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Geophysical survey for geothermal exploration at La Palma, Canary Islands, Spain

(MT and ambient seismic noise tomography)
JUN1: June 1 to 29, 2018 (days off June 2, 3, 10, 17 and 24)
JUL1: June 29 to July 27, 2018 (days off June 30, July 1, 8, 15 and 22)
4 students per month

internships_01_2018

The four selected students for this programme will perform magnetotelluric and seismic measurements. The magnetotelluric (MT) method is a passive electromagnetic geophysical exploration technique that images the electrical properties of the earth at subsurface depths. The energy for the magnetotelluric technique is from natural source of external origin. The source field is generated by variations in Earth’s magnetic field, which provide a wide and continuous spectrum of electromagnetic field waves. These fields induce currents into the Earth, which are measured at the surface and contain information about subsurface resistivity structures. MT is one of the few techniques capable of sensing through the Earth’s crust to upper mantle. MT is applied on volcanoes to identify conductive structures that might be related to fluids such as magma, often related to heat flux, important for geothermal exploration. Fieldwork will be carried out in La Palma during the first three weeks, and the final week you will be working in Tenerife.

During this period students will learn:

  • What is MT, what is the workflow for MT study (acquisition, processing, interpretation), and what are the elements of a MT station.
  • How to localize the proper place to put MT station, taking into account the complexity of volcanoes topography, man-made electromagnetic noise. Then, using digging and orientations tools, how to properly install MT station in the field.
  • How to test the MT station of the field, analyse time series to check that everything is well connected, properly set and ready for acquisition. Computer software will be used on the field for quality check.
  • How to check that recordings made by the MT station after acquisition are satisfactory. This will be done by visually checking recordings and applying commercial software to quickly process data.

MT fieldwork might imply carrying heavy equipment (~10 kg) on distances of few hundred meters to access sites and digging hard ground to install instruments, all of this under hot weather. Good physical condition is therefore required for this programme. Only acquisition and quick processing of MT time series will be done during the programme. The processing and inversion will be done at a later stage of the study, but discussions about those will be done during the fieldwork.

During this programme students will learn:

  • The theoretical basis of seismic tomography.
  • How to use ambient noise for seismic tomography
  • How to deploy temporary seismic stations
  • The processing chain from raw seismic data to seismic tomography cross-sections

Ambient noise tomography. Seismic tomography is a technique which allows imaging the interior of the earth using seismic waves emitted by natural sources (e.g. earthquakes) or artificial controlled sources (e.g. explosions). However it has been discovered that the background seismic noise produced by both natural sources and human activities, can be successfully exploited for seismic tomography. The use of seismic noise is very convenient, not requiring any specific kind of source. The use of seismic tomography in volcanoes is very useful for understanding their internal structure as well as improving the location of earthquakes. In geothermal exploration, seismic tomography enables detection of subsurface structures which can be of potential interest for exploitation.