The broadband seismometer or gravimeter installed on the surface of the earth record not only the seismic wave signal excited by the big earthquake, but also the continuous weak high-frequency vibration signal, which is the earth background noise. Therefore, the research on the background noise of the gravimeter is helpful for the site selection of the station, the adjustment of the instrument, the judgment of whether the instrument has the ability to detect the normal mode of earthquake in the near future, and the detection of weak geodynamic signals. In order to evaluate the background noise level of PET/gPhone gravimeters of the National Gravity Network of China, we collected and processed 4-yr continuous gravity data recorded by the 52 PET/gPhone gravimeters from 2014 to 2017. The power spectral density during the quiet period of the gravimeter will be used as an estimate of the background noise level. Firstly, we compared and analyzed the existing background noise level calculation methods, and proposed an improved data selection strategy (called DaySNM method) for SNM (seismic noise level). Then we calculated background noise level at seismic frequency band (200～600 s) based on the Banka method(Banka and Crossley, 1999), presenting a new low-noise level model (CN-NLNM) by fitting the lower envelope of 52 power spectral density curves. Finally, the spatial distribution map of background noise in mainland China was obtained by Continuous Curvature Spline Interpolation with the SNM data. The results show that: (1) DaySNM method can find a lower SNM, compared with the Banka and Bandpass method. (2) the variation range of SNM in mainland China is 2.013 (Yutian station)～4.654 (Yinchuan station). (3) SNM of gravimeters in coastal areas generally larger than inland, indicating that ocean noise may be the main reason for the greater noise of coastal stations; (4) SNM in North China, eastern and northern Qinghai-Tibet Plateau is huge, which may be related to the frequent tectonic activities in these areas.

Abstract Global and regional ocean tidal models were used to analyze the gravity effects of ocean tidal loading on continuous gravity observations in eastern China. The accuracies of tidal loading gravity correction results for 21 gPhone and superconducting gravity stations were evaluated. The global ocean tidal model was most effective for gravity correction at inland stations (up to 90%), but less effective at coastal stations (only 60%). Taking into account regional ocean tidal models, the effectiveness of tidal loading correction increased to 80%, and the root sum square of the residual gravity series was 0.7450 ~ 0.9733\({\times 10}^{-8}\text{m}{\text{s}}^{-2}\). The best tidal loading gravity correction was obtained by combing the global FES2004 model and regional OSU.chinasea.2010 model (root sum square of 0.7450\({\times 10}^{-8}\text{m}{\text{s}}^{-2}\)). Using this combination, the tidal loading gravity amplitude in the China Sea and adjacent areas was calculated; tidal loading gravity was. Very low gravity effects were obtained inland and for island interiors; however, tide loading gravity was relatively larger along the coast, especially the southeast coast and Bohai Bay. Moreover, the amplitude changed drastically possibly owing to offshore topography. Our results provide a reference for selecting ocean tidal models for high-precision analysis of continuous gravity observations in China.

为评估全球潮汐模型在我国潮汐改正中的适用性,本文首先对10个重力站2016—2018年的观测数据进行了精度评定,而后基于均方根、和方根、纬度依赖关系以及重力残差等指标对7个全球潮汐模型进行了精度评定.结果表明:10个重力站的一些评价指标达到甚至超越了早期超导重力仪,例如M2波潮汐因子的中误差普遍小于0.00070,其中最高精度约为0.00014,5个主要潮波的稳定度均≤0.0015.在10个观测模型和7个全球潮汐模型中,DDW-NHi和M2001模型考虑了地球扁率的影响,基于这两个模型计算的和方根较其它模型所得的和方根均小,约为0.288×10-8 m/s2.基于最高精度的乌什站数据对Molodensky,DDW-NHi,M2001与观测模型的改正精度的对比显示,DDW-NHi模型改正计算的重力残差(±0.4×10-8—±1.0×10-8 m/s2)不及观测模型(±0.1×10-8—±0.5×10-8 m/s2),但依然优于M2001模型(±0.7×10-8—±1.4×10-8 m/s2),且DDW-NHi模型改正获得的残差比传统的Molodensky模型所得残差(±0.5×10-8—±1.5×10-8 m/s2)小1×10-8—2×10-8 m/s2.

One important procedure in gravity data processing is making tidal corrections. Sound tidal corrections rely on accurate tidal gravimetric factors. The present study selects data from 12 gPhone gravimeter stations and three superconducting gravimeter stations for the period between 2009 and 2015, distributed across mainland China. Based on these data, the distribution of M2 tidal gravimetric factors over China is obtained. We also estimate the corresponding theoretical values of the M2 wave provided by the DDW99 solid Earth tide model (Dehant et al. in J Geophys Re Solid Earth 104:1035–1058, 1999) and the CSR4.0 ocean tide model (Eanes and Bettadpur in The CSR3. 0 global ocean tide model: diurnal and semi-diurnal ocean tides from TOPEX/POSEIDON altimetry, The University of Texas Center for Space Research, 1996). The results suggest that the measurements and theoretical values of the M2 wave are similar, with an average misfit no greater than 0.6% for most regions of China’s mainland, although the misfit is relatively large, about 1.1%, in northeast China. According to the latest three-dimensional tidal theory and the correction effects on the M2 gravimetric factors at Lhasa, Lijiang, and Wuhan, where the three superconducting gravimeters are located, we select the best-fitting three-dimensional Earth model, GyPSuM (Simmons et al. in J Geophys Res Solid Earth 115:B12, 2010), for mainland China. The theoretical values of the tidal gravimetric factors while considering three-dimensional inhomogeneities are calculated. The results show that the variation in gravimetric factors caused by lateral inhomogeneity across China is from −0.07 to 0.09%.

Based on the observations of 36 gPhone gravimeters in 2015, the background noise levels in the seismic frequency band (200–600s) and sub-seismic band (1–6 h) are calculated. The differences in the PSD (power spectrum density) of each band of gPhone gravimetric gauges in different surrounding environments were analyzed and compared with Peterson's NLNM (new low-noise model) which is derived from the envelope at the power spectrum density of 75 seismograph stations around the world. The results showed that: the influence of station type on the noise magnitude of gPhone gravimeter is very small; The seismic band noise magnitude (hereinafter referred to as SNM) and the sub-seismic band noise magnitude (hereinafter referred to as SSNM) in the coastal gPhone gravimeter are higher than those of inland stations. Although the local hydrological change has a great influence on the gravity observation, the rainfall is not directly relative to the noise magnitude of the instrument. Except 3 coastal stations, the eight stations which had the highest amplitudes in the SNM were located near the seismic belt. This indicates that the SNM of the gPhone Gravimeter may reflect some seismic information. Compared with the NLNM model, the PSD of the gPhone gravimeter is lower than the NLNM model in the long period band (<3×10−5Hz), indicating that the gPhone gravimeter is more suitable for detecting long-period signals (>10 h) than the seismometer.