A model for adjustment of differential gravity measurements with simultaneous gravimeter calibration

 A mathematical model is proposed for adjustment of differential or relative gravity measurements, involving simultaneously instrumental readings, coefficients of the calibration function, and gravity values of selected base stations. Tests were performed with LaCoste and Romberg model G gravimeter measurements for a set of base stations located along a north–south line with 1750 mGal gravity range. This line was linked to nine control stations, where absolute gravity values had been determined by the free-fall method, with an accuracy better than 10 μGal. The model shows good consistence and stability. Results show the possibility of improving the calibration functions of gravimeters, as well as a better estimation of the gravity values, due to the flexibility admitted to the values of the calibration coefficients. Received: 15 November 1999 / Accepted: 31 October 2000     

iGrav-007超导重力仪格值的精密测定

iGrav超导重力仪是当前世界上最新型的便携式相对重力仪,可提供最稳定和最高精度的连续相对重力测量。利用武汉九峰台站FG5-112绝对重力仪与iGrav-007超导重力仪连续3天的同址观测结果,基于最小二乘线性回归和迭代算法,精密确定iGrav-007的格值。数据处理结果表明,iGrav-007的格值为(-91.640 2±0.085 2)×10-8 m·s-2/V,相对标定精度为0.092 9%,连续1天的FG5绝对重力观测获得的格值精度优于0.2%,连续3天的FG5绝对重力观测获得的格值精度优于0.1%。     

Establishment of precise gravity network of airport stations in India

A precise gravity network of thirty-five stations based on the first order gravity station at Palam airport, New Delhi (979.13433 gals—University of Wisconsin 1969 value) was established during April–June 1971, covering the entire country, in order to use them as reference bases for any future gravity surveys in India with a repeatability of ±0.05 mgal or less. The instrument, a LaCoste-Romberg geodetic gravimeter No. G-84, was transported by air over the network of airport stations embracing Trivandrum in the south, Srinagar in the north, Bombay in the west and Mohanbari in the east. The four airport stations in New Delhi, Calcutta, Madras and Bombay which were more precisely established by a large number of repeat observations were utilised as base stations for facilitating easy occupation of the remaining thirty-one stations within their respective zones. The observations were reduced by procedure which permits automatic removal of instrumental drift from the observed readings. According to the depicted drift curve, the instrumental drift though comparatively small, is found not exactly linear due to the possible tare effect observed at the initial stage and also the resulting creep drift that might have been developed during transportation of the gravimeter by air. The final results along with their probable errors of the order of ±0.01 mgal for base stations and ±0.03 mgal for other stations relative to the adopted value at Palam airport, are given in Table 1. Fourteen of the sites occupied are reoccupations of stations already established by the University of Wisconsin in 1963, and the results of the old and the new measurements as given in Table 2, are in remarkable agreement, which ensures the correctness of the calibration factors of the present instrument relative to that of the Wollard's LaCoste-Romberg gravimeter No. G-1-A actually employed in the 1963 measurements.     

精密重力测量中相对重力仪格值系数的贝叶斯估计方法

相对重力仪的格值系数随时间会发生微小的变化,是影响精密重力测量精度的重要因素。通常需定期对相对重力仪进行专门的基线标定来评估仪器格值系数的变化。本文提出了一种利用重力观测数据进行格值系数评估的新方法,原理是利用测网中已知的多个绝对重力基准点作为先验约束,同时考虑仪器的非线性漂移变化,将格值系数作为超参数,基于贝叶斯原理和赤池贝叶斯信息准则（ABIC）估计最优值。通过对模拟数据的测试,该方法在高斯噪声和仪器非线性漂移等不确定性存在的情况下,可以获得格值系数的准确估计结果。对实测重力数据的测试表明：估计的格值系数与测量前在基线场标定的格值系数差值在5×10^-5以内;而且相较于采用标定不准确的格值系数,该方法可以获得与绝对重力测量结果差异更小的平差重力值。本文研究结果为有效提高精密重力测量的效率和精度提供了方法保障。     

Precise tidal gravity recorded with superconducting gravimeters at stations Wuhan (China) and Kyoto (Japan)

 Three long series of tidal gravity observations, totalizing approximately 24 years and recorded with three superconducting gravimeters, T004, T008, and T009, at stations Wuhan (China) and Kyoto (Japan), are studied. The tidal amplitude factors and phase differences are determined precisely using Eterna and Nsv techniques. The precision of the main tidal amplitudes is at the same level of 0.01 μGal. The atmospheric gravity signals are corrected using the coefficients determined with a regression method between tidal gravity residual and station air pressure. The oceanic gravity signals are modeled based on five global oceanic models. It is found that the oceanic models developed by the analysis of measurements from Topex/Poseidon altimeters have the best fit to the superconducting gravimeter measurements, since the observed residuals and the discrepancies between the amplitude factors and the theoretical tidal models are reduced more significantly. The long-period gravity variations are dominated by the non-linear drift phenomena of the instruments, and the short-term variations in gravity are due to the background noise at the stations. Received: 20 January 2000 / Accepted: 15 September 2000     

Comparison of the Graf sea gravimeter with the Vening Meinesz apparatus on board the submarineU.S.S. Becuna

Measurements of gravity were made on boardU.S.S. Becuna (SS 319) with a Vening Meinesz pendulum apparatus and a Graf sea gravimeter at approximately the same time. Comparison of data uncorrected for depth of submergence, E?tvos correction and second order effects of horizontal acceleration showed that there was a change related to time of observation. These corrections were not made as they would be the same for both sets of observations, and no provision had yet been made to take care of the horizontal accelerations for the Graf sea gravimeter. The variation with otime could be caused by instrumental drift or scale calibration. After removal of this effect by visually fitting the data trend with a straight line, there were three observations with large discrepancies, seventeen with discrepancies of 3–9 mgal and thirty-nine with discrepancies of 0–3 mgal, showing close approach to a statistical distribution. The three large discrepancies may be dismissed because of very bad depth control during the observations. The discrepancies 3–9 mgals are larger than expected and perhaps are attributable to depth control and inadequate observation time for the Graf sea gravimeter. It is concluded that the Graf sea gravimeter shows great promise for use on a submarine. An apparatus to take into account the horizontal acceleration effects must be added, and suitable drift characteristic obtained. Despite its advantages of ruggedness, ease of operation, ease of data reduction, reduced size and weight, many more comparisons of the Graf sea gravimeter with the Vening Meinesz pendulum equipment should be made before the latter are displaced Probably the Graf sea gravimeter will be usable on a stable platform on a surface vessel at least in calm sea states.     

A new comprison of the Italian conventional' mgal and the German pendulum values

With the small dials of two Worden gravity meters, calibrated on the Italian ‘conventional’ basis, a comparison was completed in 1956 with the German pendulum stations. The result shows a difference in the slope of −0·5%. The stations in Germany of the European calibration line line have been checked.

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Riassunto Con due gravimetri Worden, tarati sulla base italiana, é stato completato nel 1956 il confronto con le misure pendolari dell'Istituto Geodetico di Potsdam, impiegando solo le viti piccole. Ne é risultata una differenza sistematica di −0·5%. Sono state pure occupate le stazioni facenti parte del tratto tedesco sulla base di taratura europea.

     

Cross-Coupling Correction for LaCoste ＆ Romberg Airborne Gravimeter

Abstract The cross-coupling corrections for the LaCoste ＆ Romberg airborne gravimeter are computed as a linear combination of 5 so-called cross-coupling monitors. The weight factors （coefficients） determined from marine gravity data by the factory are obviously not optimal for airborne application. These coefficients are recalibrated by minimizing the difference between airborne data and upward continued surface data （external calibration） and by minimizing the errors at line crossings （internal calibration） respectively. An integrating method to recalibrate the above-mentioned coefficients and the beam scale factor simultaneously is also presented. Experimental results show that the systemic errors in the airborne gravity anomalies can be greatly reduced by using any of the recalibrated coefficients. For example, the systemic error is reduced from 4.8 mGal to 1.8 mGal in Datong test.     

国产零长弹簧原理动态重力仪首次飞行试验及精度评估

CHZ-Ⅱ重力仪是首套完全国产零长弹簧原理航空重力仪,2018年4月在陕西渭南地区进行了首次飞行试验,共完成4个架次24条测线的有效飞行,标志着我国航空重力仪在自主研发的道路上又取得了长足的进步。利用飞行地区地面重力数据对CHZ-Ⅱ重力仪的测线扰动重力和格网扰动重力数据进行精度评估,其中空中测线在10 km分辨率条件下,精度达到1 mGal。采用地形辅助法对测量形成的5'测格网重力数据进行向下延拓,经延拓至地面后精度优于5 mGal,基本满足平原地区的测量需求。     

L&R海空重力仪测量误差综合补偿方法

为了削弱各类误差源的影响,提出了一种两阶段误差综合补偿方法:第一阶段采用相关分析法对仪器厂家标定的交叉耦合改正(CC改正)的不足进行修正;第二阶段采用测线网平差对各类剩余误差的综合影响进行补偿。实际观测数据处理结果验证了该方法的有效性和可靠性。     

南极拉斯曼丘陵重力基准的建立

2008～2009年南极夏季期间,中国第25次南极科学考察队利用A-10便携式绝对重力仪和LaCoste＆Romberg G相对重力仪在南极中山站及附近拉斯曼丘陵地区建立了高精度重力基准网。该网由3个绝对重力点和10个相对重力点组成,其绝对和相对重力测量的精度分别优于7.5×10^-8 m·s^-2、20×10^-8 m·s^-2。     

北京和南宁国际绝对重力基本点上首批绝对重力测量结果

本文简要介绍了北京和南宁两个国际绝对重力基本网（IAGBN）点的点位和环境条件，列出了这些点上的首批绝对重力测量结果及所使用的仪器。在北京点的测量结果表明，不同仪器的测量结果间差异很小。将IAGBN点的1990年观测值通过换算后求得了同一城市的我国1985重力基本网（85网）点的重力值，与1981年意大利绝对重力仪IMGC的测量结果和85网的平差值作了比较。     

Scale factor calibration of a superconducting gravimeter at Esashi Station, Japan, using absolute gravity measurements

The scale factor of a superconducting gravimeter (SG) at the Esashi Earth Tides Station, Japan, was revised by repeating co-located absolute gravity measurements with an FG5 gravimeter. Although the calibration results from the absolute gravimeter (AG) show an apparent secular change in the scale factor of the SG (0.4% for the period 1993–2002), the relative scale factors, which are determined by tidal analysis with the response method, indicate that it has changed by no more than 0.01% during the above period. If the mean scale factor over the 10 years is adopted, a value of –56.082±0.029 Gal/V (1 Gal =10^–8 m s^–2) is obtained, which is about 0.4% smaller than that used in the global geodynamics project (GGP) database. Based on this newly determined scale factor, the tidal gravity factors at Esashi have been re-estimated. The observed tidal factors, corrected for the ocean tide effects with recent models, indicate that the theoretical gravity factors for an inelastic Earth model are more consistent with the observations than are those for an elastic model.     

我国重力基本网若干技术问题

本文介绍我国新的国家重力基本网1985系统(新网)。新网由57个点组成,其中包括6个绝对重力点,用拉科斯特·隆贝格(LCR—G)重力仪进行相对观测,并与国外多个已知重力基准系统作了联测,共获得各类观测数据近万个。新网重力值平均中误差为±8微伽[10~(-8)ms~(-2)],经外部检核,实际精度达20微伽。1985年通过国家鉴定。经过两年来使用证明,新网达到设计要求,具有国际先进水平。本文还讨论了重力网平差、我国旧网精度及转换、LCR—G重力仪的特性和国际重力基准等问题。     

Combination of temporal gravity variations resulting from superconducting gravimeter (SG) recordings, GRACE satellite observations and global hydrology models

Gravity recovery and climate experiment (GRACE)-derived temporal gravity variations can be resolved within the μgal (10^?8 m/s ²) range, if we restrict the spatial resolution to a half-wavelength of about 1,500 km and the temporal resolution to 1 month. For independent validations, a comparison with ground gravity measurements is of fundamental interest. For this purpose, data from selected superconducting gravimeter (SG) stations forming the Global Geodynamics Project (GGP) network are used. For comparison, GRACE and SG data sets are reduced for the same known gravity effects due to Earth and ocean tides, pole tide and atmosphere. In contrast to GRACE, the SG also measures gravity changes due to load-induced height variations, whereas the satellite-derived models do not contain this effect. For a solid spherical harmonic decomposition of the gravity field, this load effect can be modelled using degree-dependent load Love numbers, and this effect is added to the satellite-derived models. After reduction of the known gravity effects from both data sets, the remaining part can mainly be assumed to represent mass changes in terrestrial water storage. Therefore, gravity variations derived from global hydrological models are applied to verify the SG and GRACE results. Conversely, the hydrology models can be checked by gravity variations determined from GRACE and SG observations. Such a comparison shows quite a good agreement between gravity variation derived from SG, GRACE and hydrology models, which lie within their estimated error limits for most of the studied SG locations. It is shown that the SG gravity variations (point measurements) are representative for a large area within the accuracy, if local gravity effects are removed. The individual discrepancies between SG, GRACE and hydrology models may give hints for further investigations of each data series.     

海空重力测量关键技术指标体系论证与评估

技术规程是开展海空重力测量作业的重要依据。针对我国现行海空重力测量规范或标准缺乏现势性的问题,开展了海空重力测量测线布设密度、测量精度、空间分辨率、海空重力仪零点漂移与动态重复性等关键性指标分析和论证,提出了由测点重力中误差、系统差和平均误差3个指标组成的测量精度评估体系,以及由格值标定相对精度、零点月漂移量、月漂移非线性变化中误差和月漂移非线性变化限差4个指标组成的海空重力仪稳定性评估体系,给出了相关技术指标的验证和评估方法,同时对涉及船载重力测量测点归算、航空重力测量厄特沃什改正、测量平台倾斜改正及海空重力测量精度评估等关键性数学模型进行了分析和改进,旨在为下一步启动军民融合海空重力测量作业规程编制工作提供技术支撑。