During the ReunEM heliborne geophysical survey of La Réunion. © BRGM - René Carayol

Airborne geophysics: assessing solid rock quarrying potential in La Réunion

11.29.2018
La Réunion is facing a shortage of solid rock supplies, which are essential to major civil engineering projects. To address the problem, the BRGM conducted a pilot study based on data collected in 2014 during the ReunEM heliborne geophysical survey campaign.

Context

La Réunion is facing a shortage of solid rock supplies, which are essential to major civil engineering projects.  Under the current Regional Quarries Plan, supplies are restricted due to limited quarrying space with sometimes uncertain potential. To improve knowledge on potential for solid rock quarrying across La Réunion, the BRGM was commissioned by the DEAL to conduct a pilot study drawing on recent geophysical data obtained by the 2014 ReunEM geophysical survey, an approach never previously applied on this scale. The project results will feed into the new Regional Quarries Plan.

Airborne geophysical data acquisition in 2014 (ReunEM project). © BRGM

Airborne geophysical data acquisition in 2014 (ReunEM project). © BRGM

Objectives

This project aimed to use the resistivity model derived from the airborne geophysical survey to show favourable sectors for solid rock extraction (substantial thickness and sound rock), including their probable depth and minimum thickness.

Work programme

Cross-analysis of the model built up from the ReunEM data with available geological information, including over 2000 core samples documented in the BSS subsoils databank, showed the resistivity ranges likely to correspond to sound solid basalt rock.  The result was then assessed by ground reconnaissance over two zones of about 20 km2 and by comparisons with data from materials prospecting provided by the quarry managers called on for the study.

EM soundings showing the presence of a resistive body (in brown) close to the surface likely to correspond to a favourable zone for solid rock. © BRGM

 

EM soundings showing the presence of a resistive body (in brown) close to the surface likely to correspond to a favourable zone for solid rock. © BRGM

Validation on the ground. © BRGM - Nicolas Charles

Validation on the ground. © BRGM - Nicolas Charles

Results obtained

The favourability results obtained are fairly consistent despite some weakness in the comparison between ground survey results and the resistivity model due to the different scales and absence of outcrops of possible solid rock. All in all, the airborne electromagnetism survey method seems entirely relevant for acquiring imagery of sound rock formations.

The electromagnetic soundings are represented as grids showing favourability, the minimum depth of favourable layers and their cumulative thickness across a given area, reflecting the favourability of its geological environment as a whole. Simple cross-analysis of the grids can then identify the presence of solid rock across the  island, according to the criteria for target discovery and deposit thickness.

These study findings are applicable to solid rock prospecting projects and are the first step in compiling a resource map for the Regional Quarries Plan to be adopted by 1 January 2025, in accordance with government instructions issued on 4 August 2017.

Improving the resistivity ranges at the local or regional scale would require reprocessing of the ReunEM dataset, using new and robust calibration data for materials acquired from quarry managers.

PARTNERS

DEAL Réunion

BRGM - 3 avenue Claude-Guillemin - BP 36009 45060 Orléans Cedex 2 - France Tel.: +33 (0)2 38 64 34 34