Advanced three‐dimensional seismic imaging of deep supercritical geothermal rocks in Southern Tuscany

Abstract

A B S T R A C TThe seismic K-Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K-Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal-to-noise ratio. Our study aims to reveal high-resolution seismic images of the K-Horizon below a geothermal field in Southern Tuscany, using an advanced threedimensional seismic depth imaging approach. The key seismic pre-processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre-Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three-dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal-to-noise ratio data set, the migration results with the heterogeneous three-dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K-Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K-Horizon with better reflection coherency and higher signal-to-noise ratio than standard Kirchhoff Pre-Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K-Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K-Horizon.

Three-dimensional reflection seismic imaging of the iron oxide deposits in the Ludvika mining area, Sweden, using Fresnel volume migration

“…Subsequently, we applied FVM as an extension of KPSDM that limits the migration operator to the Fresnel volume around back-propagated rays and focuses the image on the physically contributing part of the two-way travel time isochrone (Lüth et al, 2005;Buske et al, 2009). FVM was applied successfully to prior hard-rock reflection seismic data in 2D and 3D (e.g., Hloušek et al, 2015b;Riedel et al, 2015;Hloušek and Buske, 2016;Jusri et al, 2018;Bräunig et al, 2019), including mineral exploration Singh et al, 2019). A key point in 3D FVM is the 3D slowness estimation from the recorded data.…”