B UREAU OF MINERAL WESOUNCESu G -E0_.OGY AIND GEOPHYSICS

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1 ' rilikir PUBLICATIONS compacrus (LENDING secnom B UREAU OF MINERAL WESOUNCESu G -E0_.OGY AIND GEOPHYSICS RECORD RECORD i983/3 LACOSTE AND ROMBERG GRAVITY METERS: LABORATORY INVESTIGATIONS INTO THE EFFECTS OF CHANGES OF AIR PRESSURE, TEMPERATURE AND METER SUPPLY VOLTAGE. b y J.W The information contained in this report has been obtained by the Bureau of Mineral Resources, Geology and Geophysics as part of the policy of the Australian Government to assist in the exploration and development of mineral resources. It may not be published in any form or used in a company prospectus or statement without the permission in writing of the Director.

2 RECORD i983/3 LACOSTE AND ROMBERG GRAVITY METERS; LABORATORY INVESTIGATIONS INTO THE EFFECTS OF CHANGES OF AIR PRESSURE, TEMPERATURE AND METER SUPPLY VOLTAGE. by J.W. Williams

3 CONTENTS PAGE ABSTRACT INTRODUCTION VOLTAGE EFFECTS TEMPERATURE EFFECTS AIR PRESSURE EFFECTS^ 2 MAGNETIC EFFECTS^ 3 CONCLUSIONS^ 3 ACKNOWLEDGMENTS^ 4 REFERENCES Figure 1 to 9 Gravity meter supply voltage tests (a), Supply voltage variation with time. (b), Effect of supply voltage variation on meter reading. (c), Change in meter reading for supply voltage variation. Meters,G20, G101, G132, G252, G460, G518, G to 16 Gravity meter temperature tests (a)variation of air temperature with time. (b),effect of air temperature variation on meter reading. (c), Change in meter reading for air temperature variation. Neters,G20, G1.01, G132, G252, G460, G518, G to 18 Gravity meter pressure tests (a),variation of air pressure with time. (b), Effect of air pressure variation on meter reading, (c) Change in meter reading for air pressure variation. Meters G460, G510.

4 ABSTRACT The effects of air temperature, air pressure and supply voltage variations on LaCoste and Romberg gravity meters are small, and for normal regional and exploration type surveys can be ignored. The effect of a decreasing power supply voltage change was, for non-electronic meters, an increase in reading by approximately ipm s-2/v, and for electronic readout meters, a decrease in reading by about 0.01' 1.005pm s-21v The effect of air temperature differed between meters, however, the reading for the non-electronic meters, in general, decreased by approximately pm s-2pc and the electronic meters increased by approximately B.Ci ±.008pm s-2/*c. Air pressure effects on the electronic meters gave an increase in reading of approximately 4' pm s-2/pa.

5 INTRODUCTION There is little quantitative information on the effects of the outside environment on LaCoste and Romberg gravity meter readings. Although these effects are likely to be small, they may be important for high accuracy surveys such as the 1981 Southwest Australian Crustal Strain Gravity Survey (to detect gravity changes in earthquake areas) and the 1980 DMR National Control Station Survey. These surveys covered large areas in sub-optimal conditions, with large variations in climate, meter supply voltage and the intensity and frequency of meter Vibration. Tests have been conducted on seven LaCoste and Romberg gravity meters to try and determine what effects, if any, changes in supply voltage, air temperature, and pressure may have on the gravity readings. All tests were corrected for instrumental drift and earth tides. VOLTAGE EFFECTS. - During normal field operations, the LaCoste and Romburo gravity meter is supplied with power from a 12V 6Ah GelCell battery. A single battery can last for up to 12hours without recharging. The battery voltage at the start of the day is greater than i3 4 J, and is often below i2v buy the end of the day. The meters were tested using an HP 6114A 0-24 V precision power supply to vary the voltage supplied to the meter. The voltage was stepped between 10 and 14 V at S minute intervals as shown in Figures ia to 9a. Figures lb, c to 9b, c show the effects the varying voltage has on the reading. The maximum daily voltage effect, for tide corrected field readings, is an increase of approximately pm s-2/v for non-electronic meters, and a decrease of approximately 0.01 j=.00s pm s-21v for electronic meters. During the day a GelCell battery can decrease in voltage by about 2 V, this would cause an apparent meter drift of approximately 0.08 pm s-2. If Nicad batteries were used, then a battery discharge for the day would be only 0.3 V, giving a 0.01 pm s-2 gravity meter drift. These latter batteries should therefore be used for surveys requiring readings of high accuracy. TEMPERATURE EFFECTS While working in temperatures in excess of 40'C, the internal temperature of EIMR LaCoste and Romberg meters G132, G460, and G518 raise above the operating temperature near 58' C. Tests show that the two electronic meters (G460, GS18) overheat at a slightly lower ambient

6 temperature than non-electronic meters. The LaCoste and Romberg manufacturers state in a letter to BMR (i0/11/8i) that this may be because of a small amount of heat generated internally by the electronic circuitry. Tests were done to determine if there is a temperature effect on the meter's perfomance while operating over a normal ambient range. The meters were put into a chamber where measurments were taken for 2, i and 0.5 hours at 20 and 40 C, as indicated in figures i0a to isa. Meter G525 was measured for i, 2 and 3 hours as indicated in figure 16a. Gravity readings from meters G20, Gi0i, G132 and G252 were made optically, while the readings from meters G460,G518, and G525, which have electronic output's, were scaled off chart recordings. The temperature variations were designed to simulate temperature conditions during survey work using aicraft. For temperature changes of 20C, the maximum tine between tests of 2 hours was insufficient for the reading to equilibrate. Equilibration is estimated to take up to 10 hours. The amplitude of the temperature effects depends on the duration of the test, and the effect of non-equilibration results in a non-linear relationship between temperature and gravity meter reading. For an increasing temperature the non-electronic meters decreased in reading by a mean of pm s-2/'c. The electronic meters recorded on a chart recorder increased in reading by a mean of pm s-2/"c. The complex nature of the meter variations with temperature makes it difficult to determine the exact correction to be applied without further testing. Figures lb & c to 16b & c show the behaviour for each temperature test. AIR PRESSURE EFFECTS. The effects of air pressure changes on gravity readings -were also investigated. These changes could be caused by the effects of passing weather fronts, large altitude differences between gravity stations,and change of aircraft cabin pressure during flight. Electronic readout gravity meters G460 and G518 were in turn placed in a small chamber sealed to the concrete floor and the air pressure varied over 30 kpa by a vacuum pump (Figures 17a and 18a). Pressure was read on a micro barometer and the gravity output recorded on a chart recorder. Unlike the voltage and temperature tests, both meters increased in reading with a linear response to the decreasing air pressure.

7 For meter G460 the effect was pm 5-2/Pa (Figures 17b&c), and for meter G518 was pm s-2/pa ( Figures 18b4c). Because the chamber was sealed for these tests, the dial could not be adjusted. Beam movement could not be read with sufficient accuracy to make optical readings on either electronic or non-electronic meters. MAGNETIC EFFECTS. The Earths magnetic field has been known to effect the readings of some LaCoste and Romberg gravity meters. Meters G460 and G5l8 were tested in a large coil which allowed a normal, zero and reversed magnetic field to be applied to the meter without having to move the meter. Preliminary magnetic tests indicated an effect of less than.01 pm s-2 for a field reversal of approximately nT and - as the equipment was required for another purpose no further magnetic tests were-conducted. CONCLUSIONS Although the testing equipment was primitive it was shown that changes in voltage, temperature and air pressure have small effects on gravity meter readings. It is difficult to determine corrections for the effects of temperature because of the irregular drift induced by the temperature change. Other reports confirm this response (Gerstenecker Oft). The mean effect of and )Jm s-2/'c for non-electronic and electronically read meters respectively, agree in part with affects reported by Boedecker (1978) of pm s-2/'1] and are much smaller than that reported by Kanngieser (1982) of pm s-2/'c. Voltage effects of (0.04 pm s-2/v for meters read optically, agree with those reported by Boedecker(1978) of 0.04 pms-2/v. These effects can be reduced by monitoring voltage and maintaining fully charged cells IG the meter. For those surveys requiring greater battery voltage stability, Nicad batteries could be used, or batteries with a larger capacity. Pressure effects appear to be one order of magnitude less than those found in other reported tests (Boedeker, 1978), and may only represent the two

8 4 meters tested. To carry out tests on the other non-electronic meters requires a pressure chamber that allows adjustment to the meters for levelling and nulling. Although no magnetic effect was noted with the two meters tested, it _^has been shown elsewhere that there can be an effect on some_meters. Further testing of BMR meters is desirable if suitable equipment is available. ACKNOWLEDGEMENTS The drafting was by J.Rayner-Sharp. REFERENCES BOEDECKER, G., ^Instrumental Investigations and improvements of LaCoste and Romberg Gravity Meters. International Association of Geodesy,^International Gravity Commission 8th Meeting, Paris i2-16 September GERSTENECKER, C., A Model for studyind temperature effects on LaCoste and Romberg gravity meters. Bureau Gravimetrique International Bulletin d'information, 49. KANNGIESER, E., ^Investigations of calibration functions, temperature and transportation effects of LaCoste and Romberg gravimeters. Presented to Symposium No 4a, General heetinq of International Association of Geodesy, Tokyo,

9 (a) 14 (c) 3! Gravity chcmge (pm. s -2) Fig. 1 Gravity meter LCR G 20, voltage effect 2 0 (pm.e4) /i2 Fig.2 Gravity meter LCR G101, voltage effect RECORD 1983/3

10 (a) It 2 to 0-3 (b) 0 2 0^02^0.4 Grovity change (pm.s-2 ) Test not completed Oc7 Part Nol = > cs, 1 Time (N) -0.2 Fig. 3 Grovity meter LCR G132 (test I) (0) : >' 4!3-0!1^0^01 b) Grovtty charge (pm.s-2) Fig. 4 Gravity meter LCR G 132 (test 2) RECORD 1983/3

11 (c) -0-1^0 (ym.. -2 ) 02 Fig.5 Grovity meter LCR G 252 (test ) (c ) (b) 2^ -0-2^0 (pm.e - 2) /14 02 Fig.6 Gravity meter LC R G 252 (test 2) RECORD 1963/3

12 (a) 14 (c) g 12 (b) _01.2 (ym.0-2 ) 0^0.1 Fig.7 Grovity meter LCR G 460 (a) 14 (c) $ (b) ^0^0-2 (vm.'s -2 )^21 /15 Fig. 8 Gravity meter LC R G 518, voltage effect RECORD 1983/3

13 14 (0) (c) & 12!! o > 10 o (b) ,1 o Grayity chanq8 (pm.s-2 ) ()2.. g'- 2':' ~ ~ O~~~~~~~~~ ~~C ~ =F~ :: =l >~ o ~ 2 Time (h) 3 21/16 -()2 Fig.9 Grovity meter LCR G525, yolloq'e effect RECORD 1983/3

14 (b) -0.3^ 0^0-1 Gravity Orange (vm.s -2) Fig. 10 Gravity meter LC R G 20, temperature test Grovi ty change (,um.s 2 )^ 21/17 Fig. 11 Gravity meter L C R G101,1emperature test RECORD 1983/3

15 (a) ii%u (c) E0 (b) (pm.s -2 ) Fig. 12 Gravity meter LC R G 132, temperature test (a) Start 6 Teat 2 3 (pm.2) 21/18 Fig. 13 Gravity meter LC R G 252, temperature test RECORD 1963/3

16 Gravity chango (onto -2 ) Tido corrected gravity value Fig. 14 Gravity motor LCR G 460, temperature test -0-2^0^0.2 (ym.13-2 )^21/ Fig.15 Gravity meter LCR G 518, temperature effect (electronic meter) IIIICOAD 1908/3

17 ... \...,... ~... " ;... j ;<'- OO~----i-----~2-----J----~4~--~L---~6~----L---~8~----L-----1~0~---L----~12~<>~O (b) ' ". '. '.. C>2..' '. «(.. =-----~----~----~----~----~----~6~----L---~8=-----L-----1~0~---L----~12~(>~0 00 Ii! 4 (C) 40 Tim. (h) ~ 30 ~ :> ;; l20 E 0-00 Gravity chanq' ("m. - 2 ) /20 FIQ. 16 Gravity meter LC R G 525, temperature effect (electronic meter) RECORD 1983/3

18 . 90,000 a- e0,000 70,000 60,000 (b) (pm.s -2 ) 0-05 Fig. 17 Gravity meter LCR G 460, pressure test 100,000 (a) 90,000 eo,000-70,000 60,000 ^ 1 0^ BO (pm. s -2 ) 21/21 Fig. 18 Gravity meter LCR G 518, pressure test