[Any questions? Contact Jonathan at email@example.com]
Team Members :Cody Rotermund (ESnet/LBNL), Inder Monga (ESnet/LBNL), Dennise Templeton (LLNL), Rob Mellors (Scripps/UCSD), Eric Matzel (LLNL), Christina Morency (LLNL), Michelle Robertson (LBNL), Bin Dong (LBNL), Avinash Nayak (LBNL)
Lawrence Berkeley National Laboratory, Energy Geosciences Division
Lawrence Berkeley National Laboratory, ESnet
Rice University, Department of Earth, Environmental, and Planetary Science
Lawrence Livermore National Laboratory
Project in a nutshell : We are exploring the use of fiber optic sensing, particularly distributed acoustic sensing (DAS), to seismically characterize geothermal systems at the basin scale in California’s Imperial Valley. We plan to use fibers that are part of the existing telecom network (“dark fiber”) to record DAS data near an active geothermal system and build subsurface models using ambient seismic noise. We will also use the same DAS network to record local and regional seismicity associated with both tectonic processes and geothermal operations. Lastly, we will attempt to use two related methods, distributed temperature sensing (DTS) and distributed strain sensing (DSS), to map deformation and heat flow associated with these systems. We hope to leverage this combination of new technology and existing infrastructure to improve our understanding of hidden geothermal systems in the western US.
The Challenge At present, large portions of western basins relevant to geothermal energy production are poorly mapped using classical high-resolution geophysical methods due to the high costs of seismic surveys, the restricted availability of archival seismic lines in the same regions, and limited coverage by dense arrays required for ambient noise imaging. The sparse spacing of permanent seismic stations also generates a high minimum seismic magnitude threshold for detecting seismicity associated with hidden systems as well as small events from existing fields. These factors likely result in both missed prospects as well as restrictions in understanding regional geological frameworks relevant to geothermal prospecting (Dobson, 2016). The Imperial Valley is one such region and combines a fascinating geologic context with extensive existing and hidden geothermal resources. One particularly interesting area is the Brawley Seismic Zone (BSZ), an on-shore spreading center which both hosts geothermal production and has several undeveloped prospects. A key component of understanding these systems is mapping subsurface reservoirs, transmissive fault zones, and local variations in thermal state.
Distributed fiber optic sensing is a family of techniques that utilize standard telecom fiber-optic cables and laser scattering to make measurements of the physical parameters including temperature, static strain, and most recently seismic wavefields. The last approach, referred to as distributed acoustic sensing (DAS) , is revolutionizing seismic acquisition in a variety of contexts (e.g. Daley et al. 2013, Dou et al. 2017, Lindsey et al. 2017, Ajo-Franklin et al. 2019); a single DAS acquisition system is now capable of recording seismic data at over 15,000 locations on a single fiber, out to 30 km. The limitation of such recording techniques is the need for fiber optic installation, a potentially costly process, particularly for basin-scale studies. Once fibers for DAS recording are available, they can be utilized for a variety of purposes including seismic imaging using ambient noise as well as monitoring for local and regional seismic events.
Dark Fibers are components of the existing telecom infrastructure which are not currently used for transmitting data. We propose leveraging advances in DAS to record continuous seismic data on several dark fiber transects in the Imperial Valley spanning the BSZ. In addition, we will work on developing paired processing approaches to transform these datasets into products useful for geothermal system characterization and mapping. LBNL has recently demonstrated the utility of dark fiber for DAS recording on a transect in the Sacramento Basin; as part of a recently completed project, we recorded ~300 TB of passive data, sampled at 2m, and utilized it for both surface wave inversion as well as recording regional and teleseismic earthquakes. We will also evaluate using dark fiber for measuring regional near-surface heat flow using a DTS interrogator across the same profile. One advantage of using permanently installed fiber for regional heat flow mapping is the capacity to record for long durations; this allows for the capture and subtraction of diurnal and seasonal anomalies related to the surface boundary condition. The limitation of such an approach is the existing geometry of the fiber transects; while they cross several relevant basins, they may not be in close proximity to every site.
The Plan : As part of this project, we will (a) adapt existing techniques for ambient noise seismic imaging to DAS, (b) plan and execute a multi-month dark fiber + DAS recording effort across the BSZ and in close proximity to existing geothermal facilities, (c) analyze the resulting datasets to extract structural information and validate against existing secondary datasets. Along the way, we will also work on building a network of more traditional sensors to better evaluate our results and improve our understanding of the basin. The key imaging targets are highly faulted zones which might provide conduits for deep hydrothermal fluid migration and zones of lower Vs and/or Qs at depth which might provide a signature of existing reservoirs. We will also process our passive data for microseismic events related to both natural and induced processes. Finally, we will record distributed temperature data on the same dark fiber cables to better understand heat flow variability across our measurement profile. When we complete the project, we hope to have a better understanding of the utility of dark fiber acquisition for geothermal exploration and monitoring.
Getting Involved: We are looking for partners/educators in the community (Brawley, Calipatria, El Centro, and surrounding) interested in participating in the project and learning about the technologies we are developing. In particular, we are looking for community members interested in hosting instruments (e.g. seismometers), helping during field deployments, providing access to existing subsurface data, or providing access to existing shallow groundwater wells for monitoring. All of the data we collect (excepting commercial data) will be made available in stages after publication on the Geothermal Data Repository . This is an opportunity to learn more about both the clean geothermal resources of the region as well as the geology of the Salton Trough and the Brawley Seismic Zone. If you are interested, please contact Jonathan (firstname.lastname@example.org) and/or Pat Dobson (PFDobson@lbl.gov).
Ajo-Franklin, J.B., Dou, S., Lindsey, N.J., Monga, I., Tracy, C., Robertson, M., Ulrich, C., Freifeld, B., Daley, T., and X.S. Li, 2019, “Using Dark Fiber and Distributed Acoustic Sensing for Near-Surface Characterization and Broadband Seismic Event Detection”, Scientific Reports, Vol 9, No. 1, pp. 1328-1339.
Daley, T.M., B.M. Friefeld, J. Ajo-Franklin, S. Dou, R. Pevzner, V. Shulakova, S. Kashikar, D. Miller, J. Goetz, J. Henninges, and S. Lueth, 2013, “Field testing of fiber-optic distributed acoustic sensing (DAS) for subsurface seismic monitoring”, The Leading Edge, Vol. 32, No. 6, pp. 699-706.
Dobson, P.F. , 2016, “A review of exploration methods for discovering hidden geothermal systems”. Geothermal Resources Council Transactions 40, 695–706.
Dou, S., Lindsey, N., Wagner, A.M., Daley, T.M., Freifeld, B., Robertson, M., Peterson, J., Ulrich, C., Martin, E.R., and J.B. Ajo-Franklin, 2017, “Distributed Acoustic Sensing for Seismic Monitoring of the Near Surface: A Traffic-Noise Interferometry Case Study”, Scientific Reports, Vol. 7, No. 1, pp. 11620-11628.
Lindsey, N.J., Martin, E.R., Dreger, D.S., Freifeld, B., Cole, S., James, S.R., Biondi, B.L., and J.B. Ajo-Franklin, 2017, “Fiber-optic network observations of earthquake wavefields”, Geophysical Research Letters, Vol. 44, No. 23, pp. 11792-11799.