Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling

Abstract

S U M M A R YSome of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10 km and geological and seismic information.We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalies.

Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)

“…We have analysed Butterworth filtering using cut-off wavelengths between 20 and 120 km, representing superficial and deep density variations, respectively ( Figure 6). According to a recent sensitivity study in a similar geological environment (Abdelfettah et al 2014), we may estimate the deep structures appearing at the applied longwavelength filters to be located in about 6-to 8-km depth; the 10-to 20-km filter typically indicates superficial often Quaternary deposits by negative anomalies. The negative residual anomalies with typical minima up to −2 mgal and locally up to < −3.5 mgal are located in the northwest, in the Buntsandstein formation of the Pfälzer Wald, directly to the east of the western Main Boundary fault (AN1a), to the east of the westernmost W-dipping fault (AN1b) within the URG and in the central part of the study area (AN2 to AN4; Figure 6a).…”

“…The interpretation of geophysical data in terms of basement structures and hydraulic properties requires a sound knowledge on possible lithological changes (Abdelfettah et al 2014;Denlinger and Kovach 1981). In this respect, in the geological setting, we will focus on both the basement lithology and the structural setting of the area of investigation.…”

“…The presented study area is centred in the extended area. A detailed sensitivity study on the potential of preferential filtering is presented (Abdelfettah et al 2014). Gravity forward modelling was carried out using Pohanka's algorithm (Pohanka 1988) applied to a finite element model.…”

Tectonics of the central Swiss Molasse Basin: Post‐Miocene transition to incipient thick‐skinned tectonics?

“…These sediments are characterized by a transparent to chaotic seismic facies (Figures a and b). The overall geographical distribution of Permo‐Carboniferous sediments (Figure ) is in good agreement with other studies based on gravity forward modeling [ Abdelfettah et al ., ] and seismic interpretation [ Pfiffner et al ., ; Meier , ], which suggest up to 4 km deep and predominantly WSW‐ENE to SSW‐NNE striking PCTs. Note that since only well Pfaffnau‐1, which lies outside of the model area, just reaches pre‐Mesozoic basement (i.e., Permo‐Carboniferous and crystalline rocks), the interpretation of PCTs in the study area is noncalibrated and thus of rather conceptual nature.…”

Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben

“…The value ε is an infinitesimal number (ε ≤ 10 −6 ) to avoid some singularities; it represents at the maximum only 1 μGal in the total gravity values. Additional information on the forward modeling and sensitivity analysis as well as the computed data uncertainty can be found in Abdelfettah et al (2014). The possible gravity effect caused by geothermal utilization is assessed using 3D forward modeling and then the misfit is computed between before and after geothermal events (e.g., hydraulic stimulation, production, water injection, etc.).…”