Geophysical study of volcanic structures
Contact : Philippe Labazuy
Technical Manager: Thierry Souriot
The internal structure of volcanoes can be studied using geophysical methods.
The LMV has a number of facilities for this purpose.
Main techniques used:
Spontaneous polarisation (PS)
Measurements of the spontaneous electrical polarisation at the surface of volcanoes allow, in particular, to highlight hydrothermal circulations.The LMV has very high impedance voltmeters, impolarisable electrodes and cables necessary for PS measurement campaigns.
PS anomaly of 2000 mV on a zone of high hydrothermal activity at Piton de la Fournaise.
The LMV has differential GNSS equipment from TopCon (off-line only) and Trimble (very recent and full RTK) brands. The accuracy of these devices can reach the millimeter. They are used for various types of measurements ranging from micro-topography to the positioning of other geophysical devices for field measurements or rock sampling. In this case, the accuracy allows for 2D and 3D tomography and inversions.
Récepteur mobile utilisé pour localiser des mesures de gravimétrie sur le volcan Yasur (Vanuatu).
Study of the electrical resistivity of volcanic edifices
The electrical resistivity of volcanic rocks can vary by several orders of magnitude depending on their water saturation, temperature, presence of mineralised fluids or clay minerals. The distribution of resistivities at depth can therefore be interpreted in terms of geological structure. This distribution can be studied by different electrical and electromagnetic methods. The LMV has two types of equipment: intruments to carry out direct surveys and 2D tomographiy and a light equipment offering the capacity to carry out electromagnetic drilling with an investigation depth of up to a few hundred metres.
The equipment includes :
- ABEM TERRAMETER SAS4000 Resistivity Meter
- ES1064 addressing box, for 4 channels
- 16 cables, 4 outputs with 40 m distance between each output, on Schill reels, 160 m / cable
- 8 Cables, 8 outputs with 20 m distance between each output, on Schill reels, Lg 160 m / cable
- Cable kit “Standard – 5 m”, 16 outlets with a distance of 5 m between them. 4 SCHILL reels,
- 2 Cables, 32 outlets with a distance of 2 m between each outlet, on Schill reels, Lg 64 m / cable
- RES2DINV interpretation software
This equipment is shared with the GeoSciences Reunion Laboratory of the University of Reunion.
Its originality is a unique line of 64 electrodes separated by 40 m. This line of 2560 m allows the acquisition of a 2D image of the resistivity distribution between the surface and 400-500 m depth.
2D DC electrical resistivity tomography measurements on Yasur volcano (field acquisition and resistivity section).
Recently acquired as part of a European CAPRICE programme, the equipment includes:
- An I-FullWeather injector from IRIS instrument and 20 sensors;
- a regulated generator;
- over 8km of cable for the infinite electrode;
Our equipment includes :
- “48 channel TEM-FAST 48HPC system with antennas, calibration unit and acquisition software.
- TEM-RES software for data analysis and inversion.
- iPAQ PC.
TDEM electrical resistivity measurements on Yasur volcano (field acquisition and resistivity section)
The LMV has two Ground Penetrating Radar (GPR) devices.
- Georadar SIR3000 with 400, 200 and 100MHz shielded antennas.
- Georadar MALA ProEx with 30MHz antennas.
After using external service providers for a few years, we have our own fleet and are developing our own sensors through the OPGC service. As the use of drones is highly regulated, we cannot lend them out, but collaboration are possible.
Our current fleet:
- Escadrone Raven HL2 X8 (2016) equipped with a Sony QX1 and various “in-house” sensors.
- DJI Phantom 2 (2015) Upgraded Hero4 Edition (Zenmuse H4-3D).
- Pixy (2001) with DSLR pod.