Calibration shifts in Scintrex CG‐3M gravimeters with an application to detection of microgravity changes at Iwo‐tou caldera,Japan

Calibration shift seriously influences gravity values measured using Scintrex CG‐3M gravimeters. We calibrated three Scintrex CG‐3M gravimeters three times (1999, 2003 and 2006) over eight years, using a calibration line with a gravity difference of 1.38 Gal. The scale factor correction coefficients (calibration factors) obtained here range from 0.9998–1.0005. The calibration factors vary with time by 89 ppm, ?102 ppm and ?126 ppm between the 1999–2003 surveys. The calibration shifts of two of the three gravimeters decreased to about 20 ppm or less in the second interval, the other remained about the same (142 ppm). The results indicate that they shifted at rates on the order of 10 ppm/year even several years after manufacturing. The large shift in calibration factors indicates that they must be corrected using calibrations done before and after the measurements to perform microgravity measurements when gravity differences between a reference gravity site and survey sites are on the order of a hundred milligals (mGal) or more. The results also indicate that the calibration factors change gradually with time, so their interpolation provides a good practical approximation for a specific survey time. We applied the time‐dependent calibration factors to microgravity monitoring at the Iwo‐tou caldera, Japan, where the gravity difference between the base site on the island and the reference site on Honshu (the mainland of Japan) is about 870 mGal mainly due to the 11° latitude difference. Gravity surveys were conducted every two years from 1998–2006. The correction of scale factors estimated from the repeated calibration surveys leads to satisfactory measurements, in which the average of the absolute differences between two Scintrex CG‐3M instruments in five surveys is reduced from 207μGal to 19μGal; for three of those surveys, it is less than 10 μGal. This result demonstrates the importance of repeated calibration surveys.     

Using ground gravity to improve ice mass change estimation from GOCE gravity gradients in mid-west Greenland

Vertical gravity gradient anomalies from the Gravity and steady-state Ocean Circulation Explorer (GOCE) DIR-3 model have been used to determine gravity anomalies in mid-west Greenland by using Least-Squares Collocation (LSC) and the Reduced Point Mass (RPM) method. The two methods give nearly identical results. However, compared to LSC, the RPM method needs less computational time as the number of equations to be solved in LSC equals the number of observations. The advantage of the LSC, however, is the acquired error estimates. The observation periods are winter 2009 and summer 2012. In order to enhance the accuracy of the calculated gravity anomalies, ground gravity data from West Greenland is used over locations where the gravity change resulting from ice mass changes is negligible, i.e. over solid rock. In the period considered, the gravity anomaly change due to changes in ice mass varies from ?5 mGal to 4 mGal. It is negative over the outlet glacier Jacobshavn Isbræ, where the mass loss corresponds to a gravity change of approximately ?4 mGal. When using only GOCE vertical gravity gradients, the error estimates range from 5 mGal at the coast to 17 mGal over the ice sheet. Introducing the ground gravity data from West Greenland in the prediction reduces the errors to range from 2 to 10 mGal.     

Absolute airborne gravimetry: a feasibility study

We report here the results obtained during a feasibility study that was pursued in order to evaluate the performances of absolute airborne gravimetry. In contrast to relative systems, which use spring‐type gravimeters, each measurement acquired by absolute systems is independent from the others and the instrument is not suffering from problems like instrumental drift, frequency response of the spring and variation of the calibration factor. After a validation of the dynamic performance of the experimental setup in a moving truck, a comparison between the experimental airborne data retrieved over the Swiss Alps and those obtained by ground upward continuation at flight altitude allow us to state that airborne absolute gravimetry is feasible. The first test flight shows a spatial resolution comparable to those obtained by relative airborne gravimetry. For a wavelength on the order of 12 km the absolute value of gravity can be evaluated with an uncertainty of 6.9 mGal.     

Gravity control network at airports in nigeria

A network of gravity of control bases located at most of the airports and airfields in Nigeria has been established as a means of controlling future gravimetric studies in Nigeriaand of integrating existing gravity surveys into a unified whole for the purpose of producing gravity anomaly maps for the country. The network will also provide a means of checking and adjusting gravimeter calibration differences to a common standard.The LaCoste and Romberg gravimeters, models G446, G464 and G468, used for the observations were transported between the base stations by means of a six-seater fixed-wing aircraft. The scale adopted of the network was the same as that of gravimeter G464 to which the other two meters were referenced. The absolute gravity values observed ranged from 978184.882 ± 0.014 mGal at Nguru to 977844.379 ± 0.020 mGal at Jos, yielding a gravity range of 340.503 mGal. The accuracy of the individual absolute gravity value is better than 0.030 mGal using a datum value of 978120.933 mGal for site 103401 estlished at the Kano International Airport. The 34 observation sites are illustrated with sufficient detail to permit reoccupation within 0.50 m to 1.0 m.     

Interpretation of 1992–1994 Gravity Changes around Mayon Volcano,Philippines, Using Point Sources

Significant gravity changes observed around the Mayon Volcano (Philippines) between 1992 and 1994 at 26 stations are interpreted in terms of an increase of mass and pressure changes at several point sources modelled using a fast inversion process. This inversion approach attempts to fit gravity and elevation changes by combining a random search for the positions of the sources and a linear least-squares fit for the incremental mass, pressure and possible common regional values for gravity or elevation changes. Some stabilizer terms are included in the misfit function. Models with one and two sources were tested against the observed changes at Mayon. Models with only one-source give a best fit for a shallow source with a positive mass increment, horizontally displaced far from the summit. The study using two sources gives a best fit that is similar to the one-source model, but in addition indicates anomalous behavior at stations in the SW. Neglecting the stations located southward from a local fracture, the best-fitting model suggests one central positive mass change source, which is likely to be an intrusion of about 0.5 MU with a depth of about 5 km beneath the volcano. Standard deviation for the residuals ranges from 7–8 μGal for one-source models to 6–7 μGal for models with two sources. Both of the cases are below the error value of 9.4 μGal estimated for the gravity data, so that it is not possible to discriminate between both possible interpretations without additional information.     

捷联式航空重力测量算法比较

捷联式航空重力测量系统与平台式系统相比具有体积小、重量轻、功耗低等许多优点,近些年来取得了显著的研究进展.本文给出了捷联式航空重力测量的两种算法模型:捷联式惯性标量重力测量(SISG)和旋转不变式标量重力测量(RISG)模型,并对其误差模型作了初步讨论.利用我国首套捷联式航空重力仪SGA-WZ01在某海域的部分试验数据,对两种算法模型进行了比较分析,表明其差值之标准差对于200s的滤波长度小于0.5mGal.同时,利用两组重复测线数据估算了不同滤波尺度下的两种算法的内符合精度,表明SISG算法略优于RISG算法.对于200s和300s的滤波长度,SISG的内符合精度分别为1.06mGal和0.80mGal.     

中国新的重力基本网

我国旧重力基本网建于1957年,约含有13.5mGal的基准误差和0.2‰的尺度误差,现已被新的国家重力基本网1985系统（新网）取代。新网由57个点组成,其中有6个绝对重力点。整体平差后点重力值平均中误差为±7.8μGal。经外部检核,实际精度达20μGal,经过实际使用,证明估计精度正确。本文讨论了国际重力基准、LCR-G重力仪特性、高精度重力网平差及我国旧网的实际精度及其转换等问题。     

Gravity changes and natural gas extraction in Groningen

Gravity observations made over the Groningen gasfield from 1978 onwards are available. The existing gravity data were reanalysed and a new survey was carried out. The observed gravity changes obtained during four surveys, spanning an 18-year period, were compared with the gravity effect due to natural gas extraction computed from the reservoir model and the production data. The random error in the gravity values is small enough to detect the effect of gas extraction after a few years (up to 2 μgal/year). The trends obtained from observed and calculated gravity changes agree well within their expected error margins after statistical data snooping. Due to the inadequate measurement set-up of the previous surveys, systematic errors present in the gravity data hampered the use of this data for refinement of the reservoir and the production models. However, with improved gravity monitoring, in particular a very systematic network set-up and well-calibrated instruments, this should now be feasible. By using the network analysis from the 1996 measurements for the planning of a network in space and time, most of the current problems can be avoided.     

张家口地震台连续重力异常分析

张家口地震台gPhone连续重力观测仪潮汐因子自2016年6月出现持续下降的趋势变化,累计变化量达到7×10-3.分析重力残差曲线,异常出现时同期残差数据未出现同步变化;将去零漂前后潮汐因子的变化情况进行对比分析,发现潮汐因子变化趋势一致,排除仪器调零影响;针对新建楼房可能引起的重力变化,建立了重力正演模型,计算结果显...     

Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×10⁹m³) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×10⁹m³ of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997     

Error sources and data limitations for the prediction of surface gravity: a case study using benchmarks

Gravity-based heights require gravity values at levelled benchmarks (BMs), which sometimes have to be predicted from surrounding observations. We use the Earth Gravitational Model 2008 (EGM2008) and the Australian National Gravity Database (ANGD) as examples of model and terrestrial observed data respectively to predict gravity at Australian National Levelling Network (ANLN) BMs. The aim is to quantify errors that may propagate into the predicted BM gravity values and then into gravimetric height corrections (HCs). Our results indicate that an approximate ±1 arc-min horizontal position error of the BMs causes maximum errors in EGM2008 BM gravity of ~22 mGal (~55 mm in the HC at ~2200 m elevation) and ~18 mGal for ANGD BM gravity because the values are not computed at the true location of the BM. We use RTM (residual terrain modelling) techniques to show that ~50% of EGM2008 BM gravity error in a moderately mountainous region can be accounted for by signal omission. Non-representative sampling of ANGD gravity in this region may cause errors of up to 50 mGals (~120 mm for the Helmert orthometric correction at ~2200 m elevation). For modelled gravity at BMs to be viable, levelling networks need horizontal BM positions accurate to a few metres, while RTM techniques can be used to reduce signal omission error. Unrepresentative gravity sampling in mountains can be remedied by denser and more representative re-surveys, and/or gravity can be forward modelled into regions of sparser gravity.     

Gravity change observed in a local gravity network and its implication to seasonal precipitation in Dali county,Yunnan province,China

This study investigates data-processing methods and examines the precipitation effect on gravity measurements at the Dali gravity network, established in 2005. High-quality gravity data were collected during four measurement campaigns. To use the gravity data validly, some geophysical corrections must be considered carefully. We first discuss data-processing methods using weighted least-squares adjustment with the constraint of the absolute gravity datum. Results indicate that the gravity precision can be improved if all absolute gravity data are used as constraints and if calibration functions of relative gravimeters are modeled within the observation function. Using this data-processing scheme, the mean point gravity precision is better than 12 μgal. After determining the best data-processing scheme, we then process the gravity data obtained in the four measurement campaigns, and obtain gravity changes in three time periods. Results show that the gravity has a remarkable change of more than 50 μgal in the first time period from Apr–May of 2005 to Aug–Sept of 2007. To interpret the large gravity change, a mean water mass change (0.6 m in height) is assumed in the ETOPO1 topographic model. Calculations of the precipitation effect on gravity show that it can reach the same order of the observed gravity change. It is regarded as a main source of the remarkable gravity change in the Dali gravity network, suggesting that the precipitation effect on gravity measurements must be considered carefully.     

中国东西重力潮汐剖面

为了检验体潮与海潮的理论模型,分析了中国东西重力潮汐剖面（1981年9月-1985年1月）。同时,为研究LaCoste ET-20和ET-21重力仪的格值系统,建立了一条由17台LaCoste G型和2台LaCoste D型重力仪观测的重力垂直基线。在基线上标定的结果表明,ET-21重力仪的格值大了1％。由标定得到的格值计算剖面上各测站的潮汐因子,经海潮改正后,接近Wahr模型值,振幅因子的残差:O₁波小于0.3μGal,M₂波小于0.4μGal。但是上海和拉萨的观测经海潮改正后,相位迟后有很大的改善,振幅因子却更偏离于模型值,其潮汐异常主要是近海的海潮模型不完善,以及在海潮计算中,所采用的地球模型未考虑地壳与上地幔的横向不均匀性所引起。     

Gravitational effect of distant Earth relief within the territory of former Czechoslovakia

We analyzed the gravitational effect of topography and bathymetry beyond the angular distance of approximately 1.5 degrees (referred to as the distant relief effect or DRE), and its impact on measured gravity values in the region of the former Czechoslovakia. Our work was strongly motivated by the contents of the pioneering contribution of outstanding Czech geophysicists Miloš Pick, Jan Pícha and Vincenc Vyskočil, which appeared at the turn of the 1950’s and 1960’s. Our numerical calculations were based upon the direct evaluation of the gravitational effects of compartments of a spherical layer, while the respective heights and depths were obtained from the 2 × 2 minutes digital elevation model (DEM) ETOPO2, taking into consideration also the influence of distant bathymetry. Our results are in close agreement with, but not identical to, those of the above cited authors. We also analyzed the influence of the grid cell size of the involved DEM upon the calculation results. We introduced an approximation of the analyzed effect, based on a simple linear relationship between the calculation point height, the DRE and its vertical gradient (VGDRE). Since when calculated at zero elevation the involved quantities DRE and VGDRE are smooth functions of latitude and longitude and can be easily interpolated, the approximation gives acceptable results in terms of desired accuracy of several μGal (1 μGal = 10⁻⁸ m/s²). In general, we can state that within the territories of the Czech and Slovak Republics the studied distant relief effect has negligible impact upon local gravity survey data. However, when applied to regional gravity studies, there could be a question of its possible influence in the form of a quasilinear W-E trend ranging approximately from −106.6 to −102.5 mGal within the territory of former Czechoslovakia. If we wanted to correct for this phenomenon, we should subtract this negative quantity from the standard Bouguer anomalies as they have been defined in the recent geophysical literature, thereby considerably increasing their values. But, instead of straightforward correcting the Bouguer anomalies for DRE only, we would rather recommend to wait until after the crustal and even lithospheric effects have been studied more carefully based upon the present day independent knowledge about the deep seated sources of those effects.     

利用震后黏弹性位错理论研究苏门答腊地震(M_w9.3)的震后重力变化

本文利用2003—2011年的GRACE RL05数据提取了苏门答腊地震(Mw9.3)引起的震后重力变化,发现断层两侧震后重力变化速率存在明显差异,断层下盘总体变化率为0.55μGal/yr,断层上盘为0.16μGal/yr.基于子断层叠加的编程思想,本文将Tanaka的黏弹球体位错理论配套计算程序(简称黏弹位错程序)加以改造,克服了其近场计算精度不足(甚至错误)的缺陷,可用来研究大地震引起的近场震后位移与重力变化.本文利用改造后的黏弹位错程序计算了2004年苏门答腊地震(Mw9.3)产生的同震重力变化,计算结果在空间分布和量级上均与利用弹性位错程序计算获得的结果一致,验证了我们对黏弹位错程序进行改造的正确性.最后,结合GRACE卫星观测数据,本文利用Tanaka的黏弹位错理论研究了苏门答腊地区的地幔黏性因子.结果表明,该地区地幔黏滞性具有显著的横向差异,当发震断层上下两盘的地幔黏滞性系数分别取8×1018 Pa·s和1×1018 Pa·s时,模拟的震后重力变化在总体空间分布和变化趋势上与GRACE卫星观测结果更接近.     

Combining relative and absolute gravity measurements to enhance volcano monitoring

To achieve a balance between uncertainty and efficiency in gravity measurements, we have investigated the applicability of combined measurements of absolute and relative gravity as a hybrid method for volcano monitoring. Between 2007 and 2009, three hybrid gravity surveys were conducted at Mt Etna volcano, in June 2007, July 2008, and July 2009. Absolute gravity data were collected with two absolute gravimeters, which represent the state of the art in recent advances in ballistic gravimeter technology: (1) the commercial instrument FG5#238 and (2) the prototype instrument IMGC-02. We carried out several field surveys and confirmed that both the absolute gravimeters can still achieve a 10 μGal or better uncertainty even when they are operated in severe environmental conditions. The use of absolute gravimeters in a field survey of the summit area of Mt Etna is unprecedented. The annual changes of the gravity measured over 2007–2008 and 2008–2009 provide unequivocal evidence that during the 2007–2009 period, two main phenomena of subsurface mass redistribution occurred in distinct sectors of the volcano, accompanying different eruptive episodes. From 2007 to 2008, a gravity change of ?60?μGal was concentrated around the North-East Rift. This coincided with a zone affected by strong extensional tectonics, and hence might have been related to the opening of new voids. Between 2008 and 2009, a North-South elongate feature with a maximum gravity change of +80?μGal was identified in the summit craters area. This is interpreted to indicate recharge of a deep-intermediate magma storage zone, which could have occurred when the 2008–2009 eruption was still ongoing.     

Gravimetric study of a potential mineral deposit in the Itapororoca region, Brazil

A semi-detailed gravity survey was carried out over an area of 650 km² localized in the Eo-Neoproterozoic coastal zone of Paraiba State where 548 new gravity stations were added to the existing database. Gravity measurements were made with a LaCoste and Romberg model G meter with a precision of 0.04 mGal. The altitude was determined by barometric levelling with a fixed base achieving a 1.2 m measure of uncertainty, corresponding to an overall accuracy of 0.24 mGal for the Bouguer anomaly. The residual Bouguer map for a 7th degree regional polynomial showed a circumscribed negative anomaly coincident with a localized aero-magnetic anomaly and with hydro-thermally altered outcrops, near the city of Itapororoca. The 3D gravity modelling, constrained by geologic mapping was interpreted as a low density, fractured and/or altered material with a most probable volume of approximately 23 km³, extending to about 8,500 m depth. This result is in accordance with a volcanic body associated with hydrothermal processes accompanied by surface mineralization evidence, which may be of interest to the mining industry.     

Accuracy of the earth's gravity field models

Accuracy tests on the most recent GEM (Goddard Earth Model) gravity models for the representation of the Earth's gravity field, using specially devised statistical techniques of comparative evaluation, show that there is steady improvement in these models with time. On this comparative basis, the accuracy of determination for the spherical harmonic coefficients of the Earth's gravity field is ～ 100% for n = 2–6, 90–99% for n = 7–10, 55–80% for n = 11–14 and ? 50% for n ? 15, deteriorating rapidly with increasing n. The higher degree coefficients corresponding to n ≥ 15 do not seem to be determined accurately enough to be useful from a geophysical standpoint, though their cumulative contribution is undoubtedly useful for specific orbital computations. The estimated errors are 0.3 mGal for n = 2–6, 1.5 mgal for the frequency range n = 2–10, 3 mGal for n = 2–14 and 5–6 mGal for n = 2–22. These error estimates, especially the ones for the higher frequency range, may have been affected by possible errors in the comparison standards used for this evaluation. Consequently, some of the higher degree coefficients of recent GEM models may be more accurate than predicted by these tests.Due to the inherent deficiency of the comparison standards, the errors given in this paper should be treated as error estimates. The steady and progressive improvement, shown by the various GEM gravity models when tested against comparison standards 10E and WGS 72, i.e. the more recent a gravity model, the better it tests against the comparison standards in contrast to its predecessors, is remarkable, as the comparison standards themselves are several years older than the gravity models tested here. This clearly validates our choice of comparison standards, as well as the premises and predictions of our evaluation techniques. It also demonstrates the power and potential of these techniques, which only seem to be limited by the level of accuracy of the available standard of comparison.     

Airborne gravimetry using a strapped-down LaCoste and Romberg air/sea gravity meter system: a feasibility study

Marine gravimeters mounted on stabilized platforms are commonly used in aircraft to perform airborne gravity measurements. The role of the stabilized platform is to level the sensor mechanically, whatever the aircraft attitude. However, this compensation is generally insufficient due to the sensitivity of modern gravity sensors. Correcting the offlevel error requires that an offlevel correction calculated from positioning data be added to gravimeter measurements, which complicates not only the processing, but also the assessment of precision and resolution. This paper is a feasibility study describing the levelling of a completely strapped‐down LaCoste and Romberg gravimeter for airborne gravimetry operation, by means of GPS positioning data. It focuses on the calculation of the sensor offlevel correction needed for the complete gravity data processing. The precision of the offlevel correction that can be achieved, in terms of GPS data precision and gravity wavelengths, is theoretically studied and estimated using the gravity and GPS data acquired during the Alpine Swiss French airborne gravity survey carried out in 1998 over the French Western Alps. While a 1 cm precision of GPS‐determined baseline coordinates is sufficient to achieve a 5 mGal precision of the offlevel correction, we maintain that this precision has to reach 1 mm to ensure a 1 mGal precision of the offlevel correction at any wavelength. Without a stabilized platform, the onboard instrumentation becomes significantly lighter. Furthermore, the correction for the offlevel error is straightforward and calculated only from GPS data. Thus, the precision and the resolution of airborne gravity surveys should be estimated with a better accuracy.     

2015年尼泊尔M_S8.1地震的地壳重力均衡背景与地表形变响应特征

对2015年尼泊尔MS8.1地震的地壳均衡背景及其引起的地表形变特征进行了研究,结果表明:(1)尼泊尔MS8.1地震震中以南的印度板块岩石圈有效弹性厚度大约为9km,加载主要来自地幔;地震以北的拉萨地块岩石圈有效弹性厚度大约为2km,加载主要来自地表.(2)尼泊尔MS8.1地震震中以南地区的地壳均衡异常大约为-100mGal(10-5 m·s-2),但其北部的地壳均衡异常则为300~400mGal,尼泊尔MS8.1地震发生在地壳均衡负异常向正异常过渡的高梯度带上.(3)尼泊尔MS8.1地震使震中周围地区的地壳整体向南运动,最大水平位移超过1.5m,分布在震中东南.震中以北的同震垂向位移总体为负值,最大下降幅度超过0.5m,同震重力变化总体为正值,最大超过60μGal(10-8 m·s-2);震中以南的垂向位移总体为正值,最大升幅超过0.7m,同震重力变化总体为负值,最大降幅超过-120μGal.(4)尼泊尔MS8.1地震使"世界屋脊"喜马拉雅山脉产生沉降,最大同震降幅超过120mm,震后松弛效应将使"世界屋脊"持续缓慢下降.该强震使世界最高峰珠穆朗玛峰降低了2~3mm,有可能被GPS、InSAR等现代大地测量工具检测到.