Abdelfettah, Yassine

Calvo, Marta

Ferhat, Gilbert

Hinderer, Jacques

Portier, Nolwenn

Riccardi, Umberto

Here  we  report  on  some  case  histories  of  geodetic  and  grav

ity  surveys  for  monitoring  of 

geothermal  sites.  The  monitoring  of  a  geothermal  reservoir  by  hybrid  gravimetry  combining 

different  types  of  instruments  (permanent  superconducting  gravimeter,  absolute  ballistic 

gravimeter and micro

-

gravimeters) is presented.

A relative gravimetric network equipped with 

levelling benchmarks was designed and surveyed at annual frequencies (2013, 2014, 2015 and 

2016  for  gravity  measurements,  2014,  2015  and  2016  for  the  leveling).  Repetition  of  high 

precision  relative  gravity  mea

surements  on  a  network  developed  around  a  reference  station, 

which  is  regularly  measured  with  both  relative  and  absolute  gravimeters,  leads  to  the 

knowledge  of  the  time  and  space  changes  in  surface  gravity.  The  observed  gravity  changes 

can  be  linked  to  the

natural  or  anthropic  activities  of  the  reservoir.  A  feasibility  study  using 

this methodology is applied to two geothermal sites of Soultz

-

sous

-

Forêts and Rittershoffen in 

the Alsace region (France) in the Rhine graben (Hinderer et al. 2015)

Ferhat, G. (Gilbert)

Portier, N. (Nolwenn)

Hinderer, J. (Jacques)

Calvo Garcia-Maroto, M. (Marta)

Abdelfettah, Y. (Yassine)

Riccardi, U. (Umberto)

univOAK

      
 
Poster Session  
 
INGEO 2017 – 7th International Conference on Engineering Surveying 
Portugal | Lisbon | October 18 - 20, 2017  
Three years of monitoring using leveling and hybrid gravimetry 
applied to geothermal sites of Soultz-sous-Forêts and 
Rittershoffen, Rhine Graben, France 
 
 
Gilbert FERHAT1, Nolwenn PORTIER1, Jacques HINDERER1, Marta CALVO 
GARCIA-MAROTO2, Yassine ABDELFETTAH1, Umberto RICCARDI3 
 
1 Institut de Physique du Glose de Strasbourg, Institute for Earth Physics 
Strasbourg Cedex France  
E-mail: gilbert.ferhat@unistra.fr, nolwenn.portier@unistra.fr, jacques.hinderer@unistra.fr, 
abdelfettah@unistra.fr 
 
2 Observatorio Geofísico Central, IGN 
Madrid, Spain 
E-mail: mcalvo@fomento.es 
 
3 Università degli Studi di Napoli Federico II, Dipartimento di Scienze della Terra 
Napoli, Italy 
E-mail: umbricca@unina.it 
 
 
Abstract 
 
Here we report on some case histories of geodetic and gravity surveys for monitoring of 
geothermal sites. The monitoring of a geothermal reservoir by hybrid gravimetry combining 
different types of instruments (permanent superconducting gravimeter, absolute ballistic 
gravimeter and micro-gravimeters) is presented. A relative gravimetric network equipped with 
levelling benchmarks was designed and surveyed at annual frequencies (2013, 2014, 2015 and 
2016 for gravity measurements, 2014, 2015 and 2016 for the leveling). Repetition of high 
precision relative gravity measurements on a network developed around a reference station, 
which is regularly measured with both relative and absolute gravimeters, leads to the 
knowledge of the time and space changes in surface gravity. The observed gravity changes 
can be linked to the natural or anthropic activities of the reservoir. A feasibility study using 
this methodology is applied to two geothermal sites of Soultz-sous-Forêts and Rittershoffen in 
the Alsace region (France) in the Rhine graben (Hinderer et al. 2015). 
 
Key words: leveling, hybrid gravimetry, geothermy, deformation monitoring 
 
1 INTRODUCTION 
 
Time-lapse gravimetry can be a monitoring tool of any underground or surface mass 
redistribution and has many applications in volcanology (magmatic chamber evolution), 
hydrology (water storage changes in the critical zone), and geothermics.  
  INGEO 2017 
 
 
  Portugal | Lisbon | October 18 - 20, 2017 
The surface at a geothermal site can be monitored by different techniques, i.e. GPS/GNSS, 
InSAR, leveling to name only a few. To get in-deep monitoring, repetitions of gravimetric 
measures can be performed. 
 
Several studies have introduced the concept of hybrid (resp. super-hybrid) gravimetry (Okubo 
et al. 2002; Sugihara and Ishido 2008; Hector et al. 2015) that is the optimal combination of 
two (resp. three) types of gravimeters (absolute AG, superconducting SG or relative 
gravimeters RG). By using different types of gravimeters, we take advantage of each 
instrument: i.e. transportable for a RG and precise and absolute measurement for an AG (see 
Table 1 of Hinderer et al. 2015). 
 
This paper reports an on-going experiment of monitoring using hybrid gravimetry (SG+RG) 
and leveling at two geothermal sites in the Rhine graben, France. 
 
2 DEEP GEOTHERMAL EXPLOITATION SITES IN THE NORTH 
EAST OF FRANCE  
 
Deep geothermal energy exploitation was defined by the French “Grenelle de 
l’Environnement” (Large Debate on the Environnement- @Grenelle-group4) as a crucial task 
for the development of renewable energy in France. Geothermics are well developed in many 
countries such as United States, Philippines, Indonesia for the first three leaders, but also in 
Europe: Italy and France to name only a few of them. A thermal anomaly is present in the 
Rhine Graben (NE of France, Figure 1) and has been exploited. 
 
Since more than 20 years, scientists and engineers worked at the site of Soultz-sous-Forêts, 
France (denoted Soultz afterwards for simplicity), in Northern Alsace (60 km of Strasbourg, 
Figures 1, 2 and 3) in order to improve our knowledge in deep geothermal and to develop 
practical experiences in exploitation of natural heat (http://labex-geothermie.unistra.fr/). 
Next to the site of Soultz site, a new geothermal exploitation facility has been built in 2012 at 
Rittershoffen at a few kilometres of Soultz (Figures 1, 2 and 3). The site is also called ECOGI 
(Figure 2). The objective consist in retrieving 24 MW of thermal energy from the heat of the 
geothermal fluid which is freely circulating in the fractured layers of the Rhine Graben 
(http://labex-geothermie.unistra.fr/). At ECOGI, the first well was drilled in 2012 as a vertical 
well reaching the depth of 2600 m. The second well is a deviated one, drilled between March 
and July 2014. 
 
Many projects associated to these two sites has been launched. In 2013, several geological 
and structural studies, observations of surface deformation, gravimetric and magneto-telluric 
surveys, installation of seismological stations has been performed (http://labex-
geothermie.unistra.fr/). 
 
Ferhat, G. et al.: 3 years of monitoring at geothermal sites: Soultz and Rittershoffen, France  
 
   
 
Fig. 1 Location of thermal anomalies in the Rhine Graben (NE France and Germany) and 
location of the deep geothermal sites of Soultz and Rittershoffen, North East of France (Albert 
Genter, pers. comm.). 
 
3 THE CONCEPT OF HYBRID GRAVIMETRY  
 
The concept of hybrid gravimetry to investigate an underground reservoir with the 
combination of superconducting gravimeter (SG), absolute gravimeter AG, and relative spring 
meter RG. The permanent gravimeter allows a precise continuous monitoring of the time-
varying gravity at a reference station located on the investigated site; in order to be able to 
retrieve the long-term behavior, one uses generally a superconducting gravimeter (SG) rather 
than a spring meter because of its very small instrumental drift (a few μGal/year) and better 
precision (0.1–0.01 μGal). We use the site of STJ9 (10 km of Strasbourg, Figure 2) which is a 
super conducting gravimeter observing since 1987 and has been replaced by modern 
instrument in 1996 and then in 2016. 
 
The ballistic absolute gravimeter (AG) allows to control the long-term gravity changes by 
repeated parallel recording over short periods of time with the SG (Sugihara and Ishido 2008), 
as well as to check the calibration stability of the SG. We performed episodic measurements 
  INGEO 2017 
 
 
  Portugal | Lisbon | October 18 - 20, 2017 
at STJ9 site and a reference site in Soultz (GPK1 site on Figure 3) and in Rittershoffen (see 
Figures 2 and 3 for location). 
 
The spring relative gravimeter (RG) is used to repeat observations on a micro-gravimetric 
network around the reference station by successive loops in order to gain more insight into the 
space-time changes in the investigated region. 
 
[j1] 
Fig. 2 The three main locations of our hybrid gravity approach. STJ9 is the Strasbourg 
Gravimetry Observatory where both a superconducting gravimeter (GWR C026) and an 
absolute gravimeter (Micro-g Solutions FG5#206) are available. There are 12 micro-gravity 
stations in the Soultz network (GEIE) and 15 stations close to Rittershoffen (ECOGI) 
(modified from Hinderer et al 2015). 
 
The micro-gravimetric network is divided in two networks for an easy transportation of the 
RG instrument, one called Soultz network and the second one called ECOGI network, close to 
Rittershoffen (Figure 3). The Soultz network consists of 13 sites around the geothermal 
exploitation sites of GPK1 and GPK2 (Figure 3). A strong effort has been done to have a 
dense network around GPK1 and GPK2. Concerning the ECOGI network, only two sites are 
very close to the ECOGI geothermal exploitation site in 2013 (2 red triangles south of ECOGI 
on Figure 3). In 2015, the ECOGI gravimetric network has been completed by ten new sites 
(Figure 3). Gravimetric surveys of Soultz network were performed in 2013, 2014, 2015, and 
in 2016. Next survey is programmed in summer 2018. 
Ferhat, G. et al.: 3 years of monitoring at geothermal sites: Soultz and Rittershoffen, France  
 
   
 
Fig. 3 Location of the relative gravimetric measurement sites (red or black triangles): 12 
surrounding Soultz-sous-Forêts and 2 sites close to Rittershoffen (ECOGI site). Permanent 
GPS stations are indicated by black triangles or black circles. Blue triangles are the sites 
monumented in 2015 around ECOGI. Three sites belongs to the Soultz and ECOGI networks. 
 
4 GRAVIMETRIC DATA PROCESSING 
 
The repetition of a micro-gravimetric network, where x0 and t0 are the reference point and 
time, leads to the following formula (1) (Hinderer et al. 2015) the gravity double differences 
Dg at point x and time t: 
 
This double gravity difference provide an interesting way of interpreting gravity changes in 
time and in space. Figure 4 shows no significant change in gravity double difference for 
Soultz network. Since any change in elevation modifies the gravity measurement, it is crucial 
to perform a vertical control, all gravimetric sites are equipped with a leveling benchmark, in 
order to measure any surface deformation at and around geothermal sites. 
  INGEO 2017 
 
 
  Portugal | Lisbon | October 18 - 20, 2017 
 
Fig. 4 Example of gravity double differences in 2014, 2015 and 2016 at the 12 sites of Soultz 
network (stations 2 to 13). The blue area is the ±2 σ uncertainty band computed from the 
uncertainties in the measurements and processing of all surveys.  
5 LEVELING NETWORK CONNECTING GRAVIMETRIC SITES 
 
In order to provide the elevations of each micro gravimetric site, a first large leveling network 
has been established in 2014 (Ferhat et al. 2014). This leveling network connects all of the 13 
gravimetric sites of Soultz network (Figure 5). For control purposes and practical observation 
procedures, the leveling network is made of five loops (Figure 5). Each loop was observed 
using a digital level (Leica DNA03) and standard leveling staff. The leveling lines include 
some National leveling benchmarks installed by the French Mapping Agency (IGN, Institut 
National de l’Information Géographique et Forestière) in order to tie the altitudes to the 
national vertical reference system (NGF-IGN69). The loop 5 was observed several times 
using digital level Leica DNA02 or Trimble DiNi 0.3, but this time using invar staffs. It 
serves as a reference benchmark for the newly constructed geothermal exploitation site of 
ECOGI. By fixing the altitudes of three leveling benchmarks BM at IGN values, we 
computed the altitudes of all other benchmarks (Ferhat et al. 2014). Uncertainties on these 
altitudes are about 2 to 5 mm. This leveling network has been surveyed in May 2014, May 
2015, May 2016 and May 2017 by students of the Department of Surveying Engineering at 
INSA, Institute for Applied Sciences, Strasbourg, France.  
Ferhat, G. et al.: 3 years of monitoring at geothermal sites: Soultz and Rittershoffen, France  
 
   
 
 Fig. 5 The leveling network connecting the 13 gravimetric sites of Soultz network (5 loops) 
and comparison of height difference for consecutive section during the period 2014 – 2015 
(after Ferhat et al. 2015). 
 
Differences in elevation between consecutive leveling benchmarks have been computed for 
the period 2014 – 2015 and are less than few mm (Ferhat et al. 2015, Figure 5). Comparison 
of the elevations during the 4 years show no significant changes, uncertainties for the 
elevation difference are mostly about 6-7 mm. Moreover, in 2016, elevations of 10 
gravimetric sites composing the ECOGI network were determined using leveling techniques 
for 5 sites. Due to long distance from Rittershoffen or steep topography or lack of time and 
operators, elevations for the 5 other sites were determined by static GPS observations 
associated to a local leveling tie. 
 
6 PRELIMINARY RESULTS 
 Almost all observed changes in the Soultz network are within the rectangular uncertainty 
zone (Figure 3) and are hence not significant. In other terms, we do not observe any gravity 
change that exceeds our measurement precision. The lack of detectable gravity changes 
indicated by our results for 2014 is in agreement with the fact that during this period, the 
geothermal activity was completely stopped in Soultz. Analysis of selected gravimetric data 
acquired in 2015 and 2016 will be presented at the conference. 
7 CONCLUSION 
 
In order to monitor surface deformation and investigate geothermal fluid circulation at depth 
at geothermal exploitation sites of Soultz-sous-Forêts and Rittershoffen located in the Rhine 
Graben (NE of France), a dense leveling network and two micro gravimetric networks has 
  INGEO 2017 
 
 
  Portugal | Lisbon | October 18 - 20, 2017 
been surveyed annually since 2013 (for gravity measures) and since 2014 (for leveling 
measures). No gravity change has been clearly observed during this period. Since no full 
exploitation operations are currently performed at Soultz and ECOGI sites, we do not observe 
clearly significant gravity changes in 2016, i.e. during the start of the exploitation. Long term 
leveling and gravimetric monitoring associated to repeated magneto-telluric surveys, InSAR 
and continuous GPS observations and seismological monitoring should provide information 
on surface deformation and processes at depth, i.e. in the geothermal reservoirs. 
 
 
ACKNOWLEDGEMENTS 
 
This work has been published under the framework of the LABEX ANR-11-LABX-0050\_G-EAU-
THERMIE-PROFONDE and benefits from a funding from the state managed by the French National 
Research Agency as part of the Investments for the future program. 
 
 
REFERENCES 
 
Ferhat G., V. Patoine & E. Clédat. 2014. Leveling network for surface deformation 
monitoring along Soultz-sous-Forêts and Rittershoffen geothermal sites, France. 
European Geothermal Workshop, Karlsruhe, Germany, 15-16 October 2014. 
Ferhat G., M. I. Lopez Barraza & E. Clédat. 2015. Vertical surface deformation monitoring 
during 2014 and 2015 using precise leveling around Soultz-sous-Forêts and 
Rittershoffen geothermal exploitation sites, Rhine Graben, France. European 
Geothermal Workshop, Strasbourg, France, 19-20 October 2015.  
Hector, B., L., Séguis, J., Hinderer, M., Wubda, M., Descloitres, N., Benarrosh. 2015. Hybrid 
gravimetry and water storage changes in a headwater basement catchment of Sudanian 
West-Africa: Water Resources Research., American Geophysical Union, 2015, 51 (10), 
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@Grenelle-group4: 
http://archive.wikiwix.com/cache/?url=http%3A%2F%2Fwww.developpement-
durable.gouv.fr%2FIMG%2Fpdf%2FG4.pdf 


Three years of monitoring using leveling and hybrid gravimetry applied to the geothermal sites of Soultz-sous-Forêts and Rittershoffen, Rhine Graben, France

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Three years of monitoring using leveling and hybrid gravimetry applied to the geothermal sites of Soultz-sous-Forêts and Rittershoffen, Rhine Graben, France

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