Heliborne geophysics in La Réunion: applications for furthering knowledge on water resources

11.29.2018

The BRGM, in partnership with the EU (ERDF), the French State and the La Réunion Region, conducted a heliborne geophysical survey of the entire island in 2014, in order to map its geological resources. Interpretation of the ReunEM campaign findings is of great interest to further knowledge on the island's groundwater resources. A thesis has been undertaken on multi-scale characterisation of geological structures in volcanic islands.

Heliborne geophysical survey in 2014 (ReunEM project). © BRGM

Context

A large-scale heliborne geophysical survey covering the island of La Réunion was conducted in 2014. Subsurface magnetic field and electrical resistivity anomalies were mapped with SkyTEM down to 300 m across the entire island. The dataset thus acquired illustrates the issue of interpretations at regional scale of electromagnetic data acquired by airborne surveys.

Objectives

Interpretation of a 3D resistivity model across 2500 km², which requires innovative methodologies to compare the results with geological, climatic and hydrogeological data. A thesis has been undertaken on the use of this geophysical dataset at the regional scale, integrating hydrogeological analysis at different spatial scales.

Work programme

To address the issues raised, the thesis is organised around a multiscale hydrogeophysical methodology applied to study the distribution and functioning of aquifers at scales never previously attempted. To interpret the 350 000 electromagnetic soundings taken during the survey, a statistical approach was developed to synthesise information on contrasts into 2D. The results are consistent with geological and hydrogeological data on the island and enable analyses of the impacts of regional geological and climatic parameters on aquifer distribution and weathering processes. Resistivity data along the coast are compared with logs of the electrical conductivity of groundwater in order to validate interpretation of the geophysical data. The resistivity model can then be used to map and analyse saline intrusion control parameters on the scale of the island.

At higher altitudes, heliborne electromagnetic and magnetic survey data are combined to characterise the geometry of geological structures and produce imagery of weathering profiles and palaeo-profiles. Combining the results obtained at different scales makes it possible to analyse the impacts of weathering on the functioning of aquifers in shield-volcano contexts.

Results obtained

Along the coastline, formations that are sound and highly permeable are vulnerable to saline intrusions. The aquifers become less vulnerable to saline intrusion as transmissivity decreases.

At higher altitudes, geophysical data shows that contact between the products of different eruptive phases are characterised by palaeo-weathering profiles that can produce permeability contrasts favourable to the formation of perched aquifers. Analysis of the geophysical response in different sectors of the island has produced a proposed weathering dynamic, governed initially by the age of the formations and subsequently by rainfall and temperature. These results show that the weathering and structuring patterns of volcanic ranges vary according to their age and the leeward or windward exposure of their slopes. Work on the thesis is thus refining the conceptual hydrogeological model by integrating the geological structuring process and the evolution of basaltic volcanic aquifers in island contexts.

AEM imagery of palaeo-valleys.