First Break - Volume 39, Issue 8, 2021

Shallow cavities, such as karstic caves in carbonate bedrock and near-surface underground mine workings or tunnels, constitute serious hazards for people and existing constructions due to the risk of collapse and subsidence. This phenomenon is growing fast with climate change causing damage to buildings and increased urban development costs. Karstic features, voids, surficial dissolution, fissuring, alteration and unconsolidated material are thus major challenges for geophysical methods that could play a major role in detection. Recent investigations have reported good results for cavity imaging using surface wave seismic methods. However, despite successful case histories, business as usual void detection is still challenging due to the complexity of near surface materials and probably a lack of understanding regarding the interaction of surface waves with voids. Recent data acquired in the context of a search for underground unpaved cavities provide new insights into the use of surface waves for the detection of these objects. Considering the findings, it appears that surface waves penetrating above and below the void may still propagate horizontally without clear features of altered propagation characteristics with respect to different actions, variations in dispersion, or both, which encourages rethinking of the systematic nature of diffraction research in operational applications.

Although the seismic technique is becoming an increasingly popular tool for mining its use tends to be restricted to exploration. Although capable of being used to characterize active mining areas, this application has tended to be restricted to underground or deep open-pit mines as acquiring high-resolution data over extensive areas has previously been prohibitively expensive. In this paper we show examples of the impact of high-resolution survey parameters on the resulting processed datasets and describe how such datasets can be acquired in a cost-efficient manner leading to an extensive 3D seismic acquisition programme.

We propose an unmanned aerial system fully dedicated to magnetic surveying in a variety of contexts ranging from small and shallow objects or pipelines detection to large-scale geological mapping: the Easymag system. Four major requirements are identified if one wishes to efficiently acquire quality magnetic data at such variable scales: 1- the accuracy of the magnetic acquisition must be better than the lowest expected signal according to the application (usually a few nT or lower); 2- the navigation and positioning must be precise enough to compete with ground operators with centimetre accuracy; 3- the flight flexibility must allow contrasting flight domains with topographic draping at any height; 4- the efficiency on the field must compete with existing carriers. Each of these aspects are investigated to demonstrate Terremys’ ability to perform efficient magnetic surveys. As a first case study, a drone magnetic acquisition for archaeological purposes is presented in comparison with a ground survey of the same area. Ground and drone surveys can lead to similar results. To demonstrate the ability to acquire magnetic data in different flight domains with the same system, a second case study focusing on a multiscale analysis is presented.

Hell Bourg landslide (Salazie, Reunion Island) is a large, inhabited, slow-moving landslide (>200 Mm³). This landslide covers and remobilizes debris from former volcanic flank collapses. The volume, lithology, structure and evolution through time of these ancient deposits make the Hell Bourg landslide particularly complex. To understand the dynamics and improve the hazard related to its displacements, geodetic, geomorpho-logic, and geologic investigations have been undertaken on this landslide for more than 20 years. Complementary investigations have been required to acquire a precise definition of this large landslide’s subsurface geometry and hydrogeology, necessary for a better understanding of its dynamics. In the present study, we combine different geophysical approaches (airborne electromagnetics, passive and active seismic data) to investigate the internal structure of the landslide. This method delivers accurate 3D images that provide useful additional information about the subsurface of the landslide. The models obtained of the subsurface are consistent with geomorphological and geological observations and allow the derivation of a new conceptual model of the 3D structure of the landslide. Thanks to this approach, new insights into the origin and mechanisms of the Hell Bourg landslide have been gained. This study shows the relevance of the multi-method approach for understanding large complex landslides. The work carried out on the Hell Bourg landslide opens up new avenues of research into large landslide mechanisms in a volcanic tropical environment.