Using Absolute Gravity Data to Measure Relative Gravity

Using Absolute Gravity Data to Measure Relative Gravity Differences for Instrument Comparisons Derek van Westrum, National Geodetic Survey NOAA, National Geodetic Survey, 325 Broadway, Boulder, CO 80305. derek. vanwestrum@noaa. gov EGU, April 2018, 18 -9967 Constraining Comparisons: Synthetic Data Modern Comparison Methodology Fit Function, Results, and Uncertainties • Five Imaginary Piers with Offsets: 0, 1, 2, 3, 4 µGal • Five Imaginary Gravimeters (ABCDE) with Biases: 0, 0, 0 µGal (no actual offsets in any instrument) • Gravimeters A through D measure Pier Offsets: 0, 1, 2, 3, on Piers 1 through 4, but do not measure on Pier 5 (i. e. they’re all “perfect”) • Gravimeter E measures Pier Offsets: -2, 1, 2, 3, 6 (i. e. , its average bias is actually 0 µGal). • With no a priori pier transfer constraints: • g 0 = 979 622 743. 1 µGal α = 0. 000047 µGal/day (17 nano. Gal/year) β = 2. 2 µGal (4. 4 µGal peak to peak signal) φ = 1. 6 days ω = 0. 0027 days (a period of 364. 8 days [!]) • Gravimeters A-D determined to have +0. 1 µGal biases • Gravimeter E determined to have -0. 4 µGal bias • Pier transfers estimated to be -0. 4, 1, 2, 3, and 6. 4 µGal With a priori pier transfer constraints imposed: • Gravimeters A-D determined to have 0 µGal biases • Gravimeter E also determined to have 0 µGal bias • Pier transfers correctly estimated to be 0, 1, 2, 3, and 4 µGal Constraining Comparisons: NACAG 16 Data Constraining Pier Transfers Pier Gravity Offset Uncert AG 756. 19 -0. 45 0. 31 AH • NACAG 16: 12 instruments in two sets of 6 over 4 days on 6 piers. 755. 58 -1. 07 0. 46 AI 763. 23 6. 58 0. 39 • AJ 765. 22 8. 58 0. 36 Agreement of instruments (standard deviation of biases) with no a priori pier transfer constraints: 1. 54 µGal. AN 759. 82 3. 17 0. 40 AO • 759. 65 3. 01 0. 34 AP Agreement of instruments with a priori pier transfer constraints imposed: 1. 51 µGal. 758. 84 2. 19 0. 45 AQ 756. 65 0. 00 0. 41 AS 754. 07 -2. 57 0. 30 AT 754. 45 -2. 19 0. 39 Conclusions • Pier-to-pier gravity differences can be determined with an accuracy of approximately 0. 5 µGal (e. g. either with FG 5 or CG 6 – but only with an FG 5 or CG 6? ). • Comparisons should be designed (scheduled) to ensure final pier gravity values have uncertainties at this 0. 5 µGal order – or constraints should be implemented • Time constraints, and/or sparse or poor observations can lead to network gaps in analysis – again, constraints should be implemented • Comparisons distributed in time and space can be “reduced” to a single value at a given time (the “Golden Pier”). • In short, a priori transfer constraints can significantly improve comparison results. Comparison with CG 6 Measurements Historical Absolute Data Three loops around the TMGO piers were performed with a Scintrex CG 6 gravity meter. After removing instrument drift, the following pier transfers were determined. Since November 2014, approximately weekly FG 5(X) observations have been collected at the ten Table Mountain Geophysical Observatory (TMGO) piers, north of Boulder, Colorado. TMGO Data Gravity - 979622000 u. Gal 760 755 750 745 740 735 730 сен-14 • • янв-16 июн-17 Date окт-18 A persistent offset is obvious between the results for each pier A periodic signal with a ~yearly period is also obvious (hydrology? ) AG AH AI AJ AN AO AP Series 8 AS AT мар-20 Note gradients must be used to transfer the gravity values to a common height. This uncertainty goes into the transfers. To minimize this, the CG 6 was located ~126 cm above the floor. Comparable to the 130 cm FG 5 measurement height. Pier FG 5 CG 6 Difference AG -0. 45 -0. 1 0. 4 AH -1. 07 -0. 5 0. 6 AI 6. 58 6. 7 0. 1 AJ 8. 58 8. 1 -0. 5 AN 3. 17 3. 1 -0. 1 AO 3. 01 2. 8 -0. 2 AP 2. 19 1. 7 -0. 5 AQ 0. 00 0. 0 AS -2. 57 -1. 6 1. 0 AT -2. 19 -1. 1 σ = 0. 53 µGal Acknowledgements A huge thanks to Dr. Jeff Kanney of DST Inc. for the CG 6 measurements and helpful discussion.