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The influence of observation environment on background noise level of gPhone gravimeter
Geodesy and Geodynamics xxx (2017) 1e5
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The influence of observation environment on background noise level of gPhone gravimeter Xiaotong Zhang a, b, *, Ying Jiang a, b, Kun Zhang a, b, Xinlin Zhang a, b a b
Institute of Seismology, China Earthquake Administration, Wuhan 430071, China Key Laboratory of Earthquake Geodesy, China Earthquake Administration, Wuhan 430071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2017 Accepted 1 June 2017 Available online xxx
Based on the observations of 36 gPhone gravimeters in 2015, the background noise levels in the seismic frequency band (200e600s) and sub-seismic band (1e6 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 ð < 3 105 HzÞ, indicating that the gPhone gravimeter is more suitable for detecting long-period signals (>10 h) than the seismometer. © 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: gPhone gravimeter Background noise level NLNM Observation environment
1. Introduction High-precision continuous gravimeter is an important means for studying Earth's free oscillations [1e3], seasonal gravity changes [4], and local hydrological changes [4,5]. Since 2007, the Crustal Movement Observation Network of China (hereinafter referred to as CMONOC) and the China Digital Earthquake Observation Network have built high-precision gravity observation stations in mainland China, and the gPhone gravimeters are used as the main observation equipment for the entire observation network (see Fig. 1), whose
* Corresponding author. E-mail address: [email protected] (X. Zhang). Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
Production and Hosting by Elsevier on behalf of KeAi
resolution reaches 0.1 mGal with long-term observation accuracy of 1 mGal. The current CMONOC gPhone gravimeters mainly include two types:cave-type and basement-type. From the perspective of tectonic areas, gPhone gravimeters can be divided into seismic and non-seismic belt types. Also, from a climate perspective, they can be divided into wet and dry types. Studying the noise levels of gPhone gravimeters in different observation environments is helpful for analyzing the source of instrument noise. At the same time, it is helpful in quantitatively analyzing the influences of observation conditions on continuous gravity observation and providing the useful information for setting up new gravity stations in the future. In this paper, we select the observation data from the eight gPhone gravimeters with different observation conditions, different regional structures, and different climatic distributions, based on the root mean square of the residual gravity after the removal of tides, atmospheric pressure, and ninth-order polynomials from the observation data. We selected the quietest five days in the year to obtain the amplitude spectrum and the power spectrum by conducting Fourier transform, calculated the average power spectral density and noise level in the seismic frequency
http://dx.doi.org/10.1016/j.geog.2017.06.002 1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
2
X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
Fig. 1. Distribution of gPhone gravimeters in study area.
band and sub-seismic frequency band, and then compared the noise level of gPhone gravimeters.
mean PSD ¼ 1=M
2. Data processing
k2 X
(2) PSD
(3)
k¼k1
Firstly, the scale factors (i.e. lGal/volt value; 1Gal ¼ 108 m/s2) were calibrated. The scale factor of gPhone gravimeter was determined by comparing its voltage amplitudes of tidal signals with the gravity amplitudes which have been observed at the observation site [6]. Here, we used the method of Liu [7] on gPhone gravimeter calibration, and then subtracted the effects of the tides and atmospheric pressure. We used the Qinwen potential [8] to calculate the effect of the gravity tide. To calculate the impact of local pressure, we used a nominal admittance of 0.3 mgal/mbar and deducted it directly from the observation results. To subtract the residual tidal signals and the instrument drift, it was necessary to perform polynomial fitting and deduction of residual gravity. Banka and Crossly [9] believe that the ninth-order polynomial is ideal, as large tidal signals cannot be deducted from a polynomial fit below the ninth-order. Finally, we use the method proposed by liu [10] to eliminate the influence of instrument tilt. After obtaining the residual gravity, we select the five quietest days with the lowest RMS values to calculate the background noise of the observed signals. In statistics, the five days could be fully representative of the cycle for a 2 mine1 h period seismic signal [11] even though the signals of the 1 h cycle were repeated 120 times. The noise level is best expressed with the PSD (power spectral density) since the amplitude of the PSD is independent of the data length and the sample rate. The data in the calmest five days were selected for the Fourier transform, and the average PSD was calculated via the following method:
X N sðkÞ ¼ f ðiÞexpð2piðj 1Þðk 1Þ=NÞ j¼1
PSD ¼ s2 Dt=N
(1)
where f ðiÞ is the observed value, j ¼ 1,…, N; Dt is the sampling interval, and sðkÞ is the amplitude spectrum. k1 and k2 are the lower and upper frequency values respectively, and M is the number of samples between k1 and k2 . The Parzen window was then used to smooth the 101 points [9]. Smoothing will not affect the overall noise level. In the 200e600s band of the average PSD, the SNM was calculated and defined as follows:
SNM ¼ lgðmeanPSDÞ þ 2:5
(4)
SNM is a quick way to compare noise levels at different points. Although SNM contains less information than PSD, it can be used as a standard to compare the noise levels of observation instruments.
3. Data analysis 3.1. Noise levels of basement- and cave-type gPhone gravimeters At present, gPhone gravimeters were installed in the basement or cave. The observations from four basement-type gPhone gravity stations with the smallest SNM (Yutian, Songpan, Enshi, and Zhongdian) and four cave-type stations (Altay, Hegang, Zhangjiakou and Nyingchi) were selected in 2015. Their PSDs are shown in Fig. 2, and the results of the noise levels in the seismic and subseismic bands are demonstrated in Table 1. Table 1 shows that the SNM ranges for the basement- and cavetype gPhone gravimeters are 2.0e2.79 (mean: 2.44) and 2.5e2.93 (mean: 2.72), respectively, whose overall differences are not big. Fig. 2 shows that two types of instruments have similar PSD. From the tidal frequency band to the sub-seismic frequency band, the
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
3
Fig. 2. Seismic and sub-seismic frequency bands in basement- and cave-type observation conditions.
Table 1 Seismic and sub-seismic noise levels of gPhone gravimeters in two observation conditions.
Basement-type
Cave-type
Yutian Songpan Enshi Zhongdian Altay Hegang Zhangjiakou Nyingchi
SNM
SSNM
2.00453 2.49585 2.57638 2.78507 2.50443 2.65068 2.80634 2.92996
5.13014 5.95073 5.97234 5.60032 5.41752 5.51757 5.81959 5.78485
tidalfrequencies, solid Earth tides are more stable than oceanic tides. Therefore, after deducting the tides from the observations of the gPhone gravimeters, the PSDs of inland-environment instruments are lower than those of coastal-environment in tidal frequencies. Due to the impact of the tides on the observation results, the PSDs of the tidal band of coastal gPhone gravimeters are significantly higher than those of the inland gPhone gravimeters. For the sub-seismic and seismic frequency bands, the PSDs of the coastal instruments are also significantly higher than those of the inland-type instruments (Table 2). 3.3. Noise levels of arid and humid gPhone gravimeters
average differences of the four instruments selected from the two types of caves are very small. However, in the seismic band, the PSDs of the basement-type equipment were lower than those of the cave-type equipment. Therefore, we believe that the caveenvironment has very few effect on the noise levels of gPhone gravimeters. But for the seismic band, the PSDs of basement-type equipment were slightly lower than those of cave-type equipment. 3.2. Noise levels of coastal and inland-type gPhone gravimeters To determine the influences of the ocean on the noise levels of the gPhone gravimeters, we selected the observation data in 2015 from four coastal stations with the minimum SNMs (Xiamen, Wenzhou, Qingdao, and Dalian) and four inland stations (Yutian, Shiquanhe, Wushi, and Golmud) to calculate the noise levels of the seismic and sub-seismic bands. We found that the SNMs and SSNMs of coastal gPhone gravimeters were all larger than those of inland stations. It can be seen from Fig. 3, the PSDs of inland stations are generally lower than those of the coastal stations, indicating that the data from inland stations are more reliable in detecting the Earth's free oscillations and other non-oceanic signals. For
An important factor affecting gravity observation is local hydrology. Therefore, according to the average annual precipitation provided by China's meteorological data network, the observation data in 2015 from four gPhone gravimeters (Ruoqiang, Yutian, Yinchuan, and Altay) with the lower rainfall and the four gPhone gravimeters (Wenzhou, Ji'an, Tengchong, and Wuzhou) with the more rainfall were selected to calculate the noise levels of the seismic and sub-seismic band (Table 3). We found that the SNMs of Ruoqiang and Yinchuan stations are significantly higher than those
Table 2 Noise levels of seismic and sub-seismic coastal and inland gPhone gravimeters.
Inland type
Coastal type
Yutian Shiquanhe Wushi Golmud Qingdao Wenzhou Dalian Xiamen
SNM
SSNM
2.00453 2.09333 2.31528 2.4151 2.93984 3.52944 3.64453 3.67242
5.13014 5.20998 5.19868 5.55775 5.42156 6.22083 6.07665 6.34727
Fig. 3. PSD of coastal and inland gPhone gravimeters.
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
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X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
Table 3 Noise levels of gPhone gravimeters in arid and humid areas of seismic and subseismic belts.
Arid
Humid
Ruoqiang Yutian Yinchuan Altay Wuzhou Tengchong Jian Wenzhou
SNM
SSNM
Annual rainfall (mm)
4.71997 2.00453 4.60018 2.50443 3.49355 3.49498 3.06309 3.52944
5.66711 5.13014 6.26639 5.41752 5.57314 5.63447 5.47058 6.22083
27.6 55.4 182.9 212.6 1452.9 1532.4 1566.2 1766.5
of other stations, while the SNMs of Yutian and Altay stations were the lowest. By comparing SSNMs, it was found that the SSNM range of the arid region was 5.13e6.27, and the SSNM range of the four stations with the highest rainfall was 5.47e6.22. And from the results in Fig. 4, there is no significant relationship between the noise levels of gPhone gravimeters and rainfall (Table 4). From the PSD results (Fig. 5), it can be seen that except the Tengchong station, the observation curves of the other four stations with more rainfall have similar PSDs. For seismic band, the PSDs of Yutian and Altay are lower than those of the other stations. The PSDs of the four stations with more rainfall are located between Ruoqiang, Yinchuan, Altay, and Yutian. The PSD results of the areas with lower rainfall may be smaller or larger than those of the areas with more rainfall, indicating that rainfall is not a major factor affecting PSD.
Fig. 4. Rainfall vs. SNM Chart. Table 4 Non-coastal stations with the highest 5 SNMs.
Ganzi Taiyuan Yinchuan Yushu Ruoqiang
SNM
SSNM
Seismic belt
3.65915 4.12775 4.60018 4.64329 4.71997
6.11135 6.02701 6.26639 6.26213 5.66711
Ganzi-Kangding seismic belt Shanxi seismic belt Yinchuan Hetao seismic belt Fenghuoshan fault belt Kunlun Mountains AltynTagh fault belt
3.4. Noise levels of gPhone gravimeters on seismic and non-seismic belts Comparing the SNMs of 36 gPhone gravimeters in 2015, we found that the eight stations with the highest SNMs are Taiyuan, Ruqiang, Yushu, Yinchuan, Xiamen, Ganzi, Dalian, and Wenzhou. Other than the three coastal stations, the remaining five are in the vicinity of the seismic belt. Comparing the PSDs of these five stations, we found that their PSDs in the seismic band are greater than 103 ðnm=s2 Þ2 Hz1 , which is an order of magnitude higher than those of the eight stations in non-seismic band (Fig. 2). The difference between the PSDs in the seismic band may mainly result from the influence of internal tectonic activity because the observation environment and precipitation have no significant influence on SNM, and all the stations are inland-environment. The PSDs of Yinchuan and Yushu in the sub-seismic frequency band are significantly higher than those of the other three stations (Fig. 6).
4. Conclusion In this paper, based on the observation data obtained by different types of gPhone gravimeters in 2015, the average power spectral densities and noise levels of the seismic and sub-seismic bands were calculated. We assumed that the noise level of the observed results mainly came from the influences of environmental factors when the noise levels of the instruments were almost identical. Comparing the influences of different observation environments on noise levels of gPhone gravimeters, we think that the influences of cave-environment on the noise levels of gPhone gravimeters were extremely small. The SNMs and SSNMs of coastal gPhone gravimeters were larger than those of inland stations. The PSDs of inland stations were generally lower than those of coastal stations, indicating that the noise levels of inland stations are smaller than those of costal stations. The noise levels were high in coastal areas due to the effects of the ocean. Although local hydrological changes have greater effects on gravity observations, noise levels have no direct relation with rainfall. Under the conditions that all stations are inland-type, the effects of rainfall and observation environment on noise level are very small, while internal tectonic activities may be the main source for the differences between the PSDs of seismic frequency-band. We found that the eight stations with the highest SNM are Ruoqiang, Yushu, Yinchuan, Xiamen, Ganzi, Taiyuan, Dalian, and Wenzhou. Other than the three coastal stations, the remaining five are in the vicinity of the seismic belt, indicating that the SNMs from the gPhone gravimeters may provide the seismic information to some extent.
Fig. 5. PSD of gPhone gravimeters of arid and humid areas on seismic and sub-seismic bands.
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
5
Fig. 6. Non-coastal station PSD with the top five SNMs.
Compared with the NLNM model, the PSDs from the gPhone gravimeters were lower in the long period band (less than 3 105 Hz), indicating that gPhone gravimeters are more suitable for detecting long-period signals than seismograph (>10 h).
[6]
[7]
Acknowledgments We are grateful to the Gravity and Deformation Sub center of China Earthquake Administration Data Sharing Center to provide the gPhone data. The work is supported by key task project in Sicence for earthquake resilience No. XH17053 and the National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2012YQ10022506).
[8]
[9] [10]
[11]
correct long periodic gravity variations, J. Geophys. Res. Atmos. 111 (B10) (2006) 239e251. T. Sato, Y. Tamura, S. Okubo, S. Yoshida, Calibration of the scale factor of superconducting gravimeter at Esashi using an absolute gravimeter FG5, J. Geodetic. Soc. Jpn. 42 (4) (1996) 225e232. Z. Liu, H. Li, J. Wei, Accurate determination of calibration factor of gPhone spring gravimeters by using M2 tidal wave amplitude factor, J. Geodesy. Geodyn. 31 (5) (2011) 146e150. Xi Qinwen, The precision of the development of the tidal generating poes Terr. 105 (1989) tential and some explanatory notes, Bull. Inf. Mare 7396e7404. D. Banka, D. Crossley, Noise levels of superconducting gravimeters at seismic frequencies, Geophys. J. Int. 139 (1) (1999) 87e97. Z.W. Liu, L.I. Hui, K.X. Kang, Y.L. Wu, Determination accurate level position parameters of gPhone Gravimeter using indirect adjustment algorithm, Prog. Geophys. 27 (5) (2012) 1957e1961. S. Rosat, J. Hinderer, Noise levels of superconducting gravimeters: updated comparison and time stability, Bull. Seismol. Soc. Am. 101 (3) (2011) 1233e1241.
References [1] Sun Heping, Xu Jianqiao, B. Ducarme, The tidal experiment model based on the global superconducting gravimeter observation data which take the effect of liquid core daily resonance into consideration, J. Chin. Sci. Bull. 48 (6) (2003) 610e614. [2] Lei Xiang'e, Xu Houze, Sun Heping, Sci. China Ser. D Earth Sci. 34 (5) (2004) 483e491, http://dx.doi.org/10.1360/zd2004-34-5-483 (in Chinese). [3] Ding, Shen, Determination of the complex frequencies for the normal modes below 1mHz after the 2010 Maule and 2011 Tohoku earthquakes, Ann. Geophys. 56 (5) (2013), http://dx.doi.org/10.4401/ag-6400. [4] J.Q. Xu, J.C. Zhou, S.C. Luo, H.P. Sun, Study on characteristics of long-term gravity changes at Wuhan station, Sci. Bull. 53 (13) (2008) 2033e2040. [5] M.V. Camp, M. Vanclooster, O. Crommen, T. Petermans, K. Verbeeck, Hydrogeological investigations at the Membach station, Belgium, and application to
Xiaotong Zhang, assistant researcher, Institute of Seismology, China Earthquake Administration. Her interests include the research on continuous gravity data processing.
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
Contents lists available at ScienceDirect
Geodesy and Geodynamics journal homepages: www.keaipublishing.com/en/journals/geog; http://www.jgg09.com/jweb_ddcl_en/EN/volumn/home.shtml
The influence of observation environment on background noise level of gPhone gravimeter Xiaotong Zhang a, b, *, Ying Jiang a, b, Kun Zhang a, b, Xinlin Zhang a, b a b
Institute of Seismology, China Earthquake Administration, Wuhan 430071, China Key Laboratory of Earthquake Geodesy, China Earthquake Administration, Wuhan 430071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2017 Accepted 1 June 2017 Available online xxx
Based on the observations of 36 gPhone gravimeters in 2015, the background noise levels in the seismic frequency band (200e600s) and sub-seismic band (1e6 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 ð < 3 105 HzÞ, indicating that the gPhone gravimeter is more suitable for detecting long-period signals (>10 h) than the seismometer. © 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: gPhone gravimeter Background noise level NLNM Observation environment
1. Introduction High-precision continuous gravimeter is an important means for studying Earth's free oscillations [1e3], seasonal gravity changes [4], and local hydrological changes [4,5]. Since 2007, the Crustal Movement Observation Network of China (hereinafter referred to as CMONOC) and the China Digital Earthquake Observation Network have built high-precision gravity observation stations in mainland China, and the gPhone gravimeters are used as the main observation equipment for the entire observation network (see Fig. 1), whose
* Corresponding author. E-mail address: [email protected] (X. Zhang). Peer review under responsibility of Institute of Seismology, China Earthquake Administration.
Production and Hosting by Elsevier on behalf of KeAi
resolution reaches 0.1 mGal with long-term observation accuracy of 1 mGal. The current CMONOC gPhone gravimeters mainly include two types:cave-type and basement-type. From the perspective of tectonic areas, gPhone gravimeters can be divided into seismic and non-seismic belt types. Also, from a climate perspective, they can be divided into wet and dry types. Studying the noise levels of gPhone gravimeters in different observation environments is helpful for analyzing the source of instrument noise. At the same time, it is helpful in quantitatively analyzing the influences of observation conditions on continuous gravity observation and providing the useful information for setting up new gravity stations in the future. In this paper, we select the observation data from the eight gPhone gravimeters with different observation conditions, different regional structures, and different climatic distributions, based on the root mean square of the residual gravity after the removal of tides, atmospheric pressure, and ninth-order polynomials from the observation data. We selected the quietest five days in the year to obtain the amplitude spectrum and the power spectrum by conducting Fourier transform, calculated the average power spectral density and noise level in the seismic frequency
http://dx.doi.org/10.1016/j.geog.2017.06.002 1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
2
X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
Fig. 1. Distribution of gPhone gravimeters in study area.
band and sub-seismic frequency band, and then compared the noise level of gPhone gravimeters.
mean PSD ¼ 1=M
2. Data processing
k2 X
(2) PSD
(3)
k¼k1
Firstly, the scale factors (i.e. lGal/volt value; 1Gal ¼ 108 m/s2) were calibrated. The scale factor of gPhone gravimeter was determined by comparing its voltage amplitudes of tidal signals with the gravity amplitudes which have been observed at the observation site [6]. Here, we used the method of Liu [7] on gPhone gravimeter calibration, and then subtracted the effects of the tides and atmospheric pressure. We used the Qinwen potential [8] to calculate the effect of the gravity tide. To calculate the impact of local pressure, we used a nominal admittance of 0.3 mgal/mbar and deducted it directly from the observation results. To subtract the residual tidal signals and the instrument drift, it was necessary to perform polynomial fitting and deduction of residual gravity. Banka and Crossly [9] believe that the ninth-order polynomial is ideal, as large tidal signals cannot be deducted from a polynomial fit below the ninth-order. Finally, we use the method proposed by liu [10] to eliminate the influence of instrument tilt. After obtaining the residual gravity, we select the five quietest days with the lowest RMS values to calculate the background noise of the observed signals. In statistics, the five days could be fully representative of the cycle for a 2 mine1 h period seismic signal [11] even though the signals of the 1 h cycle were repeated 120 times. The noise level is best expressed with the PSD (power spectral density) since the amplitude of the PSD is independent of the data length and the sample rate. The data in the calmest five days were selected for the Fourier transform, and the average PSD was calculated via the following method:
X N sðkÞ ¼ f ðiÞexpð2piðj 1Þðk 1Þ=NÞ j¼1
PSD ¼ s2 Dt=N
(1)
where f ðiÞ is the observed value, j ¼ 1,…, N; Dt is the sampling interval, and sðkÞ is the amplitude spectrum. k1 and k2 are the lower and upper frequency values respectively, and M is the number of samples between k1 and k2 . The Parzen window was then used to smooth the 101 points [9]. Smoothing will not affect the overall noise level. In the 200e600s band of the average PSD, the SNM was calculated and defined as follows:
SNM ¼ lgðmeanPSDÞ þ 2:5
(4)
SNM is a quick way to compare noise levels at different points. Although SNM contains less information than PSD, it can be used as a standard to compare the noise levels of observation instruments.
3. Data analysis 3.1. Noise levels of basement- and cave-type gPhone gravimeters At present, gPhone gravimeters were installed in the basement or cave. The observations from four basement-type gPhone gravity stations with the smallest SNM (Yutian, Songpan, Enshi, and Zhongdian) and four cave-type stations (Altay, Hegang, Zhangjiakou and Nyingchi) were selected in 2015. Their PSDs are shown in Fig. 2, and the results of the noise levels in the seismic and subseismic bands are demonstrated in Table 1. Table 1 shows that the SNM ranges for the basement- and cavetype gPhone gravimeters are 2.0e2.79 (mean: 2.44) and 2.5e2.93 (mean: 2.72), respectively, whose overall differences are not big. Fig. 2 shows that two types of instruments have similar PSD. From the tidal frequency band to the sub-seismic frequency band, the
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002
X. Zhang et al. / Geodesy and Geodynamics xxx (2017) 1e5
3
Fig. 2. Seismic and sub-seismic frequency bands in basement- and cave-type observation conditions.
Table 1 Seismic and sub-seismic noise levels of gPhone gravimeters in two observation conditions.
Basement-type
Cave-type
Yutian Songpan Enshi Zhongdian Altay Hegang Zhangjiakou Nyingchi
SNM
SSNM
2.00453 2.49585 2.57638 2.78507 2.50443 2.65068 2.80634 2.92996
5.13014 5.95073 5.97234 5.60032 5.41752 5.51757 5.81959 5.78485
tidalfrequencies, solid Earth tides are more stable than oceanic tides. Therefore, after deducting the tides from the observations of the gPhone gravimeters, the PSDs of inland-environment instruments are lower than those of coastal-environment in tidal frequencies. Due to the impact of the tides on the observation results, the PSDs of the tidal band of coastal gPhone gravimeters are significantly higher than those of the inland gPhone gravimeters. For the sub-seismic and seismic frequency bands, the PSDs of the coastal instruments are also significantly higher than those of the inland-type instruments (Table 2). 3.3. Noise levels of arid and humid gPhone gravimeters
average differences of the four instruments selected from the two types of caves are very small. However, in the seismic band, the PSDs of the basement-type equipment were lower than those of the cave-type equipment. Therefore, we believe that the caveenvironment has very few effect on the noise levels of gPhone gravimeters. But for the seismic band, the PSDs of basement-type equipment were slightly lower than those of cave-type equipment. 3.2. Noise levels of coastal and inland-type gPhone gravimeters To determine the influences of the ocean on the noise levels of the gPhone gravimeters, we selected the observation data in 2015 from four coastal stations with the minimum SNMs (Xiamen, Wenzhou, Qingdao, and Dalian) and four inland stations (Yutian, Shiquanhe, Wushi, and Golmud) to calculate the noise levels of the seismic and sub-seismic bands. We found that the SNMs and SSNMs of coastal gPhone gravimeters were all larger than those of inland stations. It can be seen from Fig. 3, the PSDs of inland stations are generally lower than those of the coastal stations, indicating that the data from inland stations are more reliable in detecting the Earth's free oscillations and other non-oceanic signals. For
An important factor affecting gravity observation is local hydrology. Therefore, according to the average annual precipitation provided by China's meteorological data network, the observation data in 2015 from four gPhone gravimeters (Ruoqiang, Yutian, Yinchuan, and Altay) with the lower rainfall and the four gPhone gravimeters (Wenzhou, Ji'an, Tengchong, and Wuzhou) with the more rainfall were selected to calculate the noise levels of the seismic and sub-seismic band (Table 3). We found that the SNMs of Ruoqiang and Yinchuan stations are significantly higher than those
Table 2 Noise levels of seismic and sub-seismic coastal and inland gPhone gravimeters.
Inland type
Coastal type
Yutian Shiquanhe Wushi Golmud Qingdao Wenzhou Dalian Xiamen
SNM
SSNM
2.00453 2.09333 2.31528 2.4151 2.93984 3.52944 3.64453 3.67242
5.13014 5.20998 5.19868 5.55775 5.42156 6.22083 6.07665 6.34727
Fig. 3. PSD of coastal and inland gPhone gravimeters.
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Table 3 Noise levels of gPhone gravimeters in arid and humid areas of seismic and subseismic belts.
Arid
Humid
Ruoqiang Yutian Yinchuan Altay Wuzhou Tengchong Jian Wenzhou
SNM
SSNM
Annual rainfall (mm)
4.71997 2.00453 4.60018 2.50443 3.49355 3.49498 3.06309 3.52944
5.66711 5.13014 6.26639 5.41752 5.57314 5.63447 5.47058 6.22083
27.6 55.4 182.9 212.6 1452.9 1532.4 1566.2 1766.5
of other stations, while the SNMs of Yutian and Altay stations were the lowest. By comparing SSNMs, it was found that the SSNM range of the arid region was 5.13e6.27, and the SSNM range of the four stations with the highest rainfall was 5.47e6.22. And from the results in Fig. 4, there is no significant relationship between the noise levels of gPhone gravimeters and rainfall (Table 4). From the PSD results (Fig. 5), it can be seen that except the Tengchong station, the observation curves of the other four stations with more rainfall have similar PSDs. For seismic band, the PSDs of Yutian and Altay are lower than those of the other stations. The PSDs of the four stations with more rainfall are located between Ruoqiang, Yinchuan, Altay, and Yutian. The PSD results of the areas with lower rainfall may be smaller or larger than those of the areas with more rainfall, indicating that rainfall is not a major factor affecting PSD.
Fig. 4. Rainfall vs. SNM Chart. Table 4 Non-coastal stations with the highest 5 SNMs.
Ganzi Taiyuan Yinchuan Yushu Ruoqiang
SNM
SSNM
Seismic belt
3.65915 4.12775 4.60018 4.64329 4.71997
6.11135 6.02701 6.26639 6.26213 5.66711
Ganzi-Kangding seismic belt Shanxi seismic belt Yinchuan Hetao seismic belt Fenghuoshan fault belt Kunlun Mountains AltynTagh fault belt
3.4. Noise levels of gPhone gravimeters on seismic and non-seismic belts Comparing the SNMs of 36 gPhone gravimeters in 2015, we found that the eight stations with the highest SNMs are Taiyuan, Ruqiang, Yushu, Yinchuan, Xiamen, Ganzi, Dalian, and Wenzhou. Other than the three coastal stations, the remaining five are in the vicinity of the seismic belt. Comparing the PSDs of these five stations, we found that their PSDs in the seismic band are greater than 103 ðnm=s2 Þ2 Hz1 , which is an order of magnitude higher than those of the eight stations in non-seismic band (Fig. 2). The difference between the PSDs in the seismic band may mainly result from the influence of internal tectonic activity because the observation environment and precipitation have no significant influence on SNM, and all the stations are inland-environment. The PSDs of Yinchuan and Yushu in the sub-seismic frequency band are significantly higher than those of the other three stations (Fig. 6).
4. Conclusion In this paper, based on the observation data obtained by different types of gPhone gravimeters in 2015, the average power spectral densities and noise levels of the seismic and sub-seismic bands were calculated. We assumed that the noise level of the observed results mainly came from the influences of environmental factors when the noise levels of the instruments were almost identical. Comparing the influences of different observation environments on noise levels of gPhone gravimeters, we think that the influences of cave-environment on the noise levels of gPhone gravimeters were extremely small. The SNMs and SSNMs of coastal gPhone gravimeters were larger than those of inland stations. The PSDs of inland stations were generally lower than those of coastal stations, indicating that the noise levels of inland stations are smaller than those of costal stations. The noise levels were high in coastal areas due to the effects of the ocean. Although local hydrological changes have greater effects on gravity observations, noise levels have no direct relation with rainfall. Under the conditions that all stations are inland-type, the effects of rainfall and observation environment on noise level are very small, while internal tectonic activities may be the main source for the differences between the PSDs of seismic frequency-band. We found that the eight stations with the highest SNM are Ruoqiang, Yushu, Yinchuan, Xiamen, Ganzi, Taiyuan, Dalian, and Wenzhou. Other than the three coastal stations, the remaining five are in the vicinity of the seismic belt, indicating that the SNMs from the gPhone gravimeters may provide the seismic information to some extent.
Fig. 5. PSD of gPhone gravimeters of arid and humid areas on seismic and sub-seismic bands.
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Fig. 6. Non-coastal station PSD with the top five SNMs.
Compared with the NLNM model, the PSDs from the gPhone gravimeters were lower in the long period band (less than 3 105 Hz), indicating that gPhone gravimeters are more suitable for detecting long-period signals than seismograph (>10 h).
[6]
[7]
Acknowledgments We are grateful to the Gravity and Deformation Sub center of China Earthquake Administration Data Sharing Center to provide the gPhone data. The work is supported by key task project in Sicence for earthquake resilience No. XH17053 and the National Key Scientific Instrument and Equipment Development Projects of China (Grant No. 2012YQ10022506).
[8]
[9] [10]
[11]
correct long periodic gravity variations, J. Geophys. Res. Atmos. 111 (B10) (2006) 239e251. T. Sato, Y. Tamura, S. Okubo, S. Yoshida, Calibration of the scale factor of superconducting gravimeter at Esashi using an absolute gravimeter FG5, J. Geodetic. Soc. Jpn. 42 (4) (1996) 225e232. Z. Liu, H. Li, J. Wei, Accurate determination of calibration factor of gPhone spring gravimeters by using M2 tidal wave amplitude factor, J. Geodesy. Geodyn. 31 (5) (2011) 146e150. Xi Qinwen, The precision of the development of the tidal generating poes Terr. 105 (1989) tential and some explanatory notes, Bull. Inf. Mare 7396e7404. D. Banka, D. Crossley, Noise levels of superconducting gravimeters at seismic frequencies, Geophys. J. Int. 139 (1) (1999) 87e97. Z.W. Liu, L.I. Hui, K.X. Kang, Y.L. Wu, Determination accurate level position parameters of gPhone Gravimeter using indirect adjustment algorithm, Prog. Geophys. 27 (5) (2012) 1957e1961. S. Rosat, J. Hinderer, Noise levels of superconducting gravimeters: updated comparison and time stability, Bull. Seismol. Soc. Am. 101 (3) (2011) 1233e1241.
References [1] Sun Heping, Xu Jianqiao, B. Ducarme, The tidal experiment model based on the global superconducting gravimeter observation data which take the effect of liquid core daily resonance into consideration, J. Chin. Sci. Bull. 48 (6) (2003) 610e614. [2] Lei Xiang'e, Xu Houze, Sun Heping, Sci. China Ser. D Earth Sci. 34 (5) (2004) 483e491, http://dx.doi.org/10.1360/zd2004-34-5-483 (in Chinese). [3] Ding, Shen, Determination of the complex frequencies for the normal modes below 1mHz after the 2010 Maule and 2011 Tohoku earthquakes, Ann. Geophys. 56 (5) (2013), http://dx.doi.org/10.4401/ag-6400. [4] J.Q. Xu, J.C. Zhou, S.C. Luo, H.P. Sun, Study on characteristics of long-term gravity changes at Wuhan station, Sci. Bull. 53 (13) (2008) 2033e2040. [5] M.V. Camp, M. Vanclooster, O. Crommen, T. Petermans, K. Verbeeck, Hydrogeological investigations at the Membach station, Belgium, and application to
Xiaotong Zhang, assistant researcher, Institute of Seismology, China Earthquake Administration. Her interests include the research on continuous gravity data processing.
Please cite this article in press as: X. Zhang, et al., The influence of observation environment on background noise level of gPhone gravimeter, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.06.002