Australian Fundamental Gravity Network Absolute Gravity Survey 2015

Transcription

1 Australian Fundamental Gravity Network Absolute Gravity Survey 2015 Gravity Survey ID: GEOSCIENCE AUSTRALIA RECORD 2016/33 A. Nakamura, S. Buckerfield and P. Wynne Australian Fundamental Gravity Network Absolute Gravity Survey

2 Department of Industry, Innovation and Science Minister for Resources and Northern Australia: Senator the Hon Matthew Canavan Assistant Minister for Industry, Innovation and Science: The Hon Craig Laundy MP Secretary: Ms Glenys Beauchamp PSM Geoscience Australia Chief Executive Officer: Dr Chris Pigram This paper is published with the permission of the CEO, Geoscience Australia Commonwealth of Australia (Geoscience Australia) 2016 With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. ( Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision. Geoscience Australia is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please clientservices@ga.gov.au. ISSN X (PDF) ISBN (PDF) ecat Bibliographic reference: Nakamura, A., Buckerfield, S. and Wynne, P Australian Fundamental Gravity Network Absolute Gravity Survey Record 2016/33. Geoscience Australia, Canberra. 2 Australian Fundamental Gravity Network Absolute Gravity Survey 2015

3 1 Summary Geoscience Australia conducted an absolute gravity survey during April and May 2015 in order to maintain and update the Australian Fundamental Gravity Network (AFGN). During the 2015 AFGN field campaign 35 absolute gravity readings were taken with an A10 gravity meter out of which 29 were new additions to the network. Six of the readings were taken over older AFGN stations in order to update and validate existing values. The re-measures found the previous gravity values agreed with the new A10 measurements within their stated uncertainties. Two ties were made with the CG5 gravity meter from a newly established station in order to resolve discrepancies with existing gravity values. 30 pre-existing stations were checked for their condition during this survey and 5 stations were found to be destroyed. GPS readings were taken at existing stations and their locations updated in the database as many of the old stations had poorly defined locations. The survey commenced on 28 April 2015 and concluded on 21 May The locations and route are shown in Figure 1.1. Figure 1.1 AFGN survey 2015 route. The survey started in Canberra via road and returned by aircraft. The red line shows the ground component and the blue line indicates the air charter component of the travel. The AFGN station localities are displayed as stars with stations known to be destroyed indicated with red stars (as at March 2015). Australian Fundamental Gravity Network Absolute Gravity Survey

4 2 Introduction The Australian Fundamental Gravity Network (AFGN) is a set of permanently marked and documented gravity base stations which allow gravity surveyors to tie their surveys to a consistent datum. The datum for the early network was defined by gravity ties to Cambridge in England, using a pendulum apparatus (Dooley et al, 1961). Relative ties to other overseas gravity stations were made in later years to further define the datum prior to the establishment of the Isogal84 datum in The Isogal84 datum was constrained by ties to five absolute gravity sites established in Australia by a Soviet absolute gravimeter in 1979 (Arnautov et al, 1979). The AFGN consists of over a thousand documented gravity base stations located throughout Australia. Each of these stations has been measured to a greater precision than in routine exploration surveys. Until the early 2000s most of the stations have been measured using two or more relative gravity meters with measurements taken at the station on two or more occasions. In recent years, this network has been supplemented with stations measured using an A10 portable absolute gravity meter. Gravity measurements taken since 2003 using the A10 highlighted a discrepancy compared to the older measurements that defined the Isogal84 gravity datum. This discrepancy dictated the need for a new gravity datum and the AAGD07 datum was created by shifting the Isogal84 values by 0.78 µm s -2 (Tracey et al, 2007). This new datum is consistent with measurements taken with modern absolute gravimeters. In a review of absolute gravimetry over the preceding thirty years, Faller (2002) showed that the accuracy of absolute gravimeters improved by almost 3 orders of magnitude during that period. Therefore, it is essential to re-measure older AFGN stations with newer instrumentation. Absolute gravity surveys are conducted by Geoscience Australia on an annual or semi-annual basis. Priority areas are identified by proposed project areas defined by State and Territory governments. State and Territory geological surveys and gravity surveying companies also inform Geoscience Australia on the condition of gravity base stations. From time to time AFGN stations are destroyed through renovation or construction work and these stations need to be re-established for gravity surveyors to tie their surveys. Out of 16 requests for absolute gravity measurements 12 sites were completed during this survey. During this survey an A10 absolute gravity meter was used as the primary instrument in measuring the gravity at each location. A Scintrex CG5 relative gravity meter was used for the measurement of the vertical gravity gradient at each site location. Two ties were also measured with the CG5. The field campaign was split into two phases. The first phase was via road transport using a Hyundai iload van hired from Thrifty Car and Truck Rental. The road component of the survey was in Victoria, Tasmania and stations within Adelaide. This consisted of about 3,000 km of travel over 13 days which included the ferry crossings to and from Tasmania. From Adelaide a Cessna 404 twin engine aircraft was chartered from Corporate Air to access remote areas more efficiently. The air charter phase covered 9,400 km over 11 days (Figure 1.1). 4 Australian Fundamental Gravity Network Absolute Gravity Survey 2015

5 3 Method The A10 absolute gravity meter used during this survey is manufactured by Micro-g Lacoste and measures gravity using the free-fall method. A test mass is dropped inside an evacuated chamber and the trajectory of the falling test mass is determined by an interferometer. Optical interference fringes are generated by the falling mass as it travels distances corresponding with the laser wavelength. The position of the falling mass is measured as a function of time by counting and timing the optical fringes (Niebauer et al, 1995). The distance standard is provided by a polarisation-stabilised helium-neon (HeNe) laser with wavelengths at nm and nm. The laser mode is switched between each group of drops and the average of these sets is used to determine the final gravity value. A rubidium atomic clock is used to measure the time interval between the optical fringes. A series of time distance pairs are generated from the falling test mass and the data are fitted to a parabolic trajectory determined from the equation of motion of a falling body. The gravity value measured in this way is absolute as the calculation is purely metrological and relies on standards of length and time (Micro-g Lacoste, 2008). A was used during this survey and this instrument has been compared with results from FG5-237 at the Geoscience Australia gravity laboratory and found to be consistent. For this survey the A10 was set up to acquire between 100 and 180 drops per set depending on the local noise levels. A minimum of 8 sets were collected with most sites having collected 12 sets. Nearby a CG5 was used to measure the gravity gradient by taking a reading at ground level, a reading on top of a tripod and then repeating five to seven times. The vertical gravity gradient was used for adjusting the gravity measured by the A10 at m above the ground down to ground level. The gradient was also used in the equation of motion of the drops as the acceleration due to gravity increases over the distance of the drop (Niebauer et al, 1995). Figure 3.1 Left: CG5 gravity meter measuring the gravity gradient on top of a tripod. Right: The A10 absolute gravity meter set up for taking a reading. Australian Fundamental Gravity Network Absolute Gravity Survey

6 The two ties at Devonport and Oodnadatta were measured by first establishing the absolute gravity station as a reference and then conducting an ABABA tie with a CG5 in order to determine the gravity difference between the reference and the test station. In the case of Devonport the test station was on the air-side of the airport, on a sloping surface and in an exposed location, hence it was impractical to set up the A10 for measurement. At Oodnadatta the test station was not on a large enough concrete surface so the absolute gravity station was established on a more suitable surface several meters away and the ties made with the CG5 in order to determine the gravity at the test station. The test station was an Atlas Geophysics/South Australian Government survey mark which was a poorly constrained gravity base station with only an ABA tie to Coober Pedy. Figure 3.2 Taking CG5 tie readings at the test station at Oodnadatta airport. 6 Australian Fundamental Gravity Network Absolute Gravity Survey 2015

7 4 Processing 4.1 Field processing Micro-g LaCoste g version 9.1 software was used to visualise and process A10 data in the field. Data from each drop were graphed immediately, incorporated into a constantly updated average and scatter. This allowed the precision and accuracy of the measurements to be assessed and the acquisition parameters updated accordingly. The data were considered noisy if the drop scatter was greater than ±3 µm s -2 (±300 µgal) and appropriate adjustments were made to the number of drops per set (increased to 120 or 180), and if necessary the number of sets was also increased to yield greater statistics. Figure 4.1 Example of a dataset with low noise levels at Melbourne University. Top: Graph showing all drops for a single set and the distribution about the mean. Bottom: The points and error bars are added live to the graph as data are recorded. The red and the blue error bars indicate sets collected at alternating laser wavelengths. Australian Fundamental Gravity Network Absolute Gravity Survey

8 4.2 Post processing QuickTide Pro, a program that calculates earth tide and ocean loading corrections ( was used to correct the CG5 gravity data in order to ensure accurate tide corrections were applied at each site. The CG5 internal tide correction values were removed as the internal tide correction does not incorporate tidal ocean loading. ETGTAB tidal acceleration predictions with ocean loading were applied to the data. The A10 data were then reprocessed using the CG5 gradient data. The processing steps were: 1. Check the station GPS positions and make corrections if necessary 2. Calculate Earth tide corrections using ETGTAB with Ocean Loading using QuickTide 3. Remove the CG5 internal tide corrections and apply the ETGTAB with Ocean Loading tide corrections 4. Calculate the vertical gravity gradient using a linear least squares approach to minimise the effects of meter drift 5. Input the measured gradient and final polar motion coordinates into g v9.1 Earth orientation parameters were obtained from the International Earth Orientation and Reference System Service, accessible at: 6. Remove drops or sets affected by excessive noise and re-process the A10 data 5 Results The absolute gravity values from the 2015 AFGN survey are listed in Table 5.1 with their corresponding location information. The uncertainties in the position are about ±5 m as the locations were taken with a handheld GPS. Uncertainties in the gravity measurements were all within 0.11 µm s - 2. Six of the sites were re-measures of existing AFGN sites and the comparison of old and new gravity values are listed in Table 5.2. The average difference between the old and new readings were 0.12 µm s -2 and the standard deviation 0.26 µm s -2. This is in line with expectations given that the accuracy of the pre-a10 AFGN sites were estimated to be 0.3 µm s -2 (Tracey et al, 2007) and uncertainties in the Isogal84 values were determined to be 0.5 µm s -2 (Wellman et al, 1985). Generally the comparison of gravity values were better in Western Australia. This is likely to be a result of the AFGN surveys of 1991 and 1993 (Williams, 2000) which were conducted with Lacoste and Romberg gravity meters tied to measurements made by a JILA instrument by the U.S. Defence Mapping Agency. The vertical gravity gradient measurement is important in transferring the height at which the gravity is measured with the A10 (0.718 m above the ground) down to ground level. The gravity at ground level is required as the most common gravity surveying datum height is at the survey marker on the ground. Errors in determining the gravity gradient can introduce significant errors in the final absolute gravity value. A 0.1 µm s -2 m -1 error in the gravity gradient will produce an error of 0.7 µm s -2 in the final gravity value. Typically the error in the gradient is less than 0.05 µm s -2 m -1. The gravity gradients measured during the 2015 AFGN survey together with the uncertainties are listed in Table Australian Fundamental Gravity Network Absolute Gravity Survey 2015

9 Table 5.1 Gravity values and location information of sites measured during the AFGN 2015 survey. Station number State Location Latitude (GDA94) Longitude (GDA94) Gravity (µm s -2 ) NSW Ivanhoe Airport NT Hooker Creek Airport Terminal SA Adelaide University Mawson Building SA Adelaide Bonython Park maintenance shed SA Adelaide Burnside Rugby Club SA Adelaide Kensington Park Toilets SA Ceduna Airport Terminal SA Cowarie Station SA Kimba Airport monument SA Oodnadatta Airport SA Waikerie Gliding club house SA Yunta highway rest stop TAS Burnie Airport carpark TAS Devonport Airport terminal TAS Hobart Princes Wharf TAS Hobart Mount Nelson Primary School TAS Hobart Mount Nelson Top CS TAS Hobart University of Tasmania CS TAS Launceston Airport Terminal TAS Ouse Picnic Ground TAS Queenstown Motel TAS Smithton Airport Terminal TAS Strahan Information Centre VIC Horsham Weir picnic ground VIC Melbourne Ferntree Gully CS VIC Melbourne Ferny Creek Primary School CS VIC Melbourne University Earth Sciences Building WA Balgo Community Airstrip Terminal WA Coolgardie Oval WA Halls Creek Airport Terminal WA Kambalda Sports Ground WA Laverton Airport shed WA Paraburdoo Airport Terminal WA Wiluna Airport Terminal WA Zanthus Airstrip Australian Fundamental Gravity Network Absolute Gravity Survey

10 Table 5.2 Comparison of old and new gravity readings. The re-measures have an average difference of 0.12 µm s- 2 between the old readings and the readings taken during this survey. The ties were conducted using ABABA ties with absolute gravity readings taken during this survey. Old station number new station number Location previous g (µm s -2 ) new g (µm s -2 ) difference (µm s -2 ) Ties Ceduna Airport Terminal Hobart University of Tasmania CS Smithton Airport terminal Balgo Community Airstrip Terminal Halls Creek Airport Terminal Zanthus Airstrip (ATLAS) Devonport Airport terminal (Airside) Oodnadatta Airport Table 5.3 Vertical gravity gradients and errors of sites measured during the AFGN 2015 survey. The Adelaide Burnside Rugby Club station used the Kensington Park Toilets gravity gradient value due to the proximity. Station number Location Gravity gradient (µm s -2 m -1 ) Uncertainty (µm s -2 m -1 ) Ivanhoe Airport Hooker Creek Airport Terminal Adelaide University Mawson Building Adelaide Bonython Park maintenance shed Adelaide Kensington Park Toilets Ceduna Airport Terminal Cowarie Station Kimba Airport monument Oodnadatta Airport Waikerie Gliding club house Yunta highway rest stop Burnie Airport carpark Devonport Airport terminal Hobart Princes Wharf Hobart Mount Nelson Primary School Hobart Mount Nelson Top CS Hobart University of Tasmania CS Launceston Airport Terminal Ouse Picnic Ground Queenstown Motel Smithton Airport Terminal Australian Fundamental Gravity Network Absolute Gravity Survey 2015

11 Station number Location Gravity gradient (µm s -2 m -1 ) Uncertainty (µm s -2 m -1 ) Strahan Information Centre Horsham Weir picnic ground Melbourne Ferntree Gully CS Melbourne Ferny Creek Primary School CS Melbourne University Earth Sciences Building Balgo Community Airstrip Terminal Coolgardie Oval Halls Creek Airport Terminal Kambalda Sports Ground Laverton Airport shed Paraburdoo Airport Terminal Wiluna Airport Terminal Zanthus Airstrip Australian Fundamental Gravity Network Absolute Gravity Survey

12 6 Conclusion 35 absolute gravity readings and two ties were measured at various locations around Australia. These measurements have uncertainties of 0.11 µm s -2 or better. Six of these readings were taken over older AFGN stations and the gravity values have been updated with more reliable figures. The final results from this survey have all been added to the AFGN database. Absolute gravity stations are necessary for calibration, drift control and datum definition for relative gravity meter surveys. The 2015 AFGN survey further defined the AAGD07 datum by the creation of 29 new absolute gravity stations which included gravity meter calibration ranges. Further information and descriptions of individual Fundamental Gravity Network stations can be obtained via the AFGN Web Application. As many of these stations are visited infrequently, please forward any information about their status to minerals@ga.gov.au so that the station descriptions can be updated. 12 Australian Fundamental Gravity Network Absolute Gravity Survey 2015

13 References Arnautov, G.P., Boulanger, Yu.D., Karner, G.D., and Scheglov, S.N., 1979, Absolute determinations of gravity in Australia and Papua New Guinea during 1979: BMR Journal of Australian Geology and Geophysics, 4, Dooley, J.C., McCarthy, E., Keating, W.D., Maddern, C.A., and Williams, L.W., 1961, Pendulum measurements of gravity in Australia : Bureau of Mineral Resources, Australia, Bulletin 46. Faller, J.E., 2002, Thirty years of progress in absolute gravimetry: a scientific capability implemented by technological advances: Metrologia, 2002, 39, Micro-g Lacoste, 2008, A10 Portable Gravimeter User s Manual. Niebauer, T.M., Sasagawa, G.S., Faller, J.E., Hilt, R., and Klopping, F., 1995, A new generation of absolute gravimeters: Metrologia, 1995, 32, Tracey, R., Bacchin, M., Wynne, P AAGD07: A new absolute gravity datum for Australian gravity and new standards for the Australian National Gravity Database. ASEG Extended Abstracts 2007, 1-3. Wellman, P., Barlow, B. C., Murray, A. S Gravity base-station network values, Australia. BMR, Geology and Geophysics, Report 261. Williams, J.W., Australian fundamental gravity network 1991 and 1993 Absolute gravity measurements and Isogal station reconstruction Western Australia : operations report. Record 2000/039. Australian Geological Survey Organisation, Canberra. Australian Fundamental Gravity Network Absolute Gravity Survey

14 Appendix A Equipment and Personnel A.1 Gravity Meters Micro-g Lacoste A10 Serial number: 004 Scintrex CG5 Serial number: A.2 Transport Hyundai iload Cessna 404 Titan Figure A.1 Geoscience Australia staff measuring gravity at Cowarie Station. The Cessna 404 used for transport is in the background. A.3 Personnel Aki Nakamura Phillip Wynne Sarah Buckerfield Joffrey Linares-Robin (Pilot Corporate Air) 14 Australian Fundamental Gravity Network Absolute Gravity Survey 2015