Michel Van Camp

Tsoft: graphical and interactive software for the analysis of time series and Earth tides

We present Tsoft, a graphical interactive analysis software package originally dedicated to the analysis and processing of gravity time series. Tsoft can also be used to process and analyze all sorts of time series like seismic or other environmental signals. The Tsoft approach has a number of important advantages in the area of pre-treatment of the data (correction of artefacts such as spikes and steps), because the user can always inspect what happened and intervene manually. The graphical environment is also very convenient for the detection, isolation and analysis of events (e.g. earthquakes). Lastly, errors in the analysis path are easily detected, because the software shows a graphical representation of the results at each stage of the analysis. In this article we describe first the general data structure and the possible data manipulations and computations. Then, we present the correcting tools dealing with unwanted steps, spikes and gaps in raw data. Afterwards, we describe the module dedicated to the computation of tidal signals. Lastly, we give an overview of the most used computing tools like spectral analysis and multilinear least-squares fit.

Accurate transfer function determination for superconducting gravimeters

The transfer function for the cryogenic gravimeter GWR-C021 operating at Membach (Belgium) has been experimentally determined by injecting known voltages into the control electronics of the system. The output of the gravimeter to the injected sine waves and step functions has been observed. This give a precise knowledge of the transfer function of the gravimeter. It allows one to reach a precision of better than 0.01 second in the phase response of the instrument, in agreement with the Global Geodynamics Project (GGP) requirements.

CCM.G-K2 key comparison

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Local stress sources in Western Europe lithosphere from geoid anomalies

We propose a method to evaluate the stress generated at the local scale by the spatial variations of the gravitational potential energy (GPE), which is related to inhomogeneous topography and mass distribution in the lithosphere. We show that it is possible to infer these local stress sources from the second spatial derivatives of a geoid height grid, used as a proxy of the GPE. The coherence of the method is validated on a passive margin, the Bay of Biscay. The result is that expected in such a geological configuration, with extensional local stress sources with the maximum horizontal principal stress parallel to the margin and compressive sources with the maximum horizontal principal stress perpendicular to the margin in the continental and oceanic lithosphere, respectively. We apply the method to Western Europe in order to provide a better understanding of the complex spatial variation of the present-day tectonic activity. Our results indicate a stress pattern from the local sources dominated by short-space-wavelength (of the order of a few tens of kilometers) variations in the tectonic style and in the direction of the maximal horizontal principal stress σ H . A comparison of the σ H orientations and tectonic style from the local sources with the ones of the World Stress Map (WSM) data set indicates that the local stress sources can be representative of the deviatoric stress state in some regions. Our results explain 71% of the faulting styles for the earthquake fault-plane solutions in the WSM, which is better than the classical compressive NW-SE stress field model. In the central part of the Pyrenees, the agreement between earthquake fault-slip directions and the direction of shear stress from the local sources acting on the associated fault planes is compatible with the extensional stress field evidenced by recent investigations.

Comment on “Nature of the recent vertical ground movements inferred from high‐precision leveling data in an intraplate setting: NE Ardenne, Belgium” by A. Demoulin and A. Collignon

[01] Using yearly leveling surveys performed from 1993 to 1998 in the Ardenne, as well as historic leveling results from 1948 and 1974, Demoulin and Collignon [2000] (hereinafter referred to as D&C) observed that the total vertical ground movement (less than 1 cm) over a 20-to 30year period is barely higher than the yearly displacements measured to a few millimeters per year. They eliminated various sources of errors from their measurements and subsequently interpreted the resulting short-term oscillating crustal displacements as true tectonic motions within the upper crust accompanying the long-term deformation of uplifting areas in intraplate settings. Their justification is that the leveling discontinuities coincide with faults they assume to be active. In their conclusions, D&C suggested that such motions and their variations could be used to monitor potential seismogenic faults and even to foretell earthquakes. [2] Unfortunately, it is our opinion that the data analysis of D&C is inadequate and does not support the subsequent interpretation. Specifically, we interpret the apparent oscillations in the leveling data as the expected expression of the noisy character of leveling differences with amplitudes ranges of 0.5 to 1 cm. If this premise is accepted then there is also no evidence to suggest that the boundary faults in the study area are active. We thus feel that the conclusions of D&C are premature until their observations can be corroborated by improved observations or by other independent measurements. In this note, we present a careful statistical analysis to support our position.

Geophysics From Terrestrial Time-Variable Gravity Measurements

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. Plain Language Summary: In a context of global change and increased human vulnerability to terrestrial hazard, monitoring the Earth system is one of the key challenges of geoscience. In particular, terrestrial gravimetry, with its precision at the level of one part of a billion, allows the monitoring of many phenomena, from water resource availability to volcanic activity. This paper reviews the technique, its advantages and limitations, how it has been used in the Earth monitoring, and the next challenges to be met by ground gravimetry.

Direct measurement of evapotranspiration from a forest using a superconducting gravimeter

Evapotranspiration (ET) controls the flux between the land surface and the atmosphere. Assessing the ET ecosystems remains a key challenge in hydrology. We have found that the ET water mass loss can be directly inferred from continuous gravity measurements: as water evaporates and transpires from terrestrial ecosystems, the mass distribution of water decreases, changing the gravity field. Using continuous superconducting gravity measurements, we were able to identify daily gravity changes at the level of, or smaller than, 10−9 nm s−2 (or 10−10 g) per day. This corresponds to 1.7 mm of water over an area of 50 ha. The strength of this method is its ability to enable a direct, traceable and continuous monitoring of actual ET for years at the mesoscale with a high accuracy.

Chapter 14 Continuous and campaign-style gravimetric investigations on Montserrat 2006 to 2009

Abstract Gravimetric time series can provide vital clues about subsurface dynamics associated with active volcanism. Here, we report on continuous and campaign-style gravimetric observations on Montserrat between 2006 and 2009. More than 240 days of continuous gravimetric records enabled us to derive a first local joint solid Earth tides and ocean loading model for Montserrat, and we report the tidal harmonics for 14 major wave groups. Compared to global predictions, the new model (MTY11) achieves an up to one order of magnitude better precision over diurnal and semi-diurnal frequencies. We anticipate that the model will help reduce the effects of tidal perturbations on other geodetic time series recorded on Montserrat. Abrupt variations in gravity accompanied Vulcanian explosions and probably reflect the response of a shallow aquifer to stress changes during pressurization and depressurization of the subvolcanic plumbing system. Campaign data enabled the quantification of mass variations during a cycle of activity including dome formation and repose. Both forward and inverse modelling of the spatio-temporal time series indicates that the source of the recorded gravity variations is situated beneath central Montserrat. Our favourite interpretation of the campaign data is that the gravity variations reflect volcano-tectonic interaction beneath the Centre Hills of Montserrat that are triggered by changes in the active magmatic system of Soufrière Hills Volcano (SHV). We also discuss our findings on subsurface mass variations in relation to annual precipitation records and active dome formation. Both continuous and discrete gravimetric observations indicate coupling between the dominant magmatic sources responsible for the ongoing eruption at SHV and shallow-seated local sources such as aquifers and fluid-saturated fault-damage zones. Our investigations demonstrate the value of including gravimetric observations over a wide frequency range for volcanic system characterization in a volcanic island arc setting. Supplementary material: Details on the inversion routine of the explorative source model GROWTH 2.0 and the resulting images from its application to time-lapse gravity data are available at http://www.geolsoc.org.uk/SUP18701.

Reply to the Comment By Crossley et al. on: ‘The quest for a consistent signal in ground and GRACE gravity time series’

(1) If SG can be compared with GRACE, considering the resolution of GRACE, there should be ‘common’ signal between the SGs themselves in Europe. The first method is to look for correlation between the SGs, of which the significance can only be tested after removing the annual component. (2) Then, we explain that, in the framework of this study, the EOF tool does not provide usable results, with or without the annual component. However, for the interannual component, we wrote that that picture may change when longer time-series are available. (3) To investigate the annual component, the appropriate method is using phasors, as it allows an accurate comparison of the phases and amplitudes, unlike the correlation or EOFs. (4) After intercomparing SGs, we compare them with the hydrological and GRACE models. (5) The last part aims at demonstrating that correcting for local effects is pointless.

Final report on absolute gravimeter intercomparison (EURAMET Project no. 1093)

An interlaboratory comparison (Euramet Project 1039) of three absolute gravimeters was carried out between the national metrology institutes of Luxembourg, Belgium and Switzerland (University of Luxembourg/ECGS, Royal Observatory of Belgium and Federal Office for Metrology METAS). The comparison was hosted in the Underground Laboratory for Geodynamics in Walferdange. The obtained results confirm a perfect agreement between the instruments used with respective expanded uncertainties (k = 2) of 4.25 µGal. Finally, a link to the Euramet project 1030 shows that the three gravimeters are coherent with the 19 other gravimeters. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by EURAMET, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).