OPERATION MANUAL - Scintrex

The SD was in the range of 0.4 mGal for at least one day (our record, out ofwhich only one tenth is shown, is only 21 hours long due to the memorycapacity restrictions while cycling at relatively fast rate). During the periodof two hours depicted on Figure B-4, the standard deviation of gravitymeasurements (the measurement error) of the meter using the seismic filterwas 0.003 mGal (just a bit higher than when the seismic noise is low), whilethe standard deviation of gravity measurements of the meter using thestandard filter was 0.0 13 mGal.Compensation and CorrectionsThe following are the types of corrections considered in this section:• Drift correction• Tilt correction• Temperature compensation• Earth tide correction• Gravity offsetSome corrections, such as temperature compensation, are performedautomatically in the system. However, the theory is provided here forcompleteness.Drift CorrectionLong term instrument drift is largely due to stress relaxation in the elasticsystem. After an initial stabilization period, it can be considered a linearfunction of time. The effect of long term drift will be to produce a uniformchange in reading with time. For example, the figure on page B-21 showsseveral days of continuous readings. If two uncorrected readings are taken attimes t j and t 21 at a point where there has been no change in gravity, then thedifference between them will depend on the long term drift rate, d, of theinstrument and is:RU( t 2) = RU( t 1) = dt ( 2– t 1)2 (B.12)1. tn is the midpoint between the start and finish of a reading.B-16CG-5 Manual - part # 867700 Revision 2