The effect of the magnetic field on the accuracy of measurements with Worden and Sharpe gravity meters

Summary The effect of an additional homogeneous magnetic field with an intensity of 0–4.5 Oe on the Worden quartz gravity meter No. 961 and on Sharpe quartz gravity meters Nos 173 and 174 was tested. Whereas no effect was observed with the Worden gravity meter, the magnetic field had a measurable effect on both the Sharpe gravity meters. The largest deviation of the reading beam is caused by the horizontal component of the magnetic field which acts in the plane of oscillation of the gravity-meter arm. The Sharpe gravity meter No. 173 is considerably sensitive; a field of 0.2 Oe intensity, corresponding to the magnitude of the horizontal component of the geomagnetic field in mid-latitudes, causes an error in the measurement of gravity of as much as 0.08 mGal. With a view to the different behaviours of the individual quartz gravity meters of the same type in a magnetic field, it should prove expedient to carry out check measurements with all gravity meters and, with regard to the sensitivity of the gravity meter to the magnetic field and the required accuracy of the gravity determination, take into account this perturbing factor in field measurements, as well as laboratory tests of gravity meters.     

Sensitivity of the Sharpe and Worden gravity meters to vertical motion

Summary Using the least-squares method the constants of both gravity meters — seismometers — were derived from the amplitude responses, observed under vertical motion on a vibrating table in the 0.3–30 s range. The values of the seismogram amplitudes, above which seismic waves cause an observable displacement of the gravity meter reading beam, were determined for the SKD and Press-Ewing station seismographs.     

Estimate of the effect of vibrations on quartz gravity meters

Summary Relations for computing the minimum amplitudes of ground vibrations which generate oscillations of the gravity meter reading beam at the limit of the resolution of the optical system, have been derived. The minimum amplitudes in the ground displacement period range of0.1 to10 s, for the assumed values of the fundamental parameters of quartz gravity meters (i.e. the periods and damping constants of the pendulum and the mechanico-optical magnification of the pendulum deflections), range from tenths of a micrometre to units of micrometres. Larger displacement amplitudes, due to, e.g., earthquakes and traffic, disrupt gravity measurements.     

拉科斯特重力仪干扰因素分析

本文根据多年重力监测工作情况，结合拉科斯特重力仪的结构特点，对仪器在观测中的干扰因素进行分析，认为影响仪器观测精度的干扰因素有两类：一类是仪器本身产生的系统误差，如格值误差、零飘及弹性后效等；另一类是外界干扰因素，如振动、温度、人为观测方法不当等，如果在观测中能消除或减小这些干扰因素影响，拉科斯特重力仪就可能达到更高的观测精度。     

成都台重力观测数据变化分析

本文通过对成都台重力仪器数据去掉固体潮汐理论值、去掉气温、气压、地下水的干扰因素来讨论重力观测曲线的变化。分析导致重力观测年变异常的因素。     

重力垂直梯度的测定及其应用与潜力

近年来,随着重力垂直梯度测量精度的提高及间接推求方法的改善,它在地球科学的研究中的作用日益扩大,本文叙述了利用高精度重力仪直接测量垂直梯度的方法与注意事项,阐明了垂直梯度在地学研究中的意义与作用,其中包括(1)可以更精确测定大地水准面与正高;(2)可以更好探测地下的物质的分布及界面起伏;(3)可根据垂直梯度及形变资料反演场源及解释引起上述变化的动力机制.关于如何进一步发掘其潜力,这里也提出了建议.     

潮汐数字记录应用软件

文中较详细地介绍了重力潮汐数字记录系统应用软件,主要包括数字采集的控制部分、观测资料的日常处理、重力仪记录格值的标定等一系列与台站观测有关的处理功能。它实现了潮汐信息从采集到成果处理的自动化,首次将48小时的Nakai潮汐数据预处理方法引入PC-1500计算机。它能有效地剔除、修正数字采集器本身无法识别的错误数据或干扰信号,使观测人员能根据拟合参量的变化,及时了解仪器的工作状况及记录系统灵敏度的变化。实践表明:此软件配合数字采集系统使用后大大提高了重力固体潮观测的整体水平。对这一软件作部分的改进后,可用于固体潮其它分量的观测。     

Analysis of beam nulling of a sharpe gravity meter

Summary The routine nulling procedure is modified on the basis of the theoretical decay of transient motion of the gravity meter beam and the time needed for its stabilizing.     

GS-15重力仪电磁反馈检测技术

GS-15型重力仪是利用摆杆位移大小来量度重力相对变化的精密仪器,摆杆位移是通过电容换能进行测量。由于该仪器为开环检测系统,而且系统的末级加接0.01Hz的有源滤波器,这样就失去了大于0.01Hz的高频成分,且使潮波产生相位失真,同时还存在着电路非线性带来的测量误差,因而测量精度难以提高。为此笔者对GS-15型重力仪加装了电磁反馈装置,将开环检测改为闭环检测,利用电磁力使重力仪摆杆始终处于零电平的动态平衡状态,以电磁反馈输出大小来度量重力的变化,这样就将开环检测系统改装为平衡电桥闭环检测系统,其时间常数为2秒。 改装后,消除了电路的非线性影响,增强了系统的稳定性,记录的固体潮含有丰富的谐波,同时系统还增加了一路稳态零位输出,为地壳形变、地震预报和地球物理研究提供了波群丰富的资料     

Regional correction of gravity data

Summary Methods for the elimination of the regional effect from gravity data are discussed in this paper. A historical review of the average gradient method used in torsion balance data is given. The connection between the averaging, or grid method, used on the gravity meter data and the average gradient method is pointed out and a criticism of these methods is given. Due to the ambiguity of the interpretation of gravity data no unique determination of the regional effect is possible and no mechanical method for its removal can be found. The determination of a regional gravity effect is an interpretation problem and for this reason it is unavoidably the subject of geological consideration and of the personal judgement of the interpreter. The method of smoothing the isogams and gravity curves is also discussed. A few suggestions are made for breaking the gravity data into two components (regional and residual) by the analysis of the gravity data. Finally the application of the second derivative methods for the elimination of the regional effect is discussed.     

重力卫星精密星间测距系统滤波器技术指标论证

本文基于重力卫星精密星间测距测量模式,从星间测距观测值与地球重力场频谱关系的角度,建立了距离观测值关于重力位系数的敏感矩阵,分析了各阶次重力场位系数对应的敏感矩阵的频谱特性,讨论了星间测距信息中能反应地球重力场信息的有效信号频带,给出了能最大限度保留地球重力场信息的低通滤波器的通带截止频率、通带增益波纹和频率采样率技术指标设计方案,可为我国首期卫星跟踪卫星重力测量计划的主要技术指标的初步设计提供参考.     

Tilt effects on GWR superconducting gravimeters

The superconducting gravimeters (SGs) are the most sensitive and stable gravity sensors currently available. The low drift and high sensitivity of these instruments allow to investigate several geophysical phenomena inducing small- and long-period gravity changes. In order to study such topics, any kind of disturbance of instrumental origin has to be identified and possibly modelled. A critical point in gravity measurement is the alignment of the gravimeter to the local vertical. In fact a tilt of the instrument will lead to an apparent gravity change and can affect the instrumental drift. To avoid these drawbacks, SGs are provided with an “active tilt feedback system” (ATFS) designed to keep the meter aligned to the vertical. We analyse tilt and environmental parameters collected near Strasbourg, France, since 1997 to study the source of the tilt changes and check the capability of the ATFS to compensate them. We also present the outcomes of a calibration test applied to the ATFS output to convert the Tilt Power signals into angles. We find that most of the observed signal has a thermal origin dominated by a strong annual component of about 200 μrad. Nevertheless, our analysis shows that even the tilt due to different geophysical phenomena, other than the thermal ones, can be detected. A clear tidal signal of about 0.05 μrad is detectable thanks to the large data stacking (>11 years). We conclude that (i) the ATFS device compensates the tilt having a thermal origin or coming from any sources and (ii) no significant tilt changes alter the gravity signal, except for the high frequency (>1 mHz) perturbations.     

Density function evaluation from borehole gravity meter data – regularized spectral domain deconvolution approach

We present a new method of transforming borehole gravity meter data into vertical density logs. This new method is based on the regularized spectral domain deconvolution of density functions. It is a novel alternative to the “classical” approach, which is very sensitive to noise, especially for high‐definition surveys with relatively small sampling steps. The proposed approach responds well to vertical changes of density described by linear and polynomial functions. The model used is a vertical cylinder with large outer radius (flat circular plate) crossed by a synthetic vertical borehole profile. The task is formulated as a minimization problem, and the result is a low‐pass filter (controlled by a regularization parameter) in the spectral domain. This regularized approach is tested on synthetic datasets with noise and gives much more stable solutions than the classical approach based on the infinite Bouguer slab approximation. Next, the tests on real‐world datasets are presented. The properties and presented results make our proposed approach a viable alternative to the other processing methods of borehole gravity meter data based on horizontally layered formations.     

成都地震台重力仪故障分析与排除

成都地震基准台重力仪20世纪90年代后期部件老化,在记录过程中出现固体潮曲线畸变现象,最后记录曲线完全变形,影响观测值可用性.2004年经过维修和调试,成功排除故障.资料内在质量检验计算,达到历史最好水平.     

基于数据空间和稀疏约束的三维重力和重力梯度数据联合反演

随着重力和重力梯度测量技术的日趋成熟,基于重力和重力梯度数据的反演技术得到了广泛关注.针对反演多解性严重、计算效率低和内存消耗大等难点问题,本文开展了三维重力和重力梯度数据的联合反演研究,该方法结合重力和重力梯度两种数据,将L0范数正则化项加入到目标函数中,并在数据空间下采用改进的共轭梯度算法求解反演最优化问题.同时,...     

Gravity compression forward modeling and multiscale inversion based on wavelet transform

The main problems in three-dimensional gravity inversion are the non-uniqueness of the solutions and the high computational cost of large data sets. To minimize the high computational cost, we propose a new sorting method to reduce fluctuations and the high frequency of the sensitivity matrix prior to applying the wavelet transform. Consequently, the sparsity and compression ratio of the sensitivity matrix are improved as well as the accuracy of the forward modeling. Furthermore, memory storage requirements are reduced and the forward modeling is accelerated compared with uncompressed forward modeling. The forward modeling results suggest that the compression ratio of the sensitivity matrix can be more than 300. Furthermore, multiscale inversion based on the wavelet transform is applied to gravity inversion. By decomposing the gravity inversion into subproblems of different scales, the non-uniqueness and stability of the gravity inversion are improved as multiscale data are considered. Finally, we applied conventional focusing inversion and multiscale inversion on simulated and measured data to demonstrate the effectiveness of the proposed gravity inversion method.     

Calibration shifts in a LaCoste-and-Romberg gravimeter: comparison with a Scintrex CG-3M

The reliability and ruggedness of LaCoste-and-Romberg gravity meters has meant that their use has extended over recent years from gravity mapping to the investigation of small gravity changes (> 10 μGal) with time. When using the instrument at the limit of its capability, limitations become apparent. Some instruments drift more than others and some are prone to excessive tares. However, with regular servicing these problems are minimized. Here we demonstrate clear evidence for calibration changes of the order of 1000 ppm for LaCoste-and-Romberg meter G513, which, for gravity differences of over 50 mGal, represents a highly significant induced error of over 50 μGal. Data collected over 15 years using nearly 4000 mGal of the 7000 mGal range of the instrument illustrates that long periods of stability may be interspersed with periods lasting several months while the calibration changes by up to 0.1%. This is comparable with the calibration drift of a new Scintrex CG-3M instrument, although the process causing the change may not be the same.     

RECENT DEVELOPMENTS IN UNDERGROUND GRAVITY SURVEYS1

Underground gravity observations in deep coal mines using the conventional gravity meters Worden (type Master) and LaCoste-Romberg (model D), both of which have been adapted to the fire damp regulations, can be accurate to ± 10 and ± 3 μgal, respectively. For underground determination of the vertical gradient of gravity the LaCoste-Romberg meter is used together with a specially designed measuring tower. Using this euipment an accuracy in tower gradient observations of ± 30 E was obtained. To apply the equipment to precision gravity observations in underground situations an additional correction, i.e. a gallery correction, is needed. High accuracy in correction is achieved by a new method of three-dimensional modelling. The gravity effect is computed for bodies with a surface approximated by triangular elements, which are generated from corner points of the body. The combination of gallery correction with tower gradient data leads to a new method for in situ density determination. It offers the possibility of horizontal instead of vertical density profiling. To demonstrate the effectiveness of the developments in underground observations the localization of a pump room is presented. Microgravity and tower gradient observations were carried out to detect the cavity. The horizontal gradient was also calculated to give a more reliable location.     

Burris型重力仪性能分析

对Burris型相对重力仪长时间断电后再次供电,分析Burris型重力仪升温过程,并使用2台Burris型重力仪（B95和B101） 2014年7月至2016年12月的实测数据,分析仪器性能。结果表明：① Burris型相对重力仪长时间断电并再次供电后,加热125 min达到恒温点,在升温过程中,读数先大幅减小后小幅增大,绝对变化率逐渐减小,通电110 h后零漂小于3×10^-8 m/（s²·h）,达地震重力观测仪器进网要求;② Burris型重力仪静态零漂存在由负变正的过程,不同仪器所需时间不同;③随着仪器使用年限加长,Burris型重力仪稳定性变差。忽略重力仪突跳读数,可获取测量精度在10×10^-8 m/s²左右的流动重力数据;④ Burris型重力仪混合零漂和静态零漂变化基本一致,呈逐渐增大趋势,但存在差别。     

PET重力仪与CTS-1EF地震计地震波记录特征分析

收集苍梧M 5.2、九寨沟M 7.0和智利M 8.2地震期间琼中基准地震台PET重力仪和CTS-1EF地震计观测数据,对重力资料进行潮汐和零漂改正获得重力残差数据,并对地震计数据求导得到加速度值。对重力残差数据和地震计加速度数据波形及频谱曲线进行对比分析,认为2种观测仪器记录的地震波震相和频谱分析显示的地震波分布频率具有一致性,但九寨沟M 7.0地震和智利M 8.2地震的地震波频率分布存在差异。