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Terrain gravimetry is a powerful geophysical tool that, through sensing changes in subsurface mass, can supply unique information on the dynamics of underground fluids, like water, magma, hydrocarbons, etc. This is critically important for energy industry (not just petroleum and natural gas, but also geothermal), resource management (particularly, with regard to water), and natural hazards (especially volcanoes).
Despite its potential, terrain gravimetry is currently underexploited, owing to the high cost of available instrumentation and the difficulty in using it under harsh environmental conditions and to the major challenge posed by retrieving useful information from gravity changes in noisy environments.
Major technology developments have recently occurred in instrumentation and methodology and are being demonstrated, opening up new perspectives to increase the capability of terrain gravimetry. On one hand, new types of sensors are being developed and ruggedized, expanding the measurement capabilities. On the other hand, methodologies and workflows are developed to exploit more efficiently hybrid networks of sensors. As an example, a recently funded H2020 project, called NEWTON-g, targets the development and field application of a “gravity imager” exploiting MEMS (relative) and quantum (absolute) gravimeters. These advancements will give new impulse to terrain gravimetry, thus helping its transition from a niche field into a cornerstone resource for geophysical monitoring and research. However, for this transition to succeed, technology developments must be complemented by constructive feedback from the gravimetry community
This session aims at bringing together instrument and tool developers and end-users of terrain gravimetry in a variety of fields, including, but not limited to, hydrology, volcanology and petroleum geology. We aim at discussing the state of the art of terrain gravimetry and the added value it provides with respect to other geophysical techniques, as well as the exciting opportunities offered by the new technologies under development.

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Convener: Daniele Carbone | Co-conveners: Hammond Giles, Jean Lautier-Gaud, Eleonora Rivalta, Filippo Greco
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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: D. Carbone and J. Lautier-Gaud
D1565 |
EGU2020-8392
Sébastien Merlet, Raphael Piccon, Sumit Sarkar, and Franck Pereira Dos Santos

Gravity measurements are performed with two different classes of instruments: gravimeters, most widely used, measure the gravity acceleration gand its variations, whereas gradiometers measure its gradient.

Quantum gravity sensors, based on cold atom interferometry techniques, can offer higher sensitivities and accuracies than current state of the art commercial available technologies. Their limits in performances, both in terms of accuracy and long term stability, are linked to the temperature of the atomic cloud, in the low µK range, and more specifically, to the residual ballistic expansion of the atomic sources in the laser beams. To overcome these limits, we use ultracold atoms in the nano-kelvin range in our sensors.

I will first present our Cold Atom Gravimeter (CAG) used for the determination of the Planck constant with the LNE Kibble Balance [1]. It performs continuously 3 gravity measurements per second with a demonstrated long term stability of 0.06 nano-gin 40 000 s of measurement. Using ultracold atoms produced by evaporative cooling in a crossed dipole trap as a source, its accuracy, which is still to be improved, is currently at the level of 2 nano-g. This makes our CAG, the more accurate gravimeter [2]. It detects water table level variations. Then I will describe a « dual sensor » which performs simultaneous measurements of g and its gradient. This offers in principle the possibility to resolve, by combining these two signals, the ambiguities in the determination of the positions and masses of the sources, offering new perspectives for applications. It uses cold atom sources for proof of principle demonstrations [3, 4] and will soon combine ultra-cold atomic samples produced by magnetic traps on a chip and large momentum beamsplitters. With these two key elements, the gradiometer will perform measurements in the sub-E sensitivity range in 1 s measurement time on the ground. Such a level of performances opens new prospects for on field and on board gravity mapping, for drift correction of inertial measurement units in navigation, for geophysics and for fundamental physics.

 References

[1] M. Thomas et al. Metrologia 54, 468-480 (2017)

[2] R. Karcher, et al. New J. Phys. 20, 113041 (2018)

[3] M. Langlois et al. Phys. Rev. A 96, 053624 (2017)

[4] R. Caldani et al. Phys. Rev. A 99, 033601 (2019)

How to cite: Merlet, S., Piccon, R., Sarkar, S., and Pereira Dos Santos, F.: Quantum Absolute Sensors for Gravity Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8392, https://doi.org/10.5194/egusphere-egu2020-8392, 2020.

D1566 |
EGU2020-9076
Cédric Champollion, Anne-Karin Cooke, and Nicolas Le Moigne

The recent advancements in gravity quantum sensors promise maintenance-free, easy to use, continuous and accurate monitoring devices. This technological breakthrough in gravity instrumentation offers new possibilities for both laboratory and field experiments in different geosciences applications. These new gravity quantum sensors allow e.g. for the monitoring of transient processes in volcanology, plate tectonics (slow slip events) or hydro-geology (pumping tests).

The first commercial field quantum gravimeters are nowadays available (AQGB, Muquans TM). The AQG#B01 is actually under validation. It is tested and compared with a superconducting gravimeter (GWR iGrav#002 and an absolute ballistic gravimeter (MG-L FG5#228) in the French Larzac Observatory () during more than 1 month. A first small (50 nm/s²) transient gravity variation caused by hydro-geological charge has been recorded by both the quantum and superconducting gravimeter.

Additionally its sensitivity to environmental noise is characterized by its Allan variance. Absolute ballistic comparison during one month allows to estimate a maximum potential drift. Sensitivity tests on instrument tilts and orientation have been done. In order to evaluate the AQG-B as a field sensor, sensitivity to external temperature changes have been tested in the range 10°C-30°C. All the tests allow a clear characterization of the AQG-B for future field experimentation.

AQG#B01 development has been funded is the frame of the grant “investissement d’avenir” EquipEx RESIF-CORE.

How to cite: Champollion, C., Cooke, A.-K., and Le Moigne, N.: Comparison and characterization of the field Atomic Quantum Gravimeter (AQG#B01), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9076, https://doi.org/10.5194/egusphere-egu2020-9076, 2020.

D1567 |
EGU2020-12317
Zhijun Du, Arif Mustafazade, Yaoguo Li, Adrian Topham, Jeremy Lofts, and Ashwin Seshia

Microgravity measurements have enabled a variety of geophysical surveying and monitoring applications including advance warning of natural hazards, slope stability monitoring, discovery of buried tunnels, pipework, and other utilities, identification of sinkholes and other natural voids, buried aquifers and in monitoring groundwater hydrology. In the civil engineering context, microgravity measurements can provide valuable information for construction projects or intervention activities by locating buried utilities, hazards or other features of relevance.

Disruptive MEMS gravity sensor technologies are poised to provide entirely new approaches for microgravity measurements in the form of portable sensors that could ultimately be mounted on remotely operated vehicles or drones, integrated into land-based distributed sensor networks, or deployed in shallow borehole configurations. Instruments based on these sensors could enable vector gravity measurements as well as full tensor gravity gradiometry.

Trials are ongoing of a single-axis MEMS surface module with a noise floor of 50 µGal/rt-Hz and a resolution of < 10 µGal while allowing for measurement over the entire +/- 1g dynamic range. This paper discusses the background and context for gravity imaging in geotechnical applications, forward modelling of case studies of relevance, and ongoing developments in the construction of a unique portable surface gravimeter.

How to cite: Du, Z., Mustafazade, A., Li, Y., Topham, A., Lofts, J., and Seshia, A.: MEMS surface microgravimetry for geotechnical surveying, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12317, https://doi.org/10.5194/egusphere-egu2020-12317, 2020.

D1568 |
EGU2020-18528
Abhinav Prasad, Karl Toland, Andreas Noack, Kristian Anastasiou, Richard Middlemiss, Douglas Paul, and Giles Hammond

Novelty / Progress Claim(s)

This paper reports a capacitive readout-based MEMS relative gravimeter which can detect sub-Hz microseismic and slowly varying gravitational Earth tide signals. The gravimeter has a noise floor of 6-7 uGal/rt(Hz) at 1Hz and a linear drift of <250 uGal/day, metrics which are on a par with the commercially available gravimeters, and are leading in the field of MEMS accelerometers. The gravimeter is packaged in a standard ceramic-carrier and interfaced to a low-power, advanced FPGA-based readout. This setup is housed within a bespoke thermal enclosure, making the platform ideal for multi-pixel array-based implementation in the field.

Background/State-of-the-Art

Gravimeters are used to measure the local acceleration due to gravity (g). One of the emerging applications of gravimetry is in volcanology where gravimeters can be used to understand magma plumbing, providing information on volcanic activity/unrest events. However, this requires multi-pixel ‘gravity-imaging’ around volcanoes, a feat which is not possible using the expensive, complex, and large form-factor commercially available gravimeters.

Recently, researchers have developed MEMS-scale accelerometers which have excellent sensitivities but not yet demonstrated good long-term stability, making them non-viable for long-term monitoring of slow gravity changes (such as produced by magma flow). In a previous work, the authors have demonstrated an optical shadow-sensor readout based MEMS gravimeter with a sensitivity of 40 uGal/rt(Hz). Building on the work, a portable version of the gravimeter was also reported previously. The devices in both the setups were limited by the displacement noise of the optical shadow-sensor and the packageability of the setup.

In this paper, we are reporting a novel gravimeter which uses a capacitive-readout for sensing the proof-mass displacement, is embedded in a MEMS IC package, and uses advanced FPGA-based electronics for signal conditioning. The improved displacement sensitivity of the capacitive readout allows designing stiffer suspension-springs making the device more robust for operations in extreme environments. The acceleration sensitivity achieved using the new gravimeter is around 6-7 uGal/rt(Hz) at 1Hz, which is a significant improvement over the previous versions of the gravimeter. The device is currently being readied for field trials in the sectors of volcano gravimetry and oil & gas, showing the maturity of the technology.

Methodology

The reported gravimeter has a microfabricated silicon proof-mass which is suspended from thin flexures. Metal-combs are patterned on top of the proof-mass and a fixed glass layer with complementary combs is assembled to be at a close separation from the proof-mass. The overlapping combs act as a capacitor, the magnitude of which is dependent on the proof-mass displacement. The multi-layered gravimeter is embedded within a standard 32-pin ceramic DIP chip-carrier and wire bonded. The MEMS package is interfaced with analog signal conditioning electronics and a digital lock-in implementation is employed for converting the capacitance change into useful units (uGals).The electronics noise of the setup is measured to be <1 uGal. To reduce temperature-related effects, a mK active temperature control is implemented around the device. The packaged device is housed within a prototype thermal enclosure making the platform field-portable.

How to cite: Prasad, A., Toland, K., Noack, A., Anastasiou, K., Middlemiss, R., Paul, D., and Hammond, G.: A High-Sensitivity, Low-Drift MEMS Relative Gravimeter for Multi-Pixel Imaging Applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18528, https://doi.org/10.5194/egusphere-egu2020-18528, 2020.

D1569 |
EGU2020-4647
Mehdi Nikkhoo, Eleonora Rivalta, Daniele Carbone, and Flavio Cannavò

The transport of magma and magmatic fluids is a key process behind the occurrence, duration and intensity of volcanic crises. Volcano gravimetry allows for unequivocal inference of the location and mass of accumulated or removed magmatic fluids at volcanoes. This task is best accomplished through collecting gravity time series at multiple stations simultaneously. The performance of individual gravimeters and the configuration of the gravimetric array, however, determine the threshold of detectable mass change and the ability of the array to minimize the uncertainty on the inferred quantities.

We utilize numerical optimization techniques to design a network including one absolute quantum gravimeter (AQG), two superconducting relative gravimeters (iGRAVs) and several microelectromechanical system (MEMS) relative gravimeters at Mount Etna. We also develop analytical solutions for simple design problems. We show that the analytical solutions are essential for validating the numerical optimization procedure. We provide practical details and caveats that should be considered in similar gravimetric network optimizations. These include 1) specifying the target zone of the network by using the history of mass transport, 2) accounting for the relative importance of  different parts of the target zone, 3) accounting for logistic and instrumental constraints in the optimizations  4) calibrating the objective functions associated with various optimizations, 5) analyzing the network sensitivities to different parts of the target zone and identifying blind zones and  6) calculating the optimal number of gravimeters as a function of the sensor sensitivity and accuracies. We show that our optimal solution for Mount Etna provides an improved detection power across the target zone as compared to an equally spaced network of gravimeters with the same existing constraints, surface topography and sensor sensitivities. Furthermore, this optimal solution ensures that a certain range of mass change anywhere in the target zone can be sensed by a given minimum number of gravimeters and at the same time minimizes the impact of random observation errors on the inferred quantities.

How to cite: Nikkhoo, M., Rivalta, E., Carbone, D., and Cannavò, F.: Analytical and numerical optimization of gravimetric networks: a case study from Mount Etna, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4647, https://doi.org/10.5194/egusphere-egu2020-4647, 2020.

D1570 |
EGU2020-2184
Benjamin Fores, Arnaud Watlet, Michel Van Camp, and Olivier Francis

Spring-based gravimeters are light and easy to install, with a precision around 5 μGal/√Hz. However, they are still not used for long-term gravity monitoring. The main reason for that is the non-linear drift of those instruments, which is very difficult to correct without removing geophysical signals. We will show that when the tilt is actively controlled, a gPhone spring-based gravimeter shows a quasi-linear drift and can reach a long-term stability at the µGal level.

This allows experiments such as the one in the Rochefort Cave Laboratory (Belgium). Thanks to the size of the gPhone and its low facility requirements, a monitoring from inside a cave was possible. Coupled with another gravity monitoring at the surface, it reveals new information on the local hydrology of this karstic site.

How to cite: Fores, B., Watlet, A., Van Camp, M., and Francis, O.: Long-term monitoring with spring-based gravimeters: tilt-control benefits and application to the Rochefort Cave Laboratory (Belgium), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2184, https://doi.org/10.5194/egusphere-egu2020-2184, 2020.

D1571 |
EGU2020-11801
Florian Schäfer, Philippe Jousset, Tania Toledo, Andreas Güntner, Tilo Schöne, David Naranjo, Kemal Erbas, Egill Juliusson, and Richard Warburton

In volcanic and hydrothermal systems, monitoring of mass and stress changes by continuous gravity field and ground motion records provides information for both volcanic hazard assessment and estimation of geothermal resources. We aim at a better understanding of volcanic and geothermal system processes by addressing mass changes in relation with external influences such as anthropogenic (reservoir exploitation) and natural forcing (local and regional earthquake activity, earth tides). Þeistareykir is a geothermal field located within the Northern Volcanic Zone (NVZ) of Iceland on the Mid-Atlantic Ridge. Geothermal power production started in autumn 2017. For the first time on a geothermal production field, we deployed a network of 4 continuously recording gravity meters (3 superconducting meter, iGrav and one spring gravity meter gPhone) in order to cover the spatial and the temporal changes of gravity and to detect small variations related to the geothermal power plant operation (e.g. extraction and injection). All gravity monitoring stations are equipped with additional instrumentation to measure parameters that may affect the gravity records (e.g. GNSS and hydrometeorological sensors). Additionally, we deployed a temporal seismic network consisting of 14 broadband stations to enhance the seismic activity monitoring of the permanent Icelandic network in this very active region of the NVZ. Results of this unique experiment contribute to determine reservoir properties and main structures and may also reveal details of active tectonic processes. Here, we present the instrumental setup at the site and first results of more than 24 months of continuous gravity and seismicity records.

How to cite: Schäfer, F., Jousset, P., Toledo, T., Güntner, A., Schöne, T., Naranjo, D., Erbas, K., Juliusson, E., and Warburton, R.: Integrated microgravimetric and seismic monitoring approach in the Þeistareykir volcanic geothermal field (North Iceland). , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11801, https://doi.org/10.5194/egusphere-egu2020-11801, 2020.

D1572 |
EGU2020-20906
Elizabeth Passey, Giles Hammond, Steven Bramsiepe, Abhinav Prasad, Richard Middlemiss, Douglas Paul, Richard Walker, Andreas Noack, and Kristian Anastasiou

Gravimetry allows us to study sub-surface structures remotely by measuring changes in Earth's surface gravitational field and using this data to infer the density of geological structures. Of its wide range of applications, it is mostly used in the oil and gas exploration industry, volcanology, civil engineering and even archaeological studies. Airborne gravimetry is a vital method of conducting a spatial gravimetric survey in areas which are difficult to access by foot, such as mountains. Generally, sensors are modified for air crafts platforms by installing them on large gimbal systems, or a strap-down gravimeter can be used as a lower-cost alternative. Now, a new MEMs gravimeter called “Wee-g” is enabling the development of a system to deploy the gravimeter on an unmanned aerial vehicle (UAV or drone). Wee-g was first developed with the objective of developing a low-cost MEMS accelerometer for gravimetric use which could be manufactured on a large scale. In 2016, Wee-g was used to measure Earth tides - the elastic deformation of the Earth caused by gravitational fields of the Moon and Sun. Since then, the device electronics have been miniaturised to make the system portable and has been tested at the Campsie Hills just north of Glasgow. Work is underway to build an isolation platform with active stabilisation on which the Wee-g can be mounted to be deployed on a drone which will reduce airborne surveys costs further and allow for more airborne gravimetric surveys to be carried out in remote locations.

How to cite: Passey, E., Hammond, G., Bramsiepe, S., Prasad, A., Middlemiss, R., Paul, D., Walker, R., Noack, A., and Anastasiou, K.: Development of a MEMs gravimeter for drone-based field surveys., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20906, https://doi.org/10.5194/egusphere-egu2020-20906, 2020.

D1573 |
EGU2020-1244
Richard Middlemiss, Giles Hammond, Richard Walker, and Abhinav Prasad

By measuring tiny variations in the Earth’s gravitational acceleration, g, one can infer density variations beneath the ground.  Since magmatic systems contain rock of differing density, changes in gravity over time can tell us when/where magma is moving. Traditional gravity sensors (gravimeters) were costly and heavy, but with the advent of the technology used to make mobile phone accelerometers (MEMS – Microelectromechanical-systems), this is changing.

At Glasgow University we have already developed the first MEMS gravity sensor and we are now working with several other European institutions to make a network of gravity sensors around Mt Etna – NEWTON-g. It will be the first multi-pixel gravity imager – enabling unprecedented resolution of Etna’s plumbing system.

While this work is ongoing, a second generation of MEMS gravity sensor is now under development. The first-generation sensor comprises a mass on a spring, which moves in response to changing values of g. This, however, can only ever be used to measure changes in gravity, which means it can be difficult to tell the difference between a geophysical signal and instrumental drift. If we could measure absolute values of gravity, then instrumental drift would become less of a concern, and we could remove the need to calibrate the sensors against commercial absolute gravimeters.

One way of making absolute measurements of gravity is to use a pendulum. This method was used for hundreds of years until the scientists and engineers essentially ran out of fabrication tolerance about 100 years ago. But now nanofabrication is at our disposal, so pendulums are a valid approach to gravimetry again. Such a gravimeter is now being designed and fabricated at the University of Glasgow. It consists of a pair of coupled pendulums, who’s oscillation period is monitored to measure gravity. Here we present the intricacies of the gravimeter design, discuss the expected performance of this new tool, and propose some implications that this sensor could have on the field of volcano gravimetry.

How to cite: Middlemiss, R., Hammond, G., Walker, R., and Prasad, A.: Pendulum MEMS gravimeters for semi-absolute gravimetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1244, https://doi.org/10.5194/egusphere-egu2020-1244, 2019.

D1574 |
EGU2020-8969
Pierre Vermeulen, Laura Antoni-Micollier, Tommaso Mazzoni, Gabriel Condon, Vincent Ménoret, Camille Janvier, Bruno Desruelle, Arnaud Landragin, Jean Lautier-Gaud, and Philippe Bouyer

The Absolute Quantum Gravimeter (AQG) is the world’s first industrial gravimeter measuring g with laser-cooled atoms [1]. Today, several units have already been delivered to end-users.

After reviewing the key principles of the AQG, we will discuss the demonstrated measurement performances of the AQG in terms of sensitivity, stability and repeatability. In particular, we report on a reproducible sensitivity to gravity at a level of 1 μGal in various types of environment (1 µGal = 1e-8 m/s2 ~ 1e-9 g). We will also present our on-going efforts towards the thorough understanding of the uncertainty budget (accuracy) of the sensor. Finally, we will share the experience that we have acquired over the past years regarding the operability of the AQG, with a specific focus on the field version of the sensor.

This new type of gravimeter is presently the only technology that allows for continuous drift-free monitoring of gravity over timescales from a few minutes to several months, which opens new perspectives for the investigation of both spatial and temporal gravity variations [2]. The AQG has been developed by Muquans in collaboration with academic laboratories LP2N and LNE-SYRTE, and RESIF (the French Seismologic and Geodetic Network, [3]).

[1] V. Ménoret et al., "Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter", Nature Scientific Reports, vol. 8, 12300 (2018)

[2] M. Van Camp, O. de Viron, A. Watlet, B. Meurers, O. Francis, C. Caudron, "Geophysics from terrestrial time-variable gravity measurements", Rev. Geophys. (2017).

[3] http://www.resif.fr/

How to cite: Vermeulen, P., Antoni-Micollier, L., Mazzoni, T., Condon, G., Ménoret, V., Janvier, C., Desruelle, B., Landragin, A., Lautier-Gaud, J., and Bouyer, P.: Operating the Absolute Quantum Gravimeter outside of the laboratory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8969, https://doi.org/10.5194/egusphere-egu2020-8969, 2020.

D1575 |
EGU2020-16329
Daniele Carbone, Flavio Cannavò, Filippo Greco, Alfio Messina, Danilo Contrafatto, Giuseppe Siligato, Jean Lautier-Gaud, Laura Antoni-Micollier, Giles Hammond, Richard Middlemiss, Karl Toland, Elske de Zeeuw - van Dalfsen, Mathijs Koymans, Eleonora Rivalta, Mehdi Nikkhoo, Costanza Bonadonna, and Corine Frischknecht

Gravimetry is the only method able to directly track redistributions of bulk masses. Hence, it can supply unique information on geophysical processes that involve subsurface fluids like water, hydrocarbons, and magma. 
Nevertheless, the high cost of currently available gravimeters and the difficulty to use them in field conditions, has limited the applicability of the gravity method, that is indeed not as widely adopted as other geophysical methods.
A new system for gravity measurements is being developed  in the framework of the H2020 NEWTON-g project. This system, called “gravity imager”, includes an array of MEMS gravimeters, anchored to an absolute quantum device. It will enable, for the first time, gravity measurements at high spatio-temporal resolution. 
After the phases of design and production of the new devices, NEWTON-g involves a 2-year phase of field tests at Mt. Etna volcano (Italy), starting in the summer of 2020.

How to cite: Carbone, D., Cannavò, F., Greco, F., Messina, A., Contrafatto, D., Siligato, G., Lautier-Gaud, J., Antoni-Micollier, L., Hammond, G., Middlemiss, R., Toland, K., de Zeeuw - van Dalfsen, E., Koymans, M., Rivalta, E., Nikkhoo, M., Bonadonna, C., and Frischknecht, C.: The NEWTON-g "gravity imager": a new window into processes involving subsurface fluids, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16329, https://doi.org/10.5194/egusphere-egu2020-16329, 2020.

D1576 |
EGU2020-13624
| solicited
| Highlight
Marvin Reich and Andreas Güntner

While studies on hydrological extremes, and floods in particular, usually take a retrospective approach, the German Helmholtz initiative MOSES (Modular Observation Solutions for Earth Systems) aims at understanding extreme events by observing the flood generation processes directly where they occur: in-situ and during the event. As part of this framework, we present a new concept of monitoring regional water storage changes by combining event-based ad-hoc field campaigns with continuous monitoring, using terrestrial gravimetry for total water storage variations and cosmic ray neutron sensing for near-surface soil moisture variations. In this concept, a key role is taken by a continuously monitoring gravimeter station: the gPhone solar cube. This station is energy self-sufficient and easily deployable at any remote location, hosting a gPhoneX, a full weather station, a GNSS antenna and receiver and a cosmic ray neutron probe. The purpose of this station is i) to provide data describing the longer term hydrological dynamics of the study area including the pre-event conditions and ii) to serve as the reference station for the gravity field campaigns during the event. These field campaigns, triggered by forecasts of extreme weather events, are carried out at least prior and after the event on a network of points across the study site. The locations are chosen with respect to the size of the area of interest, topography and travel times between the points. Measurements at each point include relative gravity with two CG-6 instruments, absolute gravity with a Muquans atom quantum gravimeter (AQG) and near-surface soil moisture using three cosmic ray neutron probes in a mobile rover setup. The same routine is strictly repeated at each point to assure uttermost comparability of the measurements. The AQG is also used to calibrate the permanently installed gPhoneX and, thus, to use the gravity reference station for correcting the high instrumental drift of the CG6 gravimeters. The monitoring concept is expected to be transferable to all areas where a similar interest in water storage dynamics at event time scales is strived for.

How to cite: Reich, M. and Güntner, A.: A concept of hybrid terrestrial gravimetry and cosmic ray neutron sensing for investigating hydrological extreme events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13624, https://doi.org/10.5194/egusphere-egu2020-13624, 2020.

D1577 |
EGU2020-9664
| solicited
Ola Eiken

Measurement techniques

High-precision aerial gravity surveys can be carried out by relative spring meters, with ties to stable reference stations or absolute measurements for time-lapse studies. Instrument drift is controlled by frequent repeat measurement and repeatability of 1-3 µGal has been common.  Free-fall gravimeters are heavier and costlier but provide absolute values and are immune to drift. Superconducting gravimeters are stationary and provide sub-µGal resolution over days and weeks, while drift uncertainty can build up to several μGal over years. Cold atom gravimeters are under development and may provide yet another survey alternative in the future.

Multiple sensors and multiple repeats are effective ways of improving survey precision, as much of the noise reduce at random noise (sqrt(N)). This holds also for the sensor drift residuals. An efficient, transparent and reproducible processing software is an integral part of such techniques.

Surface stations

Stability of measurement platforms over years is required for µGal time-lapse precision and can be achieved by installing geodetic monuments. For optimal monitoring of targets like a producing oil, gas or geothermal field, a water reservoir or a volcano, a grid of stations with spacing equal to or smaller than the overburden thickness is required. Surface subsidence or uplift requires sub-cm precision which can be obtained by optical leveling, InSAR or GPS.

Accuracy

Station repeatability is a robust accuracy measure for relative surveys with multiple occupations of each station. Together with multiple sensors they provide abundant statistics. The redundancy also allows for in-situ calibration of parameters for scale factor, tilt and temperature by minimizing residuals. Time-lapse precision can be judged at stations with minimal or known subsurface changes, and will be affected by gravity survey precision, accuracy of measured depth changes and other time-lapse effects such as benchmark stability and time-lapse signals outside interest. Groundwater variations could be one such noise term, unless the purpose is hydrology monitoring.

Efficiency and cost

Most microgravity projects have been carried out in a research or development setting, with one sensor, few stations repeat and implicit capital and personnel cost. In a more industrial setting, efficiency is likely to improve, together with reduced survey cost. More instruments and measurements will likely reduce the personnel and mobilization portion of the cost. Precision/cost tradeoffs and value of data will determine the economics of a project, whether in a scientific or commercial setting.

Conclusion

Currently proven survey repeatabilities of 1-2 µGal may be regarded state-of-the-art and become commonplace for microgravity surveys using relative gravimeters. This can widen the range of applications and reduce monitoring intervals. Further instrument developments may improve this limitation.

How to cite: Eiken, O.: Experiences with relative microgravity surveying, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9664, https://doi.org/10.5194/egusphere-egu2020-9664, 2020.

D1578 |
EGU2020-21470
Peter Vajda, Pavol Zahorec, Juraj Papčo, Massimo Cantarero, Filippo Greco, and Daniele Carbone

In situ values of vertical gradients of gravity (VGGs) are often needed when compiling residual spatiotemporal gravity changes that are interpreted in volcanic areas with the objective of drawing inferences about sources of volcanic unrest or pending eruptions. VGG values are seldom acquired by in situ observations. Their availability in 4D volcano-microgravimetric surveys and studies can be mediated by predicting the VGGs based on high resolution high accuracy DEMs and modelling the topographic component (constituent) of the VGG. Based on a modelling effort and in situ verification of VGG predicted on Etna in the summit craters area, on the north-east rift and on benchmarks of the monitoring network covering the volcano in a wider context, we learned that the VGG prediction can be improved by using drone-borne photogrammetry with GNSS ground control to produce a finer DEM in the closest vicinity of the VGG point (benchmark or field point) with resolution higher than the available high-resolution LiDAR-derived DEM, and using detailed modeling of gravity effect (on VGG) of anthropogenic objects such as walls and buildings adjacent to the VGG points. In this poster we present the methods used in the refined VGG prediction and the results of the verification of VGGs predicted on Etna.

How to cite: Vajda, P., Zahorec, P., Papčo, J., Cantarero, M., Greco, F., and Carbone, D.: In situ verification of refined predicted vertical gravity gradients on Etna, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21470, https://doi.org/10.5194/egusphere-egu2020-21470, 2020.

D1579 |
EGU2020-6955
Mathijs Koymans, Flavio Cannavò, and Daniele Carbone

We study the transient effect of groundwater mass changes on the observed gravity signal from a superconducting gravimeter deployed on Mt. Etna, Italy. Gravimeters are capable of detecting minor changes in the vertical component of gravity over time scales from minutes to years. Insight on geophysical phenomena that cause mass displacements in the subsurface can be obtained through the use of gravimetry. Gravity recordings integrate multiple components that contribute to the signal with different magnitudes. The effects of earth tides, atmospheric pressure changes, hydrological processes are among the above components. They need to be precisely evaluated, in order to isolate the signal caused by the volcanic processes. Here, we study the effect of groundwater mass changes on gravity, as a result of rainfall and snow melting, the latter estimated through GNSS interferometric reflectometry. A forward charge-discharge model is used to compare gravity recordings between 2018 - 2019 with observed precipitation events. We show that the observed gravity signal cannot be explained only through changes in groundwater mass, implying that other (volcanic) processes must have been at play.

How to cite: Koymans, M., Cannavò, F., and Carbone, D.: A strategy to study the effect of rainfall and snow melting on gravity recordings from a superconducting gravimeter installed on Mt. Etna volcano, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6955, https://doi.org/10.5194/egusphere-egu2020-6955, 2020.

D1580 |
EGU2020-3765
Meng Yang, Christian Hirt, and Roland Pail

With knowledge of geometry and density-distribution of topography, the residual terrain modelling (RTM) technique has been broadly applied in geodesy and geophysics for the determination of the high-frequency gravity field signals. Depending on the size of investigation areas, challenges in computational efficiency are encountered when using an ultra-high-resolution digital elevation models (DEM) in the evaluation of Newtonian integration. This paper presents a new MATLAB-based program, terrain gravity field (TGF), for the accurate and efficient determination of the terrain-related gravity field based on an adaptive algorithm. Depending on the attenuation character of gravity field with distance, the adaptive algorithm divides the integration masses into four zones, and adaptively combines four types of geometries and DEMs with different spatial resolutions. The most accurate but least efficient polyhedron together with the finest DEM are only considered for the innermost zone, while prism approximation for the second zone, the third zone with the more efficient tesseroid and a coarse DEM, and the most efficient but least accurate point-mass with the coarsest DEM for distant masses. Compared to some publicly available algorithms depending on one type of geometric approximation, the TGF achieves accurate modelling of gravity field and greatly reduces the computation time. Besides, the TGF software allows to calculate ten independent elements of gravity field, supports two types of density inputs (constant density value and digital density map), and considers the sphericity of the Earth by involving spherical approximation and ellipsoidal approximation. Further to this, the TGF software is also capable of delivering the gravity field of full-scale topographic gravity field implied by masses between the Earth’s surface and mean sea level. Results from internal and external numerical validation experiments of TGF confirmed its accuracy of sub-mGal level. Based on TGF, the trade-off between accuracy and efficiency, values for the spatial resolution and extension of topography models are recommended. The TGF software has been extensively tested and recently been applied in the SRTM2gravity project to convert the global 3” SRTM topography to implied gravity effects at 28 billion computation points. This confirms TGF the capability of dealing with large datasets.

How to cite: Yang, M., Hirt, C., and Pail, R.: TGF: A New MATLAB-based Software for Terrain-related Gravity Field Calculations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3765, https://doi.org/10.5194/egusphere-egu2020-3765, 2020.

D1581 |
EGU2020-18917
Filippo Greco, Daniele Carbone, Flavio Cannavò, Alfio Messina, Danilo Contrafatto, Giuseppe Siligato, Richard Reineman, and Richard Warburton

Continuous gravity measurements at active volcanoes are mostly accomplished using spring gravimeters, that can be operated under harsh field conditions. Unfortunately, these instruments do not provide reliable continuous measurements over long time-scales, due to the instrumental drift and artifacts driven by ambient parameters.

An alternative to spring devices for continuous measurements is given by superconducting gravimeters (SGs), that are free from instrumental effects and thus allow to track even small gravity changes over time-scales from minutes to years. Nevertheless, SGs cannot be deployed in close proximity to the active structures of tall volcanoes, since they need host facilities with main electricity and a large installation surface.

The mini-array of three SGs that were installed on Etna between 2014 and 2016 makes the first network of SGs ever installed on an active volcano. Here we present results from these instruments and show that, even though they are installed at relatively unfavorable positions (in terms of distances from the summit active craters), SGs can detect volcano-related gravity changes that would otherwise remain hidden, thus providing unique insight into the bulk processes driving volcanic activity.

How to cite: Greco, F., Carbone, D., Cannavò, F., Messina, A., Contrafatto, D., Siligato, G., Reineman, R., and Warburton, R.: The benefits of performing continuous gravity measurements at active volcanoes using superconducting gravimeters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18917, https://doi.org/10.5194/egusphere-egu2020-18917, 2020.

D1582 |
EGU2020-554
Alexey Shklyaruk, Kirill Kuznetsov, David Arutyunyan, and Ivan Lygin

Today, gravimetry is actively used in solving various detailed engineering problems, in researches of the underground fluid dynamics, etc. It is worth noting that the areas of large rivers and lakes are an interference in creating a regular network of observations, which is necessary to the above problems solution. For now the results of satellite, marine or aero surveys do not allow obtaining materials with the necessary resolution and accuracy parameters. The solution to this problem may be surveying in the winter period on frozen water reservoirs.

Gravimetric surveys were carried out on the surface of the ice covering the Ugra River as part of the field course of the Geological Faculty of Lomonosov Moscow State University in Kaluga region. Two high-precision relative gravimeters CG-5 Autograv by Scintrex Ltd were used.

During above-ice gravimetric observations, many factors influence the gravimeter. They can be divided into two groups: natural and human cause. The first group includes gusts of wind, melting ice, the flow of the river. The second group includes operator’s movements, interference of people passing by, etc.

The completed studies made it possible to evaluate the parameters of the standard deviation (SD) of the gravimeter records and the influence of the above factors on its tilt. An analysis of the observation results showed that high-precision relative gravimeters allow above-ice surveying with an accuracy of no worse than 5 μGal, which corresponds to the current level of ground gravity survey’s accuracy.

To minimize the influence of interfering factors, a special observation technique is required:

  • For an independent assessment of the accuracy of observations at each point, several gravimeters should be applied;
  • At each point, at least 10 measurements with one gravimeter should be performed;
  • The time of one measurement should be at least 60 seconds;
  • To minimize the influence of external factors on the measurements of gravimeters, it should be ensured that near the operator there are no outsiders, equipment, etc., creating oscillations of the surface which the device is installed on. At the same time, the operator of the gravimeter must constantly check the records of the gravimeter, the SD parameter and its levels.

How to cite: Shklyaruk, A., Kuznetsov, K., Arutyunyan, D., and Lygin, I.: Specific aspects of above-ice gravimetric observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-554, https://doi.org/10.5194/egusphere-egu2020-554, 2019.

D1583 |
EGU2020-4825
Peter Schack, Roland Pail, and Thomas Gruber

Around 100km south of Munich, the Institute of Astronomical and Physical Geodesy of the Technical University of Munich established a gravimetric-astrogeodetic testing ground over the last 20 years. Precise gravity values as well as vertical deflections exist for hundreds of points. End of 2019, a car-based strapdown inertial gravimetry survey was realized in this area along a ~25km track. For this track, a few gravity values and several vertical deflections (spacing around 200m) are available (Hirt and Flury 2008). Navigation-grade IMU (inertial measurement unit), GNSS (global navigation satellite systems) and additional relative gravimeter observations were recorded during the survey. With this setup, it is possible to evaluate the capabilities of terrestrial scalar and vector strapdown inertial gravimetry.

This contribution gives an overview about the testing ground, the recently conducted survey and the data processing. The main part treats the analyses regarding the accuracy of 1D- and 3D-strapdown inertial gravimetry. Furthermore, attention is payed to the kinematic IMU performance (noise behavior), the benefit of special IMU calibrations (Becker 2016) and a comparison of the results with pure model based gravity disturbances.

Literature

  • Becker, D. (2016). Advanced Calibration Methods for Strapdown Airborne Gravimetry. PhD thesis, Technische Universität Darmstadt, Fachbereich Bau- und Umweltingenieurwissenschaften, Schriftenreihe der Fachrichtung Geodäsie Heft 51. ISBN 978-3-935631-40-2.
  • Hirt, C. and Flury J. (2008). Astronomical-topographic levelling using high-precision astrogeodetic vertical deflections and digital terrain model data. J Geod (2008) 82:231–248, Springer-Verlag. DOI 10.1007/s00190-007-0173-x.

How to cite: Schack, P., Pail, R., and Gruber, T.: Terrestrial strapdown inertial gravimetry in the Bavarian Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4825, https://doi.org/10.5194/egusphere-egu2020-4825, 2020.

D1584 |
EGU2020-18949
Alessandro Bonforte, Filippo Greco, and Daniele Carbone

Here we present the results of repeated Absolute Gravity and GNSS measurements, collected at Mt. Etna (Italy) between 2009 and 2018. We aim at investigating the capabilities of this integrated approach for understanding the dynamics of magmatic sources over time-scales of months to years. The absolute gravity and GNSS campaign measurements were repeated roughly once a year; in order to improve the time resolution of gravity data, in some stations we performed, besides absolute gravity measurements, also relative measurements at intervals shorter than 1 year.

After being corrected for the effect of elevation changes, gravity data reveal an increase/decrease cycle, well spatio-temporal correlated with a general pattern of uplift/subsidence, during a period of intense lava fountains from the summit craters.

Our results provide insight into the processes that controlled the transfer of the magma from deeper to shallower levels of the plumbing system of Mt. Etna volcano, in periods preceding/accompanying the eruptive activity during 2009–2018.

Specifically, we propose that coupled changes in height-corrected gravity and elevation might be induced either by the magma storage/withdrawal below the volcanic pile, or by fluids pressurization/depressurization, or by a combination of both processes.

The application of the proposed approach could led to an improved capability to identify processes heralding eruptions.

How to cite: Bonforte, A., Greco, F., and Carbone, D.: An integrated approach for understanding long-term volcano dynamics based on absolute gravity and GNSS measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18949, https://doi.org/10.5194/egusphere-egu2020-18949, 2020.