- Short Time Intervals - The use of a short time interval between subsequent base station readings will produce better estimates of the tidal and drift components of the observed field so that they can be more accurately removed. More frequent base station readings, however, allow for fewer gravity stations to be surveyed during a given time period. Thus, the smaller the base station reoccupation interval, the longer it takes to complete a given survey, and the more the survey costs.
- Long Time Intervals - By reoccupying the base station less frequently, more gravity stations can be surveyed during a given time period. Hence, the cost of the survey can be reduced. If the time interval between subsequent base station readings is too long, however, the drift and the tidal component of the observed field can not be correctly removed from the data set. Thus, residual* drift and tides will corrupt the final data.
To estimate what the optimum time interval between subsequent base station reoccupations given the cost/accuracy tradeoff, we will use the earth tide data that was presented in the notes and reproduced to the right. Given that this data set was acquired at a single location, it directly shows the gravitational variation resulting from tides and instrument drift without geologic complications. Thus, we can use this data set to estimate the optimal base station reoccupation interval.
One common method of reducing these errors is to make multiple readings
at a single location and average the result. If the errors associated with
the readings are random, it can be shown that the reading error is reduced
by the square root of the number of observations made. 
Thus, if the error associated with one reading is 0.02 mGals, averaging
4 readings will reduce the error to 0.01 mGals, averaging 16 readings will
reduce the error to 0.005 mGals, etc. Notice, that because this technique
requires multiple observations at each station, relying on reading averaging
to reduce reading errors will increase the cost of the survey.
Shown to the left is a series of gravity observations made at a single location by a novice gravity crew. We will use these observations to make conservative estimates of the errors inherent in our gravity observations. If there were no reading errors in this data set, we would expect all of the observations to lie approximately along a straight line. The slope of this line depends on where in the tidal cycle these observations were collected (Remember all of the observations will not have the same value because they will still contain an earth tide contribution).
