Marta Calvo

Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben

The study is devoted to the monitoring of a geothermal reservoir by hybrid gravimetry combining different types of instruments (permanent superconducting gravimeter, absolute ballistic gravimeter, and micro-gravimeters) and different techniques of measurements (both time-discrete and recording data collection). Using a micro-gravimetric repetition network around a reference station, which is regularly measured, leads to the knowledge of the time and space changes in surface gravity. Such 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 in the Alsace region (France) of the Rhine graben. We show the results in terms of gravity double differences from weekly repetitions of a network of 11 stations around the geothermal reservoir of Soultz-sous-Forêts, separated into 5 loops during July–August 2013 and 2014 as well as preliminary results from 2 stations near Rittershoffen (ECOGI). We point out the importance of a precise leveling of the gravity points for the control of the vertical deformation. A first modeling of surface gravity changes induced by realistic geothermal density perturbations (Newtonian attraction) is computed in the frame of the existing geological model and leads to gravity changes below the μGal level being hence undetectable. However, and for the same case, borehole gravity modeling showed a significant anomaly with depth that can be used as a complementary monitoring method. We show that in the limit of our uncertainties (SDu2009~u20095xa0μGal), we do not detect any significant gravity change on the geothermal site of Soultz in agreement with the fact that there was indeed no geothermal activity during our analysis period. On the contrary, the measurements near Rittershoffen show a signal above the noise level which correlates in time with a production test but cannot be explained in terms of Newtonian attraction effects according to our basic numerical simulation.

More Thoughts on AG–SG Comparisons and SG Scale Factor Determinations

AbstractWe revisit a number of details that arise when doing joint AG–SG (absolute gravimeter–superconducting gravimeter) calibrations, focusing on the scale factor determination and the AG mean value that derives from the offset. When fitting SG data to AG data, the choice of which time span to use for the SG data can make a difference, as well as the inclusion of a trend that might be present in the fitting. The SG time delay has only a small effect. We review a number of options discussed recently in the literature on whether drops or sets provide the most accurate scale factor, and how to reject drops and sets to get the most consistent result. Two effects are clearly indicated by our tests, one being to smooth the raw SG 1xa0s (or similar sampling interval) data for times that coincide with AG drops, the other being a second pass in processing to reject residual outliers after the initial fit. Although drops can usefully provide smaller SG calibration errors compared to using set data, set values are more robust to data problems but one has to use the standard error to avoid large uncertainties. When combining scale factor determinations for the same SG at the same station, the expected gradual reduction of the error with each new experiment is consistent with the method of conflation. This is valid even when the SG data acquisition system is changed, or different AG’s are used. We also find a relationship between the AG mean values obtained from SG to AG fits with the traditional short-term AG (‘site’) measurements usually done with shorter datasets. This involves different zero levels and corrections in the AG versus SG processing. Without using the Micro-g FG5 software it is possible to use the SG-derived corrections for tides, barometric pressure, and polar motion to convert an AG–SG calibration experiment into a site measurement (and vice versa). Finally, we provide a simple method for AG users who do not have the FG5-software to find an internal FG5 parameter that allows us to convert AG values between different transfer heights when there is a change in gradient.n

Hybrid Gravimetry as a Tool to Monitor Surface and Underground Mass Changes

This paper is devoted to an overview of the use of hybrid gravimetry in Earth and Environmental Sciences. We first recall the concept of hybrid gravimetry which relies on the simultaneous use of different types of gravimeters either superconducting, absolute or relative spring gravimeters. This combination of instruments provides a complete tool for time-lapse gravimetry: while superconducting gravimeters and/or absolute gravimeters are used to obtain temporal gravity changes at one or several base stations, relative gravity surveys provide spatial differences with respect to these base stations, and allow to cover a much wider area than base stations only. Hybrid gravimetry therefore provides time-lapse gravity changes at a survey scale. We present here an overview of different published applications in hydrology, glaciology, volcanology and geothermics in order to point out that hybrid gravimetry is a powerful tool to monitor spatially and temporarily surface and underground mass changes.

Comparison of the performances of different spring and superconducting gravimeters and STS-2 seismometer at the Gravimetric Observatory of Strasbourg, France

Since 1973, the Gravimetric Observatory of Strasbourg (France) is located in an old fort named J9 and has been the place for various gravity experiments. We present a comparison of the noise levels of various instruments that are or were continuously recording at J9, including the LaCoste&Romberg Earth-Tide Meter ET-5 (1973–1985), the GWR Superconducting Gravimeter TT-T005 (1987–1996), the Superconducting Gravimeter C026 (since 1996), the STS-2 seismometer (since 2010) and the LaCoste&Romberg ET-11 (continuously since October 2010). Besides these instruments, the J9 Observatory has hosted temporary gravity experiments with the Micro-g LaCoste Inc. gPhone-054 (May–December 2008 and May–September 2009) and the Micro-g LaCoste Inc. Graviton-EG1194 (June–October 2011). We include also in the comparison the absolute gravimeter Micro-g FG5 #206 which is regularly performing absolute gravity measurements at J9 since 1997 and a spring gravimeter Scintrex CG5 which recorded at J9 between March 2009 and February 2010. We present the performances of these various instruments in terms of noise levels using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The different responses to atmospheric pressure changes of all the instruments are also investigated. A final part is devoted to the instrumental self-noise of the SG C026, STS-2 and L&R ET-11 using the three channel correlation analysis method applied to 1-Hz data.

Preliminary Results from the Superconducting Gravimeter SG-060 Installed in West Africa (Djougou, Benin)

A GWR superconducting gravimeter of the new generation (OSG-60) has been installed in July 2010 in sub-humid West Africa, at the Djougou station in Benin. This station is located in the AMMA-CATCH long term hydrological observing system. We present the first results in terms of instrumental drift as well as the calibration results using FG5 absolute gravity measurements. We show that geophysical contributions due to hydrological load can bias the initial drift estimate. The noise level is compared to the Strasbourg SG as well as to the reference New Low Noise Model (NLNM) used in seismology. We also investigate the gravity response to atmospheric pressure changes and show that, because of the presence of large thermal tides, the gravity response to mass changes in the atmosphere is more complex than in the simple case of a constant barometric admittance.

Detailed Analysis of Diurnal Tides and Associated Space Nutation in the Search of the Free Inner Core Nutation Resonance

We propose a comparison of the tidal analysis results obtained from the continuous records of time-varying surface gravity collected by a worldwide network of Superconducting Gravimeters with the analysis results of space nutation observed by the international Very Long Baseline Interferometry (VLBI) network. The length of the surface gravity time series (20 years for the longest) enables now to look for additional diurnal tides that were previously not analyzed. In parallel, we now possess 35 years of VLBI data permitting to look for additional nutation terms. We focus our analysis on the diurnal prograde frequency band in the search for a possible resonance effect linked to the Free Inner Core Nutation. This Earth’s normal mode has never been clearly observed. Its direct deformation effect at the Earth’s surface is theoretically predicted to be too small to be detected. However, the tidal forcing at a frequency close to its eigenfrequency could enhance some tidal or nutation amplitude resulting in the characterization of this mode through its resonance effect.

Tidal Spectroscopy from a Long Record of Superconducting Gravimeters in Strasbourg (France)

We present a comparison in the various tidal bands, between two different spectral analyses of long gravimetric time series. The first one is performed using a long gravity series recorded by superconducting gravimeters at J9 Observatory (Strasbourg) and the second one uses a theoretical series of the same length, almost 28 years, computed for the same location according to the Hartmann and Wenzel tidal potential development.

Study of the Time Stability of Tides Using a Long Term (1973–2011) Gravity Record at Strasbourg, France

The Gravimetric Observatory J9 located near Strasbourg (France), has a long history of recording solid Earth tides. We present here one of the longest available gravity records (1973–2011) from three different instruments operating at the J9 station: a LaCoste–Romberg spring gravimeter (ET005) from 1973 to 1985, a GWR (TT70–T005) superconducting gravimeter from 1987 to 1996 and a GWR compact superconducting gravimeter (C026) since 1996.