Analysing Seismic Anisotropy in a Geothermal Field from a Three-Component Nodal Array using Beamforming

Geothermal energy is an essential low-carbon energy resource for the energy transition. The subsurface characterization of geothermal fields is vital for the expansion of this resource. Often, faults and fractures provide the secondary permeability required for hydrothermal circulation. Anisotropy in the ambient seismic noise wavefield is a novel method for constraining these faults and fractures within geothermal settings at an inexpensive cost.

We use the code package B3AM to perform three-component (3C) beamforming of ambient noise. It is an array-based method which extracts the polarisation, azimuths and phase velocities of coherent waves as a function of frequency from ambient seismic noise, providing a comprehensive understanding of the seismic wavefield. B3AM can be used to determine surface waves and their corresponding velocities as a function of depth and the direction of propagation of waves. Through previous studies, from real and numerical examples, a relationship between anisotropic velocities and present faults has been inferred from real and numerical examples, conveying the depth of permeability necessary for hydrothermal flow within a geothermal field.

The St Austell granite in Cornwall is one of the hottest granite plutons in the UK and, thus, has become a promising source of geothermal energy; the Eden Geothermal Project aims to utilise this naturally occurring heat using the fractures within this radiogenic granite. As part of the growing geothermal research in the area, a seismic node array of 450 STRYDE nodes was deployed from November to December 2022 to provide insight into this complex geological area. Utilizing a sub-array of three-component nodal stations, we investigate the anisotropy of the St Austell granite at depth.

Using 3C beamforming on the ambient noise data, we characterize the wavefield and assess Rayleigh wave velocities as a function of azimuth. To get an accurate representation of anisotropy within the geothermal field, anisotropy was corrected for any array effects. Rayleigh wave velocities were calculated based on wavenumbers, and anisotropy estimates were compared to numerical estimations and shear wave splitting results. Preliminary results indicate fast directions of Rayleigh waves, potentially indicating fractures, in the NNW – SSE orientation. Similarly, shear wave splitting analysis found that the fast S-wave polarisation is dominantly North-South, corresponding to fracture orientation from borehole imagery. This work shows that Rayleigh wave anisotropy is promising for characterising geothermal systems during the exploration process