Authors : Sylvain Bonvalot, Jean-Daniel Bernard, Jean-Paul Boy, Cédric Champollion, Alain Coulomb, Michel Diament, Germinal Gabalda, Thierry Gattacceca, Jacques Hinderer, Thomas Jacob, Sylvain Lucas, Marie-Françoise Lalancette-Lequentrec, Nicolas Le Moigne, Laurent Longuevergne, Guillaume Martelet, Sébastien Merlet, Gwendoline Pajot-Metivier, Franck Pereira Dos Santos, Lucia Seoane
Résif-GMOB, French national park of mobile gravimetric instruments coordinated by CNRS/INSU and the instrumental component of Résif, offers French scientific teams a set of means, measurements and services for gravity field studies. This instrumental park includes various complementary sensors allowing measurements with an accuracy ranging from some 10-8 to 10-11 g , spatial or temporal variations in gravity. Used mainly in the field for reconnaissance surveys and the establishment of networks, but also occasionally in the laboratory for permanent to semi-permanent measurements, these various sensors help to characterize and quantify variations in mass or density occurring near the Earth’s surface or inside the Earth. Supplemented by permanent observatories and also by other gravimetric measurement means available in France from various organizations or from other research programs, Résif-GMOB contributes today to the consolidation of a first-rate national gravimetric infrastructure built over several decades and allowing to answer many applications of fundamental or applied research in geodesy, geophysics, geology, hydrology, oceanography, metrology.
From the ’50s to today…
Initiated in the mid-1990s, GMOB was born out of several decades of investment by French teams in the acquisition of gravimetric measurement means for academic research purposes and the establishment of reference networks. These investments followed the technical evolutions that took place in the 1950s in the design of miniaturized gravimetric sensors, developments that came almost exclusively from the United States and Canada (North American manufacturers, Worden LaCoste & Romberg, Scintrex, GWR, Micro-g LaCoste) for half a century. The first relative field gravimeters were thus acquired in the mid-1950s by the IGN and ORSTOM/IRD and made it possible to carry out reconnaissance surveys and to establish the first base networks in metropolitan France, in the DOM-TOMs (French overseas departments and territories) and in the United States , on the African continent and in the South-West Pacific (work carried out mainly until the mid-1970s with about ten instruments). This work included the establishment of so-called “absolute” bases, which were in fact carried out by relative measurements attached to rare absolute measurement stations available in Europe. Investments in measurement facilities then continued in the 1960s and 1970s, with the acquisition of more portable relative field gravimeters (Worden or LaCoste & Romberg gravimeters) by various research institutes or laboratories (CNRS, BRGM, IPGP, IRD, University of Montpellier, etc.) to complete the gravimetric coverage on the national territory (metropolis and DOM-TOM) and to develop research applications in structural geology, geophysical prospecting, volcanology, etc. At the same time, studies on the temporal variations of gravity and earth tides were undertaken leading to the installation in 1954 of a first permanent recording gravimeter (LaCoste & Romberg gravimeter) at the Strasbourg Observatory / Institute of Globe Physics. This instrument was later replaced in 1987 by the first cryogenic gravimeter acquired in France (GWR superconducting gravimeter), an instrument with higher sensitivity and very low instrumental drift, allowing to initiate research on the dynamics of the Earth and its liquid core over a spectral range from a few minutes to several years.
As for observatory gravimeters, a technological leap forward was made during the 1990s in mobile gravimetric instrumentation with the advent of digitally acquired field gravimeters (Scintrex CG3, predecessor of the current CG5 and CG6) and absolute gravimeters based on the free fall of a body (cubic wedge) in vacuum allowing absolute measurements (Micro-g LaCoste FG5 and A10 gravimeter in its portable field version marketed at the beginning of the 2000s). Most of the national organizations (CNRS, CNES, BRGM, IRD, IPGP, IGN, IFREMER, SHOM, etc.) and university laboratories (Paris, Toulouse, Montpellier, Strasbourg, Rennes, La Rochelle, Brest, Trappes, Clermont-Ferrand, etc.) involved in gravimetry in France then supported the acquisition of laboratory or field instruments which constituted the beginnings of the present national instrument park. These resources, recently completed by the Equipex programme – Investissement d’Avenir 2012 – (see below) now provide an original gravimetric infrastructure, highly complementary to space observatories (GOCE, GRACE, GRACE-FO missions), used to address a wide range of scientific issues both on the national territory and on numerous foreign sites where teams are involved. Finally, it should be noted that the relative gravimeters of Résif-GMOB are also used for the training of students during field training courses.
New Means of Measurement
Resif’s gravimetric measurement means are divided into 3 types of instruments: highly portable relative field gravimeters used for reconnaissance surveys at different scales, from microgravimetric prospecting to reconnaissance surveys, portable or transportable absolute gravimeters used in laboratories or in field conditions for precise and absolute gravity determinations (reference network, monitoring of temporal variations, metrology, calibration of relative instruments, etc.), and absolute gravimeters used for the measurement of the gravity of the earth’s surface (reference network, monitoring of temporal variations, metrology, calibration of relative instruments, etc.). ) and superconducting relative gravimeters installed for a temporal monitoring of gravity field variations in a given location over a period of a few months to a few years (even several decades for the Strasbourg gravimetric observatory).
The Equipex Résif-Core has enabled to undertake a rejuvenation of the mobile and permanent measurement means with the acquisition of two new field gravimeters (Scintrex CG5) and a cryogenic gravimeter (GWR iOSG) to replace in 2016 the one at the Strasbourg Observatory, which has been in operation for more than 20 years. Through a public-private partnership with the French company Muquans, created in 2011, it also supported the development of the first portable and commercially available AQG (Absolute Quantum Gravimeter) absolute gravimeters in the world (see Newsletter Résif n°1).Thanks to additional funding (IDEX Toulouse, IRD, CNRS/INSU), two new AQG gravimeters will equip the French groups.
Dans le domaine de l’observation permanente à semi-permanente, deux autres Equipex, CRITEX and MIGA also acquired several superconducting gravimeters (GWR iOSG and iGrav) for projects applied respectively to the study of the critical zone (redistribution of water masses in underground environments) or to fundamental physics (study of gravitational waves), thus significantly increasing the capabilities of continuous gravimetric measurements on the national territory. Figure 1 shows the main types of mobile or permanent instruments currently available to French teams for academic research applications. More detailed information on the available resources and the modalities of use are provided on the Résif-GMOB website.
Figure 2 : Inventory of the gravimetric infrastructure in metropolitan France. Left) Distribution of terrestrial and marine gravimetric data available at the IGN (with contributions from BRGM, SHOM and IFREMER); Middle) RGF reference gravimetric network established by the IGN from hybrid campaigns of absolute and relative measurements; Right) Network for temporal monitoring of gravity variations by permanent measurements (superconducting gravimeters) and repeated absolute measurements. The active (violet) and planned (light violet) permanent stations include the contributions Résif (circles), CRITEX (hexagons), MIGA (pentagon) and LNE (square).
New players in gravimetric instrumentation
The last decade has seen the emergence in France of 3 new players recognized at the international level for their R&D activities in the field of gravimetric instrumentation. These actors, public or private, are involved in the design of sensors (gravimeters or gravimeters) using the properties of matter waves to determine the modulus or gradients of gravity. These measurement techniques based on the use of cold atoms represent the main technological innovation over the last 20 years in the measurement of the gravitational field. They constitute a new field of research, now being explored in many countries, which should lead in the coming years to the development of original sensors for a wide range of gravity measurements: spot or continuous high-frequency measurements, absolute or gradiometric measurements, in situ measurements (surface, well or submarine measurements) or on-board measurements on different platforms (vehicles, ships, submarines, satellites). In this international competition, these laboratories were among the first to propose practical operational realizations of absolute measurement instruments that could be scientifically tested, in particular in connection with Résif-GMOB :
- The LNE/SYRTE (UMR 8630), with the development of the first transportable cold atom gravimeter (CAG) at the beginning of this decade whose performance could be compared with other reference instruments during international intercomparison campaigns of absolute gravimeters. The LNE site in Trappes will also become the main site for the intercomparison of national absolute gravimeters.
- MUQUANS with the development of the first miniaturized cold atom gravimeters (AQG) carried out within the framework of a partnership with Résif.
- ONERA with the development of a mobile cold atom gravimeter for dynamic measurements (GIRAFE/GIRAFE2) tested in collaboration with SHOM during marine campaigns. This instrument will soon be tested under airborne measurement conditions as part of a project supported by CNES.
A network of complementary observations
The gravimetric work carried out for several decades by the French teams has led to the definition of a gravimetric reference frame of a very good level (among the best at the European level), whether by terrestrial (under the responsibility of BRGM) or marine (under the responsibility of IFREMER and SHOM) gravimetric coverage or by materialized networks (under the responsibility of IGN). In addition, there is a growing number of sites where relative or absolute gravimetric observations are acquired on a permanent, semi-permanent or repeated basis, increasingly co-located with other networks (geodetic, seismological, tide gauge), in particular within the framework of the IR SOERE Résif. Figure 2 shows the state of the art of the currently available observations over metropolitan France, reflecting 3 strong components of the gravimetric observation system in terms of (i) knowledge of the static field by point measurements, (ii) temporal monitoring by permanent and semi-permanent observations and (iii) gravimetric reference networks and sites. Finally, it should be noted that many other observations are also available in the ROM-COMs  and in different regions of the world (Europe, South America, Africa, Asia, Pacific, polar zones) where French teams actively contribute to the acquisition of gravimetric data on land (relative or absolute measurement campaigns, reference networks) or at sea (oceanographic measurement campaigns). All these data, as well as products and other tools for scientific users, are accessible through the international scientific services of the IAG (also recognized as part of the National Gravimetry-Geodesy Observation Service), namely the International Gravimetric Bureau (BGI) in charge of archiving the distribution of all relative or absolute gravimetric measurements acquired on the surface of the globe and the International Geodynamics and Earth Tide Service (IGETS) collecting and redistributing time series of gravity and other associated geodynamic sensors.
The use of innovative cold atom technologies for gravity measurement and the advanced positioning of French metrological laboratories in the design of innovative sensors (gravimeters or gravimeters) also opens new avenues for gravimetric observation in situ but also in space. This dynamic now makes it possible to better structure the community around common means and tools, and to have at one’s disposal a growing set of complementary observations that are increasingly homogeneous and precise, to study the spatial and temporal variations of gravity and to respond to various scientific issues (physics of the globe, natural hazards, earth fluid envelopes, resources, fundamental physics, metrology, reference systems and networks, etc.). One of the consequences is also an enlargement of the community of gravimetry users in France towards a larger number of specialists (geophysicists, geodesists, geologists, hydrologists, volcanologists, seismologists, modellers, etc.) and the development of new synergies with metrology laboratories and industrial developers.
Sylvain Bonvalot, GET, Toulouse : email@example.com
Article published in the Newsletter Résif n°13, January 2018