Imagerie à l'échelle d'un massif, par méthodes électromagnétiques actives et passives

Présentation de Jean-François Girard (Eost) aux 3èmes Rencontres Scientifiques et Techniques Résif | 10-12 octobre 2017, St Jean de Monts

Transcript

1. Imagerie à l'échelle d'un massif, par méthodes électromagnétiques actives et passives Liens avec Resif Contributeurs: Jean-François Girard (IPGS-EOST, Strasbourg) Stéphane Garambois (ISTerre, Grenoble) Pascal Tarits (IUEM, UBO, Brest) Pascal Sailhac (GEOPS, Paris Orsay) 1
2. Plan  The team… EM groups in France today  The current instrumental pool  Some major recent experiments  Projects & methodological developments  Why combining seismic & resistivity ?  Future instrumental pool… 2
3. The team… EM groups in France today IPGS-EOST IUEM/UBO GEOPS ISTerre IPGP Univ. Clermont Ferrand BRGM … non-exhaustive list 3 Contributors to this presentation MT: passive EM CS: active EM
4. The current instrumental pool in France Where What ? INSU / IPGS 2 tensor MT IPGS / Labex 2 tensor MT + 1 permanent Remote UBO/IUEM 2 tensor LF MT + 5 marine BRGM 4 tensor MT + 2 TX Metronix ADU 07 4 Tensor MT = 5 components GPS synchro. Broad-band 1000 sec -> 100 kHz Magnetic sensors Electric sensors
5. Locations of MT surveys in the past 5 years (Performed by the authors team) 5 (with a small instrumental pool) Today: most deep EM surveys are for geothermal resources exploration…
6. Some major recent experiments: MT Tanzania (IUEM/UBO) (From Ebinger, 2005) B B' (Mulibo & Nyblade, 2013) S wave tomography model The East African Rift is controlled by the interaction of 1 (or several) deep mantle plume and the heterogeneous lithosphere 6
7. Some major MT recent experiments: Tanzania (IUEM/UBO) Tanzanian Craton Mozambique Belt rift rift rift rift rift Plasman, 2017 Hautot et al., 2017 7 Part of ANR CoLiBrEA and Electrolith projects
8. Some major recent experiments: MT monitoring of geothermal site (Iceland) 8 Þeistareykir network INSU pool / Metronix ADU07 Acquisition system Follows a first experiment in Alsace (Abdelfettah et al. 2017, Geothermics 71) MT Sounding curve Apparent resistivity vs period ryx rxy GFZ pool / SPAM-IV Acquisition system Collaboration: GEOPS Paris-Saclay, EOST Strasbourg, GFZ Potsdam, ISOR Reykjavik, and Univ. of Iceland
9. Some major recent experiments: CSEM in Alsace (june 2017) 9 ANR BRGM, IPGS, LHYGES, ES-Géothermie Power generator Transmitter Electrodes for 30 Amp… CS EM = Controlled – source EM (kilometric electric or magnetic dipole)
10. Some major recent experiments: CSEM in Martinica (BRGM / ADEME / Region) 10 Fort de France Bay, Lamentin plain Geothermal exploration 2015 Girard et al, WGC 2015 (8x8 km)
11. Some major recent experiments: CSEM in Martinica (BRGM / ADEME / Region) 32 Hz app. Resistivity map 8 Hz app. Resistivity 2 Hz app. Resistivity Mapping resistivity (8x8 km) at depth changing frequency using the in-phase electric field recorded at the CSEM stations numbered on the map. 11 d/2 = 140m d/2 = 280m d/2 = 560m Note (here): sea water + hydrothermalism -> very conductive zone -> limit penetration depth
12. Some major recent experiments: CSEM in Belgium (BRGM) Comparison of large 2D DC ERT vs section in 3D CSEM inversion Givet site, Belgium (Bretaudeau et al. 2016) 12 Depth (m) Depth (m) Deeper, larger, 3D, less logistics… Standard DC imaging + Large DD DC imaging 3D CSEM inversion
13. Methodological developments  3D MT inversion: in-land & marine acquisitions (UBO/IUEM)  Creation of startup IMAGIR sarl (3D imaging, since 2010)  Experiments on gravi / MT monitoring of geothermal sites (Alsace, Iceland 2017, GEOPS / IPGS-EOST / Labex G-Eau-Thermie profonde)  3D joint inversion: MT / seismology / gravimetry (Tanzania, M Plasman, PhD 2017, ANR CoLiBrEA and Electrolith projects).  3D MT/gravi/ seismic noise interferometry for deep geothermal exploration – (Massif Central, PhD JM Ars, exp. 2018 – UBO/IUEM & ISTerre)  Induced Polarization & 3D joint inversion CS / MT (ANR EXCiTING, 2018-2022 IPGS / UPMC / BRGM / IC2MP / IRIS-Instruments) 13
14. Relationship between fluid & seismicity Link with Resif 14 (Amoroso et al., 2017) Seismic tomography approach Irpinia fault zone (Italy) 3D MT approach San Andreas fault (Becken et al., 2008, 2017)
15. US array includes a MT survey plan Footprints of MT-TA (by year), MT-BB (now demobilized) and MT- FA (iMUSH and MOCHA on-land) stations. Updated 7/15/15 http://www.usarray.org/researchers/obs/magnetotelluric 15
16. Future instrumental pool… … inspired from Sismob? A mobile MT network: why ? … necessary for large, deep MT / CS surveys No problem to use X simultaneous stations (2000 sec * 1000 periods = 1 month) … new possibilities in cross remote filtering.. … coupled seismic / MT approach But a structure is needed - Scientific management board - Planning management - Equipment maintenance - Low cost for academic - Data storage & policy of distribution - Maintenance of permanent remote MT stations & data Note… 1 MT station = 1 RX in Controlled Source 16 Depth (m) Sketch from Resolve initiative (ISTerre)