Imaging the Earth with Distributed Acoustic Sensing: 3D Migration Geothermal and a MASW example

Distributed acoustic sensing (DAS) provides an alternative for measuring ground vibration from the conventional nodal devices. The DAS system, consisting of the interrogator and fiber optic cable, measures perturbations in the subsurface via phase differences of returned light signals, which can be related to the strain along the fiber. Placing the fiber in the subsurface can therefore measure seismic vibrations for exploration purposes. DAS can provide adequate resolution in both time and space for seismic exploration, while the relatively low cost of the fiber-optic environment deployment allows for long-term recordings over large distances for seismic surveys. The first example describes how vertical DAS recordings were able to measure shear waves reflecting off previously modeled faults. Brady Natural Lab is a geothermal reservoir that has numerous faults that have been previously mapped and modeled in 3D. In March 2016 at Brady, a continuous active seismic survey collected 191 3-mode vibe points, while a vertical DAS (distributed acoustic sensor) cable was in place 150 to 280 meters below surface. Reverse-time migration of both synthetic and the DAS field data was performed to assess which fault dips and strikes would be detectable given the shot geometry. The second story describe the 2-D orthogonal distributed acoustic sensing (DAS) array that was buried horizontally beneath the Kafadar Commons geophysical lab on the Colorado School of Mines campus at Golden, Colorado. The DAS system using straight fiber-optic cables is a cost-efficient technology that enables dense seismic array deployment for long-term seismic monitoring and favors both earthquake-based and ambient-noise-based surface wave analysis for near-surface characterization. The horizontally orthogonal DAS array recorded ambient noise data for a period of about two months from November 2018 to January 2019. During this time, the array detected seismic signals from an M3.6 earthquake at Glenwood Springs, Colorado, which exhibit opposite signal polarities in orthogonal DAS cable recordings. Multimodal Monte Carlo inversion of the earthquake-based Rayleigh wave and Love wave dispersion measurements and the noise-based Rayleigh wave measurement reveals a 1-D layered structure that agrees qualitatively with geologic surveys of the site. Our study demonstrates that although straight fiber-optic cable lacks broadside sensitivity, using appropriate DAS array configuration and seismic array method can extend the seismic acquisition ability of DAS and enable its application to a broad range of scenarios.

Speaker Bio

Whitney Trainor-Guitton

Whitney Trainor-Guitton graduated from Colorado School of Mines in 2001 with a bachelor’s degree in geophysical engineering. From 2002 to 2004, Whitney served in the Peace Corps in the Republic of Panama, advocating sustainable agricultural techniques, advising entrepreneurial endeavors by women’s groups, and promoting HIV/AIDS awareness. In 2006, Whitney completed her master’s degree in geophysics from Stanford University. She continued in Stanford’s Program of Earth, Energy and Environmental Science, of which she received her PhD in June of 2010. For her dissertation, she developed transferable value of information (VOI) methodologies for spatial earth problems. She worked as a risk analyst at Risk Management Solutions, and then a staff scientist at Lawrence Livermore National Laboratory for 4 years, on quantifying the reliability of geophysical measurements for monitoring of CO2 sequestration and geothermal exploration. In August of 2015, Whitney returned to the Mines Geophysics Department as an assistant professor, where she works with machine learning and geostatistical techniques for integrating data stochastically.