Project Details
Projekt
Synergy of passive seismic methods for geothermal applications (SynPaTh)
Applicant
Dr. Katrin Löer
Subject Area
Geophysics
Term
from 2016 to 2021
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 298820846
Conventional seismic methods used to create images of the subsurface require active sources, which are very costly and not always feasible, for example in urban environments. Analysing the information contained in recordings of ambient seismic noise, passive seismic methods provide a means to monitor the subsurface continuously at lower cost and with least environmental impact. The main aim of this project is to establish an explicit link between two such methods, namely frequency wavenumber (fk) analysis and seismic interferometry, in order to enable their mutual application for geothermal reservoir characterization and monitoring. FK analysis extracts backazimuth and slowness of a wave front crossing a seismic array and thus allows one to estimate the location of noise sources and the local velocity structure. Additionally, it provides information on anisotropy parameters that relate to the density and orientation of faults and fractures in the subsurface. This aspect of the method has only been suggested recently and a systematic approach to quantify its potential is yet missing. Indeed, anisotropy plays a crucial role in the characterization of geothermal reservoirs, where faults and fractures act as natural propagation paths for induced fluids between the injection and the production borehole. Exploiting their natural occurrence can reduce the need for creating artificial pathways by hydraulic fracturing. A better understanding of anisotropy in geothermal reservoirs thus supports decision-making regarding the depth and location of boreholes. This project evaluates the suitability of fk analysis to detect anisotropy from ambient noise measurements and determines its sensitivity with respect to changes of anisotropic properties. Using synthetic data, acoustic laboratory data, and data from seismic networks, I will specify the conditions under which the method is most feasible. A second rather novel technique used to interpret ambient noise is seismic interferometry. Similar to fk analysis it relies on the cross-correlation of ambient noise wavefields recorded at different receivers to infer information about the subsurface. As a result it provides an estimate of the Green's function (the impulse response of the medium) between two receivers. In this project I will describe the advantages of both methods and how they complement each other. Finally, a combination of both methods will be used to obtain the first anisotropy measurement at the International Geothermal Center in Bochum from an ambient noise data set. Although passive methods will not be able to compete against the accuracy of active methods, I will show that they provide a reasonable estimate of subsurface structures that can help to conduct prospective active surveys more efficiently and more economically, facilitating applications also in sensitive environments.
DFG Programme
Research Grants
International Connection
United Kingdom, USA
