3D Imaging of Geothermal Faults from a Vertical DAS Fiber at Brady Hot Spring, NV USA

Abstract

In March 2016, arguably the most ambitious 4D (3D space + over time) active-source seismic survey for geothermal exploration in the U.S. was acquired at Brady Natural Laboratory, outside Fernley, Nevada. The four-week experiment included 191 vibroseis source locations, and approximately 130 m of distributed acoustic sensing (DAS) in a vertical well, located at the southern end of the survey area. The imaging of the geothermal faults is done with reverse time migration of the DAS data for both P-P and P-S events in order to generate 3D models of reflectivity, which can identify subsurface fault locations. Three scenarios of receiver data are explored to investigate the reliability of the reflectivity models obtained: (1) Migration of synthetic P-P and P-S DAS data, (2) migration of the observed field DAS data and (3) migration of pure random noise to better assess the validity of our results. The comparisons of the 3D reflectivity models from these three scenarios confirm that sections of three known faults at Brady produce reflected energy observed by the DAS. Two faults that are imaged are ~1 km away from the DAS well; one of these faults (middle west-dipping) is well-constructed for over 400 m along the fault’s strike, and 300 m in depth. These results confirm that the DAS data, together with an imaging engine such as reverse time migration, can be used to position important geothermal features such as faults.

Investigation of the Time-Lapse Changes with the DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry

“…It is weaker than the first two signals. The source of this signal can be a reflection from nearby faults or bedding planes [26,48]. This the most likely case that we can have an up-going wave here.…”

“…Patterson et al [24] and Patterson [25] analyzed the borehole DTS and pressure data at different stages of operations. Trainor-Guitton et al [26] imaged features on two nearby steeply dipping faults using a portion of the DASV data. Miller et al [15] investigated the DASV data to find the signatures of earthquakes, vibroseis sweeps, and responses to different borehole processes.…”

Investigation of Time-Lapse Changes with DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry

“…Hence, we assume the pressure measurements can represent the co-located pressure changes with the DASV and DTS at the measured depth of 219 m. Figure 2 shows the evolution of the pressure and temperature, and an overview of the DASV DC values and root-mean-square (RMS) amplitudes. We focus on the eight days (18)(19)(20)(21)(22)(23)(24)(25)(26) where the DASV was actively recording. Initially on 18 March, the pressure dropped drastically due to the well resuming operation after a shutdown period (yellow to blue shade in Figure 2a).…”

“…Patterson et al [23] and Patterson [24] analyzed the borehole DTS and pressure data at different stages of operations. Trainor-Guitton et al [25] imaged features on two nearby steeply dipping faults using a portion of the DASV data. Miller et al [14] investigated the DASV data to find the signatures of earthquakes, vibroseis sweeps, and responses to different borehole processes.…”

Investigation of the Time-Lapse Changes with the DAS Borehole Data at the Brady Geothermal Field Using Deconvolution Interferometry

“…It is weaker than the first two signals. The source of this signal can be a reflection from nearby faults or bedding planes [26,47]. This the most likely case that we can have an up-going wave here.…”