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The distinction of a fluctuation from a long-term change in Earth processes is a key question in the assessment of the Earth's Climate change and in general geo- risk assessment. The distinction of a fluctuation from a steady change requires knowledge on the time variability of the signal and long term observations. Due to the decadal variability of sea level, reliable sea level trends can only be obtained after about sixty years of continuous observations. Reliable strain rates of deformation require a minimum of a decade of continuous data, due to the ambient factors leading to fluctuations. The session invites contributions that demonstrate the importance of long term geophysical, geodynamic, oceanographic and climate observatories. Advances in sensors, instrumentation, data analyses, and interpretations of the data are welcome, with the aim to stimulate a multidisciplinary discussion among those dedicated to the accumulation, preservation and dissemination of data over decadal time scales or beyond. With this session, we also would like to provide an opportunity to gather for representatives from observatories in Europe and also world-wide.

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Co-organized by G6
Convener: Nina Kukowski | Co-conveners: Carla Braitenberg, Hans-Peter Bunge, Stuart Gilder
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| Attendance Fri, 08 May, 14:00–15:45 (CEST)

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Chat time: Friday, 8 May 2020, 14:00–15:45

Chairperson: Nina Kukowski
D1245 |
EGU2020-1785
Kelly Brunt and Robert Hawley

The Greenland Geodetic Network (GNET) consists of 58 global navigation satellite systems (GNSS) installed on the bedrock around the perimeter of the island. Much of the network was installed between 2007 and 2009, providing a long time series of GNSS data for much of Greenland. The network is currently owned and maintained by the Danish Agency for Data Supply and Efficiency (SDFE), while the National Science Foundation (NSF) provides support for data transport from the deep field. Here, we present a new resource (go-gnet.org) intended to be a clearinghouse to foster international collaborations and to encourage new and innovative use of these data.

How to cite: Brunt, K. and Hawley, R.: The Greenland Geodetic Network (GNET), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1785, https://doi.org/10.5194/egusphere-egu2020-1785, 2019

D1246 |
EGU2020-3753
| solicited
| Highlight
Roland Pail, Henryk Dobslaw, Annette Eicker, and Laura Jensen

Gravity field missions are a unique geodetic measuring system to directly observe mass transport processes in the Earth system. Past and current gravity missions such as CHAMP, GRACE, GOCE and GRACE-Follow On have improved our understanding of large-scale mass changes, such as the global water cycle, melting of continental ice sheets and mountain glaciers, changes in ocean mass that are closely related to the mass-related component of sea level rise, which are subtle indicators of climate change, on global to regional scale. Therefore, mass transport observations are also very valuable for long-term climate applications. Next Generation Gravity Missions (NGGMs) expected to be launched in the midterm future have set high anticipations for an enhanced monitoring of mass transport in the Earth system with significantly improved spatial and temporal resolution and accuracy. This contribution will present results from numerical satellite mission performance simulations designed to evaluate the usefulness of gravity field missions operating over several decades for climate-related applications. The study is based on modelled of mass transport time series obtained from future climate projections until the year 2100 following the representative emission pathway RCP8.5 Numerical closed-loop simulations will assess the recoverability of mass variability signals by means of different NGGM concepts, e.g. GRACE-type in-line single-pair missions, Bender double-pair mission being composed of a polar and an inclined satellite pair, or high-precision high-low tracking missions following the MOBILE concept, assuming realistic noise levels for the key payload. In the evaluation and interpretation of the results, special emphasis shall be given to the identification of (natural or anthropogenic) climate change signals in dependence of the length of the measurement time series, and the quantification of robustness of derived trends and systematic changes.

How to cite: Pail, R., Dobslaw, H., Eicker, A., and Jensen, L.: Recovering climate-related mass transport signals by current and next-generation gravity missions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3753, https://doi.org/10.5194/egusphere-egu2020-3753, 2020

D1247 |
EGU2020-4198
Alvaro Santamaría-Gómez and Jim Ray

Chameleonic: readily changing color or other attributes.

Chameleon: a lizard that changes skin color to match what surrounds it so that it cannot be seen.

The error spectrum of decadal long GPS position time series is typically represented by a combination of flicker (pink) noise at long periods and white noise at short periods. It is known that when fitting a linear trend to the series, part of the flicker noise at the longest observed period will be absorbed by the trend. Here, using real and synthetic GPS position series, we show how the error spectrum is even more altered by the position discontinuities that populate the series. The fitted position offsets at the discontinuity epochs absorb a significant portion of the power spectrum at periods longer than the separation between the discontinuity epochs. The resulting error spectrum is flattened at long periods and this implies that:

  • the estimated content of colored noise is biased low and can even apparently change its color towards whiter noise, i.e. the true noise color is not observable due to the discontinuities,
  • the red (random walk) noise , most probably present in the series in small quantity, becomes undetectable even if long series are used,
  • the pink (flicker) noise is not the best color noise to represent the error spectrum in long series containing discontinuities,
  • the colored noise content cannot be compared between series with different sets of discontinuities.

These findings need to be considered when comparing the noise levels between series from different solutions, networks or monuments. In particular, and contrary to a recently published recommendation, station operators should make every effort to avoid adding new discontinuities into their station time series if reliable velocity estimates are expected.

How to cite: Santamaría-Gómez, A. and Ray, J.: Chameleonic noise in GPS position series: what is the true color of the GPS error spectra?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4198, https://doi.org/10.5194/egusphere-egu2020-4198, 2020

D1248 |
EGU2020-4432
Michel Van Camp, Olivier de Viron, Bruno Meurers, and Olivier Francis

Being sensitive to any phenomena associated with mass transfer, terrestrial gravimetry allows the monitoring of many phenomena at the 10-10 g level (1 nm/s²) such as Earth tides, groundwater content, tectonic deformation, or volcanic activity. This sensitivity is richness, but also a source of problems because data interpretation requires separating the signatures from the different sources, including possible measurement artefacts associated with high precision. Separating the signal from a given source requires a thorough knowledge of both the instrument and the phenomena.

At the Membach geophysical laboratory, Belgium, the same superconducting gravimeter has monitored gravity continuously for more than 24 years. Together with 300 repeated absolute gravity measurements and environmental monitoring, this has allowed us to reach an unprecedented metrological knowledge of the instrument and of its sensitivity to hydrological and geophysical signals.

Separation is possible whenever the phenomena exhibit distinct time/frequency signatures, such as (pseudo)periodic phenomena or long-term processes, so that the signatures from other sources average out by stacking. For example, when performing repeated gravity measurements to evidence slow tectonic deformation, the easiest way to mitigate hydrological effects is to accumulate measurements for many years, at the same epoch of the year: the impact of seasonal variations is then minimized, and the interannual variations cancel out. Using 10 repeated absolute gravity campaigns at the same epoch of the year, we showed that the gravity rate of change uncertainty reaches on average 3–4 nm/s²/yr. Concurrently, using superconducting gravimeter time series longer than 10 years, we also investigated the time variations of tidal parameters.

It is also possible to separate phenomena by observing them by both gravity and some other techniques, with a different transfer function. By using 11 year-long times series from the gravimeter and soil moisture probes, and by stacking the observations, we measured directly the groundwater mass loss by evapotranspiration in the forest above the laboratory of Membach. Always with a precision better than 1 nm/s² (<=> 2.5 mm of water), we also monitored ground partial saturation dynamics and combining the gravity data with a weather radar allowed measuring convective precipitation at a scale of up to 1 km².

Extracting and interpreting those elusive signals could only by achieved throughout multi-instrumentation, multi-disciplinary collaborative studies, and 25 years of hard work.

How to cite: Van Camp, M., de Viron, O., Meurers, B., and Francis, O.: Measuring gravity changes for decades, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4432, https://doi.org/10.5194/egusphere-egu2020-4432, 2020

D1249 |
EGU2020-5789
Nina Kukowski, Thomas Jahr, Andreas Goepel, and Cornelius Schwarze

The Geodynamic Observatory Moxa of Friedrich-Schiller University Jena, assigned to the Chair of General Geophysics of the Institute of Geosciences, is located about 30 km south of Jena in the Thuringian slate mountains. Due to its isolated location and the possibility of subsurface installations in a gallery or in boreholes, Moxa observatory provides excellent conditions for long term observations.
Moxa observatory is equipped with various geophysical sensor systems to observe transients signals of the local gravity field (superconducting Gravimeter CD-034, LCR-ET-18), deformation (altogether three laser strain meters with base length of 28 and 36 m, respectively, which also enable to estimate areal strain; ASKANIA borehole tilt meters, Ilmenau tilt meter,) and of subsurface temperatures (optical glass fibre in a 100m deep borehole). These systems are complemented e.g. through temperature sensors placed within the gallery, water level gauges and a climate station to record environmental parameters. Most sensor system are recording with a resolution in the nano- or subnano range, which allows to study very small parameter changes and thus to identify even very faint natural signals. All recorded time series show high signal to noise ratios for a large range of frequencies.
Some of our long-term observations already have led to more than two decades of continuous time-series, whereas other sensors now have been recording for about five to ten years. Here we provide a  concise overview about important goals and results of the records of the individuals instruments at Moxa: The analyses of Earth tides over the last 20 years show variations of the tidal parameters for the main tidal constituents, which may be caused by changes of the ocean loading effect, due to a worldwide redistribution of water masses probably linked to the increase of the sea surface hight (SSH). Investigations regarding gravity effects of storm surges show that e.g. for the North Sea a significant gravity signal which is detectable in the data of the superconducting gravimeter at Moxa observatory. Both results are based observations independent of satellite data and therefore they are an important complement to findings e.g. from satellite altimetry. Deformation signals like tilt and strain are very sensitive to hydrological signals, e.g. pore pressure fluctuations, and enable to detect both, global and local groundwater flow effects. However, as it is often difficult to clearly identify the cause of hydrological signals, these records need to be complemented by independent observations. This is done via recording temperature along a borehole which enables to detect local thermal anomalies, which can be related to groundwater movements. Further, the temperature-depth time series also mirror seasonal solar contributions to the subsurface thermal inventory as well as environmental effects. Besides contributing to geophysical research topics, the improvement and further development of sensor technology and the configuration of data acquisition systems, with emphasis on tilt, strain, and temperature recording sensor systems is a further important goals of our group, which is realised in cooperation with other institutions and companies in and close to Jena.

How to cite: Kukowski, N., Jahr, T., Goepel, A., and Schwarze, C.: The FSU Jena Geodynamic Observatory Moxa (Thuringia, central Germany): Instrumentation, observations and results from different sensor systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5789, https://doi.org/10.5194/egusphere-egu2020-5789, 2020

D1250 |
EGU2020-7288
| Highlight
Cornelius Schwarze, Thomas Jahr, Andreas Goepel, Valentin Kasburg, and Nina Kukowski

Longterm geophysical recordings of natural Earth’s parameters besides other signals also may contain past and ongoing temperature fluctuations, as they are occurring e.g. when groundwater moves or when climate changes. Similarly, repeated logs or continuous recordings reveal the amount of ongoing climate fluctuations. However, such thermal signals in the subsurface also may have other causes, e.g. groundwater motion or fluid infiltration after strong rainfall events. The Geodynamic Observatory Moxa of the Friedrich-Schiller University Jena, Germany, is an ideal test site for long-term monitoring of the subsurface temperature distribution in boreholes using optical fibre temperature-sensing, as it is equipped with a variety of complementary sensors.

A 100 m deep borehole on the ground of the Observatory, is equipped with an optical fibre and a water level gauge. Clearly shown in the records of the first five years of continuous recordings are seasonal temperature fluctuations. Seasonal fluctuations could be identified down to a depth of about 20 m and diurnal temperature signals down to 1.2 m. Precipitation events may influence subsurface temperature still in a depth as deep as 15 m. Besides these, temperature anomalies were detected at two depths, 20 m and 77 m below the surface. These anomalies most probably result from enhanced groundwater flow in aquifers. Recordings of deformation from laser strain meter systems installed in a gallery at Moxa, which are highly sensitive to pore pressure fluctuations, and measuring the physical properties during drilling the borehole, help to identify and quantify the causes of the observed  temperature fluctuations.

For more than 20 years variations of the Earth’s gravity field have been observed at the Observatory Moxa employing the superconducting gravimeter CD-034. Besides the free oscillations of the Earth and hydrological effects, the tides of the solid Earth are the strongest signals found in the time series. Tidal analysis of the main constituents leads to obtaining the indirect effect for all tidal waves which is mainly controlled by the loading effect of the oceans. Satellite altimetry revealed a mean global sea level rise of about 3.3 mm/a which may be caused amongst others mainly by ice melting processes in the polar regions. However, more detailed analyses and resulting global maps show that the sea level rise is not uniformly distributed over all oceans. This means that actual and future tidal water mass movements could vary regionally and even locally.  As a consequence, the tidal parameter controlled by the ocean loading effect could change over long-term observation periods and it should possibly be detectable as a trend or temporal variation of the tidal gravity parameters locally. Actually, a long-term change of the tidal parameters is observed for the main tidal waves like K1 and O1 in the diurnal and for M2 and K2 in the semi-diurnal frequency band. However, it is not clear if these changes can be correlated with sea level changes as observed from satellite data. On the other hand, surface and subsurface temperature fluctuations directly reveal the size of the thermal signal inherent to climate change.

How to cite: Schwarze, C., Jahr, T., Goepel, A., Kasburg, V., and Kukowski, N.: Long-term observations at the Geodynamic Observatory Moxa: Can we identify evidence for climate change?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7288, https://doi.org/10.5194/egusphere-egu2020-7288, 2020

D1251 |
EGU2020-7823
| solicited
Vincent Lesur and Aude Chambodut

In magnetic observatories the Earth’s magnetic field is continuously recorded and the acquired data are calibrated so that the evolution of the field can be studied on time scales ranging from seconds to decades. The French network (the so called BCMT) includes 18 observatories around the world and the different types of data produced are freely accessible on several data centres. We will describe a typical infrastructure of a magnetic observatory, the measurement techniques, the instruments used, the general processing applied to obtain calibrated data and finally the environmental constraints that have to be respected in order to acquire suitable data. If magnetic observatories were originally set to monitor the slow variations of the Earth’s main magnetic field, they are more and more often used for space-weather monitoring and to study signal generated in the ionosphere and magnetosphere. This new range of applications implies an evolution of the network, of the acquisition and distribution techniques. The strategy we developed to respond to these new challenges will be also presented.

How to cite: Lesur, V. and Chambodut, A.: The French network of magnetic observatories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7823, https://doi.org/10.5194/egusphere-egu2020-7823, 2020

D1252 |
EGU2020-8534
Stuart Gilder, Michael Wack, Elena Kronberg, and Ameya Prabhu

We developed a new technique based on differences in instrument responses from ground-based magnetic measurements that extracts the frequency content of the magnetic field with periods ranging from 0.1 to 100 seconds. By stacking hourly averages over an entire year, we found that the maximum amplitude of the magnetic field oscillations occurred near solar noon over diurnal periods at all latitudes except in the auroral oval. Seasonal variability was identified only at high latitude. Long-term trends in field oscillations followed the solar cycle, yet the maxima occurred during the declining phase when high-speed streams in the solar wind dominated. A parameter based on solar wind speed and the relative variability of the interplanetary magnetic field correlated robustly with the ground-based measurements. Our findings suggest that turbulence in the solar wind, its interaction at the magnetopause, and its propagation into the magnetosphere stimulate magnetic field fluctuations at the ground on the dayside over a wide frequency range. Our method enables the study of field line oscillations using the publicly available, worldwide database of geomagnetic observatories.

How to cite: Gilder, S., Wack, M., Kronberg, E., and Prabhu, A.: Solar cycle variations in differential instrumental responses from ground‑based geomagnetic records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8534, https://doi.org/10.5194/egusphere-egu2020-8534, 2020

D1253 |
EGU2020-11965
Dorothee Rebscher

Mont Terri rock laboratory, located in the Swiss Jurassic Mountains, was established with the focus on the investigation and analysys of the properties of argillaceous formations. The scope of Opalinus Clay as a safe, potential option for nuclear waste disposal was broaden, as the behaviour of claystone is of high interest also in the context of caprocks, and hence, for many dynamical processes in the subsurfaces. Extensive research has been performed already for more than 20 years by the partners of the Mont Terri Consortium. These close cooperations cover a broad range of scientific aspects using numerical modelling, laboratory studies, and last not least in-situ experiments. Here, included in the long-term monitoring programme, new investigations apply tiltmeters. Since April 2019, platform tiltmeters have been installed at various locations within the galleries and niches of Mont Terri. The biaxial instruments have resolutions of 1 nrad and 0.1 µrad, respectively (Applied Geomechanics and Lippmann Geophysikalische Messgeräte). The tilt measurements are embedded within various experiments contributing to specific, multiparametrical studies. However, the growing tilt network as a whole will also provide novel information of the rock laboratory. The different time-scales of interest include long-term observations of yearly and decadal variability. So far tilt signals were identified due to excavations during the recent enlargement of the laboratory, earthquake activity (Albania), and local effects. First results of these quasi-continuous recordings will be presented.

How to cite: Rebscher, D.: Tiltmeter Measurements at the Underground Rock Laboratory in Mont Terri, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11965, https://doi.org/10.5194/egusphere-egu2020-11965, 2020

D1254 |
EGU2020-14897
Carla Braitenberg, Barbara Grillo, Alberto Pastorutti, and Tommaso Pivetta

The long term monitoring of crustal deformation in NE-Italy derives from tilt and strainmeter observations since 1960. The stations have been maintained by three generations of scientists starting with the geodesist Antonio Marussi, keeping the instrumentation active and up to date. The decade-long time series have given observations of rare events, as the free oscillations recorded by the largest earthquakes ever recorded (Chile 1960, Sumatra 2004, Tohoku 2011) and climatic extreme events leading to extremely intense rainfalls that generate underground flooding and surface deformation (Braitenberg et al., 2019; Braitenberg, 2018). The stations have the characteristic of being representative of geodetic monitoring in karst geologic formation, that they are placed in a seismically active area which has experienced a magnitude M 6.4 earthquake in the past (1976 Gemona), and that they are influenced by the ocean loading deformation of the Adriatic Sea. The seismic area implies that the strain accumulation is an ongoing process, presently activating the elastic energy of the next earthquake. We show some relevant observations, which could hardly have been caught without such a long time series. Between 1973 and 1976 the long base horizontal pendulums of the Grotta Gigante cave gave episodic disturbances, that seized 6 months after the Gemona main shock. The hydrology of the karst is made of an underground channel system that is completely flooded during extreme rainfall and is pressurized close to simultaneously over a distance of 30 km, leading to an observable uplift and deformation of the surface (Braitenberg et al., 2019). It has been possible to extract and model this type of deformation.

The tilt and strainmeters have high accuracies and precision in the detection of crustal deformation, with the drawback to be point measurements. InSAR acquisitions cover thousands of points on the surface, but with coarser accuracy. One major problem is in the correction of atmospheric effects in the InSAR signal, which produces apparent movement in the direction of Line of Sight, uncorrelated to the real soil movement. Our present research objective is the transfer of knowledge from the signals known in the tilt and strainmeter observations to the detection of these signals with InSAR. 

 

Braitenberg C. (2018). The deforming and rotating Earth - A review of the 18th International Symposium on Geodynamics and Earth Tide, Trieste 2016 , Geodesy and Geodynamics, 187-196, doi::10.1016/j.geog.2018.03.003 .

Braitenberg C., Pivetta T., Barbolla D. F., Gabrovsek F., Devoti R., Nagy I. (2019). Terrain uplift due to natural hydrologic overpressure in karstic conduits. Scientific Reports, 9:3934, 1-10, doi.:10.1038/s41598-019-38814-1.

How to cite: Braitenberg, C., Grillo, B., Pastorutti, A., and Pivetta, T.: Long term observation of crustal deformation in NE-Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14897, https://doi.org/10.5194/egusphere-egu2020-14897, 2020