OPERATION MANUAL - Scintrex

Maintaining Your CG-5 and Trouble-shootingRe-Calibrating the CG-5As explained on page B-9, the calibration factors GCAL1 and GCAL2 areapplied to the gravity signal, according to Equation (B.4). The calibrationfactor GCAL2, which accounts for a small quadratic nonlinearity, isevaluated in the Scintrex gravity laboratory Since 1991, it has been reducedto zero by an electronic adjustment. The GCAL2 need not be re-evaluated.The main calibration constant, GCAL1, is determined on the OrangevilleCalibration Line, 70 km north of Toronto. This line, which includes twoabsolute stations, was established and is maintained by the GeologicalSurvey of Canada. The section used for the CG-5 calibration and testingconsists of 5 stations spread over a distance of 70 km covering a gravityinterval of 106 mGal (recently modified to 119 mGal). Instruments aretransported by car, strapped onto the back seat with no special transport caseor fixtures.Readings are taken at each station on the way up the line and repeated on theway down. Readings of DUR = 60 seconds are taken. Earth tide correctionsare made automatically with the instrument's built-in program.A linear residual drift correction is applied, off line, on the basis of repeatreadings at station 1. The difference S ij , between drift corrected readings atstation 1 and station is then calculated. The measurement error for a gravitydifference is defined asE ij = k * S ij - g ijwhere g ij are the reference gravity differences and k is a constant (the scalecorrection factor) which is varied until the standard deviation of themeasurements errors is minimized. The new scale factor GCAL1' is thencalculated from the old value GCAL1 used during the calibration test asGCAL1'= k * GCAL1The accuracy of the GCAL1 determination is about 0.01% (0.01 mGalmeasurement error in 119 mGal gravity test range).The GCAL1 stability depends on the dimensional stability of the capacitivedisplacement transducer, and the stability of the internal D.C. referencevoltage. After the initial period of a few months, during which the GCAL1may change, due to the stress relaxation effects in the newly fused quartzsensor, by up to 0.1%, the drift rate of the scale factor GCAL1 is commonly1 to 2 ppm (parts per million) per day. After a few years, users requiring5-32CG-5 Manual - part # 867700 Revision 2