On the Use of Aeromagnetism for Geological Interpretation: 2. A Case Study on Structural and Lithological Features in the Northern Vosges

Abstract

In many grabens of the West European Rift System, several drilling projects are targeting fluid conductive faults to produce electricity with geothermal energy. In order to reach the adequate temperatures, the targeted faults are located in the basement rocks or at the basement/cover interface. Unfortunately, the basement rock facies and structural data useful for reservoir modeling are often poorly constrained because of the lack of boreholes deeper than 2,000 m and seismic information on the deep subsurface. With the aim to outline the basement structure in the Upper Rhine Graben, two aeromagnetic surveys were conducted where the basement outcrops in the Northern Vosges, with different flight parameters and types of magnetometers. An interpolator using equivalent sources for gridding these two surveys is described by Gavazzi et al. (2020). Here, we describe the geological interpretation of these surveys. We demonstrate the consistency between the total magnetic field intensity and the outcropping lithological units organized in ENE‐WSW subvertical bands. In detail, three main blocks were delineated, separated by N‐S faults linked to the graben formation and ENE‐WSW‐inherited Hercynian faults. The latter faults, together with more local NE‐SW and NW‐SE faults, compartmentalize the N‐S‐oriented basement rocks. This is relevant for geothermal prospection where the N‐S faults are the main targets because fluid flow through other fault systems is deemed restricted. With a focus on the interface between the Northern Vosges and the Upper Rhine Graben, we also demonstrate considerable segmentation on the main graben boundary faults.

Three-Dimensional Magnetic Inversion Based on an Adaptive Quadtree Data Compression

“…In this paper, the regularized conjugate gradient (RCG) method was used to solve the objective function [16]. Since the magnetism in the crust tends to be within a specific range, the addition of a physical upper and lower bound constraint can ensure that the values of the model parameters are within a reasonable range during iterative calculations [46], as shown in Equation 9, where m min and m max denote the upper and lower bounds of the magnetic parameter values, which can be obtained from geological data or logging data, respectively. if m j < m min , then m j = m min if m j > m max , then m j = m max (9) It is worth noting that the amount of observational data is often very large, resulting in a large sensitivity matrix A.…”

“…To separate the local field from the background field, the upward continuation method was used. Upward continuation was used to calculate the abnormal value of a certain height above the observation plane according to the measured anomaly [46,47]. Since the potential field value is inversely proportional to the square of the distance from the field source to the observation point, the field value of the field source with different depths has different changes in speed when the same height is extended.…”

Magnetic Surveys With Unmanned Aerial Systems: Software for Assessing and Comparing the Accuracy of Different Sensor Systems, Suspension Designs and Compensation Methods

“…It also enables magnetic surveys in areas difficult to access or that would pose a hazard to pilot and crew on manned flights (e.g., above or near active volcanoes). These systems offer a great deal of flexibility (e.g., since the flight plan can be modified while the survey is being collected), can be flown at a lower cost than commercial airborne surveys, and are now used for a wide range of applications (e.g., Bertrand et al., 2020; De Smet et al., 2020; Gavazzi et al., 2016; Gavazzi et al., 2019; Le Maire et al., 2020; Parvar et al., 2017; Walter et al., 2020).…”

Non-Destructive Techniques for Building Evaluation in Urban Areas: The Case Study of the Redesigning Project of Eleftheria Square (Nicosia, Cyprus)

“…Applied geophysics, through the employment of different methodologies, represents a useful tool to map subsurface features in a non-invasive way, and up to date, successful documented researches in rural settings dealing with different research objectives are available in the literature. Geological investigations for characterizing faults [1][2][3], landslides [4,5], paleo-morphologies [6,7], litho-stratigraphies [8,9], acquifers [10], sinkoles [11,12], and seepage detection [13] are generally carried out through electrical resistivity tomography (ERT) [1][2][3]5,7,9], ground penetrating radar (GPR) [8,10], seismic [2,[4][5][6], magnetic [14][15][16][17] and gravity [18] methods. Archae-geophysical prospections are particularly common in applications for the detection of buried structures, tombs, and channels through the application of electromagnetic methods [19,20], magnetometry [21], ERTs [22,23], GPR [24,25] or the combination of them.…”