^ '•* "^^il^-^* the instrument used and on the

FIG. 20.-—Sequence of "looping" technical policies of the opcrat-

gravimeter observations. jng company. Commonly a net

of base stations is set out in advance to which other stations are

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MEASUREMENT OF BASE STATION
GRAVITY DIFFERENCES
FIG. 21.—Drift curves for "loop" gravimeter observations,
referred. The bases may be set out by "looping" (Fig. 20),
i.e., by an observation, say, at station I, ahead to station