We are thankful to have received approval to do limited field work as Berkeley Lab continues working under COVID-19 requirements. As an example, last month research scientist Verónica Rodríguez Tribaldos along with Michelle Robertson and Todd Wood of the Geosciences Measurement Facility deployed a distributed acoustic sensing (DAS) system on a “dark fiber” at a telecommunication Intermediate Line Amplification hut in Calipatria, California. The purpose is to explore the potential for DAS in characterizing geothermal systems at the basin scale.
The project, a collaboration across several Berkeley Lab divisions (Energy Geosciences, Computing Sciences, and ESnet), universities (Rice University, UC San Diego), and national laboratories (Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory) is particularly targeting so-called “hidden” geothermal resources which cannot be observed directly at Earth’s surface.
Verónica, a geologist and geophysicist by training, applies expertise in DAS and classic seismic techniques to advance imaging of Earth’s subsurface. She talked with us about the work in Calipatria and how it can make an impact on the sourcing of clean energy from underground geothermal reservoirs. With better understanding of the subsurface, scientists can improve technologies for extracting geothermal energy from deep underground — which has the potential to power 100 million American homes.
Q: What is dark fiber, and what does it have to do with your fieldwork?
A: This fieldwork was for our project “Imperial Valley Dark Fiber Project,” funded by the Department of Energy Geothermal Technologies Office (GTO). The main objective of this project is to explore the use of fiber optic sensing, and in particular distributed acoustic sensing (DAS), to characterize geothermal systems at the basin scale in Imperial Valley, Southern California.
DAS is a novel, photonic-based sensing technology that repurposes standard telecommunication fiber-optic cables as arrays of thousands of strain-rate sensors, enabling measuring seismic vibrations at spatial samplings of 1 m or less along distances of several 10s of kilometers. DAS systems can be deployed on so-called “dark fibers” which are existing, telecommunication fiber-optic cables that are not being used for data transfer. By connecting these fibers to a DAS system, they can be repurposed as sensing arrays, enabling efficient acquisition of high-resolution seismic data.
The fieldwork that we conducted in the Imperial Valley consisted of deploying a DAS system on one of these “dark fibers” at a telecommunication Intermediate Line Amplification (ILA) hut in the town of Calipatria. This dark fiber runs near an active geothermal area, and crosses several tectonically-interesting areas with structures that potentially control the geothermal system. The array will be used to continuously acquire high-resolution seismological and geodetic data for several months, which can help characterize these subsurface structures as well as surface deformation associated with the active geothermal area.
Q: What exactly are you looking to measure using DAS?
A: The purpose of this experiment is to use DAS deployed on dark fiber to record ambient seismic noise generated by vehicles, trains, etc. that can be analyzed to build high-resolution subsurface velocity models. These models can be used to characterize the tectonic features linked to the geothermal system, and can also help identify anomalies related to “hidden” resources that do not have a surface expression. We will also be recording local and regional earthquakes linked to tectonic processes and possibly geothermal operations. We are also hoping that the same DAS data can be analyzed to map surface deformation associated with the geothermal system.
Q: How extensive of an area are you hoping to assess?
A: The reach of our system is about 28 km from Calipatria southwards, crossing the Brawley Seismic Zone and running very close to the North Brawley Geothermal Power Plant. We are recording data at a spatial sampling of 4 meters, which gives us over 6,000 measurement points along the array.
Q: What is the real-world impact of the research?
A: At present, large portions of Western basins associated with geothermal resources remain poorly understood. One of the main reasons is the challenges associated with acquiring high-resolution geophysical data at regional scales, including high costs of active seismic surveys and long-term deployments and the limited coverage of dense arrays. The sparse spacing of permanent seismic stations also result in a high minimum seismic magnitude threshold for detecting natural and induced seismicity associated with hidden and existing geothermal fields. These factors can result in a lack of understanding of regional structures relevant to geothermal prospecting, as well as in missed resources.
DAS is a very promising technique for high-resolution, basin-scale imaging, as it enables acquisition of seismic data along a single fiber out to 30 km at spatial densities of a few meters, providing over 15,000 measurement points. Moreover, using dark fibers eliminates the challenges of sensor deployment and provides a cost-effective way of acquiring regional, high-resolution seismic datasets. Our study area in Imperial Valley, combines a complex geologic context with extensive existing and hidden geothermal systems, which makes it an excellent site to test this technology. A key target in our project is to use DAS on dark fiber to image highly faulted zones which might provide conduits for deep hydrothermal fluid migration, and zones of lower seismic velocity at depth that could provide a signature for hidden reservoirs. Moreover, we intend to process our passive seismic data to investigate microseismicity associated with these systems. By the end of the project, we hope to have a better understanding of this active geothermal area and of the utility of DAS and dark fiber as a tool for geothermal exploration and monitoring.