Tidal and Drift Corrections: A Field Procedure
Let's now consider an example of how we would apply this
drift and tidal correction strategy to the acquisition of an exploration
data set. 
Consider the small portion
of a much larger gravity survey shown to the right. To apply the corrections,
we must use the following procedure when acquiring our gravity observations:
- Establish the location of one or more gravity base
stations. The location of the base station for this particular survey
is shown as the yellow circle. Because we will be making repeated gravity
observations at the base station, its location should be easily accessible
from the gravity stations comprising the survey. This location is identified,
for this particular station, by station number 9625 (This number was chosen
simply because the base station was located at a permanent survey marker with
an elevation of 9625 feet).
- Establish the locations of the gravity stations appropriate for the particular survey.
In this example, the location of the gravity stations are indicated by the blue circles.
On the map, the locations are identified by a station number, in this case 158 through 163.
- Before starting to make gravity observations at the gravity stations, the survey is
initiated by recording the relative gravity at the base station and the time at which the gravity
is measured.
- We now proceed to move the gravimeter to the survey stations
numbered 158 through 163. At each location we measure the relative gravity at the station and the
time at which the reading is taken.
- After some time period, usually on the order of an hour, we return to the base station and
remeasure the relative gravity at this location. Again, the time at which the observation is
made is noted.
- If necessary, we then go back to the survey stations and continue making measurements, returning
to the base station every hour.
- After recording the gravity at the last survey station, or at the end of the day, we return
to the base station and make one final reading of the gravity.
The procedure described above is generally referred to as a looping procedure
with one loop of the survey being bounded by two occupations of the base station.
The looping procedure defined here is the simplest to implement in the field.
More complex looping schemes are often employed, particularly when the survey,
because of its large areal extent, requires the use of multiple base stations.
