Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments

Caribbean areas are particular volcanic territories in tropical environments. These territories juxtapose several landslide-prone areas with different predisposing factors (poorly consolidated volcanic materials, superimposition of healthy materials on highly weathered materials, high heterogeneity of thicknesses, etc.). In these environments, where rapid development of slopes and land use changes are noticeable, it is necessary to better characterize these unstable phenomena that cause damage to infrastructure and people. This characterization has to be carried out on the materials as well as on the initiation conditions of the phenomena and requires complementary investigations. This study, focusing on La Martinique, proposes a landslide analysis methodology that combines new information about landslide-prone materials acquired by an airborne electromagnetics survey with a physical-based model. Once the data are interpreted and compared with field observations and previous data, a geological model is produced and introduced into the physical model to test different instability scenarios. The results show that geophysical investigations (i) improve the knowledge of the internal structure of landslides and surficial formations, (ii) specify the spatial limits of the materials that are sensitive to landslides, and (iii) give a better understanding of landslide initiation conditions, particularly hydrogeological triggering conditions.

Application of Airborne Electromagnetics and Magnetics for Mineral Exploration in the Baishiquan–Hongliujing Area, Northwest China

Airborne electromagnetics is an effective and efficient exploration tool in shallow mineral exploration for its high efficiency and low cost. In 2016, airborne electromagnetic and airborne magnetic surveys have been carried out at the border of Xinjiang Uygur Autonomous Region and Gansu Province, the Northwest China. With an integrated system, the airborne electromagnetics and airborne magnetic data were collected simultaneously by AreoTEM-IV system from Aeroquest International Limited in Vancouver, BC, Canada, and the CS3 Cesium Vapor magnetometer from Scintrex in Concord, ON, Canada. About 3149 line-km of both data with 250 m line space were acquired. After data processing, the comprehensive analysis and interpretation of resistivity and magnetic anomalies has been carried out to infer lithological structure and outline the potential ore deposits. Verified by the ground surveys, seven outlined anomalies are consistent with the known ore sites, and one new gold deposit and several mineralization clues were found. The prospective reserves of gold are expected to exceed 10 tons. Besides, some prospecting target areas were outlined as the possible locations of copper–nickel deposits. The successful case shows the airborne magnetic data accords with geological structures, and the airborne electromagnetic method is effective in finding metal mineral resources, which can help to quickly identify potential ore targets with no surface outcrop.

Structural complexity inferred from anisotropic resistivity: Example from airborne EM and compilation of historical resistivity/induced polarization data from the gold-rich Canadian Malartic district, Québec, Canada

Structurally complex zones within orogenic terranes typically correspond to areas where there is interference between multiple fold generations and are known to be favorable pathways for fluid flow because of their higher permeability. In the Canadian Malartic district, gold anomalies have been linked with zones of structural complexity that have been quantified by outcrop bedding orientation measurements and calculation of bedding variance maps. In this work, historical apparent resistivity and induced polarization data in the Canadian Malartic district were reprocessed and combined with new surveys to create a compilation of inverted chargeability and resistivity, which were then interpreted together with airborne electromagnetics and outcrop structural data. The results indicate chargeability anomalies, up to five times the background value, associated with the sulfide mineral content in monzodioritic dikes that are thickened in folds and hydrothermally altered. Although the airborne apparent half-space resistivity is mostly sensitive to conductive surficial cover, the inverted ground resistivity method is sensitive to deeper structure and likely represents bedrock signal at depths greater than 25 m. Inverted ground resistivity exhibits strong anisotropy in areas of subvertical bedding, where measured resistivities can vary by up to a factor of two, over the same location, depending on whether the survey lines are perpendicular or parallel to the strike of bedding. This result is observed at scales of 50 cm up to 100 m. Analysis of inverted ground resistivity together with bedding variance indicates a strong correlation between structurally complex zones with high bedding variance and a decrease in resistivity at depths greater than 25 m. This suggests that in places where the presence of disseminated gold cannot be directly detected, or where the outcrop exposure is limited due to overburden cover, geophysical data may still succeed in identifying structural complexity zones that could potentially host mineralization.

Combining Hydraulic Head Analysis with Airborne Electromagnetics to Detect and Map Impermeable Aquifer Boundaries

Impermeable aquifer boundaries affect the flow of groundwater, transport of contaminants, and the drawdown of water levels in response to pumping. Hydraulic methods can detect the presence of such boundaries, but these methods are not suited for mapping complex, 3D geological bodies. Airborne electromagnetic (AEM) methods produce 3D geophysical images of the subsurface at depths relevant to most groundwater investigations. Interpreting a geophysical model requires supporting information, and hydraulic heads offer the most direct means of assessing the hydrostratigraphic function of interpreted geological units. This paper presents three examples of combined hydraulic and AEM analysis of impermeable boundaries in glacial deposits of eastern Nebraska, USA. Impermeable boundaries were detected in a long-term hydrograph from an observation well, a short-duration pumping test, and a water table map. AEM methods, including frequency-domain and time-domain AEM, successfully imaged the impermeable boundaries, providing additional details about the lateral extent of the geological bodies. Hydraulic head analysis can be used to verify the hydrostratigraphic interpretation of AEM, aid in the correlation of boundaries through areas of noisy AEM data, and inform the design of AEM surveys at local to regional scales.