The Role of Airborne EM in Engineering and Environmental Geophysics

摘要

The application of Airborne EM for Engineering,Geotechnical and Environmental issues requires both high shallow resolution and detailed,precise quantitative modeling.This requirement demands the use of specific airborne systems,characterized by high performance,and of rigorous data processing,able to produce very accurate modeling.Nowadays this geophysical technique can be certainly considered mature,as demonstrated by some case studies,shown in this paper,dealing with landslides,weathering resolution,shallow geological investigation and so on.AEM offers many advantages,such as the possibility to get a very dense sampling (so to facilitate the geotechnical interpretation),the ability to investigate challenging areas (wet or remote areas,with dense vegetation or very complicated topography).The use of apriori info,coming from stratigraphic data of boreholes or from different geophysical prospections that are more often used in Engineering (seismic,ERT),can improve greatly the outcome.Our case-studies will deal with the study of landslides,in Sicily (Italy),where a composite earthflow has involved the formation of the "Argille Varicolori"; the method was able to resolve small thickness (15-20 m) and low resistivity contrast (15 ohm-m of the earthflow,while the clay substratum has a resistivity of 7-9ohm-m).Another example of great resolution of shallow structures will be provided by some comparisons between the vertical resistivity profiles derived from AEM and the 2D section derived from the Electrical Resistivity Tomography.Another case-study,coming from a hydrogeological investigation in Manitoba (Canada),shows the advantage of using apriori,so to improve the results of AEM modelling: the results of high resolution seismic reflection and ERT surveys were imported into the AEM dataset,as nothing but an extra dataset,having their own location,values,uncertainty and expected lateral variation.Adding a-priori also reduced uncertainty in the resistivity values of the overlying layers which become more resistive although no a-priori information was added directly to those layers.Hence a-priori constraints can help refining the resolution of otherwise poorly determined parameters.We can state that AEM is nowadays a mature technology for solving geotechnical problems,thanks to the huge innovation in equipments and in processing and inversion of data.