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Quantifying the Differences Between Gravity Reduction Techniques CSIRO PUBLISHING Exploration Geophysics, 2018, 49, 735–743 https://doi.org/10.1071/EG17094 Quantifying the differences between gravity reduction techniques Philip Heath Geological Survey of South Australia, 4/101 Grenfell Street, Adelaide, SA 5000, Australia. Email: [email protected] Abstract. Gravity data processing (reduction) generally utilises the best-available formulae. New and improved formulae have been introduced over time and the resulting newly processed gravity will not match the old. Additionally, mistakes made in the gravity reduction process, as well as the incompatibilities between various equations, will inevitably lead to errors in the final product. This can mean that overlapping gravity surveys are often incompatible, leading to incorrect geological interpretations. In this paper I demonstrate the magnitude of change that results when different information is introduced at various stages of the gravity reduction process. I have focussed on differences relating to calibration factors, time zones and time changes, height, geodetic datums, gravity datums and the equations involved therein. The differences range from below the level of detection (0.01 mGal) to over 16.0 mGal. The results not only highlight the need to be diligent and thorough in processing gravity data, but also how it is necessary to document the steps taken when processing data. Without proper documentation, gravity surveys cannot be reprocessed should an error be identified. Key words: error, geodesy, gravity, processing, reduction. Received 28 July 2017, accepted 5 October 2017, published online 6 November 2017 Introduction Telford et al., 1990); however, these don’t contain comparisons A common product of Australian state and territory geological between what happens when equations of different vintage are surveys is a regional Bouguer gravity map. These images are used. Talwani (1998) discusses the errors involved in Bouguer compilations of multiple gravity surveys and it is an ongoing reduction, especially relating to the gravity effect of a spherical problem that overlapping surveys rarely match. Most commonly cap and Bullard B correction, which I have not replicated here. there appears to be an offset between the surveys, leading to A series of examples have been undertaken to illustrate mismatched points and anomalies in the final image. Numerous quantitatively how much difference is introduced at various attempts have been undertaken to create a smooth image of the stages of the gravity reduction process. Each demonstration gravity in South Australia using merging tools and different takes an element of the process, inserts realistic, but incorrect, gridding algorithms (e.g. Heath et al., 2012), but to date values and presents the results in a series of tables. a perfect image hasn’t been created. There are multiple reasons that surveys might mismatch. Method and results Different resolutions from different gravity meters as well as different precision and accuracy in elevation measurement Applying the scale factor techniques are two common issues. Another issue is how The scale factor (often referred to as a Calibration Factor; Heath, the data are processed. The process of gravity reduction is 2016) is a multiplication factor that must be applied to all raw straightforward; however, at each step of the process decisions gravity readings. A typical Scintrex CG5 gravity meter measures need to be made regarding which formulae to use, which gravity relative gravity values in mGal; however, the dynamic range of and geodetic datum is required and so on. This paper attempts the meter doesn’t match reality. As an illustrative example, to quantify some of the differences involved in using different two points on the Earth’s surface with the values 980 000 and equations. 980 100 mGal, respectively, have a difference of 100 mGal. In Multiple software packages exist (e.g. Intrepid Geophysics comparison, a gravity meter might measure 4000 and 4100.1 & Geosoft) that have the ability to process raw gravity data. mGal with a difference of 100.1 mGal. Therefore, a scaling factor In order to have full control over the reduction process I have must be calculated before all survey work by recording values at constructed a simple spreadsheet to undertake reduction in the the end points of a calibration range with a significant change field or in the office with minimal effort and to avoid software (typically over 50 mGal) in the gravity values. The scale factor (c) licensing issues. (The spreadsheet has been tested by undertaking is the ratio of the actual difference in gravity (Dga) to the measured gravity reduction using similar software and comparing results. difference in gravity (Dgm): The approach has allowed me to analyse the differences Dg involved at different stages of the gravity reduction process ¼ a : ð Þ c D 1 should erroneous information be input. It is freely available gm to anyone who requests it.) Raw gravity readings must be divided by the scale factor c. Many geophysical textbooks contain information regarding One potential source of error is using the inverse of the calibration the gravity reduction process (e.g. Blakely, 1995; Parasnis, 1962; factor (effectively multiplying instead of dividing by c). Journal compilation Ó ASEG 2018 Open Access CC BY-NC-ND www.publish.csiro.au/journals/eg 736 Exploration Geophysics P. Heath For a typical scale factor of 0.9997281, a typical raw reading of 7. Scale factor using best values only, treated as a gravity loop 3785.98 mGal becomes 3787.01 mGal if applied correctly and (from second ABA loop); and 3784.95 mGal if the inverse is used, creating a difference of 2.06 8. Scale factor using best values only, treated as a gravity loop mGal. The dynamic range of a Scintrex CG5 is 1000 to 9000 (ABABA). mGal, meaning this error could range from anywhere between For each calculation technique (listed 1 to 8 above) I’ve calculated 0.5 to 5.0 mGal (for the scale factor referred to here). the scale factor and the minimum and maximum corrected value of a CG5 gravity meter, assuming a dynamic range of 1000 to Methods to calculate c 9000 mGal. I’ve then calculated the difference between the values For the simplest approach, c can be calculated from two calculated from each scenario with the final scenario (which is measurements on the calibration range. It can also be calculated arguably the most accurate). The differences range from 0.01 to from an ABABA-type loop between two points A and B. In both ~1.00 mGal. Table 2 shows these results. cases the processor must decide to assume either a linear drift in Scenario 3 produces the least difference compared to Scenario the measurements, or to undertake a more thorough calculation to 8. Scenario 4 is effectively the same as Scenario 3, as it uses the determine the scale factor. Furthermore, the processor may have same method, just at a different time. Most of the scenarios multiple readings to choose from. If a CG5 is being used, does the produce a maximum difference in the range of 0.1 to 0.2 processor choose the readings with the lowest standard deviation mGal, with the greatest difference (1.0 mGal) resulting from (s.d.) or should they average them all? A single measurement on Scenario 2, where only two values are used to calculate the a CG5 gravity meter is actually an average of a series of readings; scale factor. hence, s.d. is available for each measurement. The following To illustrate how different scale factors affect a gravity survey, demonstration shows the effects of computing different scale consider the Coompana gravity survey (Allpike, 2017). The factors from the same set of observations. dynamic range of the survey is –64.59 to 19.20 mGal, a total Table 1 shows the raw data collected from a gravity calibration of ~84 mGal. This survey used three CG5 gravity meters, one run between the Kensington Gardens Sportsfield and Norton measuring at ~2500 mGals, one at 4000 mGals and one at 5000 Summit Cemetery, South Australia. mGals. I’ll call these gravity meters 1, 2 and 3, respectively, and Eight scenarios are considered, each a different way to I designate each gravity meter with its own scale factor from the calculate the calibration factor. The scenarios are: scenarios above. I assign gravity meter 1 the scale factor from 1. Scale factor using two ‘best’ values only (from first A and B scenario 2, gravity meter 2 the scale factor from scenario 5, and readings); gravity meter 3 the scale factor from scenario 8. If gravity meter 1 2. Scale factor using two average values (from first A and B measures values from 2458 to 2542 mGal (84 mGal around – readings); 2500 mGal), the scaled values become 2456.954- 2540.921 3. Scale factor using average values, assuming linear drift (from mGal, a difference of 83.964 mGal. Performing the same fi first ABA loop); operation for gravity meters 2 and 3, we nd scaled values of 4. Scale factor using average values, assuming linear drift (from 83.972 and 83.973 mGal, respectively. The maximum difference second ABA loop); here is 0.009 mGal, just under the 0.01 mGal level of detection. 5. Scale factor using average values, assuming linear drift (ABABA); Scale factor gravity datum 6. Scale factor using best values only, treated as a gravity loop The scale factor will also be different depending on the gravity (from first ABA loop); datum used for the known values in the calculation. Assuming Table 1. Raw readings taken from a scale factor run between Kensington Gardens (A) and Norton Summit Cemetery (B) using the CG5 gravity meter, serial number 050 800 135. ABA B A Reading s.d. Time Reading s.d.
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