Heliborne geophysical survey of La Réunion.
© BRGM - René Carayol
For volcanic islands, where resources - water, building materials, energy - raise acute problems, and which are also highly exposed to natural risks such as earthquakes and landslips, access to reliable information on near-surface physical properties is vital.
From 2010 to 2015, the BRGM carried out extensive airborne geophysics survey campaigns over Mayotte, Martinique, Guadeloupe and La Réunion. The relief and vegetation in these regions make access very difficult, and these airborne campaigns have brought considerable advances in several fields, particularly for water prospecting and mapping saline intrusions into coastal aquifers.
A digital image of saline intrusion (in dark blue) in the Pierrefonds coastal aquifer (La Réunion).
“X-rays” of the subsoil from the air
BRGM is ahead of the field with this technique. It is based on electromagnetism and was developed at Denmark's Aarhus University with the SkyTEM system, to obtain images of subsurface resistivity, like 3D X-rays of the first 0 to 100 metres below the surface where most usable water resources are to be found.
Groundwater resources in islands are often overexploited and subject to seawater intrusion. The water resources sought during airborne geophysics surveys are mainly “perched” aquifers, which are not subject to these problems.
These are small discontinuous bodies of groundwater in a porous layer enclosed in a lava flow. Their conformation, which cannot be assessed with any certainty from the surface, is revealed by airborne geophysics with enough accuracy to guide subsequent investigations on the ground (geophysical soundings, test boreholes, etc.). These small aquifers are still well preserved and therefore of strategic importance to meet increasing demand for drinking water or sometimes to replace degraded resources.
The BRGM's airborne geophysics surveys in these islands have produced extremely positive results: Of the investigations to be conducted following these campaigns, 50% concern hydrogeology. In Mayotte, for example, BRGM discovered several new hydrosystems and described them with considerable precision. Eight of the ten ground surveys made on the basis of these results proved to be very promising. And by cross-referencing our data with a geographical information system, BRGM also developed a method for the local water management body to define the catchment areas that feed drinking water abstraction points, which is essential to protect and manage the resource.